LM338 (x3) power supply cuts out on high loads

** Groper Alert !!

** Is that the one using an LM307 with its + input tied to the + rail ?? ** What - no filter caps ?? ** No - it was always 40 volts. 26 volts AC produces around 40 volts DC, when the AC rectified and filtered.

This is much too high for LM338s operating at low output voltages.

** A 5 amp lamp load will initially be a **50 amp* load when the filaments are cold. With a 40 volt in-out differential, the LM338s will immediately self protect and shut down the output voltage. ** Fraid it did.

** No clear evidence of that so far.

Try a *resistor* load of 2 ohms with the voltage set to 15 volts - ie a

7.5 amp load.

If the voltage holds steady, the circuit is probably OK.

...... Phil

DC side both times.

** GROPER ALERT !!!

** What about supply filter caps - you have IGNORED my question on this issue ????

Got any ???

26 volts AC produces around 40 volts DC, when rectified and filtered.

Do you know nothing about PUS circuits ?

BTW:

NO NOT cut my posts to little pieces and write idiotic answers.

** Try learning some basic electronics and posting manners too.

....... Phil

I have a decent filter cap (I want to say it's around 10,000uF)

You might want to take a breather...in no way was I ever trying to cut your posts to little pieces. I appreciate the help, and I do not see why you were so offended.

Try being a little leniant to new posters. That was literally my first post on this list (or any list for that matter). I have posted on message boards for years but mailing lists are new to me. Sorry if I didn't realize the effects of a greater than symbol on my first reply...

Also, I've been messing with electronics since I was 5. It's only been a hobby and I've never fully mastered it. Do you think I would be asking these questions if I knew the answers?

Try having a little tact...

Again, I appreciate the help but please do not reply if you're just going to cut people down and get offended when the OP never had any malicious intent.

Kevin

Uh-oh, you're in trouble now. Phil rants like that at everyone, so dont take it personally.

Cheers Terry

** ARROGANT OVER-SNIPPING GROPER ALERT !!

** Finally, he supplies crucial info I had to chase so hard to get out of him. ** But you HAVE done it EVERY TIME !!!

DO NOT remove the prior context from view.

That FORCES me to keep putting it back.

** LISTEN ASSHOLE:

1. A " usenet group" is not some f****ng "list" !!

2. YOU are posting on Google Groups - which is NOT part of usenet.

1. Google Groups is destroying usenet by pouring in a steady stream of half witted trolls.

2. Go look up what "usenet " is and learn the etiquette.

** Grrrrrr - this is not a damn MAILING LIST !! ** No-one is making you post bullshit. ** How about YOU try not rudely contradicting and posting bullshit.

YOU are in no position to judge the replies posted here.

** No-one gives a hoot about your alleged but unknowable "intent" !!

STOP snipping posts to pieces when you reply.

Do reply to questions as fully as possible.

DON'T expect anyone here to believe impossible claims.

Capice?

....... Phil

Proper posting etiquette is to trim the original. Some people on these groups are blind and they have to listen all the way through piles and piles of crap to get to anything new. Have pity on them and trim your replies.

"Homer J Simpson"

** Only to enhance clarity of meaning - not DESTROY IT by removing the original context from view and substituting an alternative that has to be guessed at by implication.

Plenty for YOU to go learn.

...... Phil

But not from you clearly.

"Homer J Simpson"

** Anonymous ASD f***ed assholes learn nothing from anyone.

EVER.

......... Phil

My gues is something went pop,..

but 26vac will likely give 40vdc when rectified and smoothed with no load, so this isnt indicative of whats gone pop. measure all resistors, and check all devices with a diode meter etc.

As said elsewhere light bulbs arnt constant loads, the resistance is very much lower when cold.

I found an old toaster makes quite a good load, at considerably reduced power ofc, you can put the sets of elements in series or parellel to make a moderate choice of resistances.

lots of surplus 2N3055s or similar around, ive got a big box of them Use a few in parallel with suitably large emitter resistors for current balance current sense and to take some of the brunt of the surge energy. Another 2n3055 to drive them and dual op amp for voltage and current limit.

Colin =^.^=

You won't get steady 15 amps out of that setup. Your transformer is undersized and your Vin-Vout too great.

You want a filter cap installed across the bridge, so that you don't need to connect the regulator to see the voltage across the bridge go up to ~ 40.

If you are going to use a 1 ohm resistor, put it between the bridge and the Vin to the regulator, not between Vout and the cap bank. You could use a bigger value and switch it out with a 12 V relay when the voltage climbs to ~ 9 volts. Charging will be slower, but it will lower the surge current.

Add a diode between Vout and the bank to keep the caps from discharging back through the regulator, and add a

1K resistor from Vout to ground to assure some minimum current.

Ed

Would you happen to have / know of a sample schematic of what you're talking about? I've seen a small (LM317) regulator drive a bank of

3055's before, but not quite what you mentioned. Would that accomplish what you mentioned?

Thanks so much for your helpful response!

Kevin

I'm not so certain I need a steady 15 amps. The cap bank will draw a quick surge well over 15 amps. What I'm hoping for is a circuit that can handle this surge (even if it takes longer to charge than it normally would with a steady 15 amp output) without burning up.

I'll try a larger cap yet and see if that helps.

Excellent idea -- I'll try that out when I get to that point.

I have a diode in place for that. I also have a small lamp that is always connected to make sure the 100mA minimum current requirement is met.

Thanks so much for your help! Kevin

Why do you go out of your way to post off-topic rants? What has your encessant juvenile crap got to do with the LM338 and the user's circuit problem?

Bob

"Bob Scott"

** Got get f***ed - you asinine TROLL.

...... Phil

"ehsjr"

** The OP has clearly been lying about that missing filter cap. ** Huh ??

The regulator circuit concerned has no input filter cap.

It cannot work from raw, rectified AC.

...... Phil

I'm powering it with a 26v 15A

Ooooh, I have a big case of DEJA VU. Twenty two years ago the chief engineer at CKVU TV needed a high current power supply to drive several hundred tri-color LEDs. These LEDS were used as lit pixels on a few dozen signs that spelled all of the Video sources in the news studio control room, like CAM

1, Cam 2, VTR 5, VTR 6,etc. He showed me an article about power supply design in the magazine Broadcast Engineering. The article author was a condescending anal P ENG type who scorned the use of rules of thumb. Everything had to be calculated down to the last millivolt. The article had a schematic of a high current circuit utilizing 3 National Semiconductor voltage regulator chips operating in parallel. The Chief Engineer told me to build it as a supply to drive the hundreds of LEDS.

I built it, exactly as per the magazine diagram with the 3 National Semi LMwhatevers. The circuit did not work. I consulted the National data book (We had REAL data books in 1984!) and came across an application note with a circuit diagram using 3 of the same LMwhatevers. Apparently, Mr. stickler-for-proper-engineering anal P ENG author had plagiarized the app note's diagram and taken credit.

I noticed one wire from the app note diagram was missing from the magazine diagram. (Probably a mistake by the magazine art department.) I installed the wire to my built-up circuit.

The power supply still didn't work. It was then I realized (again) that example circuitry schematics found in data books are often just used as illustrations, never left the desk, are not tested and may not work.

I ended up designing my own power supply. It worked. I think the whole control room is still in operation 2 decades later because once in a while they broadcast scenes from the room and you can see the LEDS working..

Have you considered the fact that all those hundreds of microfarads in filtering are supposed to connect to the rectifier output, not the regulator output? That the voltage on your secondary will be too high? (1.414*26)?

Regards, Bob

Please tell me why I would lie about whether or not I have a filter cap. I don't lying would help solve the problem I am having.