Resistor values for transistor going from 18V to 24V

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1

I'm using TIP107 transistors for the power side (column) of a lamp matrix using high current incandescent bulbs. I'm basing it on an existing design, but that design was for 18V and the power supply I have available produces 24V. So, one TIP107 would need to provide up to 2A - each bulb is .5A at full brightness.

The existing 18V circuit has a 560 ohm resistor on the base with a 1.2K ohm resistor tying it high. How do I calculate what the resistor values should be for the increased voltage?

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2

The datasheet suggests 6mA will saturate it for a 3A load, for 0.5A that should be
plenty.

24V / 6mA = 4k, call it 3k9.

The existing 560 ohm resistor is too low, it will be generating significant heat even
at 18V, BTW.

3

I put a 2.1K in the 560 place and a 4.2K in the 1.2K place, so it sounds like I had the right idea. How do I calculate the 560 replacement?