Continue to Site

# +12 -12 supply from 24V

Status
Not open for further replies.

#### kamalavignesh

##### Newbie level 6
Hi,
I have a 24V supply from that i need +12V and -12V. wattage will be less than 1W. I tought of using 7812 & 7912. But for 7912 i have to give negative input voltage how can i get that from 24V supply. Suggestions are welcome.

Use a potential divider. Use two 10k resistors in series and use the middle voltage as ground.

Potentials are relative. -12 means it is less than 0 by 12 V. So, choose 12 V as your reference so that, 24V will become +12 and 0 will be -12.

Hi ninju,
In the ground i have to maintain zero potential because its the controller ground. according to your case in ground some potential will be there.

You say the load will be 1W. This works out to a current draw of 83 mA at 12V.

You can use a 12 V zener diode inline with a 150 ohm resistor.

You'll waste 80 mA continuously through them. Each will idle at 1 watt.

You can tap at the node between the resistor and zener. Between them will be an artificial ground at 0V.

The zener will dissipate 2W max if you connect the load parallel to the resistor.

Or the resistor will dissipate 2W max if you connect the load parallel to the zener.

The ground node will not vary more than 1/4 V from zero provided your load is always over 140 ohms.

Last edited:

you could use a buck-boost converter.

With one side of the 24V supply grounded you can only create +12V . A DC-DC converter is needed for a minus voltage. The are many small modules for sale that can put out +and- 12V from a single + input.

Use a potential divider. Use two 10k resistors in series and use the middle voltage as ground.

Potentials are relative. -12 means it is less than 0 by 12 V. So, choose 12 V as your reference so that, 24V will become +12 and 0 will be -12.

ninju,
It is advisable to use a unity gain buffer in this scheme, so that the load sees a low impedance ground. The op amp can be piowered from the 24V rails. The sinking capacity of the op amp could be a limiting factor.

A cheaper way is to use a charge pump IC to generate minus voltage.

Something like this may work.
View with LTSpice.

Version 4
SHEET 1 880 680
WIRE 80 -32 -32 -32
WIRE 208 -32 80 -32
WIRE 352 -32 208 -32
WIRE 448 -32 352 -32
WIRE 544 -32 448 -32
WIRE 608 -32 544 -32
WIRE 352 -16 352 -32
WIRE 80 0 80 -32
WIRE 448 32 448 -32
WIRE 352 80 352 64
WIRE 384 80 352 80
WIRE -32 112 -32 -32
WIRE 208 128 208 -32
WIRE 144 144 144 96
WIRE 176 144 144 144
WIRE 352 160 352 144
WIRE 352 160 240 160
WIRE 448 160 448 128
WIRE 512 160 448 160
WIRE 608 160 608 48
WIRE 608 160 512 160
WIRE 80 176 80 80
WIRE 80 176 16 176
WIRE 176 176 80 176
WIRE 352 176 352 160
WIRE 448 192 448 160
WIRE 608 192 608 160
WIRE 80 240 80 176
WIRE 384 240 352 240
WIRE 352 256 352 240
WIRE -32 336 -32 192
WIRE 16 336 16 240
WIRE 16 336 -32 336
WIRE 80 336 80 320
WIRE 80 336 16 336
WIRE 208 336 208 192
WIRE 208 336 80 336
WIRE 240 336 208 336
WIRE 352 336 240 336
WIRE 448 336 448 288
WIRE 448 336 352 336
WIRE 528 336 448 336
WIRE 608 336 608 272
WIRE 608 336 528 336
WIRE 240 368 240 336
FLAG 240 368 0
FLAG 512 160 Vfb
FLAG 144 96 Vfb
FLAG 544 -32 +12V
FLAG 528 336 -12V
SYMBOL voltage -32 96 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL res 64 -16 R0
WINDOW 0 -38 66 Left 0
WINDOW 3 -43 36 Left 0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res 64 224 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL Opamps\\LT1001 208 96 R0
WINDOW 3 -32 1 Left 0
SYMATTR InstName U1
SYMBOL npn 384 32 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL pnp 384 288 M180
SYMATTR InstName Q2
SYMATTR Value 2N2907
SYMBOL diode 336 80 R0
WINDOW 0 -29 0 Left 0
WINDOW 3 -81 28 Left 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 336 176 R0
WINDOW 0 -36 28 Left 0
WINDOW 3 -76 57 Left 0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL res 336 -32 R0
SYMATTR InstName R3
SYMATTR Value 2k2
SYMBOL res 336 240 R0
SYMATTR InstName R4
SYMATTR Value 2k2
SYMBOL res 592 176 R0
SYMATTR Value 120
SYMBOL res 592 -48 R0
SYMATTR Value 120
SYMBOL cap 0 176 R0
SYMATTR InstName C1
SYMATTR Value 100n
TEXT -66 360 Left 0 !.tran 1

Something like this may work.
It's a virtual ground generator, nothing but an extended version of the potential divider suggested in post #2.
You however ignored the requirement from post #3
In the ground i have to maintain zero potential because its the controller ground.
It can't be fulfilled by simply placing a "-12V" label at the ground node, as you did.

Also other contributions missed this point.

As said, a kind of switching voltage inverter will be needed, either with inductive (inverting buck/boost) or capacitive (charge pump) energy storage. 1 W isn't that much, but I would prefer an inverting buck/boost converter, which is available as a single chip solution. An external inductor and capacitors will be required however. Or use a ready made converter module as Einar M suggested. It can also provide a high efficiency conversion for the +12 V node.

It's a virtual ground generator, nothing but an extended version of the potential divider suggested in post #2.
You however ignored the requirement from post #3

It can't be fulfilled by simply placing a "-12V" label at the ground node, as you did.

Also other contributions missed this point.

As said, a kind of switching voltage inverter will be needed, either with inductive (inverting buck/boost) or capacitive (charge pump) energy storage. 1 W isn't that much, but I would prefer an inverting buck/boost converter, which is available as a single chip solution. An external inductor and capacitors will be required however. Or use a ready made converter module as Einar M suggested. It can also provide a high efficiency conversion for the +12 V node.

True :0( , did not read the whole post properly.You will need a switcher to generate the negative voltage.
Something like the LM2578A could be used , but this may be overkill.
Basically 24V -> 12V reg = 12V on the positive side.
24V -> invertor -> -12V reg = -12V on negative side.
GND = GND.

I get it now. A way to generate a negative supply from a positive.

Here's a link to a simple circuit that chops the supply into positive pulses, then sends them through a network made from 2 capacitors and 2 diodes. Suitable for low power needs.

2010 September « Deeaaelectron

Since you want 80 mA, you'll need to drive something more substantial than a 555.

Say a power op amp. The device must be capable of both sourcing current and sinking current.

You'll need to scale up the capacitor values to suit your current need.

Status
Not open for further replies.