For my astrophotography equipment, I need to construct a dew heater and a flat field generator.
A dew heater is simply a bunch of resistors connected in parallel such that each resistor outputs about 0.3W of power, and spaced roughly 1cm apart. Typical dew heaters run off 240V-12V AC-DC adapters and have controls to vary heating.
A flat field generator is a box with LEDs or LED strips mounted on one end, and a diffuser (white perspex) on the opposite end to create an even field of illumination. Typical flat field generators also run off 12V and have control to vary the brightness of the panel.
The design I came up with is this:
Here, the transistor is biased through a potentiometer. The same design would be used for the dew heater by replacing the LEDs with resistors. However, I have a couple of issues with this:
I would like to have a linear (or almost linear) relationship between potentiometer value and brightness/heating (current through collector). In simulations, I can tell that for this circuit, the relationship is not linear. Is there a simple way to rectify this?
For the dew heater, I require it to produce about 5W of power max. How can I calculate the values of every resistance in the diagram to ensure this? Also, since the resistors are in parallel, the sum of all currents will go through the transistor too. Is there a generic part number for a transistor that would be better suited for this task instead of having a fried transistor? Also, how much should the power supply be rated for this? I have an option to use either a 5V or a 12V power supply.
For the flat field generator, the problems are even worse. Linearity issues aside, my panel will use LED strips like THIS one. I can't find a datasheet for this LED strip but it has 18 SMDs per strip and runs on 4V. I think all 18 LEDs would be wired in parallel since white LEDs have a forward voltage of about 3.3V. If that is the case, each strip would draw about 20mA x 18 = 360mA. My panel will house up to 25 of these strips. If I connect them in parallel, they will draw 9A of power! Is there a better way of doing this? If not, are there any generic transistor part numbers that will handle this amount of current, or will I have to break the circuit up into multiple drivers, each driving a few strips? Also, I doubt I will find a 12V power adapter rated 9A.
I know the design is a basic one that you'll find anywhere on the internet. If any of you has a better/more precise solution, I would love to hear about it.