how do i get minimum or decent voltage input for a LM317 regulator for the filament for shuguang treasure cv181 z tube which draws 6.3V 0.9A.
What should be the voltage input for the LM317 before regulation?
What should be the voltage input for the LM317 before regulation?
When using the LM317 for voltage regulation you need at least 3V higher voltage as input to insure good regulation at the output. If you want 6.3V input you need around 9.3V or more as input.
NOTE: there is a maximum voltage drop (input-output voltage) that the 317 is rated for, and this is about 30V IIRC, assuming infinite heatsinking. The larger the voltage drop, the more dissipation you will have in the device. So try to make the input voltage at least 3V but as low as possible, or add a heatsink to the device if you have to use larger input voltages. This is especially the case for you, because you are sourcing a good amount of current (0.9A) in your application.
All of this can be discerned by a careful reading of the datasheet...
http://www.ti.com/lit/ds/symlink/lm317.pdf
For example, see in Table 6.3 the line "Input-to-output differential voltage".
NOTE: there is a maximum voltage drop (input-output voltage) that the 317 is rated for, and this is about 30V IIRC, assuming infinite heatsinking. The larger the voltage drop, the more dissipation you will have in the device. So try to make the input voltage at least 3V but as low as possible, or add a heatsink to the device if you have to use larger input voltages. This is especially the case for you, because you are sourcing a good amount of current (0.9A) in your application.
All of this can be discerned by a careful reading of the datasheet...
http://www.ti.com/lit/ds/symlink/lm317.pdf
For example, see in Table 6.3 the line "Input-to-output differential voltage".
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wow thank you very nice explanation..🙂
My major concern here is the heatsink here I agree that as I have observed that once tried found considerable dissipation.
My major concern here is the heatsink here I agree that as I have observed that once tried found considerable dissipation.
> considerable dissipation.
If you could maintain *exactly* the 3.0V in-out drop Charlie gave, 0.9A * 3V is 2.7 Watts. Too much for a naked TO-220 package.
But in fact you also have maybe 1V of DC ripple, and must handle 10% line-voltage changes. So you may be running nearer 5V in-out drop. Now it is 4.5 Watts.
In any case it will run Hot.
A conservative reading is at least 2"x2" (50mm x 50mm) of at least 0.1" (2.5mm) aluminum with excellent air around it. It may survive smaller. I would prefer somewhat larger.
If you could maintain *exactly* the 3.0V in-out drop Charlie gave, 0.9A * 3V is 2.7 Watts. Too much for a naked TO-220 package.
But in fact you also have maybe 1V of DC ripple, and must handle 10% line-voltage changes. So you may be running nearer 5V in-out drop. Now it is 4.5 Watts.
In any case it will run Hot.
A conservative reading is at least 2"x2" (50mm x 50mm) of at least 0.1" (2.5mm) aluminum with excellent air around it. It may survive smaller. I would prefer somewhat larger.
When cold the valve will draw much more current. Measure the cold resistance and calculate. Make sure the regulator can pass that current for a few seconds without overheating or going into current foldback mode. This often catches people who think that providing a regulated DC supply for heaters will be simpler than doing an AC supply properly.
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