LM338K module as bench supply?

[Ben N]

Anybody ever see/use one of these?

https://www.aliexpress.com/item/1005002873838088.html
Input voltage range: DC5V-35V, AC5V-22V
Output voltage range: DC1.25V-30V (continuously adjustable)
Seems like a useful gadget, with a panel mount multiturn pot subbed for R2 and a cheapo digital volts 'n' amps display.

[antonis]

Although I've build a couple of LM338 based bentchtop power supplies, I tend to encourage for moderate current regulators (like LM317/350) in conjunction with series-pass transistor(s)..

Of course, 6,18€ for a ready-to-race power supply, where a single LM338T (TO-220) costs about 4,15€, is more than attractive price..
(LM338K (TO-3) can be found at ridiculously high prices..)


P.S.
A self-respected PS should have some kind of current limiting configuration..
(easily incorporated into particular PS output..)

[Ben N]

A self-respected PS should have some kind of current limiting configuration..
(easily incorporated into particular PS output..)

I assume you mean more than a big honking power resistor, Antonis. Something like this?

[antonis]

I assume you mean more than a big honking power resistor, Antonis.

I assume your assumption is true & correct, Ben..

Something like this?

More or less, yes..
With such a simple (passive) configuration, all the load current passes through sense resistor which has to be of significant power rating for high load current..
(3W for 5A considering Q2 activated for 600mV V_BE)
Where a 0.12R/3W (better 5W) wirewound resistor is cheap, a pot (for current limiting adjustment) of respective wattage rating might cost more than the above complete power supply..
(and there is no guarrante for all the way down to zero resistance adjustment ..)

But, go on for the above linear PS and here we are for any further improvement..

P.S.
I beg for R.G.'s (or any other professional power supply designer's) empathy for anything unsubstantially said above..

[Ben N]

I came across this handy page, complete with schematics and formulae: https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php

[antonis]

Good in you wish to stay on "passive" current limniting..

But without a series pass transistor (Tr1) you'll have to modigy those circuits to affect on regulator's ADL pin..

[Ben N]

??

[antonis]

Something like below configuration..
(the idea is the current limiting transistor to "steal" current from output voltage setting point - 240R/1k1 left below)



https://www.ti.com/lit/ds/symlink/lm338.pdf

[antonis]

Focus on the effect of left IC, considering it as current limiting transistor, on point A..
(when IC output turns full-negative, ADJ pin is grounded hence output voltage is set at regulators internal voltage reference - 1.25V here..)

[Ben N]

Ah, ok, I think I get that.

[anotherjim]

On the subject of output "protection" I'm currently confronted with this thing...

Are the BJTs in the +/-15v outputs current limiting or maybe ripple reduction using the cap on the 120R-base connection.

[antonis]

IMHO, there is no current limiting configuration there, Jim..
(no "sense" resistor across BJTs B-E..)

2k2 resistors and C4/C5 are wired as capacitance multipliers..

https://sound-au.com/tcaas/jlhcapmult.htm

P.S.
IMHO, 2k2/120R/100μF node should have a resistor to GND for output voltage precise setting..

[diffeq]

IMHO, 2k2/120R/100μF node should have a resistor to GND for output voltage precise setting..

Or a zener like D10 in 48V node.

Current control is done with smps chip in this psu. R5 sets that.

[anotherjim]

Thank you all. I couldn't see a current sensing function either, not according to my knowledge anyway. It does look as though there was meant to be a Zener, however, the +15v provides the regulating feedback so there would be no need for a second voltage regulator stage but it could if they wanted +/- 12v instead.
Anyway, this one has a fault. +15v is ok but -15v is dead. All I can tell so far is the big PNP has enough collector volts but zero on base and emitter yet its junctions diode test ok. Should be simple but I've got to access under the board to go any further - and it's SMD, yuk!
Carry on.

[antonis]

Should be simple

Shorted C5, perhaps..??

[anotherjim]

Should be simple

Shorted C5, perhaps..??

I am hoping so, but it looks ok, no bulges or leaks.

[anotherjim]

Of course, it was C5 dead short. No visible signs, just plain bad. Sancon brand, which I'm not familiar with. C4 was another and while ok, I changed it also. The rest of the electro caps are Jamicon. On a mixer with multiple channels and buses on a large single board, a short supply cap in the PSU is better than one somewhere lost in the mixer boards!