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By John Clarke
16 Silicon
ilicon Chip
hip
siliconchip.com.au
TENS – Transcutaneous Electrical
Features
• Battery pow
ered
Nerve Stimulation – is pain relief
• Adjustable
voltage level
without drugs. Attach electrodes
• Adjustable
pulse rate
• Adjustable
near to the painful area and start
pulse width
• Intermittent
or continuous
up the TENS unit for a tingling
output
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.
L
IVING 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.
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.
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.
siliconchip.com.au
January 2006 17
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 relievFig.1: the block
ing substances called endorphins.
diagram for the TENS
unit. The 9V supply
Whatever the reason, a TENS unit can
from the battery is
give pain relief for many people, with
stepped up in the
minimal side effects. It does not provide a
converter comprising
cure for the underlying cause of the pain
IC1 and T1. This
but has the major benefit that the amount
provides a DC output adjustable from 12V up to 80V with VR1 providing
of pain killing drugs can be substantially
the adjustment. The resulting DC voltage is converted to a pulsed signal
reduced.
using the switching oscillator.
Nor is it addictive. Sometimes there
can be skin irritation surrounding the
electrodes and this can be reduced or alleviated by changDC voltage is converted to a pulsed signal using switching
ing the type of electrode.
oscillator IC2. Oscillator IC4 is switched into circuit via
Note that a TENS unit must not be used if you have a
S2 to gate the switching oscillator to give short bursts of
heart pacemaker. A TENS unit also should not be used if
the pulsed signal.
the cause of the pain has not been established or if you
Fig.2 shows how the basic step-up converter circuit operare pregnant. When using the TENS unit do not attach
ates. It comprises inductor L1 which is charged from the
the electrodes anywhere around the front of the neck, and
V+ supply through transistor Q1. The charging current is
be sure the TENS unit is kept out of reach of children. It
shown as I1. When the transistor is switched off, the stored
should be treated like any other medicine, by storing it in
energy in L1 is dumped via diode D1 into capacitor C1.
a childproof cabinet.
The actual voltage across C1 depends on the amount
of charge in L1 and the load current between Vout and
Features
the ground supply. We can maintain a constant Vout for
SILICON CHIP previously featured a TENS unit in the August
a variety of loads by controlling the amount of time Q1 is
1997 issue. This new version uses a very similar circuit but
switched on.
housed in a much-more-convenient pocket-sized case.
Fig.3 shows the circuit of the switching oscillator, comThree small knobs allow adjustment of the overall output
prising IC2, Q1 & Q2. This modulates the output voltage
voltage, the width of the voltage pulses and the pulse rate. A
of the step-up converter and is based on an IR2155 made
continuous/intermittent switch selects whether the pulses
by International Rectifier Corporation. It is described as a
are provided as a continuous stream or in short bursts. To
high-side self-oscillating power Mosfet gate driver.
the left of that is a power switch and a LED to indicate when
Resistor R1 and capacitor C1 at pins 2 & 3 of IC2 set the
the unit is on. Two electrodes connect to the TENS unit via
rate at which Mosfets Q1 and Q2 are alternately turned on
a lead that plugs into a socket at the top end of the box. The
and off. There is a dead time of 1.2ms between each device
electrodes are attached to the skin adjacent to the painful
switching off and the other switching on. This prevents the
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.
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.
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.
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 80V, with VR1 providing the adjustment. The resulting
18 Silicon Chip
This shows the pulse train signal at the electrodes. Here
the voltage is set at 80V and the frequency at 108Hz.
siliconchip.com.au
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.
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.
supply from being short circuited at the switchover period
when one Mosfet turns off and the other turns on.
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.
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 0.22W 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 0.22W
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
D1. Voltage feedback from the 150kW 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
Here are the pulses shown with a faster timebase. It shows
the width of each pulse at about 320ms.
Finally, this is the intermittent pulse output showing the
bursts of pulses at about 1.2Hz.
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
siliconchip.com.au
January 2006 19
Fig.4: the complete Pocket TENS circuit diagram. Its operation can be
most easily understood by comparing it with the block diagram of Fig.1.
