PSU Unit twin rail Question

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PSU Unit twin rail Question

Postby jason65r » Sun Nov 04, 2012 3:43 am

Hi All. I would like to get this model charger
http://www.hobbyking.com/hobbyking/stor ... arger.html
I was wondering if I got say this for example
http://www.trademe.co.nz/computers/comp ... 30862.html
And I joined the DC Output rails +12V1and +12V2 together would I get combined amps value of 12v 56 amp's
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Re: PSU Unit twin rail Question

Postby Gordon » Sun Nov 11, 2012 6:56 am

Hi Jason,
I'm a relative noob to RC stuff, but I flatter myself :oops: that I know a little bit about electronics.
I would not advise connecting the two 12V rails together because, generally, switch mode power supplies are not designed to allow current sharing. That is, it could happen that one of the 12V rails could supply most or all and the current, and this could upset the voltage regulation circuits.

Because you've selected a fairly powerful supply, I think that using just one of the 12V rails would be sufficient for most purposes.

Also, computer power supplies are often designed to shut down if it detects that there is not a load each of the voltage rails or if it detects that the voltage on one or more of the rails is outside normal operating range. You may need to put a small dummy load on each one of the unused voltage rails. It may even be useful to put a dummy load on the 12V rail that you are using for the charger.
As a starting point I would try putting a 50 to 100mA load on each rail. Maybe only 20mA on the -12V rail.
To select a suitable load resistor value use R=V/I. For example 12V / 0.050 A = 240 ohms - 220 ohms is a common value and is near enough.
Also check power rating P (in watts) = V^2 / R (V^2 means V squared)
For example 12V x 12V / 220 ohms = 144 / 220 = 0.65 watts. I would choose a 2 or 3 watt resistor to be on the safe side.

I suggest the following resistor values:-
approx 220 ohm, minimum 2 or 3 watt resistor on unused + & -12V rails
approx 100 ohm, minimum 1 watt resistor on unused 5V rails
approx 56 ohm, minimum 1 watt resistor on unused 3.3V rail

Connect the resistors between the voltage rail and ground.

Use a multimeter to check the voltage on each rail.
The 12V rails should go no higher than 12.2V
5V rails no higher than 5.1V
3.3V rail no higher that 3.4V
If the voltage on any of the rails is a bit high, put a slightly larger load (lower resistance value) on it.

If all of this seems a bit complicated, I suggest something like this. I believe there are Jaycar stores in NZ.

Best of luck. :)
Gordon

Those who speak don't know, and those who know don't speak.
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Re: PSU Unit twin rail Question

Postby jason65r » Mon Nov 12, 2012 3:20 am

Hi Gordon
Thank you for a detailed reply. I have a single rail PSU (400W 12V 12A) and of course I checked out this vid
http://www.youtube.com/watch?v=ityrHaBLepg&feature=plcp
Then like any male I know the old (MORE) Hit so looked up PSU and came across the twin rail PSU. So I had to ask the question!
If I had one of them, I would give your mod a shot. Great and easy to follow instructions, even for me!!!
However while I was weighting for a reply I found this PSU
http://www.maxamps.com/Power-Supply-12V.htm
So now I need to post a question on chargers which one to get!!!!
once again thanks for your reply.
jason65r
 
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Re: PSU Unit twin rail Question

Postby Gordon » Mon Nov 12, 2012 6:37 am

Hi Jason,
Bruce's video explains an aspect of making a computer PSU work that I omitted, because the push button start switch feature is something I was not so familiar with, because it's a feature of more recent PSUs (less than 15 years old) :oops: It's been some years since I've played around with computer PSUs. My bad. :cry:

I still think that having a small dummy load on each of the unused voltage rails is a good idea.

Anyhow, most of the above is pointless, because I think that the "maxamps" option is hard to beat.

Can't help with advice on LiPo chargers.
Gordon

Those who speak don't know, and those who know don't speak.
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