Car alternator

[martin1]

I'm working on voltage regulator for a petrol generator but I have a problem. The transistor which controls the field current gets really hot. I've already destroyed a few 150 W transistors mounted on a big heatsink.

I was hoping that someone here knows if it's possible to PWM the field current or will the output DC be "choppy" if you do that?

[Flux]

I don't know why you can't get a linear regulator to work, typical fields take 4A at 12v. The worst case for a linear regulator will be about 6v at 2A or about 12W. This is pretty trivial.

You certainly can pwm control it and in fact all the car regulators use some form of switching. The field is sufficiently inductive that if you switch above about 200Hz and use a flywheel diode the field current is effectively constant. With faster switching it is even better and to be honest it wouldn't matter if the dc output was modulated, batteries probably would be very happy with it.

I have used linear regulators on alternators with much higher field demands than this but unless it is under closed loop control you might as well use a rheostat. Manual control with pwm is perfectly possible and you can use pwm under closed loop if you wish.

Flux

[martin1]

Thank you, Flux!

I don't know why the transistors fail. Maybe it was bad thermal connection between the transistor and heatsink. I used some kind of silicone pad and plastic bolts to isolate the collector from the heatsink. Maybe that was a bad idea.

You say above 200 Hz is even better for PWM. I have a PWM kit from Velleman with a swicthing frequency of 5 kHz. Do you think that's too high?

[Flux]

Silpads are ok but I don't think plastic bolts are up to the job. I also suspect you are using TO220 epitaxial transistors and they are probably not really up to the job of running linear for long periods, The old 2N3055 would be far better especially if you can get one of the older hometaxial types.

5k or higher will be perfectly ok but use a fast rectifier or a Schottky for the free wheel diode.

Flux

[tanner0441]

Hi

For what it's worth when I was messing with car alternators with wind to keep the battery drain to a minimum and remove any start up load,I used a standard darlington amplifyer with a single 2N3055 and a BFY51 so I could turn the power up as the speed went up. I never had any fail.

Brian

[martin1]

It's interesting that both you and Flux had good results with the 2N3055. I use a darlington transistor called MJ3001, also in a TO-3 package and good for 10A.

Right now I think it was the nylon screws that caused the failure. They probably lose their tension when they get warm.

[tanner0441]

Hi

I used pop rivets to hold the 2N3055 to the heat sink.

Brian

[David HK]

This URL leads to an article which may be helpful to you.

http://homepower.com/files/webextras/mark8.pdf

David HK

[Ungrounded Lightning Rod]

I don't know why the transistors fail.

Do you have the freewheeling diode connected?

This is a diode across the field coil, connected so it is back-biased when the power is turned on.  When you turn the power off the voltage at the coil terminal switches sign but only goes to the diode voltage drop:  The current in the coil circulates through the diode and the coil, gradually decaying due to diode voltage drop and coil resistance.  The bulk of the energy from the decay of the magnetic field is dissipated in the coil - at a rate proportional to the square of the current, i.e. about the same heating as if the coil had a constant current supplied which gave the desired magnetization.

If you don't include the diode, when your driver transistor turns off the inductive kick raises the voltage across it until the current keeps flowing - decaying only in proportion to the back voltage across the coil.  The result is that virtually all the energy stored in the magnetic field of the coil is dissipated as heat in the transistor.  Normally a switching power supply's transistor spends nearly all its time either full-on (little voltage drop) or full off (little current) and thus has very little dissipation.  But without the freewheeling diode the transistor is forced into conduction with a high voltage drop and dissipates as much heat as if you had an analog constant-current regulator.

It sounds to me like your freewheeling diode is missing or open.  If so you just have a transistor in series with a coil across a power supply, so the transistor gets hot whether you run it class A (analog proportional control) or attempt to run it class D (switched).

[madlabs]

I'd have to agree that it is probably the diode is shot. I am using a TO-3 package PNP transistor *forget which one, junkbox special) and it get just warm. I'm guessing it would last without a heatsink for quite a while.

Jonathan