Automatic electric stimulation concern

[Blacky]

Unfortunately many cheap TENS units have insufficient safety margins regarding short-circuit safety.
You might well fry that thing if you deliberately short the output.

Been there, done that, released magic smoke.

[ruru67]

I know this is very old, but I would try to connect the relay in parallel to the electrodes, i.e. short circuit the electrodes, since these units try to deliver constant current, not constant voltage.

From my messing with these, they don't do anything of the sort. The current figure given is typically the dead-short current (there's usually an internal current limiting resistor, of 500 ohms or so), so the quoted current - if there is one - is simply the output voltage divided by the resistor value. The output voltage is in turn basically limited by the reactance of the transformer; the input is a DC pulse, and the output holds at a fairly constant voltage until the transformer reaches its capacity, then the output drops to zero. Typically we're talking about an output pulse of up to a couple of hundred microseconds - less (obviously) if the DC input pulse is shorter than that.

When the voltage to the input side returns to 0, the output goes highly negative - on good devices this is snubbed to zero by a diode across the output; simpler devices (like shock collars) don't bother, so most of the zap you actually get is this uncontrolled "flyback" voltage. The voltage decays quickly - the lower the resistance on the output, the faster the decay (and the lower the peak voltage), but there's no control over the current beyond the total power the transformer can deliver.

I've killed really cheap shock collars by shorting the contacts (the thin wires on the output side of the transformer melted). A current limiting resistor, say 1k ohms, would prevent that. Remember that skin resistance is between a hundreds of kohms or so down to a few kiloohms (thin, waterlogged skin), so 1k ohms more resistance won't change the zap much. (Realistically, getting the "dangerous" tens of milliamps or more current into you, let alone actually across a vital organ, is actually quite difficult with these sorts of devices, and if you're getting close, you're going to know. Real currents will be in the microamps.)

I wrote some stuff a while ago about TENS voltages and currents, with experiments, here: https://rurubound.wordpress.com/2012/05 ... the-waist/

[muestrom]

Well, in case the device is not protected against short circuits, the relay should switch between the electrodes and a resistor of e.g. some kOhms.

Anyway, just interrupting the output is likely to generate high voltages since the device tries (or should try) to deliver a certain current, not a certain voltage, and without load the voltage goes up to the limit of the device. That's what the first posting was talking about.

[Riddle]

Well, in case the device is not protected against short circuits, the relay should switch between the electrodes and a resistor of e.g. some kOhms.

Anyway, just interrupting the output is likely to generate high voltages since the device tries (or should try) to deliver a certain current, not a certain voltage, and without load the voltage goes up to the limit of the device. That's what the first posting was talking about.

I don’t believe that TENS units all operate the same way and you may not have the same problem with whatever TENS unit you use. Regardless, I believe that your circuit design with multiple relays is overly complicated for the purpose. Much simpler to either control the 9V or the intensity dial so that the device only runs while actually in use. Better yet, buy an estim device with either audio input or serial data control for something much better than either on or off type control.

[KinkInSpace]

I can confirm that they don't. I have a very cheap estim box and I wanted to use this as a touch-to-get-shocked device, similar as a cattle prod.

I got a bipolar wand with two points outwards, and another bipolar wand with two sticks that have contact points. Both can be connected to this powerbox.

The reality however is that when I connect the electrode to the powerbox, turn it on at any setting, for example the max, wait a bit and then touch the skin, rather than getting a shock, it slowly builds up electricity.

Given that electricity does not "flow" if there's no complete circuit, I thought, okay, I'll first touch the skin for a bit to get the estim at a higher level, and then remove from the skin, and touch somewhere else.

Nope, exact same thing happens. This cheap powerbox has a build-in safety it seems.

I tested the same with a different powerbox, and although it did not have the build-in safety mechanism, it behaved a bit erratic and I did not trust the device to use the electrode with a session as intended. Like the channels are not properly shielded and shocking myself did some internal damage.

I feared continuing would damage that powerbox. And the estim did not feel that good anyway. Not the effect I was after.

[muestrom]

My first comment refers to the very first posting of Kurbak.

I agree that different devices have different designs, and I agree that switching on/off on the output side is not the best idea. However, Kurbak tried this, and he experienced high voltages after switching on.

If he or anybody else experiences the same problem, he/she should use a bipolar relay (most relays are bipolar) and connect the other output of the relay via a resistor to the other output of the device. This is by no means difficult or expensive, it is just a few cents for the resistor (and maybe some wires), see the red addition in the drawing.

When the resistor has a value similar to the body part between the electrodes, the tens unit should not be able to notice the switching. Therefore this should work for any design of the Tens unit.

[Riddle]

My first comment refers to the very first posting of Kurbak.

I agree that different devices have different designs, and I agree that switching on/off on the output side is not the best idea. However, Kurbak tried this, and he experienced high voltages after switching on…

I understood what you meant and what you were referencing. Regardless, you acknowledge that switching on/off on the output side is not the best idea. The original post had a major issue with this. What I do not understand is why you are still suggesting relays on the output side for controlling the output. The relay configuration you just described would still have the output not connected to anything for the brief time period while the relay switches between the resistor and electrode connections.

Installing the relay at the battery would be much easier and more likely to function as intended. If someone did not want to permanently modify the battery circuit, they could simply purchase some 9V battery terminals and then wire two of those together with a relay included in that circuit. This could be plugged into the TENS battery terminal and attach the 9V battery to the other side of it. Simple solution much more likely to function correctly.

[ruru67]

I agree that different devices have different designs, and I agree that switching on/off on the output side is not the best idea. However, Kurbak tried this, and he experienced high voltages after switching on.

I have an old TENS-3000 unit that I have done this with, and it doesn't have the "stored power" problem; in fact I originally switched the battery input and found it tended to ramp up and down the power on switching rather more slowly than I would have liked, rather than the on/off I wanted, hence moving to switching the output. (That also meant I could switch the outputs individually.)

But if does have a cap on the output that is giving you a bit much initial zap, then your solution with a spdt relay connecting a high value (say 10k) resistor should work.

While that initial zap is probably a bit alarming and not what you want, I doubt that it will be in any way dangerous - the stored power will be giving you a longer pulse rather than a higher voltage one.