I need a 1.2 volt supply from a 5 volt supply in a project I'm working on. It should be able to provide what is supposed to be 100 mA to a pair of cathodes for some VFD tubes I found.
I tried using a 25% duty cycle signal from the PWM pin directly to the cathodes, but it didn't work for some reason.
I tried using the signal on an optocoupler to drive the cathodes, but that didn't work either. I think the voltage is coming up to low but even when I give a high duty cycle to the optocoupler the VFDs won't light up. If I just connect a AA battery to the cathodes, they light up just fine, so I think they draw too much current for either of those setups to supply the proper voltage or something. I'm waiting for a voltmeter to find out for sure.
How can I get this done? Do I need to find an op amp or do something with some transistors? I can't find any helpful circuits online but maybe I'm looking in the wrong place...
I would guess that you can get by using 1.25V rather than 1.2V exactly, but your other option would be to put a resistor in series with the step-down module's output, selected to drop the 0.05V.
This is an Arduino forum. The "classic" Arduino is the ATmega328. Its output pins are suitable for a nominal drive current of 20 mA - the is, twenty. At a pinch, 30 mA maximum. The datasheet specifies an absolute maximum of 40 mA, in other words, that is the value you do not want to even approach.
You, "Soup", appear to be suggesting that you have connected an Arduino output pin to a load resistance of 12 Ohms at most. And that would be the case if the filaments were in fact, hot. When not hot, their resistance might be something like 8 Ohms. This is essentially, putting a dead short circuit on the output pin, an immensely foolish thing to do, indicating a complete ignorance of what a microprocessor is. Pardon me, but have I mistaken that this is what you actually did?
And incidentally, driving a device such as this from a PWM output that could deliver the necessary current, controlled by a microprocessor, is an open invitation to disaster when due to a program error, it applies 5 V continuously to the 1.2 V filament delivering seventeen times the rated power. The filaments will promptly burn out within a couple of seconds.
You need a trustworthy "buck" converter to power the filaments at 1.2 V. Nothing else.
Or use a linear regulator, and connect the Arduino output to the ENABLE pin to control the current. Less efficient of course than a buck converter, which one is best depends on how much current you actually need.
