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.
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.
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.
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.
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.
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.
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.
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.
Thanks everyone for your help and suggestions.
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?
7915 regulator
The maximum input voltage for this chip is -30V, so it is entitled to misbehave. The fact that it seems to work for a brief period while the main reservoir caps discharge is compatible with this interpretation. Try connecting a large zener diode, capable of carrying enough current in series with the inoput to the regulator, and also add small (0.1u) caps to input and output, as already suggested.
The maximum input voltage for this chip is -30V, so it is entitled to misbehave. The fact that it seems to work for a brief period while the main reservoir caps discharge is compatible with this interpretation. Try connecting a large zener diode, capable of carrying enough current in series with the inoput to the regulator, and also add small (0.1u) caps to input and output, as already suggested.
The specific regulator I used was the ST Microelectronics L7915ACV which has a maximum input voltage of -35VDC. The data sheet can be found here:
http://www.st.com/stonline/books/pdf/docs/2150.pdf
Thanks for the info on how to find the minimum current required for regulation. I have completed the circuit and it has been working correctly.
Thanks to everyone for you help. I love this forum.
Lindsay
http://www.st.com/stonline/books/pdf/docs/2150.pdf
Thanks for the info on how to find the minimum current required for regulation. I have completed the circuit and it has been working correctly.
Thanks to everyone for you help. I love this forum.
Lindsay
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