ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

Page 1 of 3 1 2 3 >
Topic Options
Rate This Topic
#67085 - 06/25/06 09:10 PM 12 volt motor delay
BigB Offline
Member
Registered: 03/31/04
Posts: 719
Loc: 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.
Top
Work Gear for Electricians and the Trades
#67086 - 06/25/06 11:08 PM Re: 12 volt motor delay
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9036
Loc: 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
Top
#67087 - 06/26/06 03:29 PM Re: 12 volt motor delay
JoeTestingEngr Offline
Member
Registered: 11/04/05
Posts: 781
Loc: Chicago, Il.
I would strongly suggest using a more conventional time delay relay or method to drive your final output relay or contactor.
Joe
Top
#67088 - 06/26/06 05:17 PM Re: 12 volt motor delay
JoeTestingEngr Offline
Member
Registered: 11/04/05
Posts: 781
Loc: 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
Top
#67089 - 06/26/06 06:49 PM Re: 12 volt motor delay
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9036
Loc: 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
Top
#67090 - 06/26/06 06:52 PM Re: 12 volt motor delay
JoeTestingEngr Offline
Member
Registered: 11/04/05
Posts: 781
Loc: Chicago, Il.
See, I guess we old computer guys think alike.
Joe

[This message has been edited by JoeTestingEngr (edited 06-26-2006).]
Top
#67091 - 06/26/06 08:07 PM Re: 12 volt motor delay
BigB Offline
Member
Registered: 03/31/04
Posts: 719
Loc: 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.
Top
#67092 - 06/26/06 09:29 PM Re: 12 volt motor delay
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9036
Loc: Estero,Fl,usa
This is a lot easier if you are using a relay with a DC coil.
_________________________
Greg Fretwell
Top
#67093 - 06/26/06 10:13 PM Re: 12 volt motor delay
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9036
Loc: Estero,Fl,usa
_________________________
Greg Fretwell
Top
#67094 - 06/26/06 10:31 PM Re: 12 volt motor delay
JoeTestingEngr Offline
Member
Registered: 11/04/05
Posts: 781
Loc: 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
Top
Page 1 of 3 1 2 3 >

Member Spotlight
Member Since: 04/03/02
Posts: 6775
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 85 Guests and 8 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 51
HotLine1 43
gfretwell 17
Trumpy 16
Ruben Rocha 13
 
Newest Members
clee512, Jdscott2005, FAIZAN, Regitest2, sureshazhagai

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals