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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
haxor5354 wrote ...
so, a bleeder resisotr?
I think Paul Falstad's circuit simulator might be useful to you. You can design a SMPS with a buck converter driven with a 555 like your circuit and see how different values effect the circuit.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
haxor5354 wrote ...
WHY NO JUST AMPLIFY A VARIABLE RESISTOR?!! I tested it out with a 10 turn 4.7K pot and a IRFP260N mosfet heatsinked and fan cooled, and it actually works!! works great as a 12V halogen light dimmer. no op-amps,LM317,capacitor,chokes,ICs.... just a pot and a mosfet, can't get any simpler than that!! any circuit design flaws?
I want to be concise. This is essentially a large high watt variable resistor in series with source and load. (for which a depletion mode power mosfet would be best, instead of an enhancement mode mosfet.)
So the usuall rules apply: --highest efficency is only available at minimum load. --50% or less effciency at full current load. --Regulation is based on dissipation of the unwanted V, as heat. --Regulation is static. --as the V on load goes up the Amps and Power available go down.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
got any circuits for a buck converter using N channel mosfet instead of P channel and I already got the 555 PWM generator working, all I need now is an amplifier
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
haxor5354 wrote ...
got any circuits for a buck converter using N channel mosfet instead of P channel and I already got the 555 PWM generator working, all I need now is an amplifier
Maybe a LM723 could be used for your PS,... [555 PWM] -> [LM723] -> [Switch + Inductor] -> [DC caps]
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Technically this isn't a variable resistor. It's a potential divider buffered by a source follower. So, the output voltage will be quite stable, and while nowhere near as good as a purpose-designed voltage regulator, it might be adequate for powering a lot of the popular 4hv style projects. I and many others have used more or less the same circuit to adjust the plate voltage in tube guitar amps, as a way of reducing the power output to get distortion without deafening volume. Because it doesn't regulate, the hum and grunge go right through, for extra vintage mojo. I often add a current limit, this can be done quite easily and will help save the FET in the event of an accidental short on the output. But if you're using it to vary the output of something like an ATX power supply, it probably won't be needed as the PS has its own overcurrent protection.
Switching regulators aren't the only technology in the world. Linear ones are still very popular. Sometimes efficiency isn't the most important thing, you might want low noise or just simplicity.
When all's said and done the simplest variable DC supply is a heap of batteries and some clip leads.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
A simple variable linear power supply is an op amp and a power transistor... You take feedback from the output voltage and feed it through a voltage divider into an opamp, which drives the transistor. Then the other input of the op amp is variable voltage, which can be a simple pot or any other digital input, who's voltage decides the output.
This is basically what is inside of a typical 7812 regulator.
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world's simplest variable DC power supply EVER?
