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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: LM350 regulated DC PSU & HUM problem posted by Ewing on August 19, 2003 at 18:32:47:
It sounds like the LM is going into thermal shutdown, as others have written. A similar thing happened to me. I think the LM 350 has a drop-out voltage around 2.5V at 1.5 amps. So, any input voltage to the LM above 7.5V is more than you need, and has to be dissipated as heat. For example, if you are hitting the LM with 12V and the output is 5V you are dissipating 7V times 1.5A or 10.5 watts of heat. I fixed my problem by using a 294-1027-ND heatsink from Digi-Key, and by lowering the input voltage as close to 7.5V as I could get. The heat sink still gets hot, but the voltage reg. hangs in there. The heatsink is rated for 17W at 25 deg C ambient; it was about the biggest one I could find for a TO-220, so you might want to make sure you have room. One more thing - the heat sink is electrically "hot" too. It's connected to the VR's output and will have 5V on it, so don't short it to the chassis and make sure it's not touching anything (including yourself when you poke around inside the chassis). I also used heat sink compound to make sure I had a good heat transfer path. Finally, put this part of the circuit as far away from other heat sources as you can; the lower the ambient temp of the regulator the better. If you put it near a tube, for example, and the air surrounding it is at 50-60 degrees C, you won't be able to sink enough heat to make the thing work. I guess with VR's, as with life, you gotta be cool! :-)
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- I Had the Same Thing Happen to Me - MikeT 11:16:26 08/20/03 (0)
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