useful power supply

[scopemankit]

The old, thrown out, computers are a mine of small spare parts. The little speaker is a useful too to track stepper motor drive signals. Switches and led's can be re-used and the drives have small motors, some have stepper motors and some have wonderful magnets.

Recently,I saw a project on converting a computer power supply into a bench power supply. It works a charm. There are many projects, both on YouTube and on the web to show how it is done. Special attention should be paid to the switch connections, the led connections and the load required to allow the unit to switch on.

I have used it for stepper motor power and for small guitar circuits with no problems. The main thing is the education you get from such a project and the amount you save by making as opposed to buying such a power supply.

[DavidTrap]

Hi Chris,

I recall something about potentially hazardous voltages being created with some PC power supply modifications being used by model aircraft flyers to recharge their batteries.

I notice you have a big resistor on the side which "rings a bell", so it might be a similar modification process.

Not trying to be a damper on your project - just don't want to see you getting zapped!

DT

[scopemankit]

The unit will not start unless it has a load so a 10 Ohn 15W resistor does the job.

[ZeroID]

You may find, as I have done, that under load the stated voltage drops quite a bit. I made one up a few years ago as a Lab supply and it was great except it was always a half volt or so below stated voltage on the 12 volt line. This is due to the 10 ohm, 15 watt on the +5 volt line which acts as a dummy load so the supply will switch on. It was ok till I used it to run my EQ6 where the 12 volt dropped to 10.6 volts. Funnily enough the EQ6 didn't mind the lower voltage. Plenty of current and the wee red led on the mount flashes but I've had no problems with it.

I'm switching to a converted 1 KVA UPS as a supply when I get time to modify it. Pull the 12 volts direct off the battery line ( + fuses etc ) and the UPS will also protect the rest of the gear while it charges the battery and feeds the mount.

Little PC supply is good on the bench though with + & - 12 volt, + & - 5 volt and + 3.3 volt. The modification is easily found if you google and safe enough to do if you are careful and understand what you are doing.

[wasyoungonce]

I made a computer PSU into a PSU for my Gemini, similar to that of Chris. These PSU's have great Voltage regulation and bookoo I output capabilities as well as under & over voltage control.

Only difference with mine is that occasional turn on would re-set my Gemini computer, a finicky thing with power at the best of times. The cure was putting a LC filter in line with the +12V output to reduce inrush transients.

Been working a treat for the last 2 years. Oh I tired to up the V out to +14V but the controller IC was Chinese and very little data was available to change the V set and V feedback resistors.

[alistairsam]

Hi,
if the +12v is used as a scope supply, dew heater straps, cameras, etc, won't this load draw be sufficient to turn the unit on rather than the resistor?

A CCD requires 12v 4A, 3 straps around 1.2A, scope around 2A, so this PSU would be a good solution.
anyone know what part of the motherboard acts as the load to turn it on? has to be an easier way to turn it on without wasting power as heat via the resistor.

[Poita]

Most will turn on without a load, there is usually a skinny green wire (PC_ON, pin 14 on an ATX supply) and shorting it to any of the black Common wires will usually turn the supply on.

[Poita]

Some links
http://blog.makezine.com/2009/02/23/toolbox-benchtop-power-supplies/
http://tgbuilds.wordpress.com/projects/diy-bench-supply/
http://www.instructables.com/id/DIY-PSU-into-Bench-Power-supply/

[wasyoungonce]

You can fiddle with the load values to see what works with lowest power wastage. But like most Switch-modes these devices are on all the time in some manner (as they are in Mboards, I think they have one of the the +5V rails "on" (hot) all the time) so pretty much you need a load resistor of some type to maintain some system regulation.

IMHO best to use a different PSU for dew straps/dew controller than one used for scope and camera etc.

[steve000]

I haven't used a computer PSU for astronomy but I have as a workbench supply. I just hook up a computer fan to it and that seems to be enough load to let it fire up. Alternatively if you can find it an old AT PSU, be careful though as the switch on these is 240v and does bite. These often are only 200w and usually are very old.

Another thing to try is a laptop PSU then plug it into a breadboard wired with some regulators such as the LM317t or LM78xx These will give you 1.5a but there are higher current ones. This design allows you to split the power outputs and individually switch stuff easier. Laptop PSU's are also smaller but usually put out 19v 5a instead of 12v 30a.
http://en.wikipedia.org/wiki/78xx

A final thing to try but a word of warning these things seem to get quite hot so the case may need a headsink attached but dealextreme and ebay sell regulated 12v power supplys. I picked up a little 2a unit and it was like $7 and provides 240v to 12v 2a but with massive heat. Larger units are available and many guys here use them. connecting an addon board with a LM7805 for example will also give you 5v regulated for powering USB hubs etc. a LM317 with the right resistors will give you an output to make a dummy camera battery so you can run you DSLR from mains power.
http://www.ebay.com.au/itm/AC-100V-240V-12V-10A-Switch-Power-Supply-Driver-Transformer-LED-Strip-/110928921113?pt=AU_Lighting_Fans&hash=item19d3e10219