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Topic: lm338 reg issues (Read 344 times) previous topic - next topic


Hi all i have a lm388 5amp to 22o regulator,its set with 2 resistors one between the out put and the adjust pin thats 330 ohm then another of 820 ohm from the junction of r1 and the adjust pin to ground,it outputs about 4.5v which is as expected,its fed from a 10amp 24v psu,now the big but,the most i can get out of it is just over 600ma when it is rated for 5amp both resistors are 2 watt rated,these get realy hot alon with the heatsink of the reg,it seems most of the power is going into cooking the resistors,any ideas why?,its all in line with the datasheet,cheers paul m3vuv.


The LM388 is an audio amplifier!

But...  Linear regulators are limited by power AND current.    Power (in Watts) is calculated as Current (Amps) x Voltage.    The power dissipated by the regulator is the voltage dropped-across the regulator (about 20V in your case) so at 600mA it's dissipating about 12 Watts.


It would be usual to use a buck converter to efficiently provide 5V power from 24V.  Then you get efficiencies
more like 90% and less like 20%.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


i know efficency doesent bother me,i just want to use what i have to hand,doesent explain the crappy max amperage im getting out of it tho!.


i know efficency doesent bother me...
It should.
You can NEVER draw 5Amp from that setup because of the LM338's thermal limitations.

600mA, with a 19volt drop (24-5), will generate 19*0.6= 11.4watt in the LM338.
Maybe just ok with a fist-sized heatsink.
More than 20watt dissipation (~1Amp draw) is almost impossible because of the thermal resistances of the TO-220 package.
Chuck the LM338, and get some $1 switching regulator boards from ebay.


the issue is im trying to get about 1800ma out of it at around 5 volts,the heatsink is the body of the milling machine so no issues there with heat,the issue still remains why i can only get 600ma from it!!,this is the whole point of my post,is there someting wrong with the resistor values for example?.


May 09, 2019, 04:00 pm Last Edit: May 09, 2019, 04:03 pm by MarkT
(24-5) * 1.8 is about 35 watts to dissipate - crazy.  LM2596 buck converters cost peanuts on eBay.
Phone chargers capable of 2A aren't exactly scarce or expensive these days either.  But a milling
machine is a free heatsink I suppose...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Hi All,
The LM338T package is a TO-220, rated at 1.5 Amps.
The LM338K package is a TO-3, rated at 5 Amps.
So, the TO-220 will not give 1800 mA output.


the issue still remains why i can only get 600ma from it!!
Maybe the chip goes into thermal protection.
600ma > 12watt is a lot of heat.
Did you use thermal paste.


the datasheet for the lm338 says its good for 5 amps!


May 19, 2019, 04:51 am Last Edit: May 19, 2019, 11:58 am by Wawa
Seems you're the gullible type that believes everything written on the first page of a datasheet.

If I say:
My car can do 180km/h, and it can reverse.
Do you then think it can do 180 in reverse?


You are neglecting voltage drop. If you were dropping from 7V to 5V drop would be 2V times 1.8 Amps = 3.6 Watts, but you are dropping  19V times 1.8 Amps = 34.2 Watts.


i know efficency doesent bother me,
It should bother you, as you're going to have a hell of a time getting rid of all the excess heat you produce. I feel for the person that's handling the machine with it's hot spot - don't touch or you burn yourself. And what does the machine itself think of having part of it heated excessively? That case is quite certainly not meant to be used as heat sink, definitely not for that kind of huge dissipation.

But well, if you're dead set on wasting your time pursuing the futile, go ahead. Sooner or later you'll realise that a simple, inexpensive buck converter works a lot better, and that without all the trouble you're exposing yourself to.

Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

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