Background: Trying to modify a cheap Digital Picture Frame. Since I don't enough enough about it to modify it's firmware, I need to take a more roundabout route. I will be including a msp430 to automate some things.
One of the main parts I need to change is the backlight control. For the most part, it is controlled with a single NPN transistor from the DPF's controller. Without modifying the transistor itself or the controller side operation of it.
So the question is, can I put two NPN transistors in series, C-E to C-E, to function like an AND gate? If not, Why not? Will it damage anything?
While the schematic below is simply representative, a major question would be the different voltages present. The Backlight might be 12v or 5v, and the DPF's controller might be 5v or 3.3v, while the MSP430 I will be using is only 3.3v. The Backlight might be up to 1A in current draw.
The second major question would be if anything would change if the existing transistor is being enabled by PWM instead of simple on/off. If my added transistor is off, will there be any negative effect on the first transistor being pulsed?
simulate this circuit – Schematic created using CircuitLab
Best Answer
Theoretically you can, but this will result in:
an increased VBE of the first transistor (increased by VCE of the new transistor); this may cause the transistor to dip into the ohmic region, and you may need to change R1 in order to compensate.
possibly reduced ICE if the current is limited by a simple resistor instead of an active current limiter; this could result in reduced maximum brightness.
Fortunately these can be tested by placing a couple of diodes after the existing emitter.
Hard to say. If the existing circuit uses a resistor for current sensing then it may "overdrive" the PWM in order to compensate for the now non-existent current. This may become an issue when the new transistor is turned back on.