LM338 (x3) power supply cuts out on high loads

[[email protected]]

I recently built an adjustable power supply with three LM338s
essentially in parallel. The design was copied directly from National's
tech notes PDF; all values are exact. I'm powering it with a 26v 15A
transformer through a full wave bridge rectifier. I'm actually using
this supply to charge a large capacitor bank (700,000uF charged up to
12-20 volts). I realize this creates a large load for a short amount of
time, so to help pad the load I put a 1 ohm power resistor in series
with the capacitor bank. This worked for probably 20 charges or so,
then something changed.

I noticed the DC voltage at the bridge rectifier rose up to 40v (but
only when the regulator circuit was connected, so I know nothing is
wrong with the transformer). With just a small light bulb (less than
1/2 amp) it works fine throughout the whole voltage range. However,
when I add about 5A of light bulbs, the output drops to zero instantly
until I remove the large load. This never happened before.

I'm assuming it shuts off because it's now having to regulate from 40V
to 15V which is a rather large spread. I guess my main question is what
is causing the voltage at the bridge to rise from 26V to 40V when I
connect the regulator circuit? (it never did this initially). Is it
more than likely that the LM338s are toast? I find it hard to believe
since they still work great at very low currents.

Note: I've read the app notes pretty thoroughly, and therefore
understand the basics of the LM338 (filter caps close to the
regulators, protection diodes, reference wire connected directly to
case for low resistance, etc).

I'd appreciate any suggestions / ideas that you may have about this
situation. If the LM338 is not the way to go for charging a large cap
bank, what is? I'm trying to keep this design relatively simple and
cheap.

THANKS!
Kevin

 

[Phil Allison]

** Groper Alert !!

I recently built an adjustable power supply with three LM338s
essentially in parallel. The design was copied directly from National's
tech notes PDF; all values are exact.

** Is that the one using an LM307 with its + input tied to the + rail ??

I'm powering it with a 26v 15A
transformer through a full wave bridge rectifier.

** What - no filter caps ??

I'm actually using
this supply to charge a large capacitor bank (700,000uF charged up to
12-20 volts). I realize this creates a large load for a short amount of
time, so to help pad the load I put a 1 ohm power resistor in series
with the capacitor bank. This worked for probably 20 charges or so,
then something changed.


I noticed the DC voltage at the bridge rectifier rose up to 40v

** 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.

(but
only when the regulator circuit was connected, so I know nothing is
wrong with the transformer). With just a small light bulb (less than
1/2 amp) it works fine throughout the whole voltage range. However,
when I add about 5A of light bulbs, the output drops to zero instantly
until I remove the large load. This never happened before.

** 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.

I'm assuming it shuts off because it's now having to regulate from 40V
to 15V which is a rather large spread. I guess my main question is what
is causing the voltage at the bridge to rise from 26V to 40V when I
connect the regulator circuit? (it never did this initially).

** Fraid it did.

Is it
more than likely that the LM338s are toast? I find it hard to believe
since they still work great at very low currents.

** 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

 

[[email protected]]

** Is that the one using an LM307 with its + input tied to the + rail ??

 

[Phil Allison]

** 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

 

[[email protected]]

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

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

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

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 learning some basic electronics and posting manners too.

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

 

[Terry Given]

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

 

[Phil Allison]

** ARROGANT OVER-SNIPPING 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 PSU circuits ?


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

** Finally, he supplies crucial info I had to chase so hard to get out of
him.

You might want to take a breather...in no way was I ever trying to cut
your posts to little pieces.

** But you HAVE done it EVERY TIME !!!

DO NOT remove the prior context from view.

That FORCES me to keep putting it back.

Try being a little leniant to new posters. That was literally my first
post on this list (or any list for that matter).

** LISTEN ASSHOLE:

1. A " usenet group" is not some fucking "list" !!

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

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

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

I have posted on
message boards for years but mailing lists are new to me.

** Grrrrrr - this is not a damn MAILING LIST !!

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?

** No-one is making you post bullshit.

Try having a little tact...

** How about YOU try not rudely contradicting and posting bullshit.

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

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.

** 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

 

[Homer J Simpson]

BTW:

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

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.

 

[Phil Allison]

"Homer J Simpson"

Proper posting etiquette is to trim the original.

** 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

 

[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.

But not from you clearly.

 

[Phil Allison]

"Homer J Simpson"

But not from you clearly.

** Anonymous ASD fucked assholes learn nothing from anyone.

EVER.




.......... Phil

 

[colin]

I recently built an adjustable power supply with three LM338s
essentially in parallel. The design was copied directly from National's
tech notes PDF; all values are exact. I'm powering it with a 26v 15A
transformer through a full wave bridge rectifier. I'm actually using
this supply to charge a large capacitor bank (700,000uF charged up to
12-20 volts). I realize this creates a large load for a short amount of
time, so to help pad the load I put a 1 ohm power resistor in series
with the capacitor bank. This worked for probably 20 charges or so,
then something changed.

