Why the output of LM7915 is -19V rather than -15V

AC-DC Adapter, Transformer, Lipo battery and charger, SLA, Cell battery.....

Why the output of LM7915 is -19V rather than -15V

Postby Yeannie » Mon Feb 20, 2017 5:40 pm

I'm trying to build a regulated power adapter using LM7915.

Before everything, I built on a protoboard the schema included on the datasheet . But, instead -15v, the output is -19v...

I'm using a 18+18v/500mA transformer. After the diode bridge (1n4007) the measure is -17.7v.

Into the input (-Vi) I'm using an electrolitic capacitor of 2200uF (50v). And into the output (-Vo) I'm using an electrolitic capacitor of 10uF (63v).

After wiring everything, measuring pins 1 (ground) and 3 (out) my voltmeter shows -19v...

I did the same experiment with a LM7815 and it worked nicely, so I don't think that I missed something.

Please, could you advise how to debug the circuit? Or point me out what I did wrong?

EDIT (solution?):

I managed to solve(?) the problem. I did not posted it as an answer since I don't know if it's only a coincidence or not... I've attached a picture.

The "problem" was that I wired the transformer 0V to the 7915 ground (pin 1). As you can see into the picture, the transformer has 3 wires: 2 purple and 1 black. Measuring each purple with the black, I found 19vAC, and after the bridge, -18vDC. So, as instructed here, I did the same on my circuit...

But, when I removed the ground wire, I got the correct measures: The 7915 pin 2 (in) showed -19vDC and pin 3 (out) -15vDC (in fact a little bit more, close to -16vDC). If I put it again, the measures goes back to the values that I informed before...

Image
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Re: Why the output of LM7915 is -19V rather than -15V

Postby Idris » Mon Feb 27, 2017 12:03 pm

Hi Yeannie, could you share the schematic diagram for better understanding
1. Problem - output 19V,
2. 'Solution'.

Thanks.
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