I have a Promed TENS 1000 S dual-channel muscle stimulation device which I want to be able to control from a microcontroller. It has 3 potentiometers:
#1 pulse width (30µs - 260µs)
#2 pulse frequency (2Hz - 150 Hz)
#3 pulse strength (0V - ~80V) -> current goes up to 80mA at 80V
#1 and #2 are low-signal potentiometers meant to inform the chip how to model its PWM output. I am planning to replace these with a two-channel digital potentiometer and I don't have any questions about this yet.
#3 is my main question. The pulse strength potentiometer has the raw output signal of the transformer stage going through it (up to 80V @ 80mA). As I understand it it's actually "dimming" (dividing) the max output but because the bursts are short and the interval relatively long this doesn't pose a real threat for the pot (heat). If I want to replace this pot though, I can't find a digital potentiometer that can handle up to ~80V. I did find the MCP41HVX1 rated at 36V with at most 25mA current support but.. might this chip be able to handle this task anyway due to the low PWM duty cycle?
My calculation of PWM duty cycle: at a max 150Hz, that's 6666µs interval. With at max 260µs pulse width, that's 260 / 6666 that's a duty cycly of at most 3,9%. That's really not much...
If a digital pot is not a viable solution (which imho I'm suspecting), would it be feasible to control it through pwm'ing the output with a transistor or fet and smoothing that out with a low-pass filter?
A concern I have is that the pulses are so damn short that I would need a really really high PWM frequency to actually "catch" them right?
I would really appreciate some insights on how to approach this. Thank you!
Could you supply voltage to the transformers and IC separately? Thinking: the IC probably gets some 3.3-5v regulated input off the 9v, but the 9v powers the step-up transformers directly. Could you use a DAC on the transformer voltage input and would it give you control over the output power?
I'm super hesitant to stray outside of a consumer product with (one would imagine) proven safety, but I'll check it out regardless Thanks for the link.
While we're sharing links to DIY TENS projects, let me share this one that I found. I believe it's legit since I used it to be able to better understand the components on the Promed TENS 1000 S pcb: TENS Circuits.
That's a nice idea, that would be interesting to explore. Would probably entail striking through some traces on the PCB or such... but maybe worth it! I'll look into it.
To update on this thread: I've made progress on this by finding the right components to use when building a diy digital potentiometer solution on the output side of the TENS device. Here's my post on another thread, but fyi I've copy-pasted the gist of it:
[...] I have since received a few mosfet relays (this one), which as I see it are internally like an optocoupler but with an n-channel and p-channel mosfet istead of an NPN transistor. Effectively it enables switching AC and DC sources, and this appears to have made the magic difference for me! The rating of 400V AC/DC at 140mA is also perfect for me. Lovely to have a small package which can handle higher voltages. With these mosfet relays I was finally able to 'dim' the signal of the TENS device.
@mattgaidica: I'm still interested in exploring dimming the low-side signal though, before it goes into the transformer! That seems so much more elegant.
Thanks for the link.