# Calculation of the latency of a relay

I have a SPST relay that I do not have the full datasheet for, but I wish to calculate the time it takes to switch on a current for a particular voltage. Is there any easy general way to do this? My first thought would be to time it for a clicking noise, but I don't think that would be accurate for what I need.

My relay is labeled "9V" and rated for 250V AC. I plan to switch on a set of capacitors that will be a few miliFarads at around 350 volts - the current will be around 25A for a very short period of time. Ideally, it would be great if it would be a possible to design a simple circuit that could test this delay accurate to a few jiffies.

Thanks.

EDIT: I'm assuming the relay will have a pretty consistent off-on switching time. If it doesn't, is there any switching circuit with a latency between 10MS and 100MS that can handle the above specifications?

Do you have an oscilloscope handy? A quick way to measure the switching time would be to operate the relay with probes hooked up to the control coil and the switching side of the relay. You would need to hook up some kind of dummy load so you can see it turn on. You would need to be able to monitor both the channels on a single scope, so that you can measure the time delay between the two signals.

Switching time for relays is usually in the single digit millisecond range or lower, so it would be hard to time it yourself.

• Ah - unfortunately I don't have an oscilloscope handy. If the switching time for a relay is indeed a few miliseconds, I might have to reconsider my design... May 9, 2013 at 5:14

If you have an Arduino or any other microcontroller devboard handy, you can set up an experiment to measure the delay.

Just hook up the relay coil to a micro via transistor (don't forget the protection diode!) and put a dummy load (something like 10k resistor) on the output of the relay and connect it to input pin of a microcontroller. Then write a simple program to switch the relay on and count time it takes the relay to switch on. If you use timers in input capture mode for that, you can get pretty accurate results.

One thing to note is that relay will bounce, so you might get really deep into it, like measuring bounce time, etc.

Edit: regarding the alternative - a solid state relay with zero crossing detection probably will have latency in your range or lower. Take care to select one that can handle more than 25A instanteneous current.