How can I limit the current output of my TENS unit to 25 mA?
September 22, 2015 2:17 PM Subscribe
I recently purchased an EMS 7500 TENS device. The maximum current output is 80–100 mA. How can I limit the maximum current to approximately 25 mA instead, even if the intensity dial were to be turned all the way up?
For the sake of this question, let's say I'll be using the stimulator continuously, at 120 Hz, with a pulse width of 250 µs.
My skill level with this sort of thing could probably be described as "advanced beginner." I'm comfortable using a multimeter, soldering simple circuits together, stripping/splicing/heat-shrinking wires, etc., but that's about it; I have little to no formal knowledge of electrical engineering.
My intuition is that there might be two basic approaches: (1) wire something into the circuit (a fuse? current-regulating diode? resistor?) that prevents the current from going any higher after it reaches 25 mA, or (2) reduce the current output at every point along the intensity dial so that turning it all the way up results in an output of ~25 mA. Regarding approach #1, will any of the components I've listed work with a signal of this nature (250-µs pulse width, 120 Hz)? Or, if trying approach #2, where would the extra current go? Could I use a heatsink or something? Should I take a different approach altogether?
Alternatively, are there any good DIY-electronics forums out there that would be a better fit for this question?
For the sake of this question, let's say I'll be using the stimulator continuously, at 120 Hz, with a pulse width of 250 µs.
My skill level with this sort of thing could probably be described as "advanced beginner." I'm comfortable using a multimeter, soldering simple circuits together, stripping/splicing/heat-shrinking wires, etc., but that's about it; I have little to no formal knowledge of electrical engineering.
My intuition is that there might be two basic approaches: (1) wire something into the circuit (a fuse? current-regulating diode? resistor?) that prevents the current from going any higher after it reaches 25 mA, or (2) reduce the current output at every point along the intensity dial so that turning it all the way up results in an output of ~25 mA. Regarding approach #1, will any of the components I've listed work with a signal of this nature (250-µs pulse width, 120 Hz)? Or, if trying approach #2, where would the extra current go? Could I use a heatsink or something? Should I take a different approach altogether?
Alternatively, are there any good DIY-electronics forums out there that would be a better fit for this question?
Best answer: You can limit the current by introducing a series resistor into one of the leads. At those pulse widths, you shouldn't run into any weird high-frequency/RF issues; a plain old resistor will work fine.
The specs on the device are pretty vague; as ryanrs notes, the "500 ohm load" thing doesn't make much sense in the context of a person. See, for example, this page for info about physiological effects of electric current. Note particularly that DEATH results from 0.1 to 0.2A (which is 100mA to 200mA). A consumer device is very unlikely to be delivering anywhere near 100mA into someone, and I'll personally walk a mile to avoid more than 10mA coursing through my own flesh.
If the EMS device is more like a zapper that feels kind of like repeated static-electricity shocks, and you trust that the shocks are less than 50V, then you can limit the current by putting in a 2kΩ resistor. Even if the electrodes are shorted together (rather than through whatever a "500 ohm load" is) you won't get more than 25mA.
The resistor just makes it harder for the device to push current through the circuit. There's no excess to "go anywhere;" the energy just stays in the battery.
If you're using the device on yourself, you could try several resistors, starting at 2kΩ and working down, until you find one that gives you the range of effects you want with the dial set between 0 and "100mA".
One other thing: if the device isn't really limited to 50V, then theoretically it could just make higher-voltage shocks to get to the current level you set (overcoming any resistor you put in the circuit). This seems unlikely, but who knows? I'm really just guessing here, but maybe the device charges up a small capacitor, then (somehow) discharges it into the muscle to get a little jolt. I know I saw a prank book like that when I was a kid: you'd open the (foil covered) book and get a little shock. That would probably be safer than a solid power supply delivering 50V (or whatever) and targeting a certain number of milliamps. Instead, you'd have a fixed amount of energy set by the voltage and the size of the capacitor, and you'd get whatever current you get as the capacitor discharges in a shock cycle.
posted by spacewrench at 2:55 PM on September 22, 2015 [1 favorite]
The specs on the device are pretty vague; as ryanrs notes, the "500 ohm load" thing doesn't make much sense in the context of a person. See, for example, this page for info about physiological effects of electric current. Note particularly that DEATH results from 0.1 to 0.2A (which is 100mA to 200mA). A consumer device is very unlikely to be delivering anywhere near 100mA into someone, and I'll personally walk a mile to avoid more than 10mA coursing through my own flesh.
If the EMS device is more like a zapper that feels kind of like repeated static-electricity shocks, and you trust that the shocks are less than 50V, then you can limit the current by putting in a 2kΩ resistor. Even if the electrodes are shorted together (rather than through whatever a "500 ohm load" is) you won't get more than 25mA.
The resistor just makes it harder for the device to push current through the circuit. There's no excess to "go anywhere;" the energy just stays in the battery.
If you're using the device on yourself, you could try several resistors, starting at 2kΩ and working down, until you find one that gives you the range of effects you want with the dial set between 0 and "100mA".
