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JimmyH

Wed Oct 04 2006, 05:30AM

Registered Member #358 Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28

Hey all,

I think this project is close enough to completion to post about it on here, so here goes...

My senior year of high school I hurt my back doing squats, and then wrestling on it until it got worse (upper back- ligament stuff). Long story short, I ended up using the electrotherapy deal on in the trainers place, and I thought that thing was awesome. It was supposed to help it heal and stuff. I don't know for sure how much it really helped my back heal, but it felt really cool, especially with the heatpad. It felt almost like getting a massage or something.

Anyway, I decided to make one. The design process started over a year ago. I originally planned to do it all analog. I kept adding features until it made sense to just do it digital. I layed it out, and was about to order parts when my hard drive crashed and put me at square 1. I decided to take the opportunity to rethink the design (linear or switching power side, digital/analog smarts) and ended up going more than full circle and ending up where I was last time I layed it out.

What I settled on was a MSP430F149 controlled set up with 7 individual half bridges operating as Class-D amplifiers, and a buck-boost converter as a power supply. The MSP430 basically checks the ADC and adjusts the duty cycle on the boost to keep the voltage where it was set, and runs through a look up table so that the pulse width fed to the half bridges averages out to a nice sine wave. The crystal runs at 8MHz, so I only have 47 clock cycles before I need to update the pulse width to the next value (loop cycles at 170khz). With all the calculations and checking the ADC, I had to unroll the loop a bit to get it to run fast enough.

Since I settled on using a uC, I figured I'd have it do all the waveforms commonly used, since the smarts are flexible.

It can recreate the following waveforms:

TENS
Biphasic waveform
50-300uS On time
0-100v peak
1-200hz PRF

EMS
Biphasic waveform
300-500uS On time
0-100v peak
1-200hz PRF

Russian
2 kilohertz true sine wave
10-90% duty cycle
1-200hz PRF
0-50VRMS

Premodulated
4 kilohertz true sine wave
1-400hz beat frequency
0-50VRMS

Interferential
4 kilohertz true sine wave
1-400hz beat frequency
0-50VRMS

Hi Volt
5uS on time
twin peaks, spaced 100uS
1-200Hz PRF
0-500V peak

EMS and TENS are basically identical, with EMS using longer pulses for muscle contractions and TENS using shorter pulses to do close "pain gates" and release endorphins and whatever else they think that does

Premod and Interferential are pretty much the same, except how you get it. My "Premod" is kinda cheating, since I modulate it after, but since the stim unit is floating, it's the same thing. The only real difference is 1 channel vs 2 channels. Premod/interferential was the one that I had used on me.

The half bridges are Nch/Pch bridges, using capacitors to isolate the gates, and resistors to bias the gates at 0v. Like gate drive transformers, this means the gate voltage of the fets are both 0 at the same time, and they've completely turned off before the next one turns on, eliminating shoot through.

The current status is "near done". I had all the modes except Hi Volt working until the ceramics on the 100V rail died (were rated for <<100V) and killed a pin on the MSP430. Hi Volt was having either saturation or shoot through problems when driving a pulse transformer. Other than that, it's just minor code changes like the starting voltage and the min/max PRF; that kind of stuff.

Dave Marshall has been waiting patiently to be hooked up with one since like March, so sorry man, I'll get it done, I promise.

I entered it in TI's MSP430 ez-design contest (

along with the original DRSSTC and a SLR based high voltage capacitor charger (a SBIR project) so you guys can check that out if you want.

I'm not sure exactly what the deal is on attachments, so check out the rest of the pictures/code/layout at

Part Scavenger

Wed Oct 04 2006, 11:31AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

That's pretty awesome! I wish I could come up with junk like that.

Michael W.

Wed Oct 04 2006, 02:09PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

I have a TENS that I got at a garage sale to mess around with and it was unworking and I still can't repair it....

Dave Marshall

Mon Oct 09 2006, 05:08AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I injured my back (pinched sciatic nerve) last fall. I dont think the TENS unit I used at the chiropractor's office expedited my recovery any, but it sure did a nice job of providing temporary pain relief.

It looks great Jimmy. I'm looking forward to trying it out when you iron out the kinks.

Dave

Steve Conner

Tue Oct 10 2006, 02:47PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I like how Jimmy used himself as the "development platform"

Does it have a kill switch if the uC crashes?

JimmyH

Tue Oct 10 2006, 10:20PM

Registered Member #358 Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28

I like how Jimmy used himself as the "development platform" Does it have a kill switch if the uC crashes?

Hehe, my girlfriend was kinda scared when it made my back twitch and stuff

No kill switch, but the uC controls the boost, so if it crashes, it wont be able to supply power anyway.

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