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Relay Circuit

I have a PCB mounted relay that is driven from a transistor. So the collector of the transistor is connected to the anode of a diode and to one side of the relay coil. 5 VDC is connected to the cathode of the diode and to the other side of the relay coil. This relay switches L1 which then goes to an off board relay coil. The other side of the off board relay coil goes to L2. The off board relay has an RC (snubber) across the contacts and drives an inductive load (motor). My concern is that although I have some protection for the PCB relay on the coil side? Am I at risk for RF noise and/or contact wear by not supressing the line side of the PCB relay? I thought of using another RC across the PCB relay contacts but this would allow L1 to bleed through an activate the off board relay...no good! What about an MOV or diode? Would it be better suited here? Other ideas?

Also, this transistor is driven from a PIC via a 1k ohm resistor to the transistor base. Now, shouldn't there be a pulldown between the 1k resistor and the base of the transistor? I'm pretty certain that is the case; but what are the reasons and guidelines for doing this?

Thanks.

Reply to
richard.bair
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The remote relay has an AC coil or DC?

You can put the RC across the relay coil rather than the contacts. That means a C short won't turn the relay on, and will probably self-heal a film C.

MOV is of dubious benefit, IMHO, and could be a problem when it fails shorted. A diode would be okay if it's a DC coil (across the coil).

No, that would just be silly. The PIC output is push-pull. Assuming it's not a safety-critical system.

Watch your layout, especially around the drive transistor. You may want to kill the dv/dt at the collector with a small cap to E.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

The answer depends on the specifications of the relays involved.

Relays will have a contact rating based on voltage and current being switched.

In general you can probably switch the coil of another relay from the contacts of the first relay.

There is possibility that when the on board contact opens, the inductive kick from the off board relay will get into the PCB and affect it. A tranzorb or MOV would normally be used to take care of that.

For the trasistor it all depends on low voltage from the PIC. Now adays with most devices built from CMOS the output will go to zero and there is no need for a pull down on the base. When TTL logic was common the output would not go to zero and a pull down was needed to make sure the transistor turned OFF.

Dan

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Dan Hollands

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Reply to
Dan Hollands

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