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|
Constructional Project
Project by John Clarke
DC Supply
Protectors
Any one of these three simple, inexpensive
circuits will protect your equipment from
damage due to an incorrectly connected or
malfunctioning power supply. They protect
against a higher than expected voltage or a
reverse polarity supply and have very little
effect on the voltage applied to the device.
M
any devices are powered using a
mains plugpack or power ‘brick’. All
is well if you use the proper supply
and it is wired correctly. However,
damage can occur if the wrong supply
is used or it is miswired, applying
either the wrong polarity voltage or
an excessively high voltage to the item
to be powered.
That is an especially big problem if
you haven’t used the device for many
years, have moved, if you’ve had to
buy a replacement power supply (due
to failure or loss), or someone else is
using it who is not familiar with the
correct supply.
Our Supply Voltage Protectors prevent damage to equipment in the case of
an incorrect input voltage. They switch
off power to the equipment if the input
voltage is too high and prevent current
flow if the polarity is incorrect.
A supply that produces more voltage
than a piece of equipment expects can
damage its internal components. Applying reverse polarity to a circuit can
also irreparably damage parts, such as
ICs and electrolytic capacitors unless
the circuitry already includes reverse
polarity protection.
Such protection (eg, a diode) often
reduces the voltage available to the
device. However, the designs presented
here are different, as they use a Mosfet
instead that loses very little (basically
no) voltage.
Q1 IPP80P03P4L-07
or SUP60061EL-GE3
D
S
K
ZD1
G
15V
A
OVERVOLTAGE
ADJUST
VR1
50kW
3.9kW
REF1
TL431
+
10kW
3.9kW
A
SC
Ó2025
TL431
1N4148
K
A
REF
A
E
l
K
K
+
POWER
LED1
–
2.2kW
G
K
S
ZD 2
CON2
Q2
IPP80N06S4L OR
FDP027N08B-F102
13V
A
K
1N4148
K
Q4
BC546
10kW
A
K
B
K
A
Q1 , Q2
Q3 , Q4
LEDS
ZD1,ZD2
A
K
C
1 0 kW
B
D1
A
REF
l
D
A
Mirror Version of TL431
A
2.2kW
C
K
2.5V
TL431
10kW
Q3
BC556
10kW
CON1
REF
E
B
OUTPUT
A
REF
–
K
LED2
1 0 kW
INPUT
10kW
OVERVOLTAGE
100nF
E
C
G
D
D
S
SUPPLY PROTECTOR (ADJUSTABLE THROUGH-HOLE VERSION)
Fig.1: the adjustable TH version uses Mosfet Q1 for reverse-polarity protection and Q2/REF1 for over-voltage protection.
46
Practical Electronics | May | 2025
DC Supply Protectors
These Protectors can be built as
standalone devices. Still, as they are
relatively compact and inexpensive,
they should ideally be installed within
existing equipment. That way, nothing bad should happen unless you
connect a supply that’s outside the
usual ratings.
We are presenting three versions
of the circuit. Two have a trimpot adjustment to set the overvoltage protection threshold; both can be used with
plugpacks that produce up to 27V DC.
One of those two versions uses
surface-mounting parts, so it is smaller than the other versions and can be
squeezed into tighter spaces. It is rated
to handle up to 3A. This version can
have the overvoltage set as low as 3V.
The through-hole equivalent can handle
more current, up to 7A. However, it
needs at least 5V to operate.
The third version is slightly cheaper to build but can only be adjusted
in voltage steps determined by zener
diode reverse breakdown or avalanche
values. It can handle up to 50V. One advantage of this version is that it doesn’t
require a setup procedure; you simply
build it, install it and away you go. Its
minimum overvoltage protection setting is 7.5V.
Circuit details
Figs.1-3 show the circuits of all three
versions. They all provide reverse-
polarity protection using P-channel
Mosfet Q1. With a correct polarity connection, Q1 conducts initially via the
intrinsic diode within the Mosfet, allowing current to flow and voltage to
appear at the source.
The 10kW resistor then pulls the gate
to ground, so Q1’s channel is switched
on. This allows current to bypass the
intrinsic diode, greatly reducing the
voltage across it. Zener diode ZD1
prevents the gate-to-source voltage
from exceeding the maximum specification of 10V for the surface-mount
device (SMD) version and 15V for the
through-hole (TH) version.
On the other hand, if the polarity of
the input supply is reversed, Q1’s intrinsic diode is reverse-biased and the
gate voltage is the same as the source.
So the Mosfet remains off, and no current flows to the load.
