I have the following basic circuit:

circuit picture

As I was flipping the button to turn the relay on and off, I noticed sparking from the voltage meter, and it eventually caught fire, releasing the magic smoke which makes it work. On testing, it appeared that the 7805 was also destroyed.

I'm thinking that the relay was to blame here. Would adding a diode from ground to input help the situation? What else could I do to protect this circuit? (this is a controller driving an electric vehicle).


1 Answer 1


I'm guessing your switch was actually in position A rather than B in the diagram below (so it switched off the meter and Arduino too). If not, it's unlikely you could get 35V+ spike to kill the DVM and 7805 since the 24V battery would absorb it. If you have a bench supply or similar rather than a huge battery, then it could cause problems.

In any case, a diode as shown will kill the inductive spike. Rating depends on the relay rating but a 1N5404 will work for just about any relay.

enter image description here

  • \$\begingroup\$ this is one of the confusing things; the button really was where i put it in the circuit. the arduino/everything else remains on as soon as it's plugged in, and the button is meant to only turn the relay on/off. the 24V source is two deep-discharge marine batteries. \$\endgroup\$ Aug 13, 2014 at 21:24
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    \$\begingroup\$ If the wires are long to the batteries and not close to each other it's possible there's enough inductance to cause problems, especially if the relay is a monster. I take it that the main motor was not active.. \$\endgroup\$ Aug 13, 2014 at 21:29
  • \$\begingroup\$ yup, the controller is around 5 feet away from the relay, the battery wires are around 3 feet, and the relay is huge. glad that this makes sense. related question: should i be putting the diode closer to the relay, or is further away better to use the wire resistance to slow down the current drain? \$\endgroup\$ Aug 13, 2014 at 21:54
  • \$\begingroup\$ Right across the relay should be fine (just a few inches of wire, right?), but I don't think much of your chances of not killing the H-bridge without some significant capacitance close to the H-bridge rails when the motor is being operated. \$\endgroup\$ Aug 13, 2014 at 23:27
  • \$\begingroup\$ oh it's actually a sabertooth 2x60, which is a whole package around an hbridge. thanks for all the help! i added the diode and things seem to be working so far, and also the relay sounds smoother. \$\endgroup\$ Aug 14, 2014 at 20:37

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