I try to make a 30 V, 5 A power supply using LM338 (without heatsink) voltage regulator IC.

As given in datasheet

In my case the value of R1 is 100 Ω and R2 is 2.2 kΩ. then it gives 29.6 V as output.

when we connect a 10 Ω 10 W resistor, the current is exceeded 1 A, the IC gets hot within a few seconds output current and voltage become zero. (I think the IC thermally shuts down).

After cooling the IC when we reconnect power then we got no output voltage and no current, IC is also not getting hot.

we can't conclude why it's happening. Is Ic got damaged?

After that, we did one more experiment.

Constant Current Circuit as given in the datasheet.

Constant Curreent

In my case R1 = 10 Ω, 10 W and load is again 10 Ω 10 W.

Here again within few seconds no output voltage and current.

  • 2
    \$\begingroup\$ What is the input voltage level going into the LM338? \$\endgroup\$
    – Andy aka
    Nov 23, 2020 at 14:40
  • 3
    \$\begingroup\$ A warm welcome to the site. What is your input voltage? That's crucial to know. The power dissipated in the regulator will be: (Vin-Vout) x Iout W \$\endgroup\$
    – TonyM
    Nov 23, 2020 at 14:41
  • 5
    \$\begingroup\$ @TonyM: "A warm welcome to the site" sounds extra appropriate here. \$\endgroup\$
    – ocrdu
    Nov 23, 2020 at 14:47
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    \$\begingroup\$ @ocrdu :-) Should've said 'stay cool, don't go into meltdown' and taken it to painful. \$\endgroup\$
    – TonyM
    Nov 23, 2020 at 15:29
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    \$\begingroup\$ 7V * 1A = 7W and you be wanting the poor LM338 to supply 150W. Did you even bother to read the datasheet you took those images from? \$\endgroup\$ Nov 23, 2020 at 17:11

2 Answers 2


Here's your problem:

"We are not using any heatsink."

You are drawing a lot of current from your regulator. Linear regulators drop voltage by acting like a resistor in series with the source. The waste power as heat.

$$P = (V_{in}- V_{out}) \times I$$

You don't say what your input voltage is, but the LM338 allows a maximum of 40 V difference between input and output. At 1 ampere that's 40 watts. At 5 amperes that's 200 watts.

That amount of power is wasted as heat. You must use a heat sink when drawing high current from a linear regulator.

  • \$\begingroup\$ Input voltage is 37v. \$\endgroup\$ Nov 23, 2020 at 15:20
  • \$\begingroup\$ I have one more doubt. it (failure IC in the above case) can be used further or not. if yes ??? \$\endgroup\$ Nov 23, 2020 at 15:29
  • 1
    \$\begingroup\$ @AdityaPrajapati: So your regulator is wasting about 7 watts at 1 ampere, and 35 watts at 5 amperes. \$\endgroup\$
    – JRE
    Nov 23, 2020 at 15:29
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    \$\begingroup\$ Quick thermal excursions may damage the ic before thermal protections get in the way. Without a heatsink or large copper pad, the thermal mass is very small. \$\endgroup\$
    – crasic
    Nov 23, 2020 at 16:09
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    \$\begingroup\$ cheap (fake) 338s often omit the thermal protections. \$\endgroup\$
    – dandavis
    Nov 23, 2020 at 21:02

Old, but I'd like to put my observations.

I've been testing lm338T from texas lately. Mine goes near 0V, 1A and gets hot as a result of high input voltage (heatsink is applied). That is, 24V, ~33V rectified. With 22W carbulb it works, but with 55W 12V it fails, feels like internal protection goes on.

This is probably due to high input-output difference. I've tested same lm with input 14AC and it works ok, but R2 at 5k will be too much. I suspect it's ok until 16-18V as input. At 3 amps it doesn't go so much with temperature as it does with 0v/1A protection.

14AC works well with 12V 55W bulb.

PS: Few weeks later I added another lm338 in parallel (primitive way) connection into my grotesque build (24v transformer). Now it will light up 12v/55W, but it feels like the big cpu heatsink won't suffice as TO220 get too hot at the front anyway, feels like more than 70celc both. I used to cool them with additional fan, but i doubt it will survive long with 4-5A load :)

What I've learned here? Well, always use heatsinks - that's probably the main reason the author's build went thermal protection. Second - it's possible to connect lm317/lm338 in parallel without additional resistors. The heat might be uneven at lower power, but at high load they get hot both. TO220 seems to be not enough in this case.

As I had many spare parts I played around with lm's for fun. If I wanted an adjustable power supply I'd rather spend 50-100 bucks for 0-30v/3A psu which looks and works out of the box :)

  • \$\begingroup\$ Why do you have such a high input voltage? \$\endgroup\$
    – Audioguru
    Jan 1, 2022 at 3:55
  • \$\begingroup\$ My input voltage comes from a spare 24V 100VA transformer which is rectified with 2200u+1000u+1000u (well, 4200u in total). Good question though. It should have dropped by 1.4V after greatz and then give me something more like 31.6V as idle input at lm338 input. At lm338 output I have something like ~33/34V regulation though. \$\endgroup\$
    – Elboyler
    Jan 3, 2022 at 12:03

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