Это OCR-версия документа. Могут содержаться ошибки распознавания.

# Pocket TENS

By JOHN CLARKE

Features

- Battery powered
- Adjustable voltage level
- Adjustable pulse rate
- Adjustable pulse width
- Intermittent or continuous

TENS – Transcutaneous Electrical Nerve Stimulation – is pain relief without drugs. Attach electrodes near to the painful area and start up the TENS unit for a tingling sensation that can help to reduce pain. The pocket-sized SILICON CHIP TENS unit has adjustable controls that tailor the levels to suit each patient's requirement and is battery powered.

> Warning!

> This TENS unit (or any other similar device) must not be used on a person who has 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 run 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 a 9V NiMH rechargeable battery.

Living in constant pain is a reality for many people and how well they cope with it depends on the degree of pain and the character of the person.

While pain relief can be managed in the short term using analgesics, their long-term use can be detrimental to the user’s health. Side effects of prolonged analgesics use include liver and kidney damage and in some cases irritation to the lining of the stomach. Thankfully, in many cases there is an alternative: TENS or Transcutaneous Electrical Nerve Stimulation.

In many cases where pain is constant, a medical practitioner or physiotherapist may recommend the use of a TENS unit.

Specifications
Output Voltage | Adjustable from 12V to 80V
Pulse Rate | Adjustable from 4.6Hz to 410Hz
Pulse Width | Adjustable from between 70 and 320μs
Intermittent | 24% duty cycle at 1.2Hz (220ms pulse burst with an 800ms off period)
Battery Drain | Typically less than 20mA (31mA at 80V output, 19mA at 50V output)
Battery | 9V Alkaline (or a 9V NIMH rechargeable)
Battery Voltage | 7.2V minimum for a 12V to 80V output range, 4V minimum for a 22V to 80V output range.

These are not a gimmick or a new-age form of treatment. Tests have shown that TENS is an effective and safe way to manage chronic and acute pain with virtually no side effects. Chronic pain conditions that can be alleviated with TENS include, arthritis, lumbago, neck and back pain, post herpetic neuralgia and sciatica. Acute pain conditions such as fractures, muscular pains, post operative pain and tennis elbow can also be managed with a TENS unit.

A TENS unit provides electrical stimulation of the painful area using electrodes attached to the skin. It can cause a tingling sensation in the area where the pads are attached. How the TENS reduces pain is unknown. Some suggest that the nerves are stopped from sending signals to the brain and thus the pain is removed. Alternatively, the stimulation could induce the body to produce natural pain relieving substances called endorphins.

Whatever the reason, a TENS unit can give pain relief for many people, with minimal side effects. It does not provide a cure for the underlying cause of the pain but has the major benefit that the amount of pain killing drugs can be substantially reduced.

Nor is it addictive. Sometimes there can be skin irritation surrounding the electrodes and this can be reduced or alleviated.by changing the type of electrode.

Note that a TENS unit must not be used if you have a heart pacemaker. A TENS unit also should not be used if the cause of the pain has not been established or if you are pregnant. When using the TENS unit do not attach the electrodes anywhere around the front of the neck, and be sure the TENS unit is kept out of reach of children. It should be treated like any other medicine, by storing it in a childproof cabinet.

## Features

SILICON CHIP previously featured a TENS unit in the August 1997 issue. This new version uses a very similar circuit but housed in a much-more-convenient pocket-sized case.

Three small knobs allow adjustment of the overall output voltage, the width of the voltage pulses and the pulse rate. A continuous/intermittent switch selects whether the pulses are provided as a continuous stream or in short bursts. To the left of that is a power switch and a LED to indicate when the unit is on. Two electrodes connect to the TENS unit via a lead that plugs into a socket at the top end of the box. The electrodes are attached to the skin adjacent to the painful area and the controls are adjusted until the tingling effect becomes just a little uncomfortable. The tingling sensation will tend to decrease over the period of treatment and so the controls will need to be further increased as time goes by. The typical treatment period is about 20 minutes.

This oscilloscope screen shows the pulse train signal at the electrodes. Here the voltage is set at 80V and the frequency at 108Hz.

Generally, the continuous setting is selected but for long treatment periods, the intermittent mode can be used. This mode helps to overcome the effect where the patient becomes accustomed to the stimulation. The intermittent mode allows a higher voltage and a faster rate to be selected compared to the continuous mode.

