Negative Supply

[andy257]

Hi All,

Ive made a cock up and bought in some dc -dc converters without a negative voltage output. I CANT RETURN OR EXCHANGE THEM!

Does anyone know of a circuit that can produce negative supply from a positive?

The converts are powered from 12V and give an output of 15V, if possible i would like -15V aswel.

Thanks

 

[whiz115]

the fastest but not the most reliable way is to connect two resistors of the same value (voltage divider) and 2 capacitors also in parallel and of the same value, after the rectification bridge and you can take your ground from the center of the capacitors and resistors.... i hope i helped

 

[k7elp60]

There are several ways to accomplish your goal. Can you give us an idea on how much current in milliamps or amps you need?

 

[audioguru]

It might be easier to convert your circuit to be powered from a single +12V or +15V supply. Attach the schematic and explain what it does.

 

[Nigel Goodwin]

It might be easier to convert your circuit to be powered from a single +12V or +15V supply. Attach the schematic and explain what it does.

Yes, far too little information about the PSU and the project!.

 

[andy257]

The dc-dc converters are to power an op amp with +ve and -Ve suppply. The op amp is part of a constant current circuit. (ive posted this circuit on here years ago and i still use it today when i require a constant current source), however i made a cock up in the ordering and now i am trying to find an easy (cheap) solution.

I thought i might be able to get away with a postive and gnd connection but it appears the circuit does not perfrom as a constant current source with only a positive supply. I think its perhaps because the mosfet requires a negative supply to completly turn off the fet. Thats my analysis anyway. The circuit works fine when suppled with a +/- supply.

Hope that helps.

Andy

below is the link to the circuit. Audio Guru and Nigel may remember this circuit when i initially discovered it back in 2005.

https://www.electro-tech-online.com/attachments/voltage_comparator_366-png.6472/

 

[Roff]

That circuit only shows one power supply, and generally only needs one. If you can't get the FET to turn off, you may need to select a different op amp, one whose output can go to (or at least nearer) the negative rail.
Why don't you show us how you are really using it. I doubt you are actually using a battery or power supply for the reference. And are you actually using this circuit to bias a zener?
EDIT:
A thousand pardons, Andy. I looked at your schematic again and noticed you are using a JFET. A MOSFET would allow you to use a single supply even if the reference voltage is near zero.

 

[andy257]

Nigel / Audio i would greatly appreciate some advice here. Thanks

 

[audioguru]

It looks like the opamp and FET are making a constant current for the zener diode. If the zener diode has a fairly low voltage, the output voltage of the opamp can go low enough and the FET doesn't need a high gate voltage then two similar circuits should work with a single supply like these:

 

[Roff]

Andy, I just noticed that you are using a JFET. Is there any reason you can't use a MOSFET? It might solve your problem, since the gate-to-source voltage will be positive, instead of negative as it is on a JFET.

 

[KMoffett]

Andy,

Are the DC-DC converters the isolated type...negative input has no ohmic connection to the negative output? If so just use two of the cheap converters. One with its negative to common and the other with its positive to common. If not...or if I missed this earlier...just ignore this.

Ken

 

[JimB]

I agree with Ken, he just beat me to it!

An associated thought:
There must be billions of pounds worth of stuff hidden in the stores of various companies throughout the world, and that stuff was purchased because someones finger slipped on the catalogue page when filling out the order form, resulting in the wrong part number being ordered.

How do I know this? Probably because I am as guilty as the next engineer.

JimB

 

[mvs sarma]

I agree with Ken, he just beat me to it!

An associated thought:
There must be billions of pounds worth of stuff hidden in the stores of various companies throughout the world, and that stuff was purchased because someones finger slipped on the catalogue page when filling out the order form, resulting in the wrong part number being ordered.

How do I know this? Probably because I am as guilty as the next engineer.

JimB

Hi JimB, i fear , you are rather too outspoken. It is true that purchase once made are thrown into almairahs(lockers) as the level of obsolescence is too fast. At times the concerned persons would have deserted the said company for a jump in salary

Very common and truely said. I know many of these things are sold in auction. There are Firms that purchase them and try for the next possible buyer for further burial. When firms close in India, we get all odd components at the auction houses.

for eg a 4 lead TO220 with name LM78R05 ( a LDO regulator with adjustabler current limit.)
and ofcourse what not!!