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Main Features •  Adjustable voltage •  Adjustable pulse width •  Adjustable pulse rate (frequency) •  Intermittent (pulsed) or continuous output •  Battery operation for safety Kill pain with: Transcutaneous Electrical Neural Stimulation Do away with analgesics and alleviate pain electronical­ly with a TENS Unit. This device produces pulses of current into elec­trodes placed on the skin adjacent the painful area and has a surprising success rate on most sufferers. The SILICON CHIP TENS unit provides all the necessary features and is considerably cheaper than commercially available units. By JOHN CLARKE 36  Silicon Chip T O BE IN CONSTANT and prolonged pain is a dreadful condi­tion and while analgesics can help, they cannot be used long-term without the risk of kidney and liver damage plus other side effects. The alternative method to pain relief is with the use of a TENS Unit. These are now regularly used to help pain victims with a good success rate. TENS is an acronym for Transcutaneous Electrical Neural Stimulation. This description can be simplified to a method which passes pulses of electrical current through the skin via elec­ trodes to stimulate the nerves below. This stimulation tends to prevent transmission across the nerve junctions and so the brain does not receive the pain sign­al. An alternative suggestion of why the TENS unit works in relieving pain is that the stimulation produces endorphins which are a natural pain killing substance. The effectiveness of TENS is to some extent dependent upon the willingness of the patient to believe that the treatment will work. It is widely used by physiotherapists and certainly has a high success rate on people who approach it as a “high technolo­gy” pain relief method. Fig.1: this scope waveform shows the continuous pulse train across the electrodes. The frequency is 221Hz. How it’s used The SILICON CHIP TENS Unit comprises a medium sized plastic case with several controls on the front panel. The controls adjust the output voltage, the pulse width and the pulse rate (frequency). Two electrodes connect the TENS Unit via a lead and these are placed on the skin adjacent to the painful area. The electrodes are readily available from most pharmacies. The TENS Unit produces high voltage pulses which pass the current between the electrodes via the skin and stimulate the underlying nerves. The controls are generally adjusted until the tingling is just a little too much for comfort. The sensation tends to decrease as time goes on and so the output voltage may need to be gradually increased over the period of one treatment, usually lasting 20 minutes or so. An intermittent control sets the TENS Unit to produce short bursts of voltage once every second rather than a continuously pulsed signal. This mode is useful for long treatment sessions and when the patient has become accustomed to the effect from Fig.2: this is the same pulse train as in Fig.1 but at a faster timebase setting, in this case 500µs/div. As you can see, the pulse amplitude is 80V peak and the width is 190µs. You can adjust the peak voltage down to 12V and the frequency to as low as 2Hz. The pulse width can be altered from 40-200µs. the continuous mode. The accompanying oscilloscope waveforms show the signals that are produced by the TENS Unit. Fig.1 shows the continuous pulse train across the electrodes. The frequency is 221Hz. Fig.2 shows the same wave- form at a faster timebase setting, in this case 500µs/div. As you can see, the pulse amplitude is 80V peak and the width is 190µs. You can adjust the peak voltage down to 12V and the frequency to as low as 2Hz. Pulse width can be al­tered from 40-200µs. August 1997  37 and the load current between Vout and the ground supply. We can maintain a constant Vout for a variety of loads by controlling the amount of time Q1 is switched on. Fig.6 shows the circuit configuration of the switching oscillator which modulates the output voltage of the step-up converter. Heart of the circuit is an IR2155 made by Internation­al Rectifier Corporation in the USA. It is described as a “high side self-oscillating power Mosfet/IGBT gate driver”. It is the ideal device where Mosfets or IGBTs need to be driven in a variety of configurations. Resistor R1 and capacitor C1 at pins 2 and 3 set the oscil­lator frequency and the result is that Mosfets Q1 and Q2 are turned on and off alternately, with a