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Author Topic: Voltage regulators  (Read 2131 times)
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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




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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.
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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.
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i lifted the drawing from another web page

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

i am very confused :-S
« Last Edit: October 27, 2011, 03:27:07 pm by Gadget999 » Logged

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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.
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i am very confused
Why, you need caps.
You also need it to be the right way round, check again it is.
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except i see 20v (near enough)
Maybe your 7805 was abused in a previous life.  Have you substituted a different one?

Don
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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 ?
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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?
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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.

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could i have damaged them running without caps
No.

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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.
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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.
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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.
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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.
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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.
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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?
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As to stability, it is a relative term
No it's quite specific.
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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.

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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.
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