Convert old PC power supply to 13.5 volt battery charger

[Rolo]

This is an article on how to convert an unused AT power supply into a battery charger, and if you find anything interesting feel free to share because I want to do the same thing although I need at least 15 volts so I can connect its output in parallel with the output of the solar panels to use the same solar regulator with both of them.

http://www.siliconchip.com.au/cms/A_30705/article.html

[stevew]

Hmmm, interesting. It describes it as high current but doesn't say how high (at least not without me paying for the rest of the article!).
Most AT power supplies were relatively low power by todays standards at 250-300W. They usually only supplied 5-6A on the 12V rail but I guess if you're into rewinding the transformer then you could have all 300W. I.e. 21A at 13.8V A nice power supply.
Personally I won't be trying this as I bought a 20A 3 stage charger off Ebay a few weeks ago for peanuts But in my days as a student I used to use an old AT PSU as a 12V supply all the time and it was great, almost indesructable!

Steve

[bortaf]

I did used to use my CB power pack to run my video when the transformer went on that

[Rolo]

Most AT power supplies were relatively low power by todays standards at 250-300W. They usually only supplied 5-6A on the 12V rail

8 amps was the average in the ones I came across and that would be perfect for my use.

but I guess if you're into rewinding the transformer then you could have all 300W. I.e. 21A at 13.8V A nice power supply.
...I used to use an old AT PSU as a 12V supply all the time and it was great, almost indestructable!

Steve

Without having to do anything like soldering some loading resistors across the voltage outputs you weren't using?

For the use I want it for (to occasionally help the solar panels charge three battery banks through the solar regulator) it would be great if I could simply put the 12 volt and 5 volt outputs in series for a total output of 17 volts, and without even having to open to PC power supply.

[Rolo]

--------------------------------------------------------------------------------

This information was published by VK2EVB, Peter McAdam, in the December 2001 edition of Lo-Key #72 (CW Operators' QRP Club Inc.). As I have done nothing but reproduce his results, all the credits and kudos belong to VK2EVB.
--------------------------------------------------------------------------------
Introduction
Power supplies from old personal computers are "a dime a dozen," so to speak. Go to any hamfest/computer show and you will find tables of them. I can get all I want from a local computer vendor who has stacks of old "useless" computers. Typically, these power supplies produce outputs of +12V/9A, -12V, +5V/20A, -5V. VK2EVB's circuit converts the +12V line to +13.5V. By keeping the original rectifier in the +12V line one ends up with a +13.5V/9A power supply. (VK2EVB swapped the 20A rectifier from the +5V line with the 8A version in the +12V line to end up with a supply rated at +13.5V/15A.) I choose to keep the 9A rectifier in line and not do the swap as I do not need a high current capability while running QRP.

--------------------------------------------------------------------------------
Doing the Conversion

Use Care When Working on the Power Supply as the AC Mains Lines are Exposed When Opening the Enclosure!!! Unplug the power supply when working on it and put it back together before testing it!!!
The process is fairly simple and straight forward (refer to the schematic below).
Basically VK2EVB removed three resistors from the power sampling circuitry, and added one.
1. Start by removing all but one output lead from the +12V line and the GND line. Keep the leads with the largest wires. These will serve as the output lines to whatever jack you supply.
2. Trace Pin 1 from the TL494 through the 27k resistor (R33) to the +12V line, and insert a 2k2 resistor in series with the +12V output and R33.
3. Trace Pin 1 from the TL494 through the 4k7 resistor (R32) to the +5V line and remove the resistor.
4. There is also on that same line a 470R (R21) and a 270R (R20), going from the +5V trace on the board to -5V and -12V respectively. Remove both of these resistors.
5. Secure the circuit board back into the enclosure and plug in the AC line. You should see +13.5V where formerly there was +12V. (I didn't, at first! I had removed the wrong resistor and ended up with about +8.8V!)
--------------------------------------------------------------------------------
Improving the Filtering
1. Typically there is a 2200uF/16V electrolytic from the +12V output pad to GND. VK2EVB replaced that capacitor with two 2200uF/25V electrolytics. I didn't have those values in my junk box so I replaced the original capacitor with a 6800/35V electrolytic instead. Remember to observe the polarity of the capacitors!
2. VK2EVB also installed a filter in the output line of the now +13.5V supply to remove any stray radiation from the AC mains. He used a PI filter consisting of two 0.047uF capacitors and a 100uH inductor using a ferrite core from another junk supply. I didn't install the filter, but I did install a similar RF choke in series with the +13.5V output line. I used a ferrite core from an old computer power cord, winding as much insulated 18AWG stranded wire through it as I could. I ended up with about 550uH.
3. A 220 ohm resistor was also added across the output +12V and GND terminals to serve as a load for the supply, attempting to keep the noise to a minimum. VK2EVB used a 1 watt resistor; I used a 2 watt carbon composition.
4. VK2EVB also soldered desoldering braid on top of the +12V traces to improve current handling capability. I chose not to do this since I didn't swap the 9A and 20A rectifiers (see the note above).