typical “dead time” of 1.2µs between one Mosfet turning off and the other turning on. Fig.3: this scope waveform shows the intermittent pulse output. In this case, the waveform consists of bursts of nine pulses every sec­ond. Fig.3 shows the intermittent mode. In this example, the waveform con­sists of bursts of nine pulses every second but this can be varied. Block diagram The block diagram for the TENS Unit is shown in Fig.4. The 6V supply from the battery is stepped up in the converter com­prising IC1 and T1. This provides a DC output adjustable from below 9V up to 80V, using VR1. The resulting DC voltage is con­verted to a pulsed signal using the switchmode oscillator. VR3 and VR4 set the fre- quency and pulse width respectively. An inter­mittent oscillator comprising IC4 is switched into circuit with S2 to gate the switching oscillator. This gives short bursts of the pulsed signal. Fig.5 shows how the basic step-up converter circuit oper­ ates. It comprises inductor L1 which is charged via transistor Q1 from the V+ supply. The charging current is shown as i1. When the transistor is switched off, the stored energy in L1 is dumped through diode D1 into capacitor C1. The actual voltage across C1 is dependent upon the amount of charge in L1 Diode pump Note that the voltage at the drain (D) of Q1 is greater than the supply voltage for the IR2155. For Q1 to fully turn on, its gate (G) must be raised above the source by several volts. This is achieved using a diode pump consisting of diode D2 and capacitor C2. Initially, the Vcc supply to the IC is set at about 15.6V due to an internal regulator and the current via R2 from Vsupply. In addition, Mosfet Q2 is switched on via a 15.6V signal at pin 5 driving its gate. Capacitor C2 now charges to the 15.6V supply via D2 and the switchedon Q2. When pin 5 goes low, Q2 is turned off and pin 7 is connected internally to pin 8 to switch on Q1. Q1 pulls pin 6 up to Vsupply and pin 8 is shifted to Vsupply plus the 15.6V across C2. So the circuit bootstraps itself up to whatever the Mosfet driving voltage needs to be. Pins 6, 7 and 8 of the IR2155 are floating outputs which can be shifted to 600 volts above the pin 4 ground. In our case we are only using the circuit to switch up to 80V. Circuit details Fig.4: this is the block diagram of the TENS circuit. The 6V supply from the battery is stepped up in the converter comprising IC1 and T1 to provide a DC output of up to 80V. The resulting DC voltage is converted to a pulsed signal using the switchmode oscillator. 38  Silicon Chip The full circuit for the TENS unit is shown in Fig.7. Power from the 6V battery is switched to the circuit via S1 and the 100µF capacitor decouples the supply. IC1 is the switchmode controller. It has a switching transistor at pin 1 and a feedback input at pin 5. The frequency of oscillation rate is Fig.5: this shows how the basic step-up converter circuit works. Inductor L1 is charged via transistor Q1 from the V+ supply. When the transistor is switched off, the stored energy in L1 is dumped through diode D1 into capacitor C1. set by the .001µF capacitor at pin 3 and the current flow through the primary of T1 is limited by the 0.22Ω resistor between pins 6 and 7. Current through T1’s primary is switched off when the voltage across this resistor exceeds about 300mV. Fig.6: this is switching oscillator which modulates the output voltage of the step-up converter. D2 and C2 constitute a diode pump to boost the supply voltage to correctly switch Q1. The voltage induced into T1’s secondary when the primary field collapses charges two 0.47µF capacitors via diode D1. Voltage feedback from VR1 and the 10kΩ resistor into pin 5 and trimpot VR2 sets the output voltage. VR2 is adjusted to give 80V when VR1 is at its maximum resistance. Transformer T1 is used instead of a single inductor, as depicted in Fig.5, for two reasons. Firstly, the maximum voltage allowed at pin 1 (the collector of the switching transistor within IC1) is 40V. Since we want 80V, the Fig.7: the TENS circuit uses IC1, T1 and diode D1 to step up the battery voltage to a maximum of 80V. This is modulated by the switchmode oscillator IC2 and Mosfets Q1 & Q2 to drive the skin electrodes. August 1997  39 Fig.8: the wiring details for the case and PC board. Take care to ensure that all polarised parts are correctly installed. 