0v -3v variable power supply

[oram]

HI folks.

I'm trying to find a circuit that can let me smoothly change between 0v and 3v. I've looked at various circuits (lm317 doesn't got to 0v, and the bcw33 adaptation looks good, but can only find surface mount bcw33... maybe there is an equivalent i can use?)... I looked at some 555 timer circuits but again the transistors being used weren't available. I know that variable power supply is a complicated issue and much debated (and i'm certainly one of them that has realised you can't cut corners with it, lots of smoke, burnt ic's transistors etc) so thought i'd ask for advice on here please?.

Sorry, and to add, it's to change motor speeds. I'm working a lot with old cassette players (which usually run on 3v).

Best

Darren

[David]

The complexity imo depends on the regulation you need. I suppose the simplest circuit is to use a Darlington transistor (because of its high gain) connected as an emitter follower with a potentiometer controlling the base voltage. Put a voltmeter across the motor so that you can keep tabs on the voltage across it. You may not need more than +5 or +6v to the collector. Best to have this supply stabilised, or at least the voltage across the base potentiometer stabilised (a zener may be adequate here). A bit of experimentation is needed!

If much tighter regulation is necessary, you could use a dual-rail PSU and an op amp to drive an emitter follower. Feed the motor voltage back to the inverting input and the control pot to the non inverting input of the op amp via equal resistors. A cap across the output will assist to damp most of the motor hash. Again try it!

Hope this helps

[oram]

HI david. Thanks for your reply.

Yes I also thought of using the darlington emitter follower but was concerned it might have trouble running smoothly from 0V upwards. I put the circuit together today, very simple emitter follower. As suspected the motor needs more volts to jump start it (i'm unsure of correct terminology here), it tends to slow the motor down great and smoothly, but when gradually increasing the speed it stalls until enough power reaches the motor, (i guess this is the current that needs a boost?op amp as you suggest? I have a few op amps available and will look at that today).

On some cassette players there seems to be a common ic that is used, an AN6650, for the motor regulation. They don't seem to be readily available. Just wondered if anyone else had come across these?

cheers

[David]

Hello.
This is a common problem with small DC motors - you may well have seen it with train sets. One way round it could be to supply your motors with interrupted DC of the full voltage (this was done in some train controllers) - in other words a square wave 3 or 4v peak with a variable mark/space ratio. ( A frequency in the high audio range will probably be OK , but experimentation may be necessary.) You can arrange this with a 555 IC, and there are many circuits around which will allow easy control of the m/s ratio, but from memory you will need to supply the IC with 15v or so, to give a decent range (say from 55 to 95%). The output of the 555 can be used to switch a separate transistor which feeds the motor.
The only guess with the AN6650 is that this is a "special", and not available on the retail market. Alternatively, this may be a standard motor controller, but specially badged. Have you looked on Google for suitable motor control ICs? you may come up with something suitable.

[computer blind]

I wish I could help but I only know amp laws, I never touched volts before.