Here are the pulses shown with a faster timebase. It shows the width of each pulse at about 320μs.

Thus the stimulation is greater in short bursts and because there is a break in between pulses, the patient does not adapt too readily to the higher levels.

Finally, this is the intermittent pulse output showing the bursts of pulses at about 1.2Hz.

## Block diagram

The block diagram for the TENS unit is shown in Fig.1. The 9V battery supply is stepped up by the converter comprising IC1 and T1. This provides a DC output from 12V to BOV, with VR1 providing the adjustment. The resulting DC voltage is converted to a pulsed signal using switching oscillator IC2. Oscillator IC4 is switched into circuit via S2 to gate the switching oscillator to give short bursts of the pulsed signal.

Fig.1: the block diagram for the TENS unit. The 9V supply from the battery is stepped up in the converter comprising IC1 and T1. This provides a DC output adjustable from 12V up to 80V with VRI providing the adjustment. The resulting DC voltage is converted to a pulsed signal using the switching oscillator.

Fig. 2 shows how the basic step–up converter circuit operates. It comprises inductor L1 which is charged from the V+ supply through transistor Q1. The charging current is shown as I1. When the transistor is switched off, the stored energy in L1 is dumped via diode D1 into capacitor C1.

Fig.2: 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.

The actual voltage across C1 depends on the amount of charge in L1 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.3 shows the circuit of the switching oscillator, comprising IC2, Q1 & Q2. This modulates the output voltage of the step-up converter and is based on an IR2155 made by International Rectifier Corporation. It is described as a high-sido self-oscillating power Mosfet gate driver.

Fig.3: the circuit configuration of the switching oscillator. This 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.

Resistor R1 and capacitor C1 at pins 2 & 3 of IC2 set the rate at which Mosfets Q1 and Q2 are alternately turned on and off. There is a dead time of 1.2ps between each device switching off and the other switching on. This prevents the supply from being short circuited at the switchover period when one Mosfet turns off and the other turns on.

## Diode pump

Note that the supply voltage for IC2 is around 10V while the voltage to be switched can be up to 80V. The gate voltage for Q1 must be raised above its drain by several volts in order for it to be able to switch the 80V supply. This extra voltage is derived using a diode pump consisting of diode D2 and capacitor C2.

Initially, the supply to pin 1 of IC2 is set at about 10V by an external zener diode. When Mosfet Q2 is switched on, capacitor C2 charges to the 10V supply via D2. When Q2 is turned off, pin 7 is connected internally to pin 8 to switch on Q1. Q1 then pulls pin 6 up to Vsupply and pin 8 is level-shifted to Vsupply plus the voltage across C2. So in a few switching cycles, the circuit automatically shifts pin 8 and thereby the gate voltage to Mosfet Q1, to whatever the driving voltage needs to be.

## Circuit details

The full circuit for the TENS unit is shown in Fig.4. Power from the 9V battery comes via switch S1 and diode D6. D6 is included for reverse polarity protection but because we are running from batteries, we have specified a Schottky diode to minimise voltage losses.

Fig.4: the complete Pocket TENS circuit diagram. Its operation can be most easily understood by comparing it with the block diagram of Fig.l.

IC1 is the switchmode controller. It has a switching transistor at pin 1 and a feedback input at pin 5. Its frequency of oscillation is set by the 2.2nF capacitor at pin 3. The peak current through the primary winding of T1 is limited by the 0.22Ω resistor between pins 6 and 7 of IC1. In operation, the current through the primary winding of T1 is switched off when the voltage drop across the 0.22 Ω resistor exceeds about 300mV.

Switching off the current through T1 causes voltage to be induced into T1’s secondary when the primary field collapses. This charges two 470nF capacitors via diode Dl. Voltage feedback from the 150kΩ resistor, VR1 and VR2 into pin 5 maintains the voltage at the desired setting up to 80V.

The circuit uses a transformer instead of a step-up inductor, as depicted in Fig.2. This is included to prevent high voltages occurring at pin 1 of IC1, where the maximum allowable voltage is 40V. Since we want up to 80V, the 2.59:1 step-up ratio between primary and secondary of T1 will ensure that the pin 1 voltage will be less than 40V. The primary winding can be used to provide a 10V supply for IC2 and IC4.

