Re: LM338 (x3) power supply cuts out on high loads

keebler65@xxxxxxxxx wrote:
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
.

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