Adjustable over-voltage protector circuit
Figs.1 & 2 are the adjustable overvoltage protector circuits. The circuit
Practical Electronics | May | 2025
is the same for the TH (Fig.1) and SMD
(Fig.2) versions of the board, except
that some parts have different codes/
packages and some ½W TH resistors
are replaced with two parallel ¼W
SMD resistors (where a higher power
rating is required).
This overvoltage protection circuitry comprises N-channel Mosfet
Q2, shunt regulator IC REF1 and bipolar transistors Q3 & Q4. Q2 is usually held on via gate voltage applied
through the 10kW resistor from the
positive supply. In this case, it has a
low resistance between its drain and
source, connecting the ground terminal of CON1 to the negative side of
CON2, so current can flow between
the load and supply.
Zener diode ZD2 protects the gate
of Q2 from excessive voltage. A 10V
zener is used for the SMD version,
while a 13V zener diode is used for
the TH version, reflecting the ratings
of the selected Mosfet types.
Power indicator LED1 is lit by current flow through the 2.2kW resistor
to the negative side of the supply. For
the SMD version, the 2.2kW resistor
is instead two 4.7kW resistors in parallel, in case the supply voltage is at
the higher end of the allowable range.
The TL431 adjustable shunt voltage
reference IC, REF1, detects an overvoltage condition from the input supply.
Fig.4 shows the circuitry within the
TL431, which includes a 2.5V reference, an op amp, an output transistor
and a protection diode.
When used as a voltage reference,
the REF input connects to the cathode, placing the TL431 in a negative
feedback configuration, where it regulates its ‘cathode’ voltage to match its
internal reference voltage. If you want
a higher cathode voltage, a voltage divider is included between the cathode
and anode, with the divided voltage
applied to the REF input.
For our circuit, we instead apply a
voltage to the REF terminal via a resistance. In this case, the TL431 operates in
open-loop mode without any feedback
to maintain the reference voltage. This
arrangement uses the internal op amp
as a comparator, switching the output
transistor off if the input voltage (Vin)
is lower than the reference voltage, or
on otherwise.
When the transistor is off, the cathode connection is pulled to the supply
voltage via Rsupply. In contrast, when the
transistor is switched on, the cathode
Features & Specifications
Adjustable through-hole version
● Overvoltage protection
threshold: 5-27V (3-27V if SMD
TL431 is used)
● Input voltage range: 5-27V
● Maximum current: 7A
Adjustable SMD version
● Overvoltage protection
threshold: 3-27V
● Input voltage range: 3-27V
● Maximum current: 3A
Fixed through-hole version
● Overvoltage protection
threshold: 7.5-47.7V
● Input voltage range: 5-50V
● Maximum current: 7A
Kits from Silicon Chip
Adjustable SMD Version (SC6948,
£10.50 + P&P): includes the PCB and
all onboard parts.
Adjustable TH Version (SC6949,
£13.50 + P&P): includes the PCB and
all onboard parts with both SOT-23 &
TO-92 TL431 ICs.
Fixed TH Version (SC6950, £12 +
P&P): comes with the PCB and all
onboard parts except ZD3 and R1-R7.
47
Constructional Project
Fig.2: the SMD adjustable version of the circuit is very similar to the one shown in Fig.1. The main differences are the
use of alternative devices and the doubling of some resistors for increased power handling.
connection is held close to the anode
voltage of 0V.
The state of the output, whether high
or low, depends on the voltage applied
to the REF input. In our circuit (Fig.1
or Fig.2), this voltage is from the divider connected across the input supply
formed by trimpot VR1 and a 3.9kW
fixed resistor (or two 7.5kW resistors
in parallel, giving 3.75kW).
When this divided (reduced) voltage is below the 2.5V reference, the
cathode of REF1 is pulled high. When
the divided voltage is above 2.5V, the
cathode is pulled low, near to ground
potential.
Trimpot VR1, in conjunction with
the 3.9kW resistor, sets the overvoltage threshold. When the threshold
is reached, the cathode of REF1 goes
low, so transistor Q3 switches on. It in
turn switches on transistor Q4, which
pulls the gate of Mosfet Q2 low to
switch it off.
In this condition, the high collector
voltage of Q3 pulls the adjust terminal
of REF1 higher again via diode D1 and
the 10kW resistor. This provides voltage hysteresis, ensuring that REF1’s
cathode remains low until the supply
voltage drops significantly below the
overvoltage setting.
The 100nF capacitor between the
base and emitter of Q3 is included
to prevent the circuit from initially latching into a voltage overload
state at power-up. Immediately after
power is applied to CON1, REF1 would
momentarily conduct current that
would otherwise switch on Q3 and
latch REF1 on if it were not for the
capacitor momentarily holding Q3
off.
