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| Increasing LM317 current |
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| clay1905:
Well, thanks for all the input. Firstly, there's a typo made in haste. The input is wrong. With the resistors mentioned below, the voltage on the input leg of the regulator is 4.3VDC. This *should* be just adequate headroom. R4 in the full schematic; There is a bit of discussion on the web site I pointed to about it being inadequate to dissipate enough power. The 22 ohm resistor was found lacking by a previous builder, and I have 2 of 27 Ohms & 5 Watts in series. This drops the voltage to 4.3VDC without too much dissipation per resistor. I would have used 2 Watt jobs, but they're hard to source here. The divider network in the full schematic is not what I'm using. I will be using the values in the little sketch, of 100 and approx 500 Ohms. the heat sink is a"U" shaped piece of finned aluminium that was bought for the purpose, but probably insufficient. The back of the U shape is about the same area as the back of the chip, and the two sides are also about the same size, less the slots. There 's plenty of room in the cabinet, so a bigger heat sink wouldn't be an obstacle. Current. No, I haven't measured, but that'll happen after breakfast. It should not be much more than 350 mA when stabilised as there are effectively 6 filaments in parallel, each drawing ~50mA. The datasheet says the LM317 should be capable more than this. A larger sink may be all I need from here. Went off half cocked here, I'm afraid. When I measure the actual current drawn I'll be in a better place to assess whats next. Thanks again everybody, Clay. |
| rdl:
Sounds like you are almost there. Don't overlook thermal coupling of some type between the regulator and the heat sink, it makes a big difference. The silicone pad type is okay, not as good as actual heat sink grease but much less messy. |
| clay1905:
Indeed. I have a monstrously over sized heat sink I snaffled from a defunct T.V. set. Yes, I'll be putting a smear of that evil thermal paste between the regulator and the heat sink, as well as getting it on the upholstery, the curtains and probably inside the 'fridge too. I'm not worrying about a mica insulator as the heat sink won't be in a position to cause trouble with a short. In any case, I think I'm well on the way to having a constant power supply, so I can get on with the alignment of the radio chassis soon. Clay. |
| schmitt trigger:
Last but not least; How far away are the tubes from the regulator? They may significantly heat up the surrounding ambient. |
| Gyro:
One think that strikes me as a bit dodgy... Originally, the radio would have been powered by two separate batteries. The power supply introduces a current path between the two supplies. Maybe it varies from radio to radio, but I would have though that this might cause some issues with the directly heated filament cathode biasing (unless both batteries were always conmmoned at the chassis). Ideally you would use a transformer with two secondaries to maintain the original 'separation', maybe benefiting from more appropriate voltages too. |
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