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I'm looking for a good circuit that will allow me to vary the current to a flyback H-bridge from 0-5A at a constant 170V (rectified and smoothed mains). I've found a few circuits using an op-amp to a transistor, but I'm not sure if they would be able to handle the amount of power I'd like. Does anyone have a circuit they could recommend?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
You can make a current source or a voltage source, but not both at the same time. So you have to pick either 0-5A current source OR 170V current source, but not both.
If you're trying to regulate the total power drawn by the flyback driver I think you'd be better off just reducing the duty cycle on the H-bridge rather than trying to change your power supply.
So you're saying that I can't have a fixed voltage with a variable current? What about the variable current with fixed voltage from lab power supplies? I didn't even think about varying the duty cycle, that seems like a good idea. The end goal is to have some of volume control...
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Seeing as the mains can supply huge amounts of power without voltage sag I'd say that regulating the voltage is kind of pointless. As for the current regulation, I think a power transistor would be the best way to go.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Tightmopedman9 wrote ... So you're saying that I can't have a fixed voltage with a variable current? What about the variable current with fixed voltage from lab power supplies?
The better ones have a light that tells when you're in current regulating mode (with less than the setpoint voltage) and when you're in voltage regulating mode (with less than the setpoint current). The transition depends on where the load I/V curve intersects the power supply I/V curve -- it would be educational to draw the curves with various load resistors instead of your H-bridge.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Klugesmith wrote ...
Tightmopedman9 wrote ...
So you're saying that I can't have a fixed voltage with a variable current? What about the variable current with fixed voltage from lab power supplies?
The better ones have a light that tells when you're in current regulating mode (with less than the setpoint voltage) and when you're in voltage regulating mode (with less than the setpoint current). The transition depends on where the load I/V curve intersects the power supply I/V curve -- it would be educational to draw the curves with various load resistors instead of your H-bridge.
Assuming your load is linear like a resistor then voltage on the load is equal to current times resistance. So when you set the voltage current is set by I=V/R, or when the current limit is hit then V=I*R, so voltage must change. Considering the power level you're talking about using a linear regulator to limit the current while the H-bridge is on could be done, but it would dissipate quite a lot of heat, which is why duty cycle adjustment is nice since very little power is dissipated.
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Adjustable current sink for rectified mains.
