| Electronics > Projects, Designs, and Technical Stuff |
| Increasing LM317 current |
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| schmitt trigger:
Use a switchmode supply. As others have mentioned, your raw DC voltage is way too high for a low voltage, high current output. Any linear regulator you employ will dissipate several times more heat than the vacuum tube's heaters themselves! |
| Gyro:
--- Quote from: rdl on May 18, 2020, 03:13:36 pm ---I'm betting either a heat sink or an LM350 (or both) is all that's needed. --- End quote --- Agreed. I get the impression that the OP hasn't got any sort of heatsink fitted. An LM317 should be easily able to handle the worst case dissipation without any current boosting. The fact that it ran for a few minutes indicates that it isn't pulling anywhere near 250mA anyway. Those little directly heated 1.5V valves in portables were very frugal. |
| drussell:
Trying to run an LM317 at about 1.5v out from a ~20V DC supply is going to be an issue, even with trying to add some series resistance in front of it. That 15VAC winding feeding the filament side is way too high. In order to continue to use that transformer you're going to need to increase the value of the series input resistor significantly and use a huge heat sink, drop the input voltage with a long string of diodes, or otherwise pre-regulate / drop some serious voltage before the LM317 so it doesn't have to drop all the volts by dissipating the extra power all by itself. :) |
| drussell:
--- Quote from: Gyro on May 18, 2020, 05:55:17 pm ---Agreed. I get the impression that the OP hasn't got any sort of heatsink fitted. An LM317 should be easily able to handle the worst case dissipation without any current boosting. The fact that it ran for a few minutes indicates that it isn't pulling anywhere near 250mA anyway. Those little directly heated 1.5V valves in portables were very frugal. --- End quote --- Hmm, yeah I hadn't really thought about the fact that those battery tubes have very low current filaments, I had in my head the more typical filament requirements for, say, 6.3v tubes. Even at 250ma the LM317 should only be dissipating about 5w even without the series resistor, (right?) so then that should be doable with a moderate heatsink. The OP needs to let us know what their current heat sink is, I suppose. (if any) :) |
| edpalmer42:
Another way to deal with the overheating would be to increase the value of R4. The output of the rectifiers is going to be ~20V peak. The existing R4 (22 ohms) will drop ~5V5 @ 0.25A which results in an input voltage to the LM317 of ~15V. So the LM317 has to drop that to 1V4 and dissipate 3.4W. If you increase R4 from 22 to 68 ohms, the LM317 will see an input voltage of ~6V and will only have to dissipate ~1.2W. R4 would have to be rated for at least 5W. If the radio draws more or less current than 250 ma, adjust the new value of R4 to give the LM317 an input voltage of 2 - 3 volts more than the output. This might allow you to either retain the existing heatsink, or add a smaller one that is easier to add to the circuit. |
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