My gues is something went pop,..

I noticed the DC voltage at the bridge rectifier rose up to 40v (but
only when the regulator circuit was connected, so I know nothing is
wrong with the transformer). With just a small light bulb (less than
1/2 amp) it works fine throughout the whole voltage range. However,
when I add about 5A of light bulbs, the output drops to zero instantly
until I remove the large load. This never happened before.

I'm assuming it shuts off because it's now having to regulate from 40V
to 15V which is a rather large spread. I guess my main question is what
is causing the voltage at the bridge to rise from 26V to 40V when I
connect the regulator circuit? (it never did this initially). Is it
more than likely that the LM338s are toast? I find it hard to believe
since they still work great at very low currents.

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.

Note: I've read the app notes pretty thoroughly, and therefore
understand the basics of the LM338 (filter caps close to the
regulators, protection diodes, reference wire connected directly to
case for low resistance, etc).

I'd appreciate any suggestions / ideas that you may have about this
situation. If the LM338 is not the way to go for charging a large cap
bank, what is? I'm trying to keep this design relatively simple and
cheap.

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 =^.^=

 

[ehsjr]

I recently built an adjustable power supply with three LM338s
essentially in parallel. The design was copied directly from National's
tech notes PDF; all values are exact. I'm powering it with a 26v 15A
transformer through a full wave bridge rectifier. I'm actually using
this supply to charge a large capacitor bank (700,000uF charged up to
12-20 volts). I realize this creates a large load for a short amount of
time, so to help pad the load I put a 1 ohm power resistor in series
with the capacitor bank. This worked for probably 20 charges or so,
then something changed.

I noticed the DC voltage at the bridge rectifier rose up to 40v (but
only when the regulator circuit was connected, so I know nothing is
wrong with the transformer). With just a small light bulb (less than
1/2 amp) it works fine throughout the whole voltage range. However,
when I add about 5A of light bulbs, the output drops to zero instantly
until I remove the large load. This never happened before.

I'm assuming it shuts off because it's now having to regulate from 40V
to 15V which is a rather large spread. I guess my main question is what
is causing the voltage at the bridge to rise from 26V to 40V when I
connect the regulator circuit? (it never did this initially). Is it
more than likely that the LM338s are toast? I find it hard to believe
since they still work great at very low currents.

Note: I've read the app notes pretty thoroughly, and therefore
understand the basics of the LM338 (filter caps close to the
regulators, protection diodes, reference wire connected directly to
case for low resistance, etc).

I'd appreciate any suggestions / ideas that you may have about this
situation. If the LM338 is not the way to go for charging a large cap
bank, what is? I'm trying to keep this design relatively simple and
cheap.

THANKS!
Kevin

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

 

[[email protected]]

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.

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

 

[[email protected]]

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

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.

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.

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

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.

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

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.

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.

Ed

Thanks so much for your help!
Kevin

 

[Bob Scott]

"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.

...... Phil

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

 

[Phil Allison]

"Bob Scott"


** Got get fucked - you asinine TROLL.







....... Phil

 

[Phil Allison]

"ehsjr"

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,

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

so that you don't need to connect the regulator
to see the voltage across the bridge go up to ~ 40.

** Huh ??

The regulator circuit concerned has no input filter cap.

It cannot work from raw, rectified AC.



....... Phil

 

[Bob Scott]

I recently built an adjustable power supply with three LM338s
essentially in parallel. The design was copied directly from National's
tech notes PDF; all values are exact.

I'm powering it with a 26v 15A

transformer through a full wave bridge rectifier. I'm actually using
this supply to charge a large capacitor bank (700,000uF charged up to
12-20 volts). I realize this creates a large load for a short amount of
time, so to help pad the load I put a 1 ohm power resistor in series
with the capacitor bank. This worked for probably 20 charges or so,
then something changed.

I noticed the DC voltage at the bridge rectifier rose up to 40v (but
only when the regulator circuit was connected, so I know nothing is
wrong with the transformer). With just a small light bulb (less than
1/2 amp) it works fine throughout the whole voltage range. However,
when I add about 5A of light bulbs, the output drops to zero instantly
until I remove the large load. This never happened before.

I'm assuming it shuts off because it's now having to regulate from 40V
to 15V which is a rather large spread. I guess my main question is what
is causing the voltage at the bridge to rise from 26V to 40V when I
connect the regulator circuit? (it never did this initially). Is it
more than likely that the LM338s are toast? I find it hard to believe
since they still work great at very low currents.

Note: I've read the app notes pretty thoroughly, and therefore
understand the basics of the LM338 (filter caps close to the
regulators, protection diodes, reference wire connected directly to
case for low resistance, etc).

I'd appreciate any suggestions / ideas that you may have about this
situation. If the LM338 is not the way to go for charging a large cap
bank, what is? I'm trying to keep this design relatively simple and
cheap.

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

 

[[email protected]]

"ehsjr"




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

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.