CM300 COR output

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pman976
Posts: 1
Joined: Fri Jul 08, 2005 2:40 pm

CM300 COR output

Post by pman976 »

In the past, I have used the COR low to drive a relay.
I have tried it recently and it doesn't go low enough to activate the relay.
Measuring the voltage it will go from 13 vdc down to 6 vdc when signal is present. I am measuring on the one side of the relay coil that is going to pin 8 on the CM300. the other side of the coil is going to pin 13 ( Switched A+).

If I apply a ground to the coil the relay activates no problem.

Any ideas?
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Astro Spectra
Posts: 652
Joined: Sat Sep 22, 2001 4:00 pm

Re: CM300 COR output

Post by Astro Spectra »

How much current does the relay draw from 12 volts and how much current is the COR output of the CM300 specified at?
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jackhackett
Posts: 1480
Joined: Tue Jun 10, 2003 8:52 am

Re: CM300 COR output

Post by jackhackett »

On the CM300s Pin 8 is a programmable digital input/output. It is not meant to drive a relay directly. Internally it's a digital transistor pulled up to 3.3V through a 3.3K resistor. This is meant to work with 3.3V logic circuits.

The transistor is rated for an absolute maximum collector current of 100mA, so it's good in normal use for maybe 75mA. If you're using an automotive relay those are generally 150-200mA, and then there's the matter of the inductive spike if the relay doesn't have a coil diode.

So if you're lucky you haven't damaged the radio yet, but with the readings you're getting I'd think there's a good chance that you have.
You would need some sort of interface between the radio and a relay.
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