LM317 max amperage

[Jimmah]

I have a situation where I need around 15-17w at 12v with occaisional trips into 19-21w range. I have a pile of LM317's from previous projects and finding LM338's in town is impossible. Also I would not like to use any extra components, so sadly adding a transistor to do the current isn't available.

Heat isn't an issue, I have lots of small to large heat sinks around, mostly I'm curious if I will trip a current sensing circuit of some sort and if the little regulator can handle it.

Thanks for any help and insight, I appreciate it.

 

[Mark R]

The LM317 (TO-220 version) is specified as follows:

Current limit activation point is guaranteed at least 1.5 A, but below 3.4 A. From this you would expect most specimens of LM317 to tolerate 21 W @ 12V.

In terms of continually operating at that current - the only real issue is power dissipation, and as long as you keep your power dissipation below about 10 W, and use a suitable heatsink, you shouldn't run into problems.

 

[Jimmah]

The LM317 (TO-220 version) is specified as follows:

Current limit activation point is guaranteed at least 1.5 A, but below 3.4 A. From this you would expect most specimens of LM317 to tolerate 21 W @ 12V.

In terms of continually operating at that current - the only real issue is power dissipation, and as long as you keep your power dissipation below about 10 W, and use a suitable heatsink, you shouldn't run into problems.

Excellent, thank you very much for your help

 

[r0k]

I have a situation where I need around 15-17w at 12v with occaisional trips into 19-21w range. I have a pile of LM317's from previous projects and finding LM338's in town is impossible. Also I would not like to use any extra components, so sadly adding a transistor to do the current isn't available.

Heat isn't an issue, I have lots of small to large heat sinks around, mostly I'm curious if I will trip a current sensing circuit of some sort and if the little regulator can handle it.

Thanks for any help and insight, I appreciate it.

The LM317 (TO-220 version) is specified as follows:

Current limit activation point is guaranteed at least 1.5 A, but below 3.4 A. From this you would expect most specimens of LM317 to tolerate 21 W @ 12V.

In terms of continually operating at that current - the only real issue is power dissipation, and as long as you keep your power dissipation below about 10 W, and use a suitable heatsink, you shouldn't run into problems.

I agree with Mark R's findings with one caveat... More info on the datasheet here. That 3.4 A was assuming vin - vout was less than 15 volts and that was for the "K" package. If you don't have the K package, the max was 1.8 A.

It's not worth frying up parts to save time or gas. If the parts you have on hand aren't the K package, get the part you need from Digikey rather than put on a big heat sink and hope for the best.

 

[Jimmah]

I agree with Mark R's findings with one caveat... More info on the datasheet here. That 3.4 A was assuming vin - vout was less than 15 volts and that was for the "K" package. If you don't have the K package, the max was 1.8 A.

It's not worth frying up parts to save time or gas. If the parts you have on hand aren't the K package, get the part you need from Digikey rather than put on a big heat sink and hope for the best.

1.8 amps is more than enough, my Vin-Vout is less than 3v, probably closer to 2.5v +/- .15

I think I will order a LM338 anyway and make the circuit with the LM317 then swap out parts once it arrives.

 

[Modelworks]

I usually stick to 1.5A for the 317 parts. More than that I go with a pass transistor to make up for the difference. The 2n3055 is a well proven match that is common and cheap and can get you up to 15A.