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Topic: Voltage regulators (Read 2467 times) previous topic - next topic

Gadget999

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





CrossRoads

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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Jack Christensen

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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Gadget999

#3
Oct 27, 2011, 10:25 pm Last Edit: Oct 27, 2011, 10:27 pm by Gadget999 Reason: 1
i lifted the drawing from another web page

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

i am very confused :-S

CrossRoads

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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Grumpy_Mike

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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.

floresta

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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

Gadget999

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 ?

kkman20xx

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?

Grumpy_Mike

Quote
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.

Quote
could i have damaged them running without caps

No.

Quote
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.

kkman20xx

Quote
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.

Grumpy_Mike

Quote
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.

zoomkat

Quote
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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Grumpy_Mike

Quote
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.
Quote
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?
Quote
As to stability, it is a relative term

No it's quite specific.
Quote
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.


Jack Christensen


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.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

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