2.59:1 ratio between the primary and secondary of T1 will ensure that the pin 1 voltage will be only 30.9V. The second reason is so that the primary can provide a supply for the self-oscillating Mosfet gate driver, IC2. 40  Silicon Chip Diode D3 charges the associated 4.7µF capacitor and the voltage across it is limited to +39V by zener diode ZD1. This mechanism also limits the maximum voltage at pin 1 of IC1 to a diode drop above 39V due to D3; ie, +39.6V plus or minus the zener diode tolerance. IC2’s power is supplied via an LM334Z constant current source, IC3. The 68Ω resistor between the R and V- pins of IC3 sets the constant Capacitor Codes  Value IEC Code EIA Code  0.47µF   470n   474  0.33µF   330n   334  0.1µF   100n   104  .001µF   1n0   102 current to 1mA. IC3 has a maximum voltage rating of 30V so it might seem that a voltage of 39V from ZD1 could present a problem for this current source chip. However, an internal zener diode in IC2 regulates the supply voltage at its pin 1 to +15.6V and so the maximum voltage across IC3 will be 39V - 15.6V = 23.4V. Q1 and Q2 are 200V Mosfets which switch the voltage from the two 0.47µF capacitors to produce the requisite output pulses on the electrodes. Q1 & Q2 constitute a “totem pole” output stage with Q1 turning on to charge the 0.47µF output capacitor via the series 150Ω resistor and the load resistance (which in this case is the patient). Each time Q1 turns off, Q2 turn turns on to discharge the capacitor via the series 150Ω resistor. Putting it another way, Q1 can be regarded as controlling the pulse width of the output waveform while Q2 controls the pulse rate (ie, the frequency). In more detail, Q2 is switched on for the time set by the 0.33µF capacitor at pin 3 and the resistance between pins 3 and 2 (of IC2). VR3 adjusts this on-time between about 0.5s and 5ms, giving a pulse rate between 2Hz and 200Hz. Q1 is switched on for the time duration set by potentiome­ter VR4, the Inside the TENS unit is a battery-powered circuit which produces up to 80V DC. This is pulsed by a pair of Mosfets to drive the electrodes. Note the three screws which are used as pillars to keep the battery holder in place. series 12Ω resistor and diode D4. The pulse width ranges between 40µs and 200µs. Intermittent mode IC4 is a 7555 CMOS timer which provides the intermittent mode. It operates as a free running oscillator but in a rather unusual configuration. The normal output at pin 3 is used to charge the 10µF capacitor at pins 2 & 6 via the 47kΩ resistor and diode D5 and discharge it via the parallel 100kΩ resistor. This gives a pulse waveform at pin 3 with an uneven duty cycle; the pulses are high for 0.22s and low for 0.7 seconds. However, we don’t use the normal output at pin 3 to modulate IC2. Instead, we use the capacitor discharge pin (pin 7). The pin 7 output is a Mosfet which is open circuit when pin 3 is Resistor Colour Codes  No.   1   1   1   1   1   3   1   1   1   1 Value 100kΩ 47kΩ 18kΩ 10kΩ 2.2kΩ 1kΩ 180Ω 150Ω 68Ω 12Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown grey orange brown brown black orange brown red red red brown brown black red brown brown grey brown brown brown green brown brown blue grey black brown black red black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown grey black red brown brown black black red brown red red black brown brown brown black black brown brown brown grey black black brown brown green black black brown blue grey black gold brown black red black gold brown August 1997  41 PARTS LIST 1 PC board, code 04307971, 157 x 87mm 1 plastic case, 188 x 98 x 37mm 1 adhesive label, 95 x 185mm 1 TENS electrode set with lead (available from chemists) 1 EF25 ferrite transformer assembly with N27 (Siemens) or 3C80 (Philips) ungapped cores and horizontal mounting bobbin plus clasp and spring (Philips 2 x 4312 020 3402 4, 1 x 4312 021 2626 1 and 1 x 4312 021 2612 1 and 1 x 4312 021 2619 1 or equivalent) (T1) 1 4 AA cell holder (rectangular) 1 battery clip for holder 4 AA cells 1 3.5mm phono panel socket 1 100kΩ linear pot. (VR1) 1 2MΩ linear pot. (VR3) 1 500Ω linear pot. (VR4) 3 16mm OD knobs with pointer marks 2 SPDT toggle switches (S1,S2) 1 3mm green LED (LED1) 1 3mm LED bezel 15 PC stakes 3 small cable ties 4 self-tapping screws to secure PC board 3 3mm x 20mm screws and nuts 1 60mm length of 3mm ID tubing 1 200mm length yellow hookup wire 1 200mm length blue hookup wire 1 300mm length black hookup wire 1 200mm length green hookup wire 1 300mm length red hookup wire 1 150mm length of twin wire rainbow cable 