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Power Supply and Multimeter at 3,3 Ampere...


steev
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Hi Steev,
The 0V-30V power supply project will produce a max output voltage of about 25VDC at a max current of 2.3A with your 24VAC/3.3A transformer operating continuously at its rated power rating.

The LCD meter project is powered by up to 35VDC. The unregulated voltage with your transformer is about 32VDC fully loaded to about 34.6VDC without a load.

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Your transformer can supply only 24V x 3.3A= 79.2VA. The peak voltage is 33.9V so the max average current from the transformer is only 79.2VA/33.9V= 2.34A at its max power rating. The 79.2VA power from the transformer is divided by the rectifier diodes, the power supply circuit and the load.

The rectifier diodes get too hot when they are operating near their max current rating. Use a bridge rectifier module (10A) instead that can be bolted to the metal chassis for cooling.

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Hi Steev,
Read the big long thread about improving the 0V-30V power supply project.
Q2 will smoke and melt if Q4 has a low current gain, the output voltage is set low and the current is high.
I recommend replacing the dinky little 2N2219 transistor with a real TIP31A power transistor that has a real heatsink.
The TIP31A costs nearly half and can dissipate nearly 20 times as much heat.

C1 is the main filter capacitor and its value is much too small.

Here is the parts list with changes that I recommend:

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Once again a big thanks for your help!!

After searching a while on my german shop's site I decided to choose these parts:

transformer: RKT 12015
R1: 2W METALL 2,2K
R2: 2W METALL 82
R3: 1W 220
R4: 1/4W 1k
R5, R6, R13, R20, R21: 1/4W 10k
R7: 11W AXIAL 0,51
R8, R11: 1/4W 27k
R9, R19: 1/4W 2,2k
R10: 1/4W 270k
R12, R18: 1/4W 56K
R14: 1/4W 1,2K
R15: 1/4W 100
R16: 1/4W 1K
R17: 1/4W 33
R22: 1W 3,9K
R23, R24: 2 x 2W DRAHT 0,33 (the two resistors for Q4)
RV1: PIHER 10-S 100K
P1, P2: P6M-Lin 10k
C1: AX 10.000/63
C2, C3: RAD 47/63
C4: MKS-4 100N
C5: MKS-4 220N
C6, C9: NPO-2,5 100P
C7: RAD 10/63
C8: NPO-2,5 330P
D1, D2, D3, D4: KBU8B
D5, D6: 1N 4148
D7, D8: ZD 5,6
D9, D10: 1N 4148
D11: 1N 4001
D12: LED (LED 5MM 2MA RT)
Q1, Q2: TIP 31C MBR
Q3: BC557C
Q4: 2 x 2N 3055
U1, U2, U3: OPA 445 AP


and some heatsinks I already bought. Are the 10.000 uF of C1 enough? It's the biggest on I found...

BTW: I would really like to make a HowTo of my version of the power supply aviable on my (and another modding-) website with downloadable schematic diagramm and layout (of course selfdesigned and with the fitting parts). Are I'm allowed to do this or is there any problem because of the copyright? I DO NOT want to sell the power supply or its HowTo!
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Hi Steev,
You project will have very good performance.
10,000uF is fine for C1.
You changed the value of R7 by 8.5%. Then the max current might not be 3A, depending on the actual value of the current-setting pot and other resistors. Some people add a trimpot in series with each pot for calibration of 3A and 30V.

Each 2N3055 output transistor must dissipate about 60W if the current is set to 3A and the output voltage is set low or ithe output is shorted. So it would be best to have them on separate large heatsinks, one on each side of the case. Or one lage heatsink for both transistors and a fan. Or two medium heatsinks and a fan. Your choice.
I would not use an insulator between the output transistor and its heatsink, just thermal grease for better heat transfer. Then I would insulate the heatsink from the chassis.

I don't think there is a copyright problem since you are not selling anything.

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You changed the value of R7 by 8.5%. Then the max current might not be 3A, depending on the actual value of the current-setting pot and other resistors. Some people add a trimpot in series with each pot for calibration of 3A and 30V.


Well, the problem is, that i cannot find exactly the same resistor... I only found 9 Watt resistors from 0,33 to 0,47 Ohm, but it would be too small, wouldn't it?

And don't worry about the cooling stuff, I have some old CPU cooler. I wanted to use a fan anyway  ::)
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  • 3 weeks later...
  • 3 weeks later...

The powersupply itselves is done so far. I can change the voltage from something about 0 up to 30 Volt and the current LED seems to work fine!

Now I have to grill some AVRs to get the multimeter working  ;D and build a small case...

Want to see some pictures? Here you go:
pict4931hd2.th.jpgpict4930ek6.th.jpgpict4924zu5.th.jpg

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