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[on:]

i have a 7805



i have connected

20v to the input
gnd to gnd
I expect to see 5v on the output

except i see 20v (near enough)

do I need to put capacitors around the regulator input and output ?

like this

[Reply #1 on:]

Yes. Your drawing says Transformer - is that correct?
If so you need rectifier also to convert the AC into rough DC, the caps and regulator then make smooth DC for the circuit.

[Reply #2 on:]

Yes, capacitors are required, but their absence doesn't explain to me why the output isn't pretty close to 5V. Technically speaking, a transformer doesn't supply positive and negative, but AC. What exactly is the "transformer" being used here? DC needs to be supplied to the circuit as illustrated.

[Reply #3 on:]

i lifted the drawing from another web page

the supply to the regulator is 20v DC (not ac)

i am very confused :-S

[Reply #4 on:]

Add the caps and see if that helps.
Polarized electrolytic 3.3uF, 4.7uF, 10uF along those lines on the input.
1uF on the output.

[Reply #5 on:]

i am very confused

Why, you need caps.
You also need it to be the right way round, check again it is.

[Reply #6 on:]

except i see 20v (near enough)

Maybe your 7805 was abused in a previous life.  Have you substituted a different one?

Don

[Reply #7 on:]

i added the caps and still 20v in and 19ish out

i have tried this on two brand new regs

one is 5v the other is 15v

could i have damaged them running without caps ?

[Reply #8 on:]

I sometimes wires a 7805 on breadboard without caps and they works fine, there is no reason being damage without caps.

What I am wondering is may be:
1: you may have it wrong way round?
2: is it ground or negative voltage from the transformer?
3: One of them is not a 7805 and different pinout?

[Reply #9 on:]

i added the caps and still 20v in and 19ish out

Then it is either not connected the way you think or it is faulty.

could i have damaged them running without caps

No.

I sometimes wires a 7805 on breadboard without caps and they works fine,

Rubbish. Unless by working fine you mean at just one current and you haven't bothered to check the waveform with a scope.

[Reply #10 on:]

I sometimes wires a 7805 on breadboard without caps and they works fine,

Rubbish. Unless by working fine you mean at just one current and you haven't bothered to check the waveform with a scope.
[/quote]

i.e. breadboard, quick test circuit, my point is that it should not damage the 7805 without the caps.

[Reply #11 on:]

my point is that it should not damage the 7805 without the caps.

That was my point too.

The problem is that many people think if they have a circuit that they perceive as working then it is fine, it IS working. This is not the case. A 7805 without capacitors can never be stable. You might not have the tools to see the instability or you might have hit a quasi stable state due to the current drawn and the stray capacitance in the layout. But it is not working fine.

[Reply #12 on:]

Rubbish. Unless by working fine you mean at just one current and you haven't bothered to check the waveform with a scope.

I have several simple setups where I only use a capacitor on the regulator output and they work ok (well, for 6+ years). The last data sheet I read on the regulator chips only recommended the caps on the input side when the input was over a long wire. As to stability, it is a relative term as to the actual operating conditions and requirements.

[Reply #13 on:]

I have several simple setups where I only use a capacitor on the regulator output and they work ok (well, for 6+ years).

No you haven't you have several simple setups where you have not detected the instability.

The last data sheet I read on the regulator chips only recommended the caps on the input side when the input was over a long wire.

That was one manufacturers claim on one specific chip, did you use that chip 6 years ago?

As to stability, it is a relative term

No it's quite specific.

as to the actual operating conditions and requirements.

Hey lets redefine words, I can balance an egg on it's point, that is stable as long as nothing disturbs it.

[Reply #14 on:]

i added the caps and still 20v in and 19ish out

i have tried this on two brand new regs

one is 5v the other is 15v

could i have damaged them running without caps ?

Time to post a picture if you can, the whole circuit, maybe several pics if that works best.