Will 7915 Regulator Handle

LHMAudio · 18th March 2006, 07:37 PM

I am trying to take +-32.6 VDC and get +-15 VDC. I am using ST Microelectronics L7815ACV and L7915ACV regulators. I get +15 VDC with no problem. But the negative regulator gives me 24 VDC.

Looking at the label side of the regulator I have attached the ground to the left leg, the input to the center leg and the output to the right leg. With no other circuitry connected I get -24 VDC from ground to output. I have tried 4 different regulators all with the same result. When I remove power from the supply and the capacitors in the 32.6 VDC sypply discharge, the voltage across the regulator drops to 15 and holds there for a few seconds as though it is trying to regulate.

Is it possible that this regulator will not handle 35 VDC input even though the data sheet says it will? Is there a different regulator I should use? Thanks for any help.

jaudio · 18th March 2006, 07:56 PM

The 7915 can handle 32.6 volt,I been using 35volts for more than 2years now and they are working fine.

Check wiring

Make you caps are wired properly.

LHMAudio · 18th March 2006, 11:15 PM

I hooked power directly to the pins of the regulator. -32.5 to the center pin and ground to the left pin. I then measured from ground to the right pin. and got -24 VDC. I even took one regulator that I had removed from the circuit and tried rervsing ground and ouput thinking I was reading the data sheet wrong. But it did not work at all and got very hot in short period.

The positive regulator works fine which is why I am not sure what to do next. I am thinking of trying a different brand.

ilimzn · 19th March 2006, 01:14 AM

78xx and especially 79xx need a capacitor at the output in order to work stably, and some versions may also need a minimum load (usually a few mA will suffice). In particular do not even try to run a 79xx without at least 10uF output cap, you will get all sorts of nasty effects otherwise. It would be prudent to download a datasheet, they usually include an application schematic, which, you will notice, has at least two caps connected to the regulator. You omit these at your won risk.

neutron7 · 19th March 2006, 01:19 AM

i have used a few of these and never had a problem like that. is the output you get actually regualted?

maybe you just have a dry solder joint. look esp on the ground pin. also maybe you got a bad regulator, but i just buy el cheapo chinese ones from ebay and i never even had one of those DOA.

LHMAudio · 19th March 2006, 08:40 PM

I added a 10k resistor as a load on the negative supply and that dropped the voltage to -19.6VDC. So I am not getting regulation.

LHMAudio · 19th March 2006, 10:34 PM

My problem is solved. The ST Micro version of the 79xx requires a load of 3ma or more to begin to regulate. When I added a load of 5k the voltage dropped to 15.5VDC. What was so misleading is that the 78xx version of the regulator worked fine with no load.

Thanks everyone for your help and suggestions.

ilimzn · 20th March 2006, 12:08 AM

Quote:

Originally posted by LHMAudio
My problem is solved. The ST Micro version of the 79xx requires a load of 3ma or more to begin to regulate. When I added a load of 5k the voltage dropped to 15.5VDC. What was so misleading is that the 78xx version of the regulator worked fine with no load.
Thanks everyone for your help and suggestions.

Well, see my post above

Although it is tempting, one should not compare 78xx and 79xx, internally, they are quite different beasts.

peranders · 20th March 2006, 04:44 AM

Quote:

Originally posted by LHMAudio
What was so misleading is that the 78xx version of the regulator worked fine with no load.

79xx and 78xx are not indentical inside depending of that it's hard to make good PNP transistors. It's always a good idea to read datasheets

LHMAudio · 20th March 2006, 03:19 PM

I read through the ST Micro data sheet a number of times looking for an indication of the minimum required current draw and didn't see anything. However, I don't know the meaning of all the speicification abbreviations. Is there a shorthand notation that indicates minimum current draw for regulation?

18th March 2006, 07:37 PM	#1
LHMAudio diyAudio Member Join Date: Sep 2005	Will 7915 Regulator Handle I am trying to take +-32.6 VDC and get +-15 VDC. I am using ST Microelectronics L7815ACV and L7915ACV regulators. I get +15 VDC with no problem. But the negative regulator gives me 24 VDC. Looking at the label side of the regulator I have attached the ground to the left leg, the input to the center leg and the output to the right leg. With no other circuitry connected I get -24 VDC from ground to output. I have tried 4 different regulators all with the same result. When I remove power from the supply and the capacitors in the 32.6 VDC sypply discharge, the voltage across the regulator drops to 15 and holds there for a few seconds as though it is trying to regulate. Is it possible that this regulator will not handle 35 VDC input even though the data sheet says it will? Is there a different regulator I should use? Thanks for any help.

18th March 2006, 11:15 PM	#3
LHMAudio diyAudio Member Join Date: Sep 2005	Tried With No Caps I hooked power directly to the pins of the regulator. -32.5 to the center pin and ground to the left pin. I then measured from ground to the right pin. and got -24 VDC. I even took one regulator that I had removed from the circuit and tried rervsing ground and ouput thinking I was reading the data sheet wrong. But it did not work at all and got very hot in short period. The positive regulator works fine which is why I am not sure what to do next. I am thinking of trying a different brand.

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18th March 2006, 07:56 PM	#2
jaudio diyAudio Member Join Date: Oct 2004	The 7915 can handle 32.6 volt,I been using 35volts for more than 2years now and they are working fine. Check wiring Make you caps are wired properly.

19th March 2006, 01:14 AM	#4
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	78xx and especially 79xx need a capacitor at the output in order to work stably, and some versions may also need a minimum load (usually a few mA will suffice). In particular do not even try to run a 79xx without at least 10uF output cap, you will get all sorts of nasty effects otherwise. It would be prudent to download a datasheet, they usually include an application schematic, which, you will notice, has at least two caps connected to the regulator. You omit these at your won risk.

19th March 2006, 01:19 AM	#5
neutron7 diyAudio Member Join Date: Dec 2003 Location: Toronto Canada	i have used a few of these and never had a problem like that. is the output you get actually regualted? maybe you just have a dry solder joint. look esp on the ground pin. also maybe you got a bad regulator, but i just buy el cheapo chinese ones from ebay and i never even had one of those DOA.

19th March 2006, 08:40 PM	#6
LHMAudio diyAudio Member Join Date: Sep 2005	I added a 10k resistor as a load on the negative supply and that dropped the voltage to -19.6VDC. So I am not getting regulation.

19th March 2006, 10:34 PM	#7
LHMAudio diyAudio Member Join Date: Sep 2005	My problem is solved. The ST Micro version of the 79xx requires a load of 3ma or more to begin to regulate. When I added a load of 5k the voltage dropped to 15.5VDC. What was so misleading is that the 78xx version of the regulator worked fine with no load. Thanks everyone for your help and suggestions.

20th March 2006, 03:19 PM	#10
LHMAudio diyAudio Member Join Date: Sep 2005	I read through the ST Micro data sheet a number of times looking for an indication of the minimum required current draw and didn't see anything. However, I don't know the meaning of all the speicification abbreviations. Is there a shorthand notation that indicates minimum current draw for regulation?

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