Author Topic: how reliable is 7805 5v vr whithout capacitors  (Read 5410 times)

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Offline blackbeardTopic starter

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how reliable is 7805 5v vr whithout capacitors
« on: December 19, 2009, 10:59:46 AM »
so i've read how the 7805 vr (and most vr) can only work reliably when used with capacitors in a certain configuration. i've used them whithout capacitors as long as i've used them and they've always read 5v reliably. any input on this?
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Offline waltr

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Re: how reliable is 7805 5v vr whithout capacitors
« Reply #1 on: December 19, 2009, 11:33:45 AM »
It varies a lot with the lay-out of the circuit, input voltage and output load impedance.
I used them where they worked perfectly and other times they oscillate. This also seems to change with parts from different vendors.

So, if its work it works. If not add the caps.

Offline SmAsH

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Re: how reliable is 7805 5v vr whithout capacitors
« Reply #2 on: December 19, 2009, 03:56:02 PM »
It normally depends on the input, whether it is stable or not and what i am giving current to.
If i am giving current to something that draws 100mA then its fine.
I made a crappy ipod charger out of a 7v wall wart and a 7805 and it worked fine.
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Offline Soeren

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Re: how reliable is 7805 5v vr whithout capacitors
« Reply #3 on: December 20, 2009, 08:03:23 AM »
Hi,

So, if its work it works. If not add the caps.
Unfortunately, this is one situation where it doesn't allways work just because it initially did.
It's foolhardy and very poor design to leave out the caps, which is a very cheap way of ensuring a trouble free function.

Even if you measure a steady 5V with a DMM, you probably have lots of noise and ripple. the output impedance goes down when caps are added and it won't be sensitive to lead/track length as it would be without them.
Line and load regulation is also improved with caps.

Try probing input and output with a 100MHz (or higher) 'scope and pace it through a couple of hoops and see for yourself.


78- and 79-series voltage regulators were never designed to work without the caps!
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Offline blackbeardTopic starter

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Re: how reliable is 7805 5v vr whithout capacitors
« Reply #4 on: December 20, 2009, 01:56:42 PM »
my scope is only 40mhtz but i'll give it a go. the noise can actually explain quite a lot
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