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Help needed about voltage controller cct ?

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Hi,

I want to controll a something its max voltage is 5 to 5.6V( somethig like that ).
But my in put voltage is 12V. ( ah.., and that thing is not very hi current draining. Its current is 0.5A )

So i have to controll it between 0V & 5V

My question is, is there a way to do both jobs from one regulator ?

What is the best way to do it?

Thanks guys:)
 

ericgibbs

Well-Known Member
Most Helpful Member
blueroomelectronics said:
An LM317 should be able to do that.
hi pasan,

IIRC the LM317 can only get down to +1.2Vout, would that be acceptable?

What are you powering with 0 to +5V ??
 

Tomble

New Member
Would it be unsuitable to use just a 7805 to set the 5V maximum voltage, and then just put a potentiometer under that, centre terminal leading to an emitter follower? Maybe you don't need it to be very accurate?

One disadvantage of this is that the follower could only give 4.4V or so max, but maybe the 5v is not as important as getting 0v though? It might also need to be in a darlington pair (dunno, sorry), in which case even more voltage drop. You could also arrange it with an op-amp between the pot and the transistor(s) to give the full range, but then this is getting more and more components.

A different alternative might be similar but using a 6v2 zener to get the voltage reference, roughly cancelling out the drop from a darlington pair- but perhaps that might be really really crappy, I dunno.
 
Tomble said:
Would it be unsuitable to use just a 7805 to set the 5V maximum voltage, and then just put a potentiometer under that, centre terminal leading to an emitter follower? Maybe you don't need it to be very accurate?

One disadvantage of this is that the follower could only give 4.4V or so max, but maybe the 5v is not as important as getting 0v though? It might also need to be in a darlington pair (dunno, sorry), in which case even more voltage drop. You could also arrange it with an op-amp between the pot and the transistor(s) to give the full range, but then this is getting more and more components.

A different alternative might be similar but using a 6v2 zener to get the voltage reference, roughly cancelling out the drop from a darlington pair- but perhaps that might be really really crappy, I dunno.

Yes, i dont need very accurate voltage.

I'll try ur method,

Thanks a lot:)
 

Tomble

New Member
OH! Sorry, my brain really isn't working right this morning :eek:

I'm quite sure none of those methods I suggested limit the current at all! :( I think it's possible to adapt some of those to do simple current limiting, but otherwise it might draw too much! Probably wait for a better answer instead *sigh* :(

EDITED AGAIN: ALSO you may need flyback protection diodes somewhere because the motor is an inductive load *groan*

(I think our collective lessons for today are: wait a little bit for other people to reply before assuming someone's suggestion is any good, and don't comment with only 4h sleep :rolleyes: I think I'll just go read or something.)
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
hi pasan
Look at this method.
 
Last edited:
Tomble said:
OH! Sorry, my brain really isn't working right this morning :eek:

I'm quite sure none of those methods I suggested limit the current at all! :( I think it's possible to adapt some of those to do simple current limiting, but otherwise it might draw too much! Probably wait for a better answer instead *sigh* :(

EDITED AGAIN: ALSO you may need flyback protection diodes somewhere because the motor is an inductive load *groan*

(I think our collective lessons for today are: wait a little bit for other people to reply before assuming someone's suggestion is any good, and don't comment with only 4h sleep :rolleyes: I think I'll just go read or something.)

Hmm I think i just have to wait and see what other guys think about this.
Thanks for the reply:)
 
Tomble said:
Would it be unsuitable to use just a 7805 to set the 5V maximum voltage, and then just put a potentiometer under that, centre terminal leading to an emitter follower? Maybe you don't need it to be very accurate?

One disadvantage of this is that the follower could only give 4.4V or so max, but maybe the 5v is not as important as getting 0v though? It might also need to be in a darlington pair (dunno, sorry), in which case even more voltage drop. You could also arrange it with an op-amp between the pot and the transistor(s) to give the full range, but then this is getting more and more components.

A different alternative might be similar but using a 6v2 zener to get the voltage reference, roughly cancelling out the drop from a darlington pair- but perhaps that might be really really crappy, I dunno.
Holly amigo.:( :rolleyes: :eek: :)
 

audioguru

Well-Known Member
Most Helpful Member
Reducing the voltage or reducing the current to a DC motor is a lousy way to control its speed. Then the torque drops so low that it won't start running and it won't drive anything.

Pulse-Width-modulation is a good way to control the speed of a DC motor. the pulses are at full voltage so there is plenty od torque. But the average power is low for slower speeds.
 

ericgibbs

Well-Known Member
Most Helpful Member
audioguru said:
Pulse-Width-modulation is a good way to control the speed of a DC motor. the pulses are at full voltage so there is plenty od torque. But the average power is low for slower speeds.
hi Pasan,
This is the correct way to control the speed, you could add some back emf circuitry to sense/control the torque at the lower speeds.
 
audioguru said:
Reducing the voltage or reducing the current to a DC motor is a lousy way to control its speed. Then the torque drops so low that it won't start running and it won't drive anything.

Pulse-Width-modulation is a good way to control the speed of a DC motor. the pulses are at full voltage so there is plenty od torque. But the average power is low for slower speeds.
PWM is a good way to do it. But its mean hell of parts.

Any how at the moment I have to move on to PWM.

Thanks for the info:)
 
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