--------------------------------------------------------------------------------
Almost Finished!
1. In my supply there were two holes where the original output wiring and computer on/off switch wires exited the enclosure. These holes were on the end of the enclosure opposite the fan and AC jack. The end of the enclosure was slotted for ventilation.
2. I bolted a rectangular piece of circuit board on this back side of the enclosure, using the slots for screw holes, and mounting two banana plug terminals in the space provided by the larger holes.
3. I also enlarged one end of one ventilation slot to accept an SPST toggle switch. (This switch should be rated for the AC line current and it should be installed in one of the AC lines coming into the power supply. Use fairly heavy wire.)
4. VK2EVB also installed a power on indicator LED. He picked up +12V for the LED from the original +5V circuit board pad which after the conversion is at about +12V.
5. Stick four rubber feet on the bottom of the enclosure and test the supply at various frequencies while listening on your radio. Hopefully, you'll hear only signals and background noise, and no spurious signals from the switching supply.

--------------------------------------------------------------------------------
My Results
My converted supply output is +13.49V/9A and, thus far, I haven't heard any unwanted signals from the power supply itself. The conversion took me about 2 hours to complete. Have fun and credit VK3EVB for a fun homespun project!
--------------------------------------------------------------------------------


For a diagramme click on:

http://www.qsl.net/aa3sj/Pages/PC-Supply.html

[stevew]

Without having to do anything like soldering some loading resistors across the voltage outputs you weren't using?

For the use I want it for (to occasionally help the solar panels charge three battery banks through the solar regulator) it would be great if I could simply put the 12 volt and 5 volt outputs in series for a total output of 17 volts, and without even having to open to PC power supply.

Nope, I just left the outputs I wasn't using discconnected.

You can't series up the 12V and 5V supplies because they have a common ground connection. You could go between the -5V and 12V outputs but the -5V supply is usually very small, like 500mA so it wouldn't be much use.

Interstingly you can also use an ATX power supply. To turn it on you just ground one of the connections (I can't remeber which off hand) and these are available at much higher power ratings.
EDIT: Look here http://www.techpowerup.com/articles/other/22

The supply you finish up with won't have any current limiting so at 17V it would be easy to cook it if your batteries are big.

Steve

[Rolo]

You can't series up the 12V and 5V supplies because they have a common ground connection. You could go between the -5V and 12V outputs but the -5V supply is usually very small, like 500mA so it wouldn't be much use.

Steve:

How about using two PC power supplies with the 12 volt 10 amp and 5 volt 20 amp outputs in series with each other for a total output of 17 volts 10 amps?

And if I use three PC power supplies (a have plenty of old ones) I could connect two of them with their 12 volt 10 amp outputs in paralell and then a third one using its 5 volt 20 amp output in series with the other two for a total output of 17 volts 20 amps, and without even having to open any of them, but what about their 230 volt primaries?

Is there likely to be any funny interaction between them and their now non earthed outputs in a motorhome?

The supply you finish up with won't have any current limiting so at 17V it would be easy to cook it if your batteries are big.

A 50 watt 1 ohm resistor (or less - made from electric heating wire) or a headlamp in series could take care of that current limiting but surely if I connect that 17 volt output directly in parallel with the solar panels the solar panel regulator should take care of that while at the same time giving the batteries exactly the boost and float voltages they need.

My solar panels produce between 17 and 21 volts, they already have isolating diodes (so they don't drain on each other when one of them is in the shade) and another 40 amp 65 volt Schottky diode on the output of this PC power supply setup should take care of the rest to prevent any voltage feedback from the solar panels into the ps regulating circuits, I think,.. or I used to think therefore I used to be.

[Sundowner]

Is this some kind of home made auto battery charger
how much would it cost in time and parts???

[Rolo]

Is this some kind of home made auto battery charger
how much would it cost in time and parts???

In the way I want to do things, without even having to open them up to add or change any components, it would cost me absolutely nothing; if you ask around you'll find some people who cannot even give away their old PC power supplies, or look around for unwanted computers on the side of the road at council clean up time in your area.

Mounting these two PC power supplies into the camper and wiring it all up should take me no more than half a day and that's including plenty of time to have a long breakfast, drink three or four slow cups of cofee while reading the newspaper, answer the phone, maybe spend one hour on the internet, ..

[stevew]

Steve:

How about using two PC power supplies with the 12 volt 10 amp and 5 volt 20 amp outputs in series with each other for a total output of 17 volts 10 amps?

