lm7815 & 7915 output problem

gerben00 · 6th November 2011, 10:27 PM

Hi all, i put this in the analogue source forum since i'm building a supply for a phone pre amp.

Basically my problem (?) is this, the lm7815 gives 14.9 volts; the lm7915 gives 15.1 volts. Is this normal, and is this a problem when driving an op amp?

After rectification i get +-21.9 volts, so the toroid and rectifier seem ok.. Also i rebuild the whole thing a second time with fresh parts and a cleaner layout, but still the same difference.

I build a basic power supply, mix from the VSPS power supply and sound westhost pre amp supply. It has a 2*15 vac, 50VA toroid on it.

Thanks for any input!

kevvywevvywoo · 6th November 2011, 10:52 PM

The tolerance on the standard 7815/7915 is not the greatest. Make sure you have the decoupling capacitors fitted as shown in the datasheets, they should be mounted very close to the actual regulators. If they're not fitted, the output voltage can drift in and out of spec or even oscillate.

If you want a more accurate balanced supply, use a fixed 7915 for -ve and a variable LM317 for the +ve rail, then adjust the trimmer on the LM317 to exactly match the output from the LM7915.

counter culture · 6th November 2011, 10:56 PM

Datasheet says +/- 2% and no, it's not a problem.

gerben00 · 6th November 2011, 11:12 PM

Thanks guys! I did reread the spec sheet a couple of times this weekend but missed that one..;-)

Little erratum for the archives, i meant +14.9 volts for the lm7815, and -15.1 for the lm7915.. if anybody didn't get it but does care.

6th November 2011, 10:27 PM	#1
gerben00 diyAudio Member Join Date: Jun 2011 Location: amsterdam	lm7815 & 7915 output problem Hi all, i put this in the analogue source forum since i'm building a supply for a phone pre amp. Basically my problem (?) is this, the lm7815 gives 14.9 volts; the lm7915 gives 15.1 volts. Is this normal, and is this a problem when driving an op amp? After rectification i get +-21.9 volts, so the toroid and rectifier seem ok.. Also i rebuild the whole thing a second time with fresh parts and a cleaner layout, but still the same difference. I build a basic power supply, mix from the VSPS power supply and sound westhost pre amp supply. It has a 2*15 vac, 50VA toroid on it. Thanks for any input!

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6th November 2011, 10:52 PM	#2
kevvywevvywoo diyAudio Member Join Date: Aug 2008	The tolerance on the standard 7815/7915 is not the greatest. Make sure you have the decoupling capacitors fitted as shown in the datasheets, they should be mounted very close to the actual regulators. If they're not fitted, the output voltage can drift in and out of spec or even oscillate. If you want a more accurate balanced supply, use a fixed 7915 for -ve and a variable LM317 for the +ve rail, then adjust the trimmer on the LM317 to exactly match the output from the LM7915.

6th November 2011, 10:56 PM	#3
counter culture diyAudio Member Join Date: Jun 2011	Datasheet says +/- 2% and no, it's not a problem.

6th November 2011, 11:12 PM	#4
gerben00 diyAudio Member Join Date: Jun 2011 Location: amsterdam	Thanks guys! I did reread the spec sheet a couple of times this weekend but missed that one..;-) Little erratum for the archives, i meant +14.9 volts for the lm7815, and -15.1 for the lm7915.. if anybody didn't get it but does care.

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