| Electronics > Beginners |
| Lm317 vs LM338 |
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| queennikki1972:
So I played around with the LM338 5 amp variable IC in the T0-220 package and I literally wasted 3 of them and still cant get them to work the same as a Lm317.. It would immediatelty short them out. According to Google they have the same pinout so I dont understand why.. Both were being powered by the same setup.. 40VA transformer - Bridge Rectifer - Smoothing Capacitors - Resistor and Variable Resistor (Pot) and one works and the other doesnt. Am I missing something here... The circuit below is the basic circuit used for learning. |
| queennikki1972:
But that would short it out like that? |
| Bud:
Try reducing the output capacitor to the value recommended in the datasheet. Also the datasheet recommends using a prote tive diode across the 317. |
| Ian.M:
--- Quote from: queennikki1972 on August 25, 2019, 02:31:37 pm ---But that would short it out like that? --- End quote --- Nope. However overvoltage at the input could be an instant failure. --- Quote from: Bud on August 25, 2019, 03:03:02 pm ---Try reducing the output capacitor to the value recommended in the datasheet. Also the datasheet recommends using a prote tive diode across the 317. --- End quote --- A large output capacitor shouldn't be a problem. The regulator will simply current limit till its charged. An anti-parallel diode to protect the pass transistor is only required if the input can ever be shorted to (or rapidly dragged down to) ground with a large output capacitor. Its internal protection is good for a reverse surge of up to 25A. There is no way short of rectifier failure for the voltage of a conventional secondary => bridge rectifier => reservoir capacitor unregulated supply, not powering other large loads, to collapse rapidly enough to damage the LM338 unless you physically short the input. One possibility is that it may be acting as a RF power oscillator due to poor layout. That can cause excessive dissipation and cause the internal thermal limiting to misbehave. Post a photo of your actual circuit! Also, where did you get them from? If it wasn't a major distributor you could have fakes. |
| queennikki1972:
I tend to order bulk from china so its a possibility of fakes. I dont always get bad stuff from china. Ill try some US ones and i will consider all of the advice. The layout is a mess do to soldering and desoldering in frustration but was basically like the drawing attached. As I said I hooked up a LM317 and it worked just fine so I doubt the layout was the issue. |
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