Extra current to a DC motor

[ChazN]

Hey I was wondering if anybody can help me out...

I have an controller which is able to supply max 1.25 amps and 0~8 volts(variable), however I need about 3amps to power a DC motor at required torque. I have used a transistor as a switch connected via a 9v battery to switch the motor on and off using the controller however then I loose speed control of the motor.

My question is this, how can I provide extra current via a 9volt battery to a my DC motor and still control the motor speed?

Thanx

 

[Nigel Goodwin]

It depends on the existing controller, do you have the circuit for it?.

Also, what type of 9V battery is it?, most won't even provide 3A.

 

[ChazN]

Thnx for the reply...

well nope, no circuit but the controller is part of fischertechnik series including the battery which is capable of providing the required current (tested it directly) ....

any ideas?

 

[Nigel Goodwin]

Thnx for the reply...

well nope, no circuit but the controller is part of fischertechnik series including the battery which is capable of providing the required current (tested it directly) ....

any ideas?

Not without the circuit - either it's current limted at 1.25A, or exceeding it will blow the controller - either way you need to know what the circuit is.

 

[ChazN]

it probably is current limited, hence my need to add extra current via an external source.

Is it not possible to just add in a buffer and somehow amplify the current or use an external source?

 

[Nigel Goodwin]

it probably is current limited, hence my need to add extra current via an external source.

Is it not possible to just add in a buffer and somehow amplify the current or use an external source?

Without knowing exactly how it works it would be very difficult.

 

[5uco]

Not entirely sure what you want, you don't say whether the controller is linear or PWM

If it is linear, what you have is a 'sort of' regulator, if so you may be able to get more poke out of it by the usual method of a bypass transistor(s).

Download the pdf for a L200, and look at fig 21. It shows a 2.5 Ohm resistor before the regulator. This will switch on the bypass transistor at around 280mA or so.
The remainder of the current drawn will go through the bypass transistor.

Just a suggestion of where you MAY find an answer. I've certainly used a L200 as a motor speed controller many times, including with the bypass wotsit. Up to 10A or so. You will need a circuit breaker/fuse, because you will lose the current limit I think. Not through the controller, but through the bypass.
Other than that, you'll have to be a lot more specific.
**broken link removed**

 

[5uco]

Apologies, should have realised. As you have a proprietory controller, I conclude you cannot easily get to the DC part.

So, not much help in that case, unless you have the knowledge to gut the thing and modify it.

Dave

 

[colin55]

Have you tried an emitter-follower?

 

[ronv]

Buffer Amp

If u want to risk $5 at radio shack try this:
It assumes a pwm fet driver in your controller (most are).
Keep the ground and gate drives close together and us a bit of a heat sink on the FET.
There is no current limit so be careful or the FET will smoke.
Oh, and the speed control will be reversed from what it is now.