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Re: [Amps] Checking relay timing with a scope

Subject: Re: [Amps] Checking relay timing with a scope
From: Manfred Mornhinweg <>
Date: Mon, 22 Jun 2009 02:35:35 +0000
List-post: <">>
Hi Pete, and all,

I'm not sure if my advice is welcome at all, given that you want to do 
something useful with your new scope. But for measuring the closure time 
of a relay, or any other such slow one-time phenomenom, I wouldn't use 
an oscilloscope (unless it has memory). Instead, for this kind of test I 
use my computer's sound card.

The system is quite simple: Wire up the relay as already suggested by 
other forum members, but to the soundcard input instead of the scope. 
You can wire the coil to one channel and the contact to the other 
channel, and you can connect the contact with a resistor network so that 
you get three different voltage levels for one position, the other 
position, and in between while the contact reed is travelling.

Then you use any sound recording and editing software. You set it to 
record, then you activate the relay, maybe just on and off again, or 
perhaps a few times, to see if the timings are repetitive. Then you stop 
the recording, and look at the waveforms recorded. Sound editing 
software usually makes it easy to precisely measure the time between any 
features you want on those oscillograms! And you can take all the time 
you want for it.

A sound card cannot record DC levels, but that's no problem. A switched 
DC that's connected to the soundcard will register as a positive pulse 
when it's switched on, and a negative pulse when it's switched off. And 
that's all you need to precisely measure the times.

I also use the soundcard for non-radio things such as measuring the 
exposure time and shutter curtain travel time of photo cameras. In that 
case I use two photodiodes wired to the soundcard inputs.


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