This supply is derived in two steps. First, diode D3 charges the associated 4.7μF capacitor. Voltage across it is limited to +39V by zener diode ZD1. Diode D3 also clamps the maximum voltage at pin 1 of IC1 to one diode drop above 39V.

IC2’s power is then derived via an LM334Z constant current source, IC3.

The 27Ω resistor between the R and V-pins of IC3 sets the constant current to about 2mA. The current source supplies a 10V zener diode (ZD2) that regulates the supply voltage to 10V. This supply also powers IC4.

Note that we need to derive the supply for IC2 in this way because the 9V directly from the battery is just not enough for satisfactory operation. This is because IC2 has an internal voltage shutdown that operates at below 8.4V. IC2 will therefore not operate when its supply drops to this level.

If we were powering this IC directly from batteries, we would need at least 8.6V from the battery to ensure operation if we include the drop across diode D6.

This would give an extremely short operation time with a 9V battery. By contrast using the power supply system described above, the battery can be used down to at least 7.2V and in most cases down to 4V.

Q1 and Q2 are 200V Mosfots and are used to switch the high voltage on and off to produce the requisite output pulses on the electrodes. Q1 & Q2 constitute a totem pole output stage with Q1 turning on to charge the 470nF 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. The amount of time Q1 is switched on determines the pulse width of the voltage output. Q2’s on time controls the pulse rate (ie, the frequency).

In more detail, Q2 is switched on for the time set by the 330nF capacitor at pin 3 and the resistance between pins 3 and 2 of IC2. VR3 adjusts this time between about 0.22 and 2.4ms, giving a pulse rate between 4.6Hz and 410Hz.

Q1 is switched on for the time duration set by potentiometer VR4, the series 12Ω resistor and diode D4. The pulse width ranges between 70μs is and 320μs.

## Intermittent mode

IC4 is a 7555 CMOS timer configured to provide the intermittent mode. It operates as a free running oscillator. The 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 100Ω resistor. This gives a pulse waveform at pin 3 with an uneven duty cycle, with the pulses being high for 0.22s and low for 0.7 seconds.

We don’t use the pin 3 output to modulate IC2. Instead, we use the capacitor discharge output at pin 7. This pin 7 output is an open drain Mosfet which is open circuit when pin 3 is high and conducts signal to ground when pin 3 is low.

Each time pin 7 of IC4 pulls low, it discharges the 330nF capacitor at pin 3 of IC2 to stop IC2 from oscillating. This prevents any output to the electrodes and provides an intermittent modulation for the electrode output.

## Construction

The Silicon Chip TENS unit is built onto a PC board coded 11101061 and measuring 85 x 64mm. It is housed in a plastic case measuring 134 x 69 x 23mm. An adhesive plastic label measuring 49 x 113mm is fitted to the lid of the case.

Three trimpots are used as controls instead of potentiometers. They provide us with suitable sized components for the small box. 10mm long spindles are inserted into each trimpot to allow adjustment and these protrude through the front panel of the box. Note that the trimpots specified are long-life components suitable for potentiometer use.