One other thing: if the device isn't really limited to 50V, then theoretically it could just make higher-voltage shocks to get to the current level you set (overcoming any resistor you put in the circuit). This seems unlikely, but who knows? I'm really just guessing here, but maybe the device charges up a small capacitor, then (somehow) discharges it into the muscle to get a little jolt. I know I saw a prank book like that when I was a kid: you'd open the (foil covered) book and get a little shock. That would probably be safer than a solid power supply delivering 50V (or whatever) and targeting a certain number of milliamps. Instead, you'd have a fixed amount of energy set by the voltage and the size of the capacitor, and you'd get whatever current you get as the capacitor discharges in a shock cycle.
posted by spacewrench at 2:55 PM on September 22, 2015 [1 favorite]
You could hypothetically increase resistance somewhere, or more likely open the circuit for some settings, but the most likely solution is in programming.
posted by Dashy at 3:13 PM on September 22, 2015
posted by Dashy at 3:13 PM on September 22, 2015
to give people an idea of current - these things have two electrodes (conductive pads with conductive glue) which you connect to either end of where a muscle is (eg top and bottom of your shin). when you turn them on, your muscle twitches. i personally find it very unpleasant, but they're fairly commonly used in physiotherapy. (i am not an expert - i am just going from experience during treatment).
this link http://www.electrotherapy.org/modality/transcutaneous-electrical-nerve-stimulation-tens says the current range is 0-80mA. it doesn't surprise me that it's close to dangerous, because if you can make a leg muscle twitch, why not the heart?
(actually, that link is TENS, not EMS, so it doesn't talk about muscles twitching, but it's the same machine afaik. here's an EMS link http://www.bengreenfieldfitness.com/2013/02/how-to-use-electrical-muscle-stimulation-to-enhance-performance/ but it doesn't have max current)
posted by andrewcooke at 3:19 PM on September 22, 2015
this link http://www.electrotherapy.org/modality/transcutaneous-electrical-nerve-stimulation-tens says the current range is 0-80mA. it doesn't surprise me that it's close to dangerous, because if you can make a leg muscle twitch, why not the heart?
(actually, that link is TENS, not EMS, so it doesn't talk about muscles twitching, but it's the same machine afaik. here's an EMS link http://www.bengreenfieldfitness.com/2013/02/how-to-use-electrical-muscle-stimulation-to-enhance-performance/ but it doesn't have max current)
posted by andrewcooke at 3:19 PM on September 22, 2015
I know nothing about wiring or this machine. If I wanted a thing with a dial not to go beyond a certain point, I'd make something physically stop the dial at that point. Like a little nubby glued to the dial that bumps into another one glued to the faceplate.
posted by daisyace at 4:50 AM on September 23, 2015
posted by daisyace at 4:50 AM on September 23, 2015
to answer more directly:
i doubt this device works by controlling current. it is most likely to control voltage. the current you get is then whatever that voltage implies for the given resistance, limited by what the power supply can provide. and the upper limit is probably from the power supply (which is stepped up from a 9v battery, probably by an IC that is likely a switching step-up converter).
to control max current you would need to measure current, and then somehow limit voltage. measuring current is easy - just measure voltage across a resistor in series. but controlling voltage is hard. i don't see an easy way to do this. sorry.
posted by andrewcooke at 5:13 AM on September 23, 2015 [1 favorite]
i doubt this device works by controlling current. it is most likely to control voltage. the current you get is then whatever that voltage implies for the given resistance, limited by what the power supply can provide. and the upper limit is probably from the power supply (which is stepped up from a 9v battery, probably by an IC that is likely a switching step-up converter).
to control max current you would need to measure current, and then somehow limit voltage. measuring current is easy - just measure voltage across a resistor in series. but controlling voltage is hard. i don't see an easy way to do this. sorry.
posted by andrewcooke at 5:13 AM on September 23, 2015 [1 favorite]
Limiting the output current is probably pretty simple if you know the circuit. It's probably a flyback booster and you can change the voltage setpoint by altering the feedback resistors. Basically you make it think the voltage it is outputting is 4x the actual voltage, so the 50V setting becomes 12.5V, etc.
I might be able to tell you what to modify if you have high quality, in-focus, high resolution, macro photos of the circuit board, both sides. I need to be able to see traces and read components, so a crappy cell phone pic won't do. But a really good cell phone pic might be enough.
posted by ryanrs at 8:14 AM on September 23, 2015
I might be able to tell you what to modify if you have high quality, in-focus, high resolution, macro photos of the circuit board, both sides. I need to be able to see traces and read components, so a crappy cell phone pic won't do. But a really good cell phone pic might be enough.
posted by ryanrs at 8:14 AM on September 23, 2015
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The giveaway is the statement "0-100 mA peak into 500 ohm load". Human skin resistance is significantly higher than 500 ohms, so this statement is irrelevant to human use. (Unless maybe this device uses needles inserted under the skin? That sounds pretty unpleasant.)
Another reason I don't believe this device outputs 100mA is that amount of current has a reasonable of inducing heart fibrillations, killing the user.
So to answer your question, I guess I would suggest designing a new device from the ground up? But I suspect you don't actually want a real 25mA current limit, but something more like "prevent the user from changing the device settings above X". Is this correct?
posted by ryanrs at 2:38 PM on September 22, 2015