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What is the Purpose of this Resistor?

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=142775
Printed Date: April 19, 2024 at 7:15 AM


Topic: What is the Purpose of this Resistor?

Posted By: davep.
Subject: What is the Purpose of this Resistor?
Date Posted: March 20, 2017 at 5:47 PM

I have used this circuit on several occasions. It works. I always configure it with the capacitor and resistor values shown. I believe the resistor is to discharge the capacitor. But I have a question: The relay coils I use are 85 Ohms. Both the coil and the resistor are in parallel. Do I really need the 10K resistor as well? It seems to me that with the 82 Ohm resistance, the circuit won't even know the 10K is there too.

Thanks in advance. Both for the explanation, AND the diagrams. I've used a bunch of them as food for thought over the years and come up with some cool installs.

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Replies:

Posted By: astuff
Date Posted: March 30, 2017 at 10:44 PM
Just guessing but the capacitor is there to produce a slight delay and/or reduce chatter. Resistor is not needed if relay has internal diode or resistor to take care of the coil current/voltage spike when turned off.




Posted By: davep.
Date Posted: April 10, 2017 at 2:58 PM
Thanks for the reply. The cap and resistor in this circuit are not related to "anti spike" protection.
The capacitor is there to keep the coil energized until the cap discharges, which is about 3 seconds with these values. I was asking about the purpose for the resistor that is in parallel with both the cap and the relay coil.




Posted By: astuff
Date Posted: April 12, 2017 at 4:59 PM
Why do you think that the resistor is not related to spike protection? If there is not a resistor or diode built into the relay the recommendation is to add one.

Start with page 11: https://www.autoshop101.com/forms/hweb2.pdf




Posted By: the12volt
Date Posted: May 08, 2017 at 8:11 AM
When I started adding capacitors to relay coils in the late 70's, early 80's, I frequently had to add the resistor to eliminate the relay chatter I would get. Since then, I've always added it to play it safe.

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