# Help with a simple LM338 circuit

Discussion in 'General Electronics Discussion' started by quirkymac, Aug 4, 2009.

1. ### quirkymac

1
0
Aug 4, 2009
Hi there,
I was hoping someone here could help me with this simple circuit. I have made the circuit listed on www dot national.com/mpf/LM/LM338.html

to help me regulate the output voltage to a nichrome wire hot wire cutter - for cutting polystyrene foam wings for model aircraft.

I have scavenged a couple of old computer power supplies and have a number of different input voltages to choose from (with amps to spare). The circuit appears to work perfectly and I now have an adjustable output voltage.

My issue is that when I try and turn up the voltage the circuit appears to max out then go into a fluctuating state - when I hooked up the ammeter to see what current I was getting, it ramps up beautifully to 2.85amps then starts to alternate between 2.85 and about 1.5. Am I right in thinking that the LM338 should output up to 5 amps?

If I was using the full rated 5 amps for the LM338 do I need a heatsink?
Could it be that the LM338 is limiting the current due to overheating?
Or could there be something else going on?

Any help greatly appreciated!
QM

2. ### Resqueline

2,848
2
Jul 31, 2009
It'll put out 5A provided it has a voltage drop of 3-15V AND a proper heatsink.
The device by itself in free air will not be able to dissipate more than around 1W before it shuts down due to overheating.
Simple math: 3V drop times 5A equals 15W, minimum. Thats a lot of heat, and it has the potential to give off 5 times more..
Look in the data sheet. You'll find the thermal resistance junction-to-case there. Add to that the case-to-heatsink mounting resistance (typ. 0.1-0.5 depending). Max junction temp is 125 deg C, and ambient is around 25, so you have a 100 deg C rise to play with. Take these 100 and divide them by the Wattage the device gives off, then subtract the result with the junction-to-heatsink resistance. The end result is the specification for the heatsink. A number lower than 0.5 means you have to use fan cooling to achieve, and if it's negative then you have to improve the mounting method and/or reduce the Wattage (= lower Voltage drop).
So measure the Voltages involved for starters.

3. ### neon

1,325
0
Oct 21, 2006
You realy do not need voltage what for ? you need current and regulate it. for that kind of power think abaout a bypass power transistor. heatsink becomes massive to dissipate if continious duty cycle. think of adding a small fan to the heatsink to get rid of heat..Most people when need power think of voltage but in essence is the product of the two that you want.