Bench PSU Build

[weirdlookinguy]

I'm building a Bench PSU for a buddy of mine. He is an Aerospace Engineer and has needed a good high-amperage power supply for some time now for his tinkering with small motors and such and also to power a programmable battery charger.

We went PSU hunting at UCSD and found a 1996, Made In USA (!) 300w ATX power supply out of an old server. According to a sticker on the case it belonged to NASA. I took it home and cracked it open. NCC, Marcon, and JPCE-TUR caps. Nidec fan.

I did a ton of wire snipping, wired up a switch for power, am currently working on a two-color LED to indicate power state (standby/power on). Today I am going to drill holes for the binding posts and give the case a sanding and blast of paint because it is rather old and has a fair amount of rust.

Here is my question to you guys: I have some engine enamel in a can type of stuff. Can I give the case a light coat and throw it in the oven to cure it? I have some clearcoat stuff too, is it necessary or is cured engine enamel enough? I want a strong, durable finish not something that's going to chip and look like shit in a year.

If curing engine enamel is the way to go how long should i let it cure and how what temperature?

[weirdlookinguy]

Re: Bench PSU Build

Alright guys, she is all done.

I ended up curing the enamel in a toaster oven @ 250F 25 Min.

I think it came out really good. Looks pretty sharp considering it was grimy and rusty around the vents when I first got it. I did my best to keep the interior wiring clean and I have to say I think I did pretty damn good. I ran two wires each to the 12v and 5v binding posts for all that high-amperage goodness.

The LED above the switch is dual-color. It is green whenever the PSU is plugged in and orange when the PSU is on.

The only thing left to do is to fix that loud Beta fan. I'm thinking a 3v zener diode.

Let me know what you guys think.

[PCBONEZ]

Re: Bench PSU Build

Looks good.

[Th3_uN1Qu3]

Re: Bench PSU Build

Does your PSU have the typical fan controller with a green thermistor and a transistor in what looks like an oversized to-92 case? If so, remove the zener from c-e, this will make the fan's minimum speed to be 'off'. If more adjustment is required, play with the bottom resistor in the divider - the one from the base of that transistor to gnd.

[weirdlookinguy]

Re: Bench PSU Build

No fan controller on this one (it's a '96!). The fan gets straight 12v.

[Th3_uN1Qu3]

Re: Bench PSU Build

The fan control circuit is fairly trivial to build, look here: https://www.badcaps.net/forum/showpo...37&postcount=5

Play with the bottom resistor according to your NTC.

[Rulycat]

Re: Bench PSU Build

I'm surprised you found one made in the USA. I suppose NASA has to order only from U.S suppliers.

Looks tough.

[ratdude747]

Re: Bench PSU Build

I'm surprised you found one made in the USA. I suppose NASA has to order only from U.S suppliers.

Looks tough.

yes, its called "buy american" laws. in many cases, the gov't can only import is there is no suitable domestic substitute... and at the time there was a made in USA comapany for PSUs, so that is what was bought...

[PCBONEZ]

Re: Bench PSU Build

Wrong Thread . LOL

[weirdlookinguy]

Re: Bench PSU Build

EDIT: post no longer relevant.

[weirdlookinguy]

Re: Bench PSU Build

Alright guys, a video of it in action. I used it to test out a Dell soundbar I had laying around.

http://www.youtube.com/watch?v=083IjQQuWGE

Also I've decided to leave the fan running full blast. It may be loud, but the high airflow is why it has lasted 15 years and counting.

[shovenose]

Re: Bench PSU Build

Looks wonderful!

[weirdlookinguy]

Re: Bench PSU Build

Why thank you sir!

In a few months I'll be making a lab psu that has a variable (0-24v) output in addition to the standard 12v,5v,3.3v outputs. I found a very simple schematic online that uses a small regulator and the -12v line. Stay tuned!

[momaka]

Re: Bench PSU Build

Looks wonderful!

Agreed. Very awesome paint job.

Shouldn't the LED colors be backwards, though? (orange for OFF and green for ON?).

In a few months I'll be making a lab psu that has a variable (0-24v) output in addition to the standard 12v,5v,3.3v outputs. I found a very simple schematic online that uses a small regulator and the -12v line. Stay tuned!

LM317 regulator and 24v between the 12v and -12v rails?

[weirdlookinguy]

Re: Bench PSU Build

LM317 regulator and 24v between the 12v and -12v rails?

Yup.

[dumpystig]

Re: Bench PSU Build

Great job on the PSU and paint finish

In a few months I'll be making a lab psu that has a variable (0-24v) output in addition to the standard 12v,5v,3.3v outputs. I found a very simple schematic online that uses a small regulator and the -12v line. Stay tuned!

I'm interested in building something like this - any chance of sharing the schematic?

I also think the dual colour LED would be more logical the opposite way round...?

[weirdlookinguy]

Re: Bench PSU Build

I originally wanted orange for standby and green for on, BUT, here is why that didn't work:

Two color LED: green and red. Turn both on and you get orange.

To get orange when it's in standby, I would need to have green on constantly (wired to standby) and red wired to a voltage that is only there when the PSU is in standby. I tried the power_on signal wire, since it sits at 5v until it gets pulled to ground to start the PSU, but the LED would not light.

I could have done red when in standby and orange when on, but I don't like red.

Dympystig, here is the schematic:
http://www.wikihow.com/Add-Variable-...h-Power-Supply

I have a graphical version of the schematic stored on my server, but that's down for another week or so. Once I bring it back online I'll be happy to post it. It shows pictures of the actual components instead of the schematic symbols. For an engineering-challenged dunce like myself, it's very helpful.

[mariushm]

Re: Bench PSU Build

Be careful with that as very few power supplies do 1.5A on -12v, which seems to be required by that schematic (well, more as a consequence if you plan to go over 10-11v at high loads, other chips won't do better) .
Also, I'm not sure you'd get 24v as the chip has about 1.3v dropout when in use, going up to 1.5v when heated and under 1-1.5a load - you'd probably only be able to get about 1-1.3A at ~ 22v

Consider using LT108x (1083 - 3a, 1084 - 5a , 1085 - 7.5a) :

digikey:
1085 http://search.digikey.com/us/en/prod...3PBF-ND/891129
1084 http://search.digikey.com/us/en/prod...3PBF-ND/891835


They use the same 1.25v reference voltage so they're probably drop in replacements to LMx17 but it has better load regulation (0.1% vs 0.3%) and has guaranteed dropout voltage betwen 1-1.5v compared to about 2.2v on LMx17.

lm x17: lt 1083:

and it doesn't make much difference but resistors are cheap enough to use 1% rated ones instead of 5% as in the graphs there.

... and keep in mind they'll generate a lot of heat, make sure they're heatsinked properly.

[momaka]

Re: Bench PSU Build

Be careful with that as very few power supplies do 1.5A on -12v

It's pretty easy to go around that limitation - just install bigger diodes . 3A fast rectifiers should work pretty well. Even the cheap PSUs that use the diodes-on-a-bracket method to get the 12v rail can withstand quite a bit of abuse before they fail.
Probably also a good idea to install a bigger cap on the 12v rail to minimize the ripple. 1x 1000uF should be adequate.

[weirdlookinguy]

Re: Bench PSU Build

I'll definitely need your guys' help when I get this project started. Señor Ratdude is going to hook me up with a PSU that has a 1 amp -12v rail.