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power supply issue

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With good reason. 

Since each of the supplies is connected to the transformer as a
full-wave center-tapped supply, the rectifiers will only drop about
0.7V in front of the filters, so with 27VRMS into the rectifiers the
peak voltage (the voltage across each filter cap) will be:


      Vp = (Vrms * sqrt(2)) - 0.7V =  37.7V

Which is pretty close to what you got.
Reply to
John Fields
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Hi. I have built this dual power supply. Transformer rating: 2 x 24 V 60VA Pretty standard with a bridge rectifier and ground connected to the center tap. So i got + 24 and -24. The i have two 4700uF 50v caps, one connected at each powerline to provide filtering. Then there is a 7815 and and a 7915 as regulators. Ok here is the issue. When i measure the voltage over the filtercaps i see

+37,5V and - 37,5V! How is this remotly possible?. Even the peak to peak voltage isent that high. The 7815 and 7915 is only rated up to 35V so i am scared they might blow or something.

Another problem is that since there is a voltage drop 37.5 -15 = 22 V over the regulators. It cant provide much current before they shut down because of the heat. I know from the datasheet they can handle up to 10W with a decent heatsink and a max current of 1,5A. I dont need that much current anyway since the transformer can only deliver 30VA / 24 = 1,25A per secondery anyway. I have another question too. How much current goes through the primary side of the transformer with max load? I was thinking of having a fuse there instead of buying a fuse to each of the secondery outputs.

Thanks in advance

Anders Vinje

Reply to
Anders Nesheim Vinje

I took a new measurement and found out the the ac value was acutally 27V not

  1. So when i muliply with the square of two i roughly get 37,5V peak to peak. But why do i see the peak to peak voltage after it has been rectified??? Even with load 40ohms load.

Anders

Reply to
Anders Nesheim Vinje

Why are you seeing 27 V instead of 24 V at the output of the transformer? Is your line voltage 27/24 * 120 = 135 V?

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Reply to
jgreimer

Thank you for all answers.

I see i need another transformer. Something like 2X 15V instead. The reason i see 27 V is under no load. I think its 24 V when under max load. I live in Norway and we got 220V 50HZ mains here.

Anders

Reply to
Anders Nesheim Vinje

That is strange, I think Norway has 230V like almost all west-European countries. If the transformer is rated for 220V this will explain half the overvoltage. The secondary voltage is indeed rated at max. AC-current. This will explain the other half. Bigger transformers(M85b) have only 106% output voltage at no load.

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ciao Ban
Bordighera, Italy
Reply to
Ban

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