Overvoltage damage to relay which breaks an inductive current?

[treez]

Hello,

Please can you help to solve the problem of overvoltage damage to a relay which breaks inductive current in a LC filter?

I am doing a retrofit of LED reverse lights into a car. -Previously, these used incandescent bulbs in this car.

The power to the LED lights is the same as when the incandescants were used…that is, an upstream relay simply switchs off power to the LED lights, just as it did with the previously fitted incandescent bulbs.

However, the LEDs are powered by an SMPS LED driver, and as such , we need to have an LC filter at the input to the Reverse Light module.

When the relay switchs off, the inductive current in the filter inductor rings with the input capacitor and the input node to the reverse light, which is also the relay node, rings up to 120V.

The high voltage ringing at the relay node will damage the relay, do you agree? (due to sparking at its contacts as it opens)
Should we put a freewheeling diode around the filter inductor to reduce this voltage?

Please find attached,
1…a picture of the ringing voltage waveform at the input node.
2…The LTspice simulation
3…The schematic.

 

[Tunelabguy]

Assuming high-side switching by the relay, just connect a diode between the output of the relay and ground, pointing away from the ground. When the relay opens, current will flow up from ground into the inductive power supply. You do not have to get into the innards of the power supply itself.

And after looking at the schematic, I see that it is inded high- side switching.

 

[treez]

yes but the relay is in the part of the car that we retrofitters do not have access to......we cannot make changes there.

 

[FvM]

yes but the relay is in the part of the car that we retrofitters do not have access to......we cannot make changes there.

No, you didn't read the suggestion thorougly. Tunelabguy is suggesting a simple free-wheeling diode. It would be connected in parallel to C6.

 

[chuckey]

The diode if connected across C6 would have to have its cathode to the +ve line, so it would not conduct on +v rings. On way would be use use a zener diode with a breakdown voltage greater then the expected maximum on the 12V battery line, I would say 20V, connect cathode to +ve line anode to earth.
Frank

 

[FvM]

The diode if connected across C6 would have to have its cathode to the +ve line, so it would not conduct on +v rings.

There won't be no "+v rings" when the relay current is commutated to a free wheeling diode on switch-off. If you place a zener-diode, it must be able to absorb load dump events without damage.

 

[Tunelabguy]

There won't be no "+v rings" when the relay current is commutated to a free wheeling diode on switch-off. If you place a zener-diode, it must be able to absorb load dump events without damage.

True. The voltage at C6 will merely go down to -0.7v until the coil is done drawing current, then it will drop to 0v. No ringing will occur.