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 4.7mF 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 27W 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
Specifications
from the battery is just not enough for
Output Voltage........... Adjustable from 12V to 80V
satisfactory operation. This is because
IC2 has an internal voltage shutdown that
Pulse Rate................. Adjustable from 4.6Hz to 410Hz
operates at below 8.4V. IC2 will therefore
Pulse Width................ Adjustable from between 70 and 320ms
not operate when its supply drops to this
Intermittent................. 24% duty cycle at 1.2Hz
level.
(220ms pulse burst with an 800ms off period)
If we were powering this IC directly
Battery Drain.............. Typically less than 20mA
from batteries, we would need at least
(31mA at 80V output, 19mA at 50V output)
8.6V from the battery to ensure operation
Battery....................... 9V Alkaline (or a 9V NIMH rechargeable)
if we include the drop across D6.
This would give an extremely short
Battery Voltage........... 7.2V minimum for a 12V to 80V output range,
operation time with a 9V battery. By
4V minimum for a 22V to 80V output range.
contrast using the power supply system
20 Silicon Chip
siliconchip.com.au
Fig.5: the PC board
component overlay
with same-size photo at
right. Note how the 10mF
capacitor (between VR3
and IC2) is laid parallel
to the PC board.
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 Mosfets 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 150W 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 150W 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 12W resistor and diode D4. The
pulse width ranges between 70ms and 320ms.
Intermittent mode
IC4 is a 7555 CMOS timer configured to provide the in-
Fig. 6: winding
details for
the toroidal
transformer, T1.
siliconchip.com.au
termittent mode. It operates as a free running oscillator. The
output at pin 3 is used to charge the 10mF capacitor at pins
2 & 6 via the 47kW resistor and diode D5 and discharge it
via the parallel 100kW 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.
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!
January 2006 21
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.
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
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 1m 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 for VR1, VR3 & VR4
2 200mm long cable ties
2 PC stakes
4 M3 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 100mF 16V PC electrolytic
3 10mF 16V PC electrolytic
1 4.7mF 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 150kW
1 100kW
1 47kW
1 10kW 1 2.2kW
1 1kW
1 180W
1 150W
1 27W 1 12W
1 0.22W 5W
2 1MW horizontal trimpot (Piher PT10MV10 105A 202E) (VR1,VR3) (OR
2MW for VR3 for a 2.3Hz minimum rate) (Farnell 868-437 for 1MW)
1 100kW multi-turn top adjust trimpot (VR2)
1 1kW horizontal trimpot (Piher PT10MV10 102A 202E) (VR4)
(Farnell 868-383)
22 Silicon Chip
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.
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 10mF capacitor adjacent to VR3 must be laid on its side.
The switches are mounted on cutdown 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.
As mentioned earlier, the leads
Capacitor Codes
Value
470nF
330nF
100nF
2.2nF
mF Code
0.47mF
0.33mF
0.1mF
.0022mF
IEC Code EIA Code
470n
474
330n
334
100n
104
2n2
222
siliconchip.com.au
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!
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.
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.
Indicator LED1 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 M3 screws
directly into the integral standoffs in
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.
Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
No.
1
1
1
1
1
1
1
1
1
1
Value
150kW
100kW
47kW
10kW
2.2kW
1kW
180W
150W
27W
12W
siliconchip.com.au
4-band Code (1%)
brown green yellow brown
brown black yellow brown
yellow violet orange brown
brown black orange brown
red red red brown
brown black red brown
brown grey brown brown
brown green brown brown
red violet black brown
brown red black brown
5-band Code (1%)
brown green black orange brown
brown black black orange brown
yellow violet 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
red violet black gold brown
brown red black gold brown
Drill a hole in the end panel for the
output socket to allow access for the
DC plug.
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,
then use heatshrink tubing (the small
diameter length cut in half) to cover
the PC stakes and wire.
Note that 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
LED1 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
January 2006 23
IC2 is around +10V DC. 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 0V 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.vitalityweb.com/backstore/tensplacement.htm
Before switching on the TENS unit
be sure that the output voltage is
turned down to the minimum.
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
24 Silicon Chip
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.
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 sug-
gest 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
SC
and makes it more bearable.
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.
siliconchip.com.au
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