LED2 is the overvoltage indicator
and it lights under two conditions. If
the input supply exceeds ZD2’s breakdown voltage, current will flow through
LED2, its 2.2kW series resistor and ZD2.
However, if the overvoltage threshold
is set below ZD2’s breakdown voltAn enlarged photo
of the underside of
the Adjustable SMD
version of the DC
Supply Protector.
Compared to the
other versions,
this one has
components
mounted on both
sides of the PCB.
48
age, LED2 will only light when there
is an overvoltage shutdown, via NPN
transistor Q4.
Overvoltage shutdown is indicated
when LED2 is on and LED1 is off. When
reverse polarity protection is active,
both LEDs will be off despite the input
power supply being switched on.
TL431 limitations
One thing to note when using the
TL431 in the TO-92 through-hole
package is that the threshold between
switching high or low is closer to 2V
than 2.5V. The likely reason is that
the reference requires a minimum
current to produce the 2.5V reference,
which is only available in closed-loop
mode. In open-loop mode, the reference is operating further down the
threshold knee of the voltage versus
current curve.
This threshold also varies with temperature, although provided the temperature does not vary over a wide
range, the resulting accuracy will be
satisfactory. For more information on
using the TL431 in open-loop mode
for undervoltage and overvoltage detection, see the Texas Instruments Application Report SLVA987A PDF at
www.ti.com/lit/pdf/slva987
The SMD version of the TL431 does
not appear to suffer the same problem,
as it shows a very sharp voltage-versuscurrent threshold voltage curve even
at very low currents. For this reason,
the TH PCB has provision for using
Practical Electronics | May | 2025
DC Supply Protectors
Q1 IPP80P03P4L-07
or SUP60061EL-GE3
D
S
K
K
ZD1
15 V
G
A
ZD3
A
R3
R4
R5
R6
A
10kW
–
Q3
BC546
K
CON1
D
B
A
SCR1
C106D
R2
A
K
G
LEDS
K
CON2
10k W
E
150 W
ZD1,ZD2, ZD3
–
R7
R1
C
+
l
LED1
9. 1k W
+
SC
LED2
OUTPUT
A
POWER
K
INPUT
Ó2024
l
OVERVOLTAGE
K
10nF
G
S
ZD2
13V
K
Q2
IPP80N06S4L OR
FDP027N08B-F102
A
470W
SUPPLY PROTECTOR (FIXED OVERVOLTAGE)
B
E
G
C
Q 1, Q 2
C106D
BC546
A
A
G
K
D
D
S
Fig.3: the fixed overvoltage version requires you to select values for resistors R1-R7 depending on the threshold voltage
you want; see Table 1 overleaf. It uses an SCR and zener diode for the over-voltage function rather than a TL431.
the surface-mount version instead of
the TO-92 package version.
Fixed overvoltage
protector circuit
Fig.3 is the fixed overvoltage protector circuit. This circuit includes
reverse polarity protection using Pchannel Mosfet Q1 in the same way
as Figs.1 & 2. The overvoltage protection also uses N-channel Mosfet Q2,
although Q2 is controlled differently
in this circuit.
Instead of an adjustable overvoltage threshold controlled by a TL431
shunt reference IC and trimpot, the
threshold is set and detected by zener
diode ZD3. If the voltage applied to the
zener is above the overvoltage threshold and it conducts, silicon-controlled
rectifier SCR1 is triggered to switch off
Mosfet Q2.
A 10nF capacitor is included across
the SCR to prevent it from latching on
due to a rapid rise in voltage (dV/dt)
as power is initially applied to CON1.
Any voltage rise faster than 8V/μs will
likely switch the SCR on. The 10nF
capacitor slows down the voltage rise.
Mosfet Q2 is normally held on via
the gate voltage applied by the 10kW
gate resistor and paralleled resistors
R3-R6. Zener diode ZD2 protects the
gate from excessive voltage. With Q2
on, a low-resistance connection exists
between the drain and source, connecting the ground of CON1 to the negative side of CON2.
Practical Electronics | May | 2025
In that case, the power LED (LED1)
lights due to the current flow through
R7 to the negative side of the supply.
Transistor Q3 is typically switched
on by the bias current from the positive supply via resistors R3-R6 and
R1. With Q3 on, current can flow
through ZD3 at its collector and the
150W resistor at its emitter, but only
if the supply voltage exceeds ZD3’s
breakdown voltage.
