12V to 5V

dome406 · 29th April 2007, 02:39 PM

This is a rather trivial question, if I have 12V from a power supply how can I drop this to 5V. The current drawn is going to be 0.5A so potential diveder not practial ?. I thought you could use a 5V voltage regulator which I tried (L78M05) however get an output of 7V.

Thanks
Will

Sonusthree · 29th April 2007, 02:53 PM

Hi,
It's a little strange that you had 7 volts. How was it connected? Did you try another reg in cas that one was faulty?
You say that you need 500mA but the L78M05 is only rated at that. You should probably try a 1/1.5/2 amp 5 volt reg instead.

http://www.maplin.co.uk/search.aspx?...enu=y&doy=29m4

dome406 · 29th April 2007, 03:03 PM

Unfortunatley I only ordered one

it was connected as in the datasheet. Its for a project which has to be handed in on friday and i was doubting if what i had done was correct. Will have another look at it when go back.

Thanks very much

Minion · 29th April 2007, 08:37 PM

When you tested the Voltage of the 7805 regulator dwas there a Load on the Supply??

I believe that proper regulation can only be accomplished with these regulators if there is a Minimal Load on the Regulator, So maybe try putting a LED or a resistor on the 5v supply and then test the Voltage and it should be closer to the correct 5v.....

Cheers

PS the LM117/217/317 Positive regulators are adjustable so you could Dial in the correct Voltage useing a trim pot....

cpemma · 29th April 2007, 08:55 PM

It's probably testing without a load, the 5V±0.25V output is only guaranteed with loads over 5mA and multimeters are way less than that.

XEAGLEKEEPER · 29th April 2007, 09:31 PM

http://www.sound.westhost.com/project05a.htm
This may be of some help.

I_Forgot · 30th April 2007, 12:28 AM

Quote:

Originally posted by Minion
When you tested the Voltage of the 7805 regulator dwas there a Load on the Supply??

I believe that proper regulation can only be accomplished with these regulators if there is a Minimal Load on the Regulator, So maybe try putting a LED or a resistor on the 5v supply and then test the Voltage and it should be closer to the correct 5v.....

I've never seen one that puts out anything more than maybe 100mV away from spec'd value under no-load to full-load conditions. Those ICs have an internal reference. I'm betting it was connected wrong.

I_F

cpemma · 30th April 2007, 03:09 PM

Another possibility. From here

Quote:

A while back I hooked up a 5V regulator using long wires and no capacitors and got 10V out of it! Adding the output capacitor fixed the problem instantly. When I say 10V I don't necessarily mean DC. I assume the regulator was oscillating like crazy.

So when you say "It was connected as in the datasheet" you mean including the recommended capacitors?

A 317 will usually regulate fine with no load, as the typical 240R pot divider resistor between Output and Adjust pins gives a 1.25/240 = 5mA loading.

TwoSpoons · 2nd May 2007, 07:47 AM

12 - 5 = 7. Its just possible it was working fine and you connected your meter to the wrong points.

29th April 2007, 02:39 PM	#1
dome406 diyAudio Member Join Date: Mar 2004 Location: uk	12V to 5V This is a rather trivial question, if I have 12V from a power supply how can I drop this to 5V. The current drawn is going to be 0.5A so potential diveder not practial ?. I thought you could use a 5V voltage regulator which I tried (L78M05) however get an output of 7V. Thanks Will

29th April 2007, 09:31 PM	#6
XEAGLEKEEPER diyAudio Member Join Date: Jan 2006 Location: central Iowa	http://www.sound.westhost.com/project05a.htm This may be of some help. __________________ Ret. USAF... AKA- Avionic * Solder slinger for hire...*

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29th April 2007, 02:53 PM	#2
Sonusthree diyAudio Member Join Date: Dec 2005 Location: Brighton,UK	Hi, It's a little strange that you had 7 volts. How was it connected? Did you try another reg in cas that one was faulty? You say that you need 500mA but the L78M05 is only rated at that. You should probably try a 1/1.5/2 amp 5 volt reg instead. http://www.maplin.co.uk/search.aspx?...enu=y&doy=29m4

29th April 2007, 03:03 PM	#3
dome406 diyAudio Member Join Date: Mar 2004 Location: uk	Unfortunatley I only ordered one it was connected as in the datasheet. Its for a project which has to be handed in on friday and i was doubting if what i had done was correct. Will have another look at it when go back. Thanks very much

29th April 2007, 08:37 PM	#4
Minion diyAudio Member Join Date: May 2006	When you tested the Voltage of the 7805 regulator dwas there a Load on the Supply?? I believe that proper regulation can only be accomplished with these regulators if there is a Minimal Load on the Regulator, So maybe try putting a LED or a resistor on the 5v supply and then test the Voltage and it should be closer to the correct 5v..... Cheers PS the LM117/217/317 Positive regulators are adjustable so you could Dial in the correct Voltage useing a trim pot....

29th April 2007, 08:55 PM	#5
cpemma diyAudio Member Join Date: Jul 2003 Location: S Yorkshire OK	It's probably testing without a load, the 5V±0.25V output is only guaranteed with loads over 5mA and multimeters are way less than that.

2nd May 2007, 07:47 AM	#9
TwoSpoons diyAudio Member Join Date: Oct 2004	12 - 5 = 7. Its just possible it was working fine and you connected your meter to the wrong points.

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