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#67085 - 06/26/06 12:10 AM 12 volt motor delay  
BigB  Offline
Member
Joined: Mar 2004
Posts: 719
Tucson, AZ USA
This is slightly off topic but I can't seem to locate a 12 volt motor delay for a project I am working on. It needs to be rated at least 30 amps 12 volts DC and be able to delay starting for one to two seconds.


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#67086 - 06/26/06 02:08 AM Re: 12 volt motor delay  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
How hard do you want to work? You can buy a Potter Brumfeld time delay relay for a one part fix or you can cobble something up yourself.


Greg Fretwell

#67087 - 06/26/06 06:29 PM Re: 12 volt motor delay  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
I would strongly suggest using a more conventional time delay relay or method to drive your final output relay or contactor.
Joe


#67088 - 06/26/06 08:17 PM Re: 12 volt motor delay  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
OK, I might get in trouble for this BUT:
I looked at some of the TDRs in the range that you need and they are very expensive and, I think, overkill. So at the risk of a DIY label, I would probably solve the delay problem with an SCR. I would use a flyback diode across the relay coil and return the coil to the - supply through an SCR. Add 1 resistor, 1 cap, and maybe a trigger diode for style points. The 12 volt supply will charge the cap through the coil and resistor. The SCR fires after a short time delay, flowing enough current to pick the relay. The cap discharges, at least partially, when the SCR fires. When you kill the supply the current from the back EMF flows through the flyback diode, the SCR turns off and you're ready for the next cycle.

I have seen a small, metal encased relay with just 2, .250" quick connect terminals on it. You put it in series with the load for a short on delay. I think that it is for 120AC loads though. I'll post it if I find one that will work at 12VDC.
Joe


#67089 - 06/26/06 09:49 PM Re: 12 volt motor delay  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
I did something like this for the contactor on my spa. I really only needed a part of a second but all I used was a capacitor and a resistor to the gate of the SCR. The voltage resistor to the gate is limited by the charging capacitor.


Greg Fretwell

#67090 - 06/26/06 09:52 PM Re: 12 volt motor delay  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
See, I guess we old computer guys think alike.
Joe

[This message has been edited by JoeTestingEngr (edited 06-26-2006).]


#67091 - 06/26/06 11:07 PM Re: 12 volt motor delay  
BigB  Offline
Member
Joined: Mar 2004
Posts: 719
Tucson, AZ USA
Actually I could easily use a relay, allowing me to get away with a much lower amperage rated delay. I am not sure how to build a delay, I am weak in electronics. I figured to just buy one. Altho seeing what you guys posted has got me fired up enough to look for some good learning resources.


#67092 - 06/27/06 12:29 AM Re: 12 volt motor delay  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
This is a lot easier if you are using a relay with a DC coil.


Greg Fretwell

#67093 - 06/27/06 01:13 AM Re: 12 volt motor delay  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa


Greg Fretwell

#67094 - 06/27/06 01:31 AM Re: 12 volt motor delay  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
OK BigB, I just sent you an Email with a screen capture of a CADD drawing. There are alot of reasonably priced automotive style relays out there that will handle 30A. The time delay is what makes things get pricy. It is easy to tweak this circuit but a couple of things should be considered. If you were to use a very low value resistance and a very high capacitance, it would be possible to pulse the relay with the cap charging current. If you add your coil resistance and resistor value and multiply by the capacitance in Farads, you get a "time constant" in seconds. In theory, the voltage on the gate of the SCR would reach about 63% of your 12 volt supply in this time. But the SCR will fire in < 1 time constant unless you add something to the gate circuit. Why don't you start by grabbibg a 10k & 100k resistor and 100uF & 470uF cap., plug and chug, and see what you come up with. Feel free to Email me with comments, questions, or descriptions of the smoke rings.

My apologies for morphing this into a theory post.
Joe


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