4mA needs to flow through ZD3
before the voltage across the 150W resistor reaches 0.6V, which is just sufficient to trigger SCR1 via its 470W gate
resistance.
Thus, when the SCR switches on and
disconnects the load, the supply voltage is ZD3’s rated breakdown voltage
plus the 0.6V required across the 150W
resistor. When SCR1 latches on, there
is about 1V between its anode (A) and
cathode (k), so Q3 switches off.
With SCR1 on, the low voltage at
Q2’s gate switches it off, disconnecting
the ground supply at CON2. LED1 is
now off, while the low voltage across
SCR1 causes LED2 to light, with current flowing through the 9.1kW resistor to the switched-on SCR.
Note that LED2 will also light when
the voltage across ZD2 reaches its breakdown of 13V. As the supply voltage
rises, LED2 brightens as more current
flows through the LED via the 9.1kW
resistor and ZD2. Overvoltage shutdown is indicated when LED2 is lit
but LED1 is off.
The voltage divider formed with
R1 and R2 ensures that Q3’s base is
well below 0.6V, keeping Q3 off when
SCR1 is on. With Q3 off, the gate drive
to SCR1 is off, but the SCR remains
latched on due to the current flowing
through it. Resistors R3 to R6 provide
the required 5mA latching and holding current to ensure it stays on in this
condition.
Fig.4: the basic circuitry
within a TL431 voltage
reference. Usually, the REF
terminal is connected to a
divider between the anode
and cathode (closed-loop
mode). Here, we are using
it in open-loop mode, as a
voltage detector.
49
Constructional Project
Table 1 – resistance values for fixed TH version
13V
13.7V
30kΩ
10kΩ
8.2kΩ
8.2kΩ
6.8kΩ
×
1.2kΩ
If you are wondering why we need
Q3 instead of ZD3 connecting directly in series with the 150W resistor, it
is because ZD3 could be damaged
by excessive current as the supply
voltage rises well above its breakdown voltage. For example, if ZD3
is a 12V zener diode, it will conduct
4mA when the supply is at 12.6V
but 186mA at 40V. In that case, it
would be running well above its power
rating.
Additionally, the 150W resistor
would be dissipating just over 5W.
Having transistor Q3 means that all
this current stops once the overvoltage threshold is reached, preventing
high dissipation in ZD3 and the 150W
resistor.
12V
12.7V
27kΩ
8.2kΩ
6.8kΩ
6.8kΩ
6.8kΩ
×
1kΩ
11V
11.7V
24kΩ
8.2kΩ
5.6kΩ
6.8kΩ
6.8kΩ
×
1kΩ
Zener diode biasing
10V
10.7V
18kΩ
6.2kΩ
6.8kΩ
5.6kΩ
5.6kΩ
×
910Ω
ZD3
Vovl
R1
R2
R3
R4
R5
R6
R7
47V
47.7V
130kΩ
13kΩ
18kΩ
18kΩ
×
×
8.2kΩ
43V
43.7V
110kΩ
13kΩ
16kΩ
16kΩ
×
×
6.8kΩ
39V
39.7V
100kΩ
13kΩ
15kΩ
15kΩ
×
×
5.6kΩ
36V
36.7V
91kΩ
13kΩ
16kΩ
13kΩ
×
×
4.3kΩ
30V
30.7V
75kΩ
13kΩ
12kΩ
12kΩ
×
×
3.0kΩ
27V
27.7V
68kΩ
13kΩ
5.6kΩ
×
×
×
2.4kΩ
24V
24.7V
62kΩ
13kΩ
4.7kΩ
×
×
×
2.2kΩ
22V
22.7V
56kΩ
13kΩ
8.2kΩ
10kΩ
×
×
2.0kΩ
20V
20.7V
51kΩ
13kΩ
8.2kΩ
8.2kΩ
×
×
1.8kΩ
16V
16.7V
36kΩ
10kΩ
10kΩ
10kΩ
8.2kΩ
×
1.3kΩ
15V
15.7V
33kΩ
10kΩ
10kΩ
8.2kΩ
8.2kΩ
×
1.3kΩ
9.1V
9.8V
15kΩ
4.3kΩ
5.6kΩ
5.6kΩ
5.6kΩ
×
820Ω
8.2V
8.9V
12kΩ
4.3kΩ
4.7kΩ
4.7kΩ
4.7kΩ
×
750Ω
7.5V
8.2V
7.5kΩ
2.4kΩ
5.6kΩ
5.6kΩ
5.6kΩ
5.6kΩ
620Ω
6.8V
7.5V
3.6kΩ
1.2kΩ
4.7kΩ
4.7kΩ
5.6kΩ
5.6kΩ
560Ω
White = ½W, yellow = 1W, × = not fitted
Fig.5: a typical V/I curve for a zener diode.
