Dual Coil Relay Problems

[dbbaker2007]

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.

Thanks

 

[FvM]

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.

 

[chuckey]

"The BJT's were dying out after switching" seems that either their Ic/Vc or the freewheel diode are wrong! :-(
Frank

 

[dbbaker2007]

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.

Thanks

 

[FvM]

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