Standard logic controlling a PNP transistor

Hello, I was wondering if I need anything else for this type of setup; like pullup resistors. The transistor needs to be either fully on or fully off. Thanks.

To turn the transistor off you will have to make the base and emitter voltages very similar, probably below 0.5V of each other. Since the emitter is tied to 12V you would have to make the base almost 12V too and that's impossible when you are driving it from 5V logic.

The trick to making it work is to place a resistor between the base and emitter so they assume a close potential and keep the transistor turned off. Then a second resistor goes from the base to the logic output. You select the values so when the logic is high, the base voltage is less than 0.5V lower than the emitter and when the logic is low, it is more than 0.6V lower than the emitter. The transistor will conduct when the logic output is low.

There are better ways of achieving this but the suggestion above uses the least number of components. You might run into problems if you are trying run the transistor in saturation, depending on the drive capability of the logic gate.

Brian.

Like this? It seems as if 12 volts will go through the gate in the opposite direction. Wont that damage the gate?

Pay attention to the load resistor value, when you turn on the 3906, you may need to sink the base current out by using the standard logic.

Yes, your new schematic is correct. the logic gate should be able to withstand the voltage, the resistor in it's output should be as high a value as possible though to give it as much protection as possible. The 2N3906 is only a small signal transistor so the base current only needs to be very low.

A far better way of doing this is to keep the resistor you added between the base and emitter, instead of connecting the other resistor to the logic gate, connect it to the collector of an NPN transistor. The emitter of that transistor goes to ground and the base is fed from the logic gate through another resistor. Sorry but I am unable to draw a schematic for you at the moment. It means two extra components, the NPN transistor and its new base resistor but it does make all the voltages fall within safe limits. It will however, invert the signal so you might need to change your logic functions slightly.

Brian.

Ok, I think I understand what you have explained.

hi drwily

in last drwaing u have problem u drive the the base emiter of first transistore
without any resistore and first transistore driver second transistore without any
rb resistore i prefere to use that circuit

Nashwan is correct, you missed out the resistor between the logic gate and the base of the new transistor in your schematic.

Brian.

hi, I was wondering, using an active current source (PNP) on the SDA and SCL lines, where do I connect the base of the transistor? I believe the Collector is connected to the lines and the emitter is connected to the 3.3V source

Thanks