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LM350 Variable Power Source


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Hello All,

Just finished building a modification of the following circuit.


The changes I made were to use a 24v 3a transformer as this was available at the time. I also used an LM350 instead of LM317 as it is rated at 3A. Also used a 2A fuse and some other small component changes due to availablility. The LM350 is a metal can type on a large heat sink.

So, the unit works and will power a 12v 10 watt light. The voltage varies nicely under this load and also without load. However, when I try to power a 35 watt halogen light, nothing!

I suspect that the LM350 might be cutting out due to current overload. Could this be the problem?

Would fitting an LM338 (with 5A rating) be the cure?

I originally built this as a general power source for use in cutting foam with a NiCr wire, so think that I will need to draw quite high current.

By the way, I am an electronics beginner so any help is greatly appreciated

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:)hi there DerFly, you hit it rite, you have current overload on your 350. my suggestion is this, determine first your current need, if its 8A then get the needed parts you need, if its 12 A, then do it, there are plenty of circuits in the web, but if you want some allowance for some experiments then make a 25A transfo at 13.8v. this is usually used in a base transceiver.
here are some sites you can visit:
1. http://www.sam.dk3.com
2. http://www.discovercircuits.com
3. http://www.geocities.com
happy hunting! ;)

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  • 1 month later...


What happened with LM350 is as follows:

As its designed to dissipate a power of 30 watts,and it can be calculated using the formula:


So when you are using 24 volts transformer,and want 12 volts,the maximum current that you can draw is 2.5 Amp.

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