50
The 150W resistor could be increased
in value, but that would mean that the
overvoltage threshold would occur at
a much lower voltage than the zener
diode breakdown voltage. This would
be less consistent than using the zener
at the steeper region of its conduction curve.
Fig.5 shows a typical zener diode
V/I curve. In the forward direction
(current flowing from anode to cathode), it acts like a regular diode, conducting current with 0.6-0.7V voltage across it. In the reverse direction,
the zener initially acts like a diode,
blocking current with minimal leakage current.
However, beyond a certain voltage,
the ‘leakage’ current increases and
then it begins conducting significant
reverse current. This is the reverse
breakdown mode, which provides a
relatively steep VI curve beyond the
knee region.
Each zener diode is characterised at
a particular current for its zener voltage. If the zener diode is operated at a
current much less than that, the voltage across it will also be lower. For
our circuit, we want the zener diode
operating more in the linear region,
where the V/I curve is steep, rather
than in the knee region and preferably near to the reference current for
the zener.
The recommended BZX79Cxx series
of zener diodes for our circuit are characterised for a 5mA reference current
between 2.4V to 24V, or 2mA above
that. The 4mA current for the zener
diode in our circuit is a reasonable
compromise between those.
Practical Electronics | May | 2025
DC Supply Protectors
Resistance value calculations
The resistance values required for
resistors R1 to R7 depend on the overload voltage (Vovl), the maximum input
voltage (Vmax) and the latching and holding current for SCR1. Resistor power
ratings, LED currents, transistor Q3’s
base current and ZD3’s current need
to be considered.
Table 1 shows the resistor values and
wattage ratings for various overvoltage
thresholds and a 50V maximum applied input voltage. A panel describes
the calculations used to formulate that
table in more detail.
SMD adjustable version
The SMD adjustable version is built
using a double-sided plated-through
PCB coded 08106241 that measures 51
× 23mm. As shown in the overlay diagrams (Fig.6), all the SMD parts except
the two LEDs mount on one side of the
PCB, with the through-hole parts such
as the two screw terminals and trimpot on the other side.
Begin by soldering the SMDs. That
can be done by soldering one lead of
the component first, holding it in place
with tweezers. Once it is aligned and
positioned correctly (by remelting
the solder if necessary), the remaining lead(s) are then soldered. A good
light and a magnifying glass are very
useful for this task.
You will need to identify the parts
first. The resistors are marked with
three or four digit codes as shown in
the parts list. The 100nF capacitor will
not be marked. The smaller semiconductors in SOT-23 packages also have
component markings, as per the parts
list (although they can vary).
The 10V zener diodes are cylindrical with blue markings at the cathode
(k) end. Diode D1 also has a polarity
stripe at the cathode end.
Note that the TL431 can have alternative pinouts, with the standard
pinout having the cathode at left and
reference at right when the anode pin
is at the top. The mirrored pinout has
the cathode and reference pins transposed. We have provided for both orientations on the PCB by having a 6-pad
footprint instead of just the three required for one pinout of the device.
The TL431 must be orientated according to the pinout of the device
used. We have marked the pins on the
PCB overlay showing the anode, cathode and reference pads. The parts supplied in our kit should be the mirror
pin version. The way to check this is
to use a multimeter on its diode test
across the cathode and reference pins.
You should get a reading of one
diode drop (around 0.7V) when the
red probe is on the REF pin and the
black probe on the cathode pin. You
can then orientate it correctly on the
PCB and solder it in place. While
doing that, be careful not to let solder
bridge the used and unused pads. If
that happens, use a bit of solder wicking braid can be used to remove the
excess solder (adding flux paste will
make it easier).
When installing the diodes, make
sure these are orientated correctly. The
anode (A) and cathode (k) orientations
are marked on the PCB overlay.
Once all the surface mount parts
have been soldered in place on the
one side, flip it over and fit the LEDs,
taking care to place each with its cor-
rect orientation (checked as mentioned
earlier) and in the correct position
with regard to colour. These are green
for power and red for overvoltage, although you are free to customise the
colours if desired.
Ideally, the surface mount LEDs
should be tested using the diode test
mode of a multimeter. With the red
probe on the anode and black lead
on the cathode, the LED should light
and show its colour. We used green for
power and red for overload. There is
often a stripe or dot on the cathode but
we have seen LEDs with a marking on
the anode, so it’s better to test them.
The trimpot is installed with the top
screw adjustment orientated as shown.
