LM350 problem

[Wheresmyhammer]

Hi – I recently built a simple voltage regulator circuit using LM350, straight off the datasheet. (I’m looking to power a piece of equipment requiring 12.6V at 2.6 Amps.) I first tested it with about a 1 Amp load and everything was fine. I then hooked it to an automotive bulb (printed rating 27W). It stayed lit for a few seconds and then the chip’s thermal and/or current protection kicked in. I had the 350 on what I thought was a reasonable heatsink for the job, but to make sure I replaced it with a large 3”x4” heatsink and added a fan too. The bulb stayed on a few more seconds until it dimmed out. The big heatsink wasn’t that hot.

The LM350 says it’s “guaranteed” to put out 3 Amps. What am I doing wrong?

- In a perfect world, 27W / 12.6V would be 2.1A. I understand that AC Watts are derated based on efficiency, but I thought DC Watts equaled V x A? My ammeter shows just over 2 Amps while the bulb is lit.

- Do you think I might just have a bad LM350, or that there is some reason why it’s acting like a regular 1.5A LM317? Or do I really need gi-mongous cooling for such a seemingly modest circuit?

- Sure, I could redo the circuit with a pass transistor to share the load, but there would seem no reason to – the 350 says it can handle the Amps.
I would appreciate any ideas you might have. Thanks!

- Steve

 

[blueroomelectronics]

There will be conditions to that 3A output, under ideal conditions it will put out 3A. It really boils down to thermal. How high is your input voltage.

 

[schmitt trigger]

The actual current limit varies with both the input-output voltage differential and the junction temperature...check the device's datasheet and on page three there is a very useful plot.

 

[Wheresmyhammer]

Input is 24V. I suppose I could regulate that down to ~16V first with a sturdy transistor/Zener combo to take some stress off the 350?

 

[Hero999]

You could connect a power resistor in series to drop the extra voltage.

 

[schmitt trigger]

Possible solutions:

1) zener/transistor pre-regulator
2) power resistor
3) Most efficient, but most complex and expensive; use a switchmode supply.

 

[RCinFLA]

Put your thumb on back of LM350, then turn on power supply. It will be an indelible learning exercise.

The LM350 will put out 3 amp. They also have thermal shut down protection. If you are going from 24 vdc to 12 vdc with 2 amps of load the LM350 will be dissipating 24 watts. This requires a very large heat sink. It will do it if you provide the necessary heat sink. I would try to keep the dissipation to less then 5-6 watts and that is a pretty large heat sink too, about 10 square inches heat sink with 1" fins. You can add a fan to cut the heat sink size down.

A discrete series pass transistor will have the same issue. Since it does not have a smart thermal shut down, it will just blow out instead.

 

[Hero999]

A simple switching regulator can be made with a couple of transistors.

The current can be increased to 3A with the help of a MOSFET, it's possible to include current limiting, the site linked below gives more information.

2-transistor Black Regulator

 

[schmitt trigger]

A good online thermal calculator:

Heat Sink Temperature Calculator

 

[Wheresmyhammer]

Thanks for the great help, guys! I was thinking in terms of load and not anything else. I understand the issues much better now!

Cheers!

- Steve