Making a Bluetooth adapter for a Car Phone from the 90's

Only the relay current passes through the flywheel diode, as long as the circuit is intact. An open circuit in the power connection could cause an overvoltage pulse, though.

Close to the load is preferred, but a remote diode can still work.


Another simple solution is to add a VDR across the switch device in the phone? We use those in a product to balance a fast turn-off with limited voltage spiking in a product we've been making for around 30 years.
The relay flyback pulse should be tiny compared to the energy from the 24V clutch & brake our units switch continuously.

eg. These look suitable:

rjenkinsgb said:

eg. These look suitable:

ERZV20D180 | Panasonic, V Surge Absorber 40nF 20A, Clamping 36V, Varistor 18V | RS

uk.rs-online.com

Click to expand...

That's close to what I already selected to protect the main power input from surges: ERZV10D220

I need the 10mm size rather than 20mm size to fit inside the car phone transceiver case. The "180" part you linked has a "maximum allowable" DC voltage of 14V, which is too close for comfort on an automotive electric system with the engine running, so I'd probably just use another ERZV10D220 to protect the MOSFET?

The ERZV10D220 has a "maximum allowable" DC voltage of 18V, and clamping voltage of 43V (at 5A).

I can re-use the same UMW 50N06 MOSFET that I'm using for the battery protection circuit, which has a max drain-source voltage of 60V.

That seems like it should work well. Thanks for the suggestion!

UselessPickles said:

Everything I read recommends placing the flyback diode as close to the and directly across the inductive load as possible. If the diode was in the car phone, then would that expose other circuits/components in the vehicle to some kind of side effects?

Click to expand...

In between the inductive load and the car phone is a load of wiring. The inductance of that could affect the car phone output, which is a reason to put the suppression in the car phone.

There is no harm in suppressing in more than one place.

I would definitely not use a simple diode to suppress the coil on a relay that is driving a car horn. See the thread here:- https://www.electro-tech-online.com/threads/diode-on-relay-coil-downside.167257/

Car horns are highly inductive and slowing the rate of change of current in the relay coil by a lot runs the risk of the contact burning. The peak current that a horn takes may be well over the rated current. I've just measured the resistance of the two horns on my car, and it's around 0.3 Ohms for the two in parallel, so peak current around 40 A. The horns themselves have contacts that open and close as the diaphragm vibrates, so the average current is far less.

If you run a horn from a relay that is opening slowly, you risk welding the contacts. That will leave the horn turned on which will annoy the neighbours, may burn the horn out and will kill the battery if left running.

You can put a 30 V zener in parallel with the MOSFET (as long as your mosfet is over 30 V). It will also mean that there is no risk of powering the car phone from the horn feed.

Actually, I suggest a higher voltage zener. Use a zener a bit lower voltage than the MOSFET's peak rating.