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Author Topic: Voltage Regulator Issues  (Read 2525 times)

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Offline spizzakTopic starter

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Voltage Regulator Issues
« on: February 25, 2012, 06:31:51 PM »
Hey all, I'm trying to make a board that takes power from a 7.8V niMh battery and regulates it to 5V to power a beagle board. The issue is that the Vreg doesn't appear to be doing anything. I'm measuring 8V on input and output pins. I ordered two of them and I tested the other one with a 12V supply and I see the same thing.

When I test another regulator it behaves as I expect it. For my tests I'm just wiring Input and Ground to a 12V battery and probing the output pin. Using this setup the old regulator is giving 5V out and the new one I'm trying to use is giving 12.

Not working regulator: http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=ca&keywords=576-1121-ND&x=0&y=0

Old regulator that works (From $50 bot): http://search.digikey.com/ca/en/products/KA7805ETU/KA7805ETU-ND/965381


Any ideas what might be wrong other than a defective Reg?

Thanks

Offline Pogertt

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Re: Voltage Regulator Issues
« Reply #1 on: February 26, 2012, 01:47:30 AM »
You might have to put capacitors on input and out pins.
Check the data sheet for values to use.
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Offline diskman

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Re: Voltage Regulator Issues
« Reply #2 on: February 26, 2012, 04:27:32 AM »
Use the simple 7805 regulator.  If you need more than 1 amp, then you can piggyback 2 of em together.  Dead simple to hook up.  Use a piece of aluminum for a heatsink and you are all set.


Offline joe61

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Re: Voltage Regulator Issues
« Reply #3 on: February 26, 2012, 08:58:41 AM »
Hey all, I'm trying to make a board that takes power from a 7.8V niMh battery and regulates it to 5V to power a beagle board. The issue is that the Vreg doesn't appear to be doing anything. I'm measuring 8V on input and output pins. I ordered two of them and I tested the other one with a 12V supply and I see the same thing.

I had a problem like that with a Micrel regulator as well, but I didn't figure it out so can't help there. Most of the time now I use one of these. I don't typically need over 500mA, and this has a really low dropout. I can give it power from a wall wart outputting 5.16V, and get 5.03V out of the regulator (no load).

Joe

Offline Soeren

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Re: Voltage Regulator Issues
« Reply #4 on: February 26, 2012, 10:08:41 AM »
Hi,

When I test another regulator it behaves as I expect it. For my tests I'm just wiring Input and Ground to a 12V battery and probing the output pin. Using this setup the old regulator is giving 5V out and the new one I'm trying to use is giving 12.

[...]

Any ideas what might be wrong other than a defective Reg?
Micrel makes some of the very best LDO regulators, but you need to use them correctly.
Your -300 device need to be loaded with a minimum of 7mA.

Either add a resistor of maximum 680 Ohm, or use a red LED and a 330 Ohm resistor over the output (ground to out) and you should get the nominal 5V output.

To keep it from oscillating, you need 10µF on the output as well, but that is not your imminent problem. Likewise, if you have more than an inch of wire from battery to input terminal, you should have a cap on the input as well, but that is for keeping the regulator from misbehaving as well. It will still test good without the caps, as long as you apply the minimum load.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline spizzakTopic starter

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Re: Voltage Regulator Issues
« Reply #5 on: February 28, 2012, 11:00:24 AM »
Thanks for the replies. I had the capacitors hooked up but maybe I didn't have any load. I think I wanted to probe the output before hooking up my loads. I calculated that I'd need roughly 1A but I wanted to be safe which is why I got the bigger reg. I've since swapped the 7805 into my circuit but I'll test the new one with a load to see if that works.

Thanks again!

Offline spizzakTopic starter

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Re: Voltage Regulator Issues
« Reply #6 on: March 04, 2012, 09:30:39 PM »
Just to wrap this us, it looks like the issue was in fact that there was no load.

Thanks!
« Last Edit: March 06, 2012, 12:37:43 PM by spizzak »

Offline Soeren

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Re: Voltage Regulator Issues
« Reply #7 on: March 05, 2012, 09:54:51 PM »
Hi,

Just to wrap this us, it looks like the issue was in fact that there was load.
Eh? Don't you mean "...the issue was in fact that there was NO load."  ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline spizzakTopic starter

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Re: Voltage Regulator Issues
« Reply #8 on: March 06, 2012, 12:39:03 PM »
Haha thanks. Fixed.

Offline jkerns

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Re: Voltage Regulator Issues
« Reply #9 on: March 07, 2012, 01:58:15 PM »
Just to wrap this us, it looks like the issue was in fact that there was no load.

Thanks!


Glad you got it fixed, but FWIW it is typically worth reading the datasheet - page 18 of http://www.micrel.com/_PDF/mic29150.pdf tells you that a minimum load is required to avoid having leakage current result in a high output voltage.

(Oh - I see I have to pass a quiz in order to post!)
I get paid to play with robots - can't beat that with a stick.

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