LM338 High Current question

[Peter1960]

Hi,

I am using a LM338 with a set no load o/p of 7.1 volts, mounted on a large heatsink.

I need 4.2 amps out, however I am wondering if 20v in is to large,

due to that when I place the load on the output the volts drop to ~3.5 volts.

I am using 120R as the feedback resistor, ie out to variable in, and a 560R to ground.

any suggestion appreciated.

Peter

 

[IanP]

LM338 is rated to 40V and has guaranteed output current of 5A.
If your regulator is not faulty, then check voltage at Vin pin under 4.2A load.
This voltage has to be at least 3V higher then the output .. so you should measure Vin>10V @ 4.2A ..
Regards,
IanP

 

[Peter1960]

The voltage at Vin remains at approx 20v.

I dont believe the LM338 to be faulty, but I have a spare.

I also tried a lower load, ~ 2 amps, still have the same issue,

ie drops to ~ 3.5 volts.

Peter

 

[IanP]

An input bypass capacitor is recommended. A 0.1 µF disc or
1 µF solid tantalum on the input is suitable input bypassing
for almost all applications. The device is more sensitive to
the absence of input bypassiing when adjustment or output
capacitors are used but the above values will eliminate the
possiblity of problems.

. . . . . .

Although the LM138 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values between
500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF
aluminum electrolytic) on the output swamps this effect and
insures stability.

So if you say it is not faulty and the input voltage is stady at 20V then you will need to try the above ie. instal input and output capacitors (and maybe 10µF to ADJ pin) with protective diodes.
It is possible that it just oscillates ..

Regards,
IanP