Increasing the voltage without changing the current

hgmjr

Joined Jan 28, 2005
9,027
What you have described is a constant current source?

What is the magnitude of the constant current that you need to generate?

hgmjr
 

SgtWookie

Joined Jul 17, 2007
22,221
In a circuit with a purely resistive load;
I = E / R, or Current = Voltage / Resistance (Ohm's Law)
In order to change the voltage without changing the current, you must change the resistance a porportional amount.

If the load resistance varies, you can use a constant current source to maintain the current level, as hgmjr implied. If the load resistance increases, a constant current source will increase the voltage to maintain current (within limits, of course).
 

Audioguru

Joined Dec 20, 2007
11,249
Are you the guy with a little fan in your helmet powered from a little 9V battery and you want to double the voltage, quadruple the power but not increase the battery's current?
The poor little battery will curl up and die. The poor little motor might die too.
 

Thread Starter

gloriouss89

Joined Oct 17, 2008
5
What you have described is a constant current source?

What is the magnitude of the constant current that you need to generate?

hgmjr
Actually; what i'm trying to do is increasing the voltage of my output current which came from my current limiting circuit .. In some way i used , i could limit the current to the specific value but while doing this i also limited voltage ..I want my dc motor to take the voltage it requires but not giving the current over a specific value like 200-300 mA..so, i thought maybe i can increase the voltage later i limited the current..It might be very stupid idea indeed but what can i say i'm just a beginner :)
 

Thread Starter

gloriouss89

Joined Oct 17, 2008
5
Are you the guy with a little fan in your helmet powered from a little 9V battery and you want to double the voltage, quadruple the power but not increase the battery's current?
No, I'm not that guy but that seems interesting though :D To do something like that tunnel diodes might be handy, i guess..
 

scubasteve_911

Joined Dec 27, 2007
1,203
We talked about this recently and it isn't a simple problem to solve. The function you speak of requires two separate parts, and something to switch between them. Namely, a voltage regulator, then switch to a current source when there is maximum current being drawn.

Since your load is inductive, it might be a good idea to use a simple comparator circuit to switch off your motor when the current is being exceeded. Then, after a short delay, you can try to switch it on again. This way the current through the winding remains fairly constant and you get to keep efficiency by using on or off devices.

To implement this, use a current sensing resistor from the return path of the motor to ground. Then, amplify the signal (optional) and run it into a simple voltage comparator. The comparators output will fire a one-shot timer delay, which will shut off a power FET to your motor. The motor will turn on again, and if it hasn't stopped exceeding the current limit, the circuit will trip again. You might need a good low-pass RC filter on the input to your amplifier or comparator.

Steve
 
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