1 3.5-metre length of 0.5mm diameter enamelled copper wire Semiconductors 1 MC34063 DC-DC converter (IC1) 1 IR2155 Mosfet driver (IC2) 1 LM334Z current source (IC3) 1 ICM7555CN, LMC555CN, TLC555CP CMOS 555 timer (IC4) 2 IRF610 N-channel Mosfets (or equiv 200V <at>>1A, TO-220) (Q1,Q2) 1 39V 1W zener diode (ZD1) 2 1N4936 500V fast recovery diodes (D1,D2) 3 1N914, 1N4148 diodes (D3-D5) Capacitors 1 100µF 16VW PC electrolytic 1 10µF 25VW PC electrolytic 2 10µF 16VW PC electrolytic 1 4.7µF 63VW PC electrolytic 3 0.47µF 100VW MKT polyester 1 0.33µF 63VW MKT polyester 1 0.1µF 63VW MKT polyester 1 .001µF 63VW MKT polyester Resistors (0.25W, 1%) 1 100kΩ 1 180Ω 1 47kΩ 1 150Ω 1 18kΩ 1 68Ω 1 10kΩ 1 12Ω 1 2.2kΩ 1 0.22Ω 5W 3 1kΩ Fig.9: winding details for the transformer. Both the primary and secondary are wound using 0.5mm-diameter enamelled copper wire. 42  Silicon Chip high and conducting when pin 3 is low. Each time pin 7 of IC4 pulls low, it discharges the 0.33µF capacitor at pin 3 of IC2 and this stops IC2 from oscillating. This prevents any output to the electrodes and is an effective method of modulation. Construction The SILICON CHIP TENS Unit is built onto a PC board which is coded 04307971 and measures 157 x 87mm. It is housed in a plastic case measuring 188 x 98 x 37mm. An adhesive plastic label measuring 95 x 185mm is fitted to the lid of the case. Begin construction by checking the PC board for any defects such as shorted tracks or hairline breaks in the copper pattern. Repair these before assembly. The full wiring details are shown in the diagram of Fig.8. Insert the 15 PC stakes first. These are positioned at all the wiring points. Next, insert and solder in all the resistors. You can use the accompanying resistor colour code table when selecting the resistors although it is also a good idea to check each value with a digital multimeter before it is installed. Next, install the five diodes, making sure that the 1N4936s are used for D1 and D2. Three of the ICs are 8-pin DIP devices so don’t mix them up when installing them. Make sure that the ICs and Mosfets are correctly orientated when they are installed. The capacitors come next and the accompanying table shows the codes which may be on the MKT style devices to indicate their values. The electrolytic types must be oriented as shown and with the correct voltage rating. Higher voltage rated capacitors can be used. The winding details for the transformer are shown in Fig.9. Start by stripping the end of the 0.5mm enam­ elled copper wire and solder it to pin 1 on the bobbin. Wind on 44 turns in the direction shown and terminate the end to pin 4. The secondary is wound by soldering a 0.5mm wire to pin 8 and winding on 17 turns in the direction shown. Finish on pin 5. You can then wrap the windings in a few layers of insulation tape. The transformer is assembled by sliding the cores into place in the bobbin and securing them with the supplied clips. If no clips are supplied then you can secure the cores togeth-er with a cable tie around the core’s An effective alternative to analgesics can be provided by TENS in many situations. TENS stands for Transcutaneous Elec­trical Neural Stimulation and is widely used by physiotherapists for treatment of sports injuries and back pain. The skin electrodes can be readily purchased from your local pharmacy. former is wound correctly. If the primary and secondary are out of phase, the correct voltage cannot be obtained. Check that the voltage at pin 1 of IC2 is around +15V DC. With the pulse width pot (VR4) set fully clockwise and continuous mode selected, you should measure about +40V DC at pin 6, indi­cating that switching is taking place. If you have access to an oscilloscope, the output pulses can be observed to verify that the pulse width and frequency are to specification. The output can also be tested with a multimeter set to read AC volts. Connect your multimeter leads to the output socket and measure the voltage. You should obtain about 7VAC with all pots set to maximum when the continuous mode is selected. Note that this is only an indication of the Warning! perimeter. Insert the transformer into the PC board with the orienta­tion shown in Fig.8. Pin 1 of the bobbin is adjacent to the 0.47µF capacitor furthest from diode D1. To secure the battery holder, we used three 25mm-long 3mm screws and nuts in the positions shown near the transformer. These locate the 4-AA cell holder at the end of the