This provides an increasing overvoltage threshold with clockwise rotation.
The two screw terminals should be
mounted with the wire entry toward
the outside of the PCB at each end.
TH adjustable version
The through-hole adjustable version is built on a double-sided plated-
through PCB that’s 08106242 and measures 70.5 × 35.5mm. Refer to Fig.7,
the PCB overlay diagram, during the
assembly process.
If you are using the SMD TL431 version, install it first, but be careful as
they can have alternative pinouts with
the reference and cathode transposed.
See the instructions a few paragraphs
above on determining which pinout
you have, aligning it with the PCB
markings and soldering it.
The zener diodes and diode D1 can
be fitted next. ZD1 is a 15V type, while
ZD2 is rated at 13V. These each need
to be orientated as shown in Fig.7,
Fig.6 (left): the
overlay diagrams
for the SMD
adjustable version
of the Supply
Protector (shown
at 150% actual
size).
Fig.7 (upper right):
the PCB overlay
diagram for the
through-hole
adjustable version.
Fig.8 (lower right):
the PCB overlay
diagram for the
through-hole
fixed overvoltage
version.
Practical Electronics | May | 2025
51
Constructional Project
Resistance value calculations
Table 1 shows the required resistance values and power ratings for the Fixed Protector for
overvoltage thresholds from 7.5V to 47.7V with a maximum input voltage of 50V. There
are no satisfactory resistance values to meet all requirements for overvoltage thresholds below 7.5V, so if you require a threshold that low, build one of the other versions.
R3 to R6 calculations
The total resistance for R3 to R6 is calculated first. This resistance provides current
for SCR1 and the base of transistor Q3 via R1. Up to four resistors can be paralleled for
a sufficient power rating and to achieve the required resistance.
The latching and holding current required for SCR1 to remain in conduction is 5mA.
This satisfies the worst-case latching current and the worst-case holding current at 25°C.
The total resistance, R, required is the overload voltage threshold (Vovl) minus one
volt (the SCR anode-to-cathode on-voltage), divided by 5mA, ie, R = (Vovl − 1V) ÷ 5mA.
The total power rating required is the maximum operating voltage for the circuit (eg,
50V) minus 1V squared and then divided by the resistance, ie, (Vmax − 1V)2 ÷ R.
The required power rating can be reduced by spreading it between two to four resistors in parallel. If all those resistors have the same value, they share the dissipation
equally. If different, each resistor will need to be assessed for its share of the dissipation.
R1 & R2 value calculations
Resistor R1 drives the base of Q3, which must saturate when conducting 4mA. This
4mA is the current that flows through ZD3, Q3 and the 150Ω resistor at the overvoltage threshold. We drive Q3’s base with 250μA (1/16th the collector current) or more to
ensure Q3 goes into saturation.
Resistor R2, between the base of Q3 and ground, is necessary since it reduces the
base voltage to less than 0.3V due to divider action with R1 once SCR1 is latched. Typically, SCR1 will have about 1V across, so provided that R1 is at least triple R2’s value,
that will be reduced to 250mV or less. That prevents Q3 from conducting through ZD3
once overvoltage has been detected and SCR1 latches on.
For overvoltage settings of 20.7V and above, we set R2 so 100μA flows through it at
the overload voltage threshold. At this threshold, there will be 0.66V between the base
and emitter of Q3 and 0.6V at the emitter of Q3, giving a total of 1.26V across R2. For
an approximate 100μA current, R2 needs to be 12.6kΩ (13kΩ is the closest E24 value).
13kΩ gives 96.9μA, close enough to 100μA.
When calculating the value for R1, this 100μA needs to be included since this bypasses
the current from Q3’s base. So, instead of R1 supplying 250μA to Q3’s base, it needs to
supply 350μA in total.
R1 is calculated as the overload voltage threshold (Vovl) minus the 1.26V at Q3’s
base, divided by 350μA. Since R1 is in series with the R3-R6, the parallel value of R3-R6
is then subtracted from this to get R1’s value, ie, R1 = (Vovl − 1.26V) ÷ 350μA − (R3 ||
R4 || R5 || R6).
If the calculated value for R1 is less than three times the value of R2, the current
through R2 needs to be increased and the equations reworked. For example, to get
200μA through R2, R2 = 1.26V ÷ 200μA = 6.3kΩ (use 6.2kΩ). Then R1 = (Vovl − 1.26V)
÷ 450μA, where 450μA is the 200μA R2 current plus the 250μA required for Q3’s base.
