12 volt dc oil burner

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12 volt dc oil burner

Post  jjtjr on Thu 23 Feb 2017, 1:28 am

This is for the cars that use a 12 volt dc oil burner. I've been driving the car often and had a problem that came up a few days ago. The relay that controls the burner on/ off cycle failed. What I found was the relay contacts that the 12 vdc go thru for the burner, started to pit and not pass the 12 volt thru them. This is due to the back emf (collapsing magnetic field) from the motor when the contacts open, you get a arc across the contacts which causes pitting and carbon on the contacts. The next time the boiler calls for the burner to turn on the relay closes but the contacts are non-conductive and the burner wouldn't run. This is what happened to me. The fix is a simple one, you can clean contacts or replace the relay then you have to add a snubber circuits across the relay contacts. The circuit is a 12 ohm resistor 1 watt and 12 mfd 25 vdc capacitor which are wired in series and connected across the open contacts.


Theory of operation.
The contacts are normally open, the snubbing circuit does nothing at this time, then power is applied. The boiler calls for pressure the relay closes and the burner motor turns on. When the pressure is reached the relay contacts open and the back emf charges the capacitor, ( this action is absorbing the back emf from the motor) which eliminates contact arcing. When the relay closes again calling for heat, the contacts short the capacitor & resistor and discharges it. It is now ready for the next opening cycle of the relay contacts. This simple circuit will save the relay and not leave you broke down. As what happened to me.

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Re: 12 volt dc oil burner

Post  lynnr on Thu 23 Feb 2017, 10:19 am

Nice fix. I was thinking the same, Snubber, as I was reading.
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Re: 12 volt dc oil burner

Post  Ian on Thu 23 Feb 2017, 3:11 pm

jjtjr wrote:This is for the cars that use a 12 volt dc oil burner. I've been driving the car often and had a problem that came up a few days ago.  The relay that controls the burner on/ off cycle failed. What I found was the relay contacts that the 12 vdc go thru for the burner,  started to pit and not pass the 12 volt thru them. This is due to the back emf (collapsing magnetic field) from the motor  when the contacts open, you get a arc across the contacts which causes pitting and carbon on the contacts. The next time the boiler calls for the burner to turn on the relay closes but the contacts are non-conductive and the burner  wouldn't run. This is what happened to me. The fix is a simple one, you can clean  contacts or replace the relay then you have to add a snubber circuits across the  relay contacts.  The circuit is a 12 ohm resistor 1 watt  and 12 mfd 25 vdc capacitor which are wired in series and connected across the open contacts.


Theory of operation.
The contacts are normally  open, the snubbing  circuit does nothing at this time,  then  power is applied. The    boiler calls for pressure  the relay closes and the burner motor turns on. When  the pressure is reached the relay contacts open and the back emf charges the capacitor, ( this action is absorbing the back emf from the motor) which eliminates contact arcing. When the relay closes again calling for heat, the contacts short the capacitor & resistor and discharges it. It is now ready for the next opening cycle  of the relay contacts.  This simple circuit will save the relay and not leave you broke down. As what happened to me.

If its the standard Modelworks or STW relay you are replacing dont forget the diode across the relay coil.

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relay

Post  jjtjr on Thu 23 Feb 2017, 8:40 pm

yes the diode is across the coil
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Re: 12 volt dc oil burner

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