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# Schematics for a TENS

## Schematics for a TENS

(OP)
I am currently working with schematics for a muscle stimulator. The muscle stimulator will be used in a rehabilitation technique. One of the basics for this rehabilitation technique is to use many independent output channels with three degrees of freedom, amplitude (voltage set), frequency and pulse duration. A muscle stimulator uses electrical pulses to stimulate muscles in a body, the transfer between the apparatus and the muscle is usually not invasive, but is done using surface mounted electrodes on the skin.

I currently have a solution which I would appreciate some thoughts and comments on. The schematic is shown below (one channel only).

http://files.engineering.com/getfile.aspx?folder=2d08886e-a31d-42d9-8490-042a41f70713&file=tens1.png

V + is a constant voltage source to 40V +.

OK1 is an opto couplers with triac-output. This will disconnect from the anode when the channel is not in use.

T1 is a high-frequency transistor, whose function is to limit the flow of current to a desired level by using a PWM signal (4bit, 50MHz)

RL is the load, in this case a muscle.

IC1A, IC1B is comparators which send signals to logic to give indication of high or low current flow through the circuit. They should indicate below 1mA and above 40 mA.

RC1, RC2, RC3, RC4, are all resistors to give the comparators reference signals.

I have three questions; first, will T1 be able to limit the voltage? Since the pwm is 4bit it will provide 16 steps and this will lead to 16 linear steps in the amplitude, correct?

Second, how similar will the voltage-drop over T1 be? Is it possible to have the same RC1, 2, 3 for allt channels?

Thee, is there anything else I have completely missed?

Kind regard/ Johan, Sweden

### RE: Schematics for a TENS

How would T1 limit the voltage?  It's a switch.

Even using PWM, it can't limit current, either... that's the resistor's job.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Schematics for a TENS

And after reading all that, I still do not know what is a TENS.

### RE: Schematics for a TENS

If you place an RC Low Pass filter (with a cutoff frequency less than your PWM frequency) in the net named LOGIC3 then the base current of T1 would be controlled by your average voltage due to the PWM waveform and the resistor of the LPF.  The collector current of T1 is then limited to base current times beta.  The beta of a transistor in not generally well controlled so you would would have to decide if the current limit is accurate enough for your application.

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