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-12v cheap and simple power supply

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Llamarama

Member
Hello everyone, I'm trying to make a simple -12v power supply that can supply up to 800mA using only simple components so I don't have to order online as I don't have a credit or debit card.

I found a circuit on Colin Mitchell's site showing how this could be done with a 555, and lower down the page, how to increase current using transistors in a push-pull arrangement. Would I be able to use this to get such a result?

The final regulation will be done by a 7912, will the 555 and transistors put out enough current to drive this reliably, or will I have problems with the transistors saturating?

Many thanks in advance

(Schematics lightly modified from Colin Mitchell's site)

View attachment 64970
 
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JimB

Super Moderator
Most Helpful Member
The power supply you have shown looks ok at first sight but depending on the oscillator frequency, the 22uF capacitors could be a bit on the low side for an 880mA supply and depending on the ripple you can tolerate.

Before building a PSU like this, I have to ask why you want a negative supply? and what psu do you have to start with?

This is one of these odd questions like "If I was going there, I would not start from here!"

A bit more info about what you are trying to do would be helpfull before making any recomendations.

JimB
 

Llamarama

Member
I'm making a small ATX power supply for a Mini ITX board I have. I don't really want to spend £30-50 + P&P on a power supply that will fit in the enclosure I want to use, so I decided to make one myself. I've heard of a few who have done this but didn't give any schematics or even the ICs used. The motherboard I'm using helpfully gives it's current consumption in the manual. Fast forward a bit of rummaging and I've got the 3.3v, 5v, 12v and 5vsb sides sorted out, including PW_ON and PW_GOOD signals. It's just the -12v that I need now.

Total ripple allowed is 120mv, if I increased the 22uF capacitors to 47uF and added a 220 in parallel with the 7912 output help reduce ripple to an acceptable level?
 

colin55

Well-Known Member
You will not get -12v from +15v. There are too many voltage-drops. You need 2,200u for 880mA current.
 

JimB

Super Moderator
Most Helpful Member
Having seen your requirements and studied the proposed circuit, my opinion is that it wll not be adequate for what you want.

The 7912 requires an input voltage of at least 13.1 volts in order to work correctly.
Your proposed circuit feeds the 7912 via two 1N4004s, each will have a volt drop of about 0.7 volts.
There will be some volt drop across the 2.2 ohm resistors and switching transistors, lets just pretend that it is 1volt across each, it WILL be more.

So, we need a supply to this circuit of 13.1 + 2 x 0.7 + 2 x 1 = 16.5 volts.

Your existing supply has at most 12 volts to feed this circuit, you are a bit short of volts Bonny Lad! :(

JimB
 

Llamarama

Member
Oh well, I'll try building the circuit on a bit breadboard, see what values work and which don't, mess around with supply voltages and stuff. Something to do in the evening anyway :) There's more than one way to skin a cat as they say :)
 

bountyhunter

Well-Known Member

bountyhunter

Well-Known Member
I'm making a small ATX power supply for a Mini ITX board I have. I don't really want to spend £30-50 + P&P on a power supply that will fit in the enclosure I want to use, so I decided to make one myself. I've heard of a few who have done this but didn't give any schematics or even the ICs used. The motherboard I'm using helpfully gives it's current consumption in the manual. Fast forward a bit of rummaging and I've got the 3.3v, 5v, 12v and 5vsb sides sorted out, including PW_ON and PW_GOOD signals. It's just the -12v that I need now.

Total ripple allowed is 120mv, if I increased the 22uF capacitors to 47uF and added a 220 in parallel with the 7912 output help reduce ripple to an acceptable level?
Typical commercial power supply designs use an offline flyback converter so making multiple outputs is very cheap and adding another output just means adding a winding to the transformer.

Building up another separate supply will cost many times as much as what the incremental cost is in the ATX supply.
 
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