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0-20 (3A) and 0-40 (1.5)A Power Supply (Elenco)


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Hi Suraj,
A simple calculation shows that Ante's circuit has 60V input and 5A output, so each Mosfet dissipates up to 30W. If you use a 40V supply, you need only 7 Mosfets. Mosfets with a lower on-resistance such as the IRF5210 would need only 2 or 3 for about the same total cost.
It is hard to see in the schematic, but Ante's circuit uses P-channel Mosfets, because they must pull-up. Your IRF540 N-channel Mosfets pull-down, and cannot be used in this circuit.

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Hi Ante,
You are absolutely correct that RDS-on resistance doesn't matter much in a linear circuit. I guess I have "switchers" on my brain lately.
But using fewer of the low RDS-on Mosfet IRF5210 is still true because they also have a much lower thermal resistance than your original Mosfets. In the original 60V circuit, 10 of your Mosfets are needed, but only 4 of mine. In the 40V one, 7 of yours, but only 3 of mine.

1000 posts? Good, then I'll win an LED!

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Thanks Ante,
But I'll have to swim nearly half-way around this planet to pickup the purple LED from Greece. And anyway I already have some purple ones that were shipped to me by mistake!
I have some HP "cool blue" and green ones that are wide-angle but still are blindingly bright.

Your power supply will have full dissipation when driving a high current into a short. A guy on another chat line was having trouble with his rocket-launcher ignitor. I think it was a paper-clip that he was trying to feed a few Amps through.
You could also use it to see how hot a 0.1 ohm, 5W resistor gets with 5A through it (only 2.5W), to see if it will char a PCB.

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Hi Ante,
HP sold their LEDs manuf. to Agilent, but I think Fairchild makes them too.
Cool Blue (clear case) HLMP-CB30. The last 2 numbers are the angle.
Its datasheet:

Thanks for the avitar, but I dislike those shrieking megaphones. I'm still looking for something that is more refined.

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Has anybody ever tried connecting a resistor to the power supply through a MOSFET. If you set the VGS to maximum then you can get the power supply voltage at the drain. Then you can apply a signal through that resistor and the drain will hold it's voltage. So you can have a changing current with a constant voltage. I think it will work by changing the GM even though you really can't change the current without changing VGS.

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Hi Kevin,
If you apply a Vgs of only 10V to a Mosfet, then it would be turned-on so hard that the drain would seem like it is welded to the source.
But if you apply a Vgs of just a few volts, then a Mosfet would be turned-on just a little, and would act like a constant-current source.
The only devices that I know of that can "hold its voltage" are a zener diode and a voltage regulator IC.

You are correct if you are talking about a P-channel Mosfet as in Ante's power supply. The Mosfet will be turned-on hard and "then you can get the power supply voltage at the drain".
But if you "apply a signal through a resistor" that you connect to its drain then of course the voltage across the resistor will change and its current will change too. The same thing will happen if you connect that resistor to any DC voltage including 0V.

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