Parts List – Pocket TENS Unit
1 | PC board coded 11101061,85 x 64mm
1 | plastic case, 134 x 69 x 23mm, with 9V battery compartment (DSE Cat ZA-4731)
1 | front panel label, 49 x 113mm
1 | TENS electrode set (available from pharmacy suppliers and chemists)
1 | Neosid ferrite core, 25 x 15 x 10mm (28-780-36P)
1 | 9V battery clip lead
1 | 9V alkaline or 9V NiMH rechargeable battery
1 | 2.5mm PC-mount DC socket
1 | 2.5mm DC line plug with long shaft
2 | 2mm plugs for electrodes
1 | 1 m length of figure-8 light duty flexible cable
2 | PC-mount SPDT slider switches (S1,S2)
2 | DIP-8 low-cost IC sockets to mount switches
3 | 15mm spindles forVFH, VR3 & VR4 2 200mm long cable ties
2 | PC stakes
4 | М3 x 6mm screws
1 | 2m length of 0.5mm enamelled copper wire
1 | 12mm length of 9.5mm heatshrink tubing
1 | 15mm length of 3.3mm heatshrink tubing
Semiconductors
1 | MC34063 DC-DC converter (IC1)
1 | IR2155 Mosfet driver (IC2)
1 | LM334Z current source (IC3)
1 | 7555 CMOS timer (IC4)
2 | STP6N60E N-channel Mosfets or similar rated at 200V 1A minimum (Q1.Q2)
1 | 39V 1W zener diode (ZD1)
1 | 10V 1W zener diode (ZD2)
2 | 1N4936, UF4004 fast diodes (D1,D2)
3 | 1N4148 switching diodes (D3-D5)
1 | 1N5819 Schottky 1A diode (D6)
Capacitors
1 | 100pF 16V PC electrolytic
3 | 10gF 16V PC electrolytic
1 | 4.7pF 63V PC electrolytic
3 | 470nF MKT polyester
1 | 330nF MKT polyester
1 | 100nF MKT polyester
1 | 2.2nF MKT polyester
Resistors (0.25W 1%)
1 | 150kΩ
1 | 100kΩ
1 | 47kΩ
1 | 10kΩ
1 | 2.2kΩ
1 | 1kΩ
1 | 180Ω
1 | 150Ω
1 | 27Ω
1 | 12Ω
1 | 0.22Q 5W
2 | 1MΩ horizontal trimpot (Piher PT10MV10 105A 202E) (VR1.VR3) (OR 2MΩ for VR3 for a 2.3Hz minimum rate) (Farnell 868-437 for 1 MΩ)
1 | 100kΩ multiturn top adjust trimpot (VR2)
1 | 1kΩ horizontal trimpot (Piher PT10MV10 102A 202E) (VR4) (Farnell 868-383)

All components must be placed so that they sit no more than 13mm above the top surface of the PC board. This means that one electrolytic capacitor is mounted on its side and the two Mosfets (Q1 & Q2) are bent over at right angles. In contrast, the switches must be raised above the PC board using cut down IC sockets, to make them accessible when the lid is fitted to the case.

Begin construction by checking the PC board for any defects such as shorted tracks or breaks in the copper pattern. Repair these before assembly. The component overlay diagram is shown in Fig.5.

Fig.5: the PC board component overlay with same-size photo at right. Note how the 10μF capacitor (between VR3 and IC2) is laid parallel to the PC board.

Insert the two PC stakes at the battery wiring points first. Next, insert and solder in all the resistors. You can use the accompanying resistor colour code table when selecting the resistors and it is also a good idea to check each value using a digital multimeter before it is installed.

Next, install the six diodes and two zener diodes, making sure that the correct diodes are used in each place. Each of the ICs is an 8-pin DIP device, so don’t mix them up when installing them.

The capacitors can be mounted next. The MKT polyester types have codes stamped on them to indicate their value and we have provided a table of the different codes. The electrolytic types must be oriented as shown and the 10μF capacitor adjacent to VR3 must be laid on its side.

The PC board is a nice neat fit inside the pocket-sized case. It contains its own 9V battery – don’t be tempted to run this from a mains adaptor!

The switches are mounted on cut-down IC sockets. The sockets are made by cutting up IC sockets into strips of five contacts using a sharp utility knife. The two unused pin contacts for each switch socket are removed. Insert and solder the sockets in place and then insert the switches.

The trimpots are soldered next, taking care to place the correct value of trimpot in each position. The 10mm spindles are inserted with the pointer facing the centre pin of the trimpot. Check that the rotation to the left and right is correct, with the pointer rotation the same from each side of centre. Remove and readjust the spindle orientation if this is incorrect.

Capacitor Codes
Value | μF Code | IEC Code | EIA Code
470nF | 0.47μF | 470n | 474
330nF | 0.33μF | 330n | 334
10OnF | 0.1 μF | 100n | 104
2.2nF | .0022μF | 2n2 | 222

As mentioned earlier, the leads of Q1 and Q2 have their leads bent over at right angles as shown in the photograph above. They must lie over the adjacent components so that their bodies are no higher than 13mm above the PC board.

Fig.6 shows the winding details for the toroidal transformer T1. It is wound with 0.5mm enamelled copper wire. It is important to get the winding direction and number of turns correct.

Fig. 6: winding details for the toroidal transformer, T1.

