# TENS spike smoothing when using a relay

I'm currently messing around with a TENS unit and would like to block/unblock the signal from reaching the muscle (Alterations to the TENS unit circuitry aren't possible).

The unit uses a 2 mm jack which runs two wires to a conductive pad each.

My assumption was that splicing one of the wires into a relay would give the desired effect, which for the most part it does.

What I'm finding, however, is that if the relay is turned on while the TENS unit is generating a signal, a sharp spike is being sent to the muscles.

Is there an easy approach to solving this issue?

(I found the following at a quick glance: Preventing spikes when plugging jacks Could this possibly be the right solution to my issue?)

• post a diagram that explains your proposed method( relay based). so it will be comfortable for the people to help you – yogece Apr 11 '19 at 7:53

Your problem lies in the way TENS units operate.

This site shows how a simple one is built.

This is the circuit diagram from that site:

The output uses a capacitor in parallel with the transformer to form a resonant circuit - this makes the output voltage higher than if there were no capacitor.

When you are connected to the TENS device, your body acts as a capacitor (and as a resistor.) This damps the resonance.

Because of this, the output voltage is lower when you are connected than when you aren't. Or, put the other way around, the output voltage is higher when you aren't connected.

TENS devices usually operate in pulses. But, you can't guarantee that your relay will switch between pulses. So, sometimes you might get a smoother switchover (no jolt or weak jolt because you switched between pulses) than other times (major jolt because you switched in the middle of a pulse.)

Because of the resonance, there's probably not a time when there's no signal present at the output.

What you would have to do is to switch from "connected to you" to "connected to an electrical dummy of your body."

You would use a relay to switch between the two instead of simply open or closed. You might need something faster than a mechanical relay, since the TENS device can create pulses faster than a mechanical relay can switch.

An "electrical" dummy in this case would be a resistor. A few kiloohms ought to do. If the resistor value is too high, you will get jolts. If it is too low, you will get a sort of "fade in" effect where it takes a pulse or two to get back up to the desired intensity.

You seem to be using a real, medical TENS device. Hence your reluctance to modify it. I'd go a step further and say you shouldn't modify the treatment plan, either. Use it as prescribed, and don't experiment.

If this isn't a medical device, I'd still say "leave it alone." You can hurt yourself with those things if you mess up. The level of your question says you don't have the skill or knowledge to play with high voltage devices connected to your body.

• Wow, Thanks very much for the thorough answer that's an awfully large help. I'll have a look at sorting out an alternative switching technique and dummy body. The system i'm working with is cheap grade from amazon (£25), with me controlling the intensity and keeping it low. I fully agree about my low level understanding of electronics, so it's something that if I deem is getting anywhere near dangerous i'll make a stop to messing with the system. – calcif Apr 11 '19 at 11:18