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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?

schematic

simulate this circuit – Schematic created using CircuitLab

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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?

Theoretically you can, but this will result in:

  1. 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.

  2. 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.

If my added transistor is off, will there be any negative effect on the first transistor being pulsed?

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.

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  • \$\begingroup\$ The possibly reduced brightness I had already figured on. And from my tracing of the circuit, along with the generic oem's reference designs I have found, there is no feedback from the backlight to the controller (These are cheap 5+ year old <$70 dollar retail DPFs). **Can you expand a bit on the VBE/VCE point? I don't understand that part. ** \$\endgroup\$ – Passerby Aug 25 '13 at 3:29
  • \$\begingroup\$ If running as PWM, the current fed into the base of the transistor needs to be high enough to saturate the transistor, while still being as low as possible in order to conserve power. Since the voltage from the base to ground has been elevated by the other transistor, a lower resistance is needed in order to maintain the same current. R1 = VbeQ1 / Ib ; R1' = (VbeQ1 + VceQ2) / Ib \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 25 '13 at 3:37
  • \$\begingroup\$ And in saturation, Vce should normally be 0.2~1v (generally speaking right?) Time to mock up a test circuit. Thanks @Ignacio \$\endgroup\$ – Passerby Aug 25 '13 at 4:13
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Another approach would be to adjust the base current to the existing transistor. Wiring Q2 collector to Q1 base and turning Q2 on would give you an additional way to turn Q1 and the backlight off (which is what your "and" gate would achieve).

Wiring a second resistor from Q1 base to +V would give you an alternative way to turn Q1 on.

This bypasses questions arising from operating the transistors in series.

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