# Boost converter safety during debugging

When I was debugging my microcontroller-based boost SMPS I accidentally got 85V (instead of 15V as I intended) across the 1kΩ test load resistor. Does this spell a hazard for the one debugging the circuit?

The SMPS is powered from a 9V wall wart capable of 1A max, and the microcontroller is powered with AMS1117 drawing power from the 9V rail.

## 3 Answers

The worst case scenario for what you did would be that all 9 watts were available at the 85 volt end of that circuit. This would result in a maximum current supply of 100 milliamps across a dead short. Depending on how contact was made through the body, this does have the ability to be lethal.

In most conditions, the worst thing I would have expected to happen to you is you get finger zapped, but the point here is that you must always exercise caution with live equipment. The Navy trains that anything above 30 volts must be treated as potentially lethal, and deenergized for work whenever possible.

Yes, it does pose a hazard. 85V is unlikely to kill anyone, but people have been killed by lower voltages. Even with an output limited to somewhat less than 9W it could be dangerous. Also, there's no guarantee that the voltage wouldn't go higher than 85V given the right fault. To be safe you should have some sort of over voltage protection circuit or clamp.

you can implement something called a Crow Bar circuit, you can learn all the wonderful things about these by reading the Art of Electronics by Horowitz and Hill (1989) and they are cheap and simple to set up. The crow bar circuit clamps the node it's attached to when the voltage rises above the defined clamping voltage - and continues to conduct until the fault condition is removed. This is often used as over-voltage connections from out-side sources, and is much safer when there is a fuse or sacrificial element between the high/over-voltage source and the crow bar circuit.

In your case, it will at least pull the voltage down to the clamping voltage, but the circuit itself is not immune to damage and your crow bar (if the input source is powerful enough) may fail either to a complete short circuit, or in open circuit where your "protection" is now void.