Relay won't release...Help...Please!

[Chris305]

Hi, I hope this is not a stupid question and I would really appreciate a few kind, helping words here. I'm using a PIC micro to pic a solid state relay (crydom PF240D25) via a ULN2003. The relay completes a circuit that provides 12V to a motor. When I used my test motor (0.6 amp motor) everything worked fine (motor turned on and off when it was supposed to). When I used my real motor (2 amp motor), the relay would pick and run the motor fine, but it won't shut down. I checked to make sure the control voltage at the relay and found it to be about .005V. The relay isn't permanantly "stuck" because the smaller motor still works. The relay is rated for like 20 amps so I'm not overloading it. Please any help would be much appreciated.

 

[ScottyUK]

I this an AC or DC motor? Sounds a stupid question but...

Assuming an AC motor, there was a thread with similar problems just recently: thread956-206857


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Yep, To large an inductance and the SSR can not shut off.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Chris305]

Thanks for the replies! Both motors were 12VDC. I guess I should just go back to a mechanical relay.

 

[ScottyUK]

Chris,

It is an AC relay, which will use a thyristor or triac as the switching device. Neither a triac nor a thyristor is a good switch for DC because they don't inherently switch off when the gate signal is removed - they latch on and wait until the current reaches zero, as it will when an AC supply swings from positive to negative polarity when they switch off. Most DC SSR's use either a MOSFET or an IGBT as the switching element.

There are few DC rated PCB mounting SSR's from Crydom. The CMX60D20 is a MOSFET type good for 20A at up to 60V.

http://www.crydom.com/userResources/productFamilies/141/Crydom_CMX60D20.pdf

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If we learn from our mistakes I'm getting a great education!

 

[roydm]

Chris,
Before the advent of high power transistors It was common practice to use SCRs for speed control of DC motors in battery vehicles. As a previous poster pointed out the trick is to get the current to fall to zero. A second smaller SCR(b) in series with a charged capacitor was connected in parallel with the main SCR(a). Firing SCR(b) causes the capacitor to discharge into the load reverse biasing SCR(a) allowing it to turn off. The capacitor current quickly reaches zero allowing SCR(b) to also turn off. It's a while since I worked on DC Drives but I'm sure you could use 2 SCR based relays in a similar scheme. I suspect there may be a simpler solution now-days using a transistor based relay. I will follow this tread with interest.
Regards
Roy