making 20Amp power supply???

[buckeyes1997]

i was thinking about making a power supply rather than buy one. i need 20A at 12-13.8v and was thinking of converting 2 PC power supplies into 12v supplies. my question is if i put the two side by side and run them in parallel on the output will that double the output current.

in other words if i have two PC supplies and they are each rated 12v@10a and i put the two in parallel will that provide up to 20a @12v?? or would the fact that they are switching supplies goof it all up?

thanks

[Chippie]

The fact that they are switchers has nothing to do with paralleling them up.......but what you will need is to isolate each output with a hefty diode and ensure that there is sufficient cooling.

The other very important thing about pc power supplies, is that the +12v output diodes are not man enough ......trust me I know!......if you wish to check, look at the diode part number and do a search on the net for the electrical characteristics........most of them are rated for around 40v piv which is way too low.....you need something that will handle voltage peaks of around 80-100v.(I found this out while developing a 13.8v 15A supply). You will need to rewind the output choke with thicker gauge wire in order to maintain a decent ripple level......Although I'm no designer I learned a lot about Half Bridge switchers.....shout up if I can help.

Have a look here........http://www.rcgroups.com/forums/showthread.php?t=136133

[buckeyes1997]

i found this info very helpful. i think this is a very reasonable alternative to buying commercial power supplies.

http://www.antennex.com/preview/archive3/powers.htm

note:yes each line of the paralleled 12v needs to be diode protected against feedback. im not really sure what you are saying about the output diodes needing to be modified??

[Chippie]

im not really sure what you are saying about the output diodes needing to be modified??

The diodes used in the +12v output are low piv diodes with a moderate current rating, sometimes they are just 2 DO41 packaged diodes wired together to form a biphase rectifier, alternatively they are 2 diodes in a TO220 package....either way, they are not man enough to cater for the demands of a 10 amp supply.......If you check the link I posted, you will see I used some really heavy diodes in mine......even then I was skimping a bit on cost....I used hi speed recovery diodes simply because I could get Schottky diodes with a suitable rating/cost....very dear!

I checked the link you posted, not keen on the series idea of adding up 5v supplies....but thats my opinion....

Hope you get sorted.. ;)

[Miami Mike]

Why not wire the 5 volt outputs in series instead? I used two arcade video game power supplies, which are very similar to computer supplies. I opened them and isolated the logic grounds from the cases and safety grounds, and then wired the outputs in series and the inputs in parallel. Then I cranked up the output adjustments to about 6 volts each. I get 12 volts with all the current my Triton battery charger can use with no problems.

http://www.vvsss.com/ezone/ps.jpg

[jeffs555]

Even with series diodes in the outputs, unless the supplies are current limited, it is not a good idea to parallel supplies without some kind of load balancing to prevent one supply from trying to supply all the current. There no guarantee that one supply will not try to supply the entire 20 amps and burn up.

[Miami Mike]

Right. In other words you shouldn't have two voltage regulators fighting to regulate the same voltage.

[adamkerrnz]

i use a 12V (adjustable voltage up to 13V) switchmode PSU. it gives out 8amps which is more than enough for charging batteries. i have never heard of anyone charging higher than 5amps, i have also had 2 other friends chargers running off it, they pull around 3amps each, and im usually charging at 4. the PSU gets warm but not hot. i have plans for adding a fan and making a rig with PSU, SLAB (sealed lead acid battery), battery storage slots, and 2 chargers all in an aluminium case.

I recon u dont need the 20Amps!

just my 2 cents...

[Miami Mike]

Buckeyes1997 said that he needs 20 amps and we really ought to take his word for it. At this point it's only an assumption that he needs to power a battery charger.

[zagisrule!]

Transformers are available for that kind of current, it is only 250W or so. 1000VA toroidal transformers from www.toroid.com run under $180 or so....300VA might be $80 or so, add a beefy rectifier and some good size caps, and there you go!

Remember that rectified AC ouputs a larger DC voltage than the AC input. Multiply the AC by the square root of 2 (1.414) to get the DC output after rectification.

Hopefully you don't require a regulated supply, but there are fairly simple schematics available to do that too...be prepared to buy a hefty heatsink and some big transistors if you want to go that route.



-Matt

[Comatose]

Zagis, you can also just get computer power supplies that can source 20A if you want to drop that kind of cash. A few offhand

Antec TRU550 EPS12V claims 36A on the 12v rail (yipe!)
http://www.antec.com/us/productDetails.php?ProdID=20551

Antec TRUE430 claims 20A on the 12V rail
http://www.antec.com/us/productDetails.php?ProdID=20430

A lot of Sparkle PSUs are rated in the high teens if 20A is more of a round number than a hard and fast requirement
http://www.sparklepower.com/proPCPS_ATX.html

There are others, but thats should be a start. Not very DIY, but also very easy. Just something to think about.

[vintage1]

Look on Ebay. In the UK there is a bloke selling a 500VA toroid in a case for a few bucks - he's got lots.

Add a bridge rectifer and capacitor, and you are halfway ere. A half a dozen surplus 2N3055's and a regulator chip and robert is a relative.

Over here also you can get big 11.5v AC toroids desined to run low voltage lighting - not expensive.

[JRuggiero]

I was lucky enough to find a defunct ininterruptible power supply that had a hefty power transformer. Even though the tranny was intended as a step-up device (24V center-tapped to 110VAC) with input fed from a switched battery, it works fine when run from the 110AC in the USA.

With that tranny I built just such a power supply as buckeyes1997 is looking for. The supply puts out 20 VDC at 15 Amps for maybe ten minutes. The transformer runs cool and so do the rectifiers. I'm too chicken to try for higher current.

Jim R

[Miami Mike]

Remember that rectified AC ouputs a larger DC voltage than the AC input. Multiply the AC by the square root of 2 (1.414) to get the DC output after rectification.Sorry, no. Rectified AC has the same RMS voltage that it had before rectification. It's capacitive filtering that tends to increase the voltage to its peak value, while inductive filtering tends to hold it at its RMS value.

[Norman Adlam]

Sorry, no. Rectified AC has the same RMS voltage that it had before rectification. It's capacitive filtering that tends to increase the voltage to its peak value, while inductive filtering tends to hold it at its RMS value.

Mike,

Spot on! You beat me to it! :D

Cheers,

[Chippie]

Look on Ebay. In the UK there is a bloke selling a 500VA toroid in a case for a few bucks - he's got lots.

Add a bridge rectifer and capacitor, and you are halfway ere. A half a dozen surplus 2N3055's and a regulator chip and robert is a relative.

Over here also you can get big 11.5v AC toroids desined to run low voltage lighting - not expensive.

11.5V ac isnt really enough to give you 13.8v dc......you need sufficient headroom for the regulator to function correctly......18v is probably nearer the mark......and then the closed loop gain needs to be tight enough to maintain regulation under varying load conditions...
But then there is the problem of heat dissipation....getting rid of around 9volts at 20 amps is a good 180w.....which is why switchers are so efficient