How to view automotive relay voltage transients

[NerdyGirlyGirl]

How to view 12V automotive relay voltage transient with an Oscilloscope

I have a 5 pin 12V 30amp automotive relay and need to capture the transient voltages created when the magnetic field collapses in the coil. How do I make the connections with the oscilloscope to be able to capture these voltage spikes?

I connected my scope to pins 86 and 85 and can see the voltage and drops from the power supply but not the transient spikes across the coil.

The assignment is capture the voltage transients with the scope both with a suppressor circuit and without - just a high switching diode on the coil in reverse bias pins 86 and 85 and then adding a Zener Diode in series with the switching diode across the coil.



Thanks.....

 

[mpfisher]

Set your scope's trigger level above 12V and trigger mode to single-shot. It should detect voltage spike and trigger when you disconnect the power to the relay. If its not detecting, the scope may be too slow.

 

[KlausST]

Hi,

The voltage spike will be negative with respect to the usually applied voltage.

Depending on how you connect the scope you may set a negative trigger threshold with negative slope.

Klaus