relay coil low voltage drop out.

[o07wray]

Hello,

I have a open c frame 12vdc dpst Dayton 30amp relay I am using on my hot rod to run my dual electric fans ( these are huge mommas 28amp draw each! ) and they will smoke a standard relay fast, also solid state is out due under hood heat 120+deg.

What happens is this.. the thermo ( solid state output ) sences when the temp gets to 150deg..and supplies a clean 12vdc to the coil. the relay will close for a split sec.. then open.. then close again, and open again, but they stay closed for a little longer, then they open again, and then stay closed a little longer again.. then open, and then close and stay closed.

As fan speed incresses ( and start current goes down because they are spinning ) the closure time incresses in proprotion.

what happens I think is this: the massave draw of the 60amps of both fans causes the coil voltage to drop below the hold-in of 11vdc.

the fan has a separate leads from the batt. and the relay coil + is pulled directly form the batt.

I have seen a diag for a large cap & resistor w/ 2 diodes that will act as a voltage supply on a temporary basis and keep the coil voltage up for a 1/2 sec or less...
and we are not talking about a arc gap suppression rig.. just a brown out voltage supply..

I have strained my brain and can not remember how to do this,,

Please help, and thanks in advance,
Jeff

 

[Dr_Doggy]

get a second 12volt?

 

[o07wray]

its a single batt system in s 99 chevy S-10 good charging system and runs at 14.5v, I have 300amp power master alt... is just that sudden demand of 60+amps that makes the coil drop out.

 

[Reloadron]

You can try something like the attached. I am not sure that when calling for cooling the cap will have time to charge before the battery voltage drops. The diode across the relay coil is just there as a snubbing diode. You could likely use a 1N4003, 1N4004 or anything above 1N4002.

Ron

 

[Boncuk]

I guess it's the sensor circuit which gets "disturbed" by the high current flow and experiences a power surge.

Use a properly rated electrolytic cap of 2,200 to 4,700 µF for the sensor circuit to overcome the surge.

Boncuk

 

[o07wray]

Ron,

Thanks for that diag.. but thats simlar to what I have areay done, with the result you spoke of .. the cap take to long to charge, it will actulay make it chatter more!!

what I remember was a cap and resistor w/ a zenier diode to keep the cap charged. the zeiner passed the current to the coil and the Resistor backed up the flow of current and was before the Resistor, and would allow the cap to retain a charge at a threshold just below the zeiner's break down voltage.


Boncuk,
and the sensor singal is clean.. no problem


Thanks again
Jeff

 

[o07wray]

Ron,

Thanks for that diag.. but thats similar to what I have allready done, with the result you spoke of .. the cap take to long to charge, it will actually make it chatter more!!

what I remember was a cap and resistor w/ a zenier diode to keep the cap charged. the zeiner passed the current to the coil and the Resistor backed up the flow of current and was before the Resistor, and would allow the cap to retain a charge at a threshold just below the zeiner's break down voltage.


Boncuk,
and the sensor signal is clean.. no problem


Thanks again
Jeff

 

[ericgibbs]

Ron,

Thanks for that diag.. but thats similar to what I have allready done, with the result you spoke of .. the cap take to long to charge, it will actually make it chatter more!!

what I remember was a cap and resistor w/ a zenier diode to keep the cap charged. the zeiner passed the current to the coil and the Resistor backed up the flow of current and was before the Resistor, and would allow the cap to retain a charge at a threshold just below the zeiner's break down voltage.


Boncuk,
and the sensor signal is clean.. no problem


Thanks again
Jeff

hi Jeff,
I would consider a second open c frame 12vdc dpst Dayton 30amp relay thats powered from a delay circuit controlled by the first relay.

The sensor energises relay #1 for fan #1, after a delay relay #2 energises and turns on fan #2.

I guess the fans are 12V, so it would be fairly easy to use fan #1 voltage supply to energise relay #2 after a res/cap relay.?

 

[crutschow]

If you can, try putting Ron's diode-capacitor circuit at the input power connection to the thermo sensor. That way it has plenty of time to charge up and be available to pull-in the relay when the thermo sensor closes.

 

[o07wray]

Here is an idea of what I was trying to remember....

 

[ronv]

Drop out

You don't say how much the coil of the relay draws. That will determine to some extent the size of the cap. For the battery voltage to drop below 11 volts the internal battery resistance is .03 Ù or so. This seems a little high to me. Use a Schottky diode it will save you a little drop. Below is a simulation for a 100 ma coil. It only buys you 150 ms or so. So if the coil draws that much a bigger cap may be in order.