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Dual Coil Relay Problems

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Newbie level 2
Jun 14, 2011
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I am having difficulty driving dual coil relays. I have tried several techniques with no consistent switch. I had a switch working with just a simple PNP BJT, resistor, and diodes but the BJT's were dying out after switching. The other problem I seem to be having is that the voltage of the output of the switch is normal until the relay is connected and then there is s significant drop in voltage. The relays I am using are K850 6Vdc 60A.

I am using my microcontroller as the logic source. When a pushbutton is pressed the micro reads it and outputs a value. The relay requires at least a 50msec positive pulse on the open contact in order to open the relay or at least a 50msec positive pulse on the closed contact in order to close the contact.

I have searched high and low with no luck on finding a switching circuit that will work for a dual coil relay. There just seems to be too much current draw and too much of a voltage drop when applying voltage to the relay. If anyone has something working or any ideas that would be great.


Sounds like you're not using the right switching transistors or not a suitable freewheeling circuit. I didn't find a coil specification for the 6V type, but I guess it's around 20 ohm. So small (ICmax 0.5 - 1A) BJT or small MOSFETs should work.

Switched my design to a NPN BJT and a PNP MOSFET. Works fine now. With the MOSFET there is only a mV drop as opposed to several volts. When I originally designed the circuit I was using a NPN BJT into a darlington PNP. The calculation was 145 mA collector of darlington. When it was actually switching it was 600 mA collector of darlington. Anyway it works fine now.

Next question... For a dual coil relay how should a voltage suppression protection circuit go? Right now I just have a diode from the control contact to GND of the relay. If I have a scope on the OPEN contact, when it opens there is no transient, but watching the OPEN contact and closing the relay, there is a spike after the pulse is complete. I can post a screenshot tomorrow.


I think, the circuit you are using isn't quite clear. Can you please post it, so we don't jump into conclusions.

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