case. We used some plastic sleeving over the screw threads to prevent scratch­ing the holder. The front panel label can be affixed to the lid of the case (the half with the brass thread inserts in each corner) and the holes drilled for the two switches, the 3mm LED bezel and the three pots. Attach all these components to the lid. Note that some pots with long shafts may need to be cut to length before assembly. Drill a hole in one of the end panels for the output socket. Follow the diagram of Fig.8 to connect all the components on the lid to the PC board. The battery clip is se­ cured to the PC board with a cable tie to prevent the wires from breaking at the PC stakes. Cable ties are also used to secure the wiring into a neat loom. Testing Fit the batteries and connect a multimeter (set to the 200V DC range) •  This TENS Unit (or any other similar device) must not be used on a person with a Heart Pacemaker. •  Do not connect the electrodes to the body so that there can be a flow of current through the heart. •  Electrodes must not be placed on the neck, since this can stimulate nerves which control breathing and blood pressure. •  Do not use the TENS Unit for headaches or attach the elec­trodes to the head. •  Do not be tempted to use the TENS Unit from a mains adap­tor, plugpack or power supply. This could be dangerous if a breakdown occurs in the isolating transformer. If you want to reduce the cost of battery replacement, we suggest using re­chargeable nicad cells. between the (-) terminal of the battery and the metal tab (drain) of Q1. Switch on and check that LED1 lights and that there is voltage on Q1’s drain. Set the voltage pot VR1 fully clockwise and adjust trimpot VR2 for a reading of +80V. If you are not able to obtain the correct voltage, check that the trans­ output; some multi­meters may give different readings With intermittent mode selected, you should see the voltage changing from 0V to a higher reading. Using TENS Connect the electrodes to the TENS Specifications Output level .........................................................................................2-80V Output pulse width ........................................................................ 40-200µs Frequency ......................................................................................2-220Hz Intermittent rate ........................................................... 700ms off; 220ms on Supply Voltage ......................................................................................... 6V Current Consumption ...................................30mA <at> 80V out and 6V input (frequency and pulse width at mid setting) August 1997  43 OUTPUT CONTINUOUS OFF + + + POWER 4 5 INTERMITTENT 1 MAX 9 MIN PULSE RATE 4 MAX PULSE WIDTH 5 6 7 3 2 8 + 1 1010 8 + 1 9 MIN 7 2 8 + 6 3 7 3 2 5 4 6 9 0 10 LEVEL 10 10 Fig.10 (above): this is the actual size artwork for the PC board. Check your board carefully against this pattern before installing any of the parts. Fig.11 (right): actual the size artwork for the front panel. unit using the 2.5mm plug to 2 x 2mm probe lead as supplied with the electrodes. If you wish to make your own leads, the 2mm probes are available from Dick Smith Electronics (Cat. P-1750). The electrodes should be smeared with K-Y* jelly (*trade mark of John­son & Johnson Pacific) or salt water solution to provide a reliable skin contact. They can be attached to the skin using any of the variety of tapes used to secure wound dressings. Attach the electrodes in 44  Silicon Chip TENS (Transcutaneous Electrical Neural Stimulation) position on either side of the pain source. Before switching on the TENS Unit be sure that the voltage is turned down to the minimum. Wind the voltage up until sensa­tion can be felt and adjust the pulse rate and width for the desired effect. The voltage will need to be wound up during treatment to compensate for the body’s adaptation to the stimula­tion. The intermittent selection is used where the treatment period is long (normal treatment sessions are typically for 20 minutes) or where the user finds the continuous effect to be waning. Further details on the TENS treatment techniques can be obtained from your General Practitioner. NOTE: Electrodes may be difficult to locate. Two sources are as follows: Water Fuel, 18 Springfield Road, Springvale, Vic 3172, phone (03) 9574 0002; or, Masters Medical, 8 Palmer Street, Parramatta, NSW 2150, phone SC (02) 9890 1711.