LED current
LED1 switches off above the overvoltage threshold, so the maximum LED current will
occur with the supply at the overvoltage setting. Assuming 10mA is a suitable maximum
current, the value for R7 is the overload voltage minus the 2V across the LED, divided
by 10mA, ie, R7 = (Vovl − 2V) ÷ 10mA.
The power rating for R7 also needs to be considered, so its value needs to be greater
than (Vovl − 2V)2 ÷ 250mW, where 250mW is a conservative derating for a 500mW resistor. If this calculation gives a higher value than the above, the maximum LED current will
be below 10mA to avoid overheating the current-limiting resistor.
The overvoltage LED (LED2) series resistor value is calculated similarly; only this time,
the maximum input supply voltage is used in the calculation. That’s because LED2 will
light from the overvoltage threshold to the maximum input supply voltage, Vmax. So the
calculation is Vmax minus the voltage across LED2 and SCR1, divided by 10mA, ie, R =
(Vmax − 3V) ÷ 10mA.
Similarly, the minimum value, considering the resistor power rating, is (Vmax − 3V)2 ÷
250mW. We selected a 9.1kΩ 1/2W resistor for a Vmax of 50V.
52
with the cathode band toward the top.
The resistors can be mounted next;
check each value with a multimeter to
be sure the correct value is installed
in each place.
The two LEDs are installed with the
tops of their domes about 12mm above
the top of the PCB. Check which colour
the diode is before installing it, using
the diode test mode on a multimeter
if the lenses aren’t tinted. We used a
green LED for power (LED1) and red
for overvolage (LED2). In each case,
the longer lead is the anode.
Next, fit transistors Q1-Q4, being
careful that each is placed in the correct position (check their part codes
against Fig.7 and the PCB overlay). If
using the TO-92 package version of
the TL431 (REF1), you can also fit it
now. Follow by mounting the 100nF
capacitor.
The trimpot is installed with the
screw adjustment orientated as shown,
providing an increasing overvoltage
threshold with clockwise rotation. The
two screw terminals are mounted with
the wire entry toward the outside of
the PCB at each end.
TH fixed overvoltage version
The through-hole fixed overvoltage
version is built on a double-sided,
plated-through PCB coded 08106243
that measures 70.5 × 35.5mm. The
PCB overlay diagram for this version
is Fig.8.
First, the values for resistors R1-R7
need to be selected using Table 1, based
on the required overvoltage threshold.
The required voltage rating for ZD3 is
also listed in that table. Note that resistors R3-R6 may need to be 1W types
(if shown in yellow in Table 1), and
not all four of these resistors are necessarily used for all possible threshold voltages.
The zener diodes and diode D1 can
be fitted now. ZD1 is rated at 15V, ZD2
is a 13V type, while ZD3 is as per Table
1. These each need to be orientated as
shown in Fig.8, with the cathode band
toward the top.
The resistors can be mounted next;
check each value with a multimeter
to be sure the correct value is used in
each location.
The two LEDs are installed with
the tops of their domes about 12mm
above the top of the PCB. Check which
colour the diode is before installing it,
using the diode test on a multimeter if
the lenses aren’t tinted. We used green
Practical Electronics | May | 2025
DC Supply Protectors
for the power LED (LED1) and red for
overvoltage (LED2). In each case, the
longer lead is the anode.
Be sure when mounting Q1 to Q3
that each is placed in the correct position and orientation. The SCR goes in
with the metal tab side towards R3-R6.
The trimpot should be installed
with the screw adjustment orientated as shown, providing an increasing
overvoltage threshold with clockwise
rotation. The two screw terminals are
mounted with the wire entry toward
the outside of the PCB at each end.
Testing
If you have an adjustable power
supply, you can apply power to the
input and check that the power LED
lights and the overvoltage switch-off
function operates at the desired voltage. This is preset with the fixed version or can be changed using VR1 for
the adjustable versions.
Once the overvoltage threshold has
been reached, the power LED goes off
and the overvoltage LED lights up.The
supply will need to be switched off or
significantly reduced before power is
restored to the output.
Also remember that the overvoltage
LED may light once the supply voltage exceeds ZD2’s breakdown voltage.
Overvoltage shutdown is indicated
when the power LED (LED1) is off and
the overvoltage LED (LED2) is lit, but
not when both LEDs are alight.
For the adjustable versions, you can
set the overvoltage threshold approximately by measuring the resistance
across VR1 when the power is off.
Divide the VR1 resistance by 3.9kW,
add one, then multiply by 2V if you
used a TO-92 TL431 or 2.5V if you
used the SMD version. The formula
is Vovl = (R(VR1) ÷ 3.75kW + 1) × Vref.
