12Vdc -> +-12Vdc

[bh00]

Hi,

Another issue, is the easiest way to convert a +12Vdc input to both +12Vdc and -12Vdc to power some op-amps and video switchers. The issue is, I will also need to power some 7805/7905 regulators off those supplies for PICs and some opamps at the lower voltage. I need to be able to source quite a bit of current therefore and the usual devices I come across will only provide 100mA or so at maximum which isn't enough. Any circuits out there to generate this for me reliably with at least 200mA of current on the -12Vdc rail?

Cheers

 

[Hero999]

A 555 timer can give you a negitive voltage at the expense of some voltage drop.

Use shotky diodes to minimise the voltage drop.

Most op-amp circuits can be converted to run from a single supply though so it normally isn't a problem.

 

[Blatman Bond]

A battery mirror.
555 charge-pump is OK. Just buffer it to reach 200mA.

 

[Speakerguy]

What's a battery mirror?

BTW, the expensive way to do this is to go to astrodyne.com and get a DC/DC converter.They make 'em from 1.5 watts up to waaaay more than you need. I used a 10 watt 5V-> +/-15V unit on a project a while back. $35 or so IIRC. PCB mount.

ETA: since you already have twelve, all you need is something like this:

**broken link removed**

400ma enough?

 

[Hero999]

A battery mirror.
555 charge-pump is OK. Just buffer it to reach 200mA.

A 555 can output 200mA you know.

I presume a battery mirror just mirrors the voltage, i.e. it produces an equal negitive supply to the imput positive supply.

There are also IC that will do this, check out the MAXIM and Linear websites.

If you're using a wallwart, then go for one with an AC output, it's easy to build a rectifier that outputs both a positive and negitive voltage.

 

[mvs sarma]

Hi BH00 ,
try using a old device MC34063 ( an 8 pin device)
datasheet at www.alldatasheet.com

they offer you the design and pcb layout -- it can probably be housed in a square inch space.

Sarma

 

[Speakerguy]

An AC power pack (wall wart) with a voltage doubler on it would be a good way to get this if you have AC available. Only problem with those is you need bigger filter caps and you'll have larger ripple currents when doing a +/- supply because you only get halfwave rectified current for each polarity.

**broken link removed**

 

[Hero999]

Ripple wouldn't be a problem for op-amps as the negitive feed-back should eliminate its effects.

 

[Blatman Bond]

A 555 can output 200mA you know.

200mA at 2.5V output voltage drop.
Isn't the topology will decrease the current output?

 

[bh00]

With a DC-DC converter IC, such as the NMA1209SC, can you connect the Vin ground to the output common, or is this not good as it greats issues with grounding or whatever?