power supply issue

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

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

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

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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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

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.