That will tell you roughly what voltage it will cut out at, within about 1V.
For the reverse calculation, to determine what resistance you need across
VR1 for an approximate voltage threshold, divide the desired threshold by 2V
(TO-92 TL431) or 2.5V (SMD TL431),
then subtract one and multiply by
3.9kW (3.75kW for the SMD version)
The formula is R(VR1) = (Vovl ÷ Vref
− 1) × 3.9kW.
To set it more accurately, you will
need an adjustable power supply or
make a basic one using a wirewound
1kW potentiometer connected across
a fixed supply (but be careful not to
exceed its power rating).
PE
Practical Electronics | May | 2025
Parts List – DC Supply Protectors
Common between all versions
2 2-way PCB mount screw terminals with 5mm or 5.08mm spacing (CON1, CON2)
SMD Adjustable Version
1 double-sided, plated-through PCB coded 08106241, 51 × 23mm
1 100nF 50V X7R ceramic capacitor, SMD 3216/1206 size
1 50kΩ multiturn top-adjust trimpot, Bourns 3296W style (VR1)
Semiconductors
1 AO3401(A) 30V 4A P-channel logic-level Mosfet, SOT-23 (Q1; marking: X15V)
1 AO3400 30V 5.8A N-channel logic-level Mosfet, SOT-23 (Q2; marking: XORB)
1 BC856C 65V 100mA PNP transistor, SOT-23 (Q3; marking: 9AC)
1 BC846C 65V 100mA NPN transistor, SOT-23 (Q4; marking: 1C)
1 TL431 adjustable shunt voltage reference, SOT-23 (REF1; marking: 431) 🔴
1 1N4148WS 75V 150mA switching diode, SOD-323 (D1)
2 BZV55-C10 10V 500mW zener diodes, SOD-80C (ZD1, ZD2)
1 green SMD LED, M3216/1206 size (LED1)
1 red SMD LED, M3216/1206 (LED2)
Resistors (all M3216/1206 size 1/4W 1% SMD)
4 7.5kΩ (code 7501 or 752) 4 4.7kΩ (code 4701 or 472)
7 10kΩ (code 1002 or 103)
Through-Hole Adjustable Version
1 double-sided, plated-through PCB coded 08106242, 70.5 × 35.5mm
1 100nF 63V/100V MKT polyester capacitor
1 50kΩ multiturn top-adjust trimpot, Bourns 3296W style (VR1)
Semiconductors
1 IPP80P03P4L-07 or SUP60061EL-GE3 P-channel logic-level Mosfet, TO-220 (Q1)
1 IPP80N06S4L or FDP027N08B-F102 N-channel logic level Mosfet, TO-220 (Q2)
1 BC556 65V 100mA PNP transistor, TO-92 (Q3)
1 BC546 65V 100mA NPN transistor, TO-92 (Q4)
1 TL431 adjustable shunt voltage reference, TO-92 (REF1) OR
1 TL431 adjustable shunt voltage reference, SOT-23 (REF1; marking: 431) 🔴
1 1N4148 75V 200mA signal diode (D1)
1 15V 500mW or 1W zener diode (ZD1)
1 13V 500mW or 1W zener diode (ZD2)
1 3mm green LED (LED1)
1 3mm red LED (LED2)
Resistors (all ½W metal film, 1%)
2 3.9kΩ
2 2.2kΩ
7 10kΩ
Through-Hole Fixed Overvoltage Version
1 double-sided, plated-through PCB coded 08106243, 70.5 × 35.5mm
1 10nF 63V/100V MKT polyester capacitor
Semiconductors
1 IPP80P03P4L-07 or SUP60061EL-GE3 P-channel logic-level Mosfet, TO-220 (Q1)
1 IPP80N06S4L or FDP027N08B-F102 N-channel logic level Mosfet, TO-220 (Q2)
1 BC546 65V 100mA NPN transistor, TO-92 (Q3)
1 C106B 200V or C106D 400V 4A SCR, TO-126/TO-225AA (SCR1)
1 15V 500mW or 1W zener diode (ZD1)
1 13V 500mW or 1W zener diode (ZD2)
1 BZX79Cxx 500mW (2mA or 5mA reference current) zener diode (ZD3)
[See Table 1 for voltage rating]
1 3mm green LED (LED1)
1 3mm red LED (LED2)
Resistors (all ½W metal film, 1%)
1 9.1kΩ
1 470Ω
1 150Ω
2 10kΩ
R1-R7: see Table 1
🔴 TL431QDBZR, TL431FDT or TL431SDT have the standard pinout; TL431MFDT or
TL431MSDT have the mirrored pinout
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