You could do that but you'd have to be absolutely sure that the outputs of the power supplies were isolated from the inputs and probably the case. This isn't usually true. In PSUs I have tested the 0V output is usually connected to the case and hence mains earth. Thus when you connected them in series one PSU would be shorted out. Of course you could use a non earthed supply but I couldn't ever recomend that!

And if I use three PC power supplies (a have plenty of old ones) I could connect two of them with their 12 volt 10 amp outputs in paralell and then a third one using its 5 volt 20 amp output in series with the other two for a total output of 17 volts 20 amps, and without even having to open any of them, but what about their 230 volt primaries?

Parallel is no problem although you may find they don't share the power very well. As I said above you must have isolation somewhere in order to use them in series.

The supply you finish up with won't have any current limiting so at 17V it would be easy to cook it if your batteries are big.

A 50 watt 1 ohm resistor (or less - made from electric heating wire) or a headlamp in series could take care of that current limiting but surely if I connect that 17 volt output directly in parallel with the solar panels the solar panel regulator should take care of that while at the same time giving the batteries exactly the boost and float voltages they need.

My solar panels produce between 17 and 21 volts, they already have isolating diodes (so they don't drain on each other when one of them is in the shade) and another 40 amp 65 volt Schottky diode on the output of this PC power supply setup should take care of the rest to prevent any voltage feedback from the solar panels into the ps regulating circuits, I think,.. or I used to think therefore I used to be.

A resistor would do it I suppose although it seems like a waste of power.
You could use your regulator but only if it's the right type. It would have to be some sort of swithing/load matching regulator. The problem is that solar panels have a high open circuit voltage but it doesn't matter if you short them out. When you connect them to a flat battery the voltage is pulled down to whatever the batteries are at because of the low impedence of a lead acid battery. The panel can only supply, say for a 60W panel, 5A and does so happily. If you try that with a PSU it will try to supply how ever many Amps are needed to bring the battery up to 17V and tht could be a lot! It probably won't be able to do that but might cook itself trying.
If you are going to use a PSU with no ccurrent limiting then my advice is set it to a lower voltage that won't kill the batteries or the PSU like 13.8V. Whilst this won't get the most charge into the batteries it will last longer.
Always fit fuses! As many as possble!

Steve

[Rolo]

You could do that but you'd have to be absolutely sure that the outputs of the power supplies were isolated from the inputs and probably the case.

Why do you think I'm still waiting to hear from somebody else who's game to try my ideas so I can learn from his mistakes?

Isolating the secondary earth should be a breeze but any possible interactions between the output voltages and the 230 volt earth and neutral is another issue and I'm a coward when it comes to certain things.

One thing a man has to know is what his limitations are.

A resistor would do it I suppose although it seems like a waste of power.

Which is why I wouldn't use it and since I already have a very good battery charger regulator I don't even have to.

You could use your regulator but only if it's the right type.

I know for certain that it isn't a shunt type because when there is no charge the voltage on the regulator input goes up.

Normally you need shunt type regulators with wind type voltage regulators so that the electrical load of the shunt acts as a brake and prevents the blades from overspinning when the batteries don't need the high current and you are not using any power.

[stevew]

You could use your regulator but only if it's the right type.

I know for certain that it isn't a shunt type because when there is no charge the voltage on the regulator input goes up.

Normally you need shunt type regulators with wind type voltage regulators so that the electrical load of the shunt acts as a brake and prevents the blades from overspinning when the batteries don't need the high current and you are not using any power.

Exactly. The problem is that a lot of solar regulators are just on off type. They connect the panels to your battery untill the battery volts come up and then disconnect them. That's fine for solar panels but might draw too much from the power supply without a dropping resistor in series.
Just fit a fuse to be safe.

Steve

[UK_ANDY]

if anyone wants AT power supplys contact me i get more than a few now and again as a computer recycler ive just disposed of half a van load !!!!

[Rolo]

Exactly. The problem is that a lot of solar regulators are just on off type. They connect the panels to your battery untill the battery volts come up and then disconnect them. That's fine for solar panels but might draw too much from the power supply without a dropping resistor in series.

I just did some preliminary experimentation and that's not necessary at all.

Lifting the secondary (low voltage) PCB earth connections from the metal case is a breeze and just by putting some insulation under the PCB earth tags the negative becomes a floating one.

I checked for any resistance or capacitance between the 230 volt primary and the low voltage outputs and there is nothing there so this is going to be a piece of cake.

Just fit a fuse to be safe.

Steve

I always fit fuses everywhere in the most unexpected places, and I even mounted three 63 amp 230 volt house type circuit breakers in my camper (working as 12 volt "fusible" links), althought not through the starter circuit, of course.

[Rolo]

Old Fords never die - they just go faster

Some nasty person could reply that Fords couldn't possible go slower.