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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.
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