Start by winding on 44 turns for the secondary in the direction shown. The primary is also wound in the direction shown, with 17 turns. Strip the enamel insulation from the wire ends before soldering them to the PC pads. Then secure the finished toroid to the PC board with a cable tie, as shown.

Fig. 7: here’s how to wire the electrode leads, using a 2.5mm long shaft DC plug. The leads can be as long as you like, within reason!

Indicator LEDl is mounted with the top of its lens 15mm above the PC board. Make sure its orientation is correct. Attach the PC board to the base of the case with the four М3 screws directly into the integral standoffs in the case.

Figs 8 & 9: same-size artwork for the PC board and front panel. A photocopy of the front panel can also be used as a drilling template for the
case.

The front panel label can be attached to the lid of the case and the holes drilled and filed to shape for the two slide switches, the 3mm LED and the three trimpot spindles.

Drill a hole in the end panel for the output socket to allow access for the DC plug.

Resistor Colour Codes
No. | Value | 4-band Code (1%) | 5-band Code (1%)
1 | 150kΩ | brown green yellow brown | brown green black orange brown
1 | 100kΩ | brown black yellow brown | brown black black orange brown
1 | 47kΩ | yellow violet orange brown | yellow violet black red brown
1 | 10kΩ | brown black orange brown | brown black black red brown
1 | 2.2kΩ | red red red brown | red red black brown brown
1 | 1kΩ | brown black red brown | brown black black brown brown
1 | 180Ω | brown grey brown brown | brown grey black black brown
1 | 150Ω | brown green brown brown | brown green black black brown
1 | 27Ω | red violet black brown | red violet black gold brown
1 | 12Ω | brown red black brown | brown red black gold brown

The battery clip wires are fed through from the battery compartment side via the holes in the box. Secure these wires with a cable tie and solder them to the PC stakes using heatshrink tubing (the small diameter length cut in half) to cover the PC stakes and wire. Mosfet Q2 also has its tab covered in heatshrink tubing to avoid its tab shorting to the tab of Q1.

## Testing

Fit the battery and plug in the DC socket with the backing piece removed. Connect a multimeter (set to the 200V DC range) between the outside terminal of the plug (-) and the tab of Q1.

Switch on power and check that LEDl lights and that there is a voltage reading. 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 transformer is wound correctly. In particular, check the winding directions for each winding.

Check that the voltage at pin 1 of IC2 is around +10VDC. Set the pulse width pot VR4 fully clockwise and select the continuous mode.

Connect your multimeter set for AC volts across the DC socket terminals. You should measure about +18V AC, indicating that switching is taking place.

Note that this is only an indication of the output, as some multimeters may give different readings. The readings should alter with different control settings. With intermittent mode selected, you should see the voltage changing from OV to a higher reading.

If you have access to an oscilloscope, the output pulses can be observed to verify that the pulse width and frequency are to specification.

## Using TENS

Make up electrode leads using the 2.5mm DC plug and the two 2mm plugs. Now connect to the electrodes. The electrode sockets may need to be slightly crimped with pliers to close up the socket hole. This will hold the 2mm plugs more securely.

The electrodes are usually supplied with an adhesive back that allows them to be easily attached to the skin. If the adhesive dries out, a smear of personal lubricant will be helpful.

The electrodes can then be attached to the skin using any of the variety of tapes or bandages used to secure wound dressings. Attach the electrodes in position on either side of the pain source. (A useful chart showing typical TENS pad locations may be found at www.vitalitvweb.com/back-store/tensplacement.htm)

Before switching on the TENS unit be sure that the output voltage is turned down to the minimum.

TENS pads are normally self-adhesive and, with care, can be used many times. When not in use they should be stuck onto the backing sheet they came with. The most usual position for pads is each side of a painful area, bearing in mind the warnings published on page 17.

Wind the voltage up until a tingling sensation can be felt and adjust the pulse rate and. width for the desired effect. The voltage will need to be wound up during the period of 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.

It is possible that the TENS pads will irritate the skin, not (usually) so much from the TENS itself but the adhesive used on the pads. If so, we suggest trying a different brand or type of pad.

There is a wealth of information on the internet about TENS units and their use.

Like any treatment regimen, we suggest you ask your General Practitioner for advice before commencing treatment with the TENS unit. Remember, TENS does not treat any underlying condition; it merely masks the pain and makes it more bearable.