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# Supply help

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#### hantto

##### Member
Hello, i'm planning to build this pwr supply **broken link removed** (the 15v) but the author hasn't given the 33R and 0.1R resistors wattages. I need some help with those. I got different aswers when I tried to calculate the 0.1R's.

If W=U²/R then it should mean 28²/0.1 =7840W? no i don't think so.

Then with W=I²*R it would mean 2²*0.1=0.4W? hm only 400mW resistor, I dont think that either. (I calculated this with 2amps per transistor, to be on the safe side)

hantto said:
Hello, i'm planning to build this pwr supply **broken link removed** (the 15v) but the author hasn't given the 33R and 0.1R resistors wattages. I need some help with those. I got different aswers when I tried to calculate the 0.1R's.

If W=U²/R then it should mean 28²/0.1 =7840W? no i don't think so.

Then with W=I²*R it would mean 2²*0.1=0.4W? hm only 400mW resistor, I dont think that either. (I calculated this with 2amps per transistor, to be on the safe side)

0.4W is correct, your first calculation was totally wrong because you invented a huge voltage from somewhere - the voltage you use is the voltage dropped across the resistor - which in this case is 0.2V (at your 2A current). I would advise using a decent size resistor, probably a 2.5W wirewound would be good - and is available at 0.1 ohm.

The 33 ohm simply feeds the base of the transistors (by the voltage dropped across them) - so the voltage across them is about 0.9V - if we call it 1V (to make the maths simple), it's dissipation will only be 1/33 of a watt.

In the text:" When the current exceeds about 20mA, the power transistor will turn on," so the 33R is 0.66W, i recommend 2W resistor. The 0.1R wattage calculated from output current div2. So if output current 4A, the resistor dissipate only 0.2W so 1W type quite enough.

Hi, Nigel, we posting in same time...

Thanks both! I learned something new now, it's the voltage DROP on the resistor. Hm, I feel a bit dumb I "suffer" from the trouble that I don't read things and then screw things up. Especially in tests at school!

If I may bother you a bit more, how's the voltage drop accross a resistor calculated?

And I noticed that you say different about the 33Rs wattage, which one is the correct one?

hantto said:
If I may bother you a bit more, how's the voltage drop accross a resistor calculated?

From ohms law, V=I*R, so you need to know the value of the resistor and the current through it. For your 0.1 ohm we knew the current was 2A maximum.

For the 33 ohm we don't actually know the current through it, but can work it out from the circuit configuration.

The base emitter junction of a silicon transistor requires roughly 0.7V to turn on, and the 0.1 ohm resistor has 0.2V across it (from the previous calculation). This gives a total of 0.9V across the resistor.

And I noticed that you say different about the 33Rs wattage, which one is the correct one?

I think Sebi has made a mistake in his calculations, assuming 20mA (as he said from the text) gives 0.02*0.02*33=0.0132.

The way I did it was to work out the voltage drop (as above) and calculate from that - 0.9*0.9/33=0.0245 - for my original estimate I rounded the voltage up to 1V to make it easy, knowing that that would give a higher wattage.

Yes, Nigel right, i forgot one multiplying...

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