Simple Transistor Question

Hi everyone,

Very new to electronics and just need some once off help, I think. I am a software developer but I require some indication lights for my Windows based software. I have picked up a thing or two and programmed a pic12f683 to receive commands serially and turn pins high based on the command. Everything is working, but I obviously cannot add a real load to one of these pins since they are only 25mA and 5v.

I have 2 Strobe Beacons that I want to connect to each pin, they are 12v DC and 800mA each so I need an NPN Transistor I just have no idea which one! I dont think a relay is required, since it is only lets say 1Amp (800mA so 1Amp to be safe?) I know how to wire up the transistor and such, I just don't know which transistor and what resistor to use with it.

Can anyone be so kind as to advice me on a part number suitable for my needs?

Thanks in advance,
Ross Steytler.

Hi Ross,

I would recommend to use a standard NPN transistor with a high current gain, e.g. **broken link removed** as SMD or the BC635 as the thruhole variant.
Connect a 160R resistor between PIC portpin and Base (to limit the bias to 25mA), Emitter to GND and Collector to the load (which is connected to 12V)

Regards
rho-bot

Thanks for that reply! I will give it a try as soon as I get hold of one. The datasheet says the Collector-emitter voltage is 80v and the Collector-base voltage is 50 and the Emitter-base voltage is 5v. Would you care to explain these to me? I know I'm probably asking for a lot, but I'm just curious for learning purposes. are these maximum voltages?

ross135 said:

Thanks for that reply! I will give it a try as soon as I get hold of one. The datasheet says the Collector-emitter voltage is 80v and the Collector-base voltage is 50 and the Emitter-base voltage is 5v. Would you care to explain these to me? I know I'm probably asking for a lot, but I'm just curious for learning purposes. are these maximum voltages?

Click to expand...

Those are typically the maximum voltages you you can apply across those particular terminals before the part breaks down (and typically dies). They are probably listed under Absolute Maximums, if I had to guess.

An NPN BJT like that needs about 0.7-0.8V from emitter-to-base to turn it on. By driving the maximum current into the base (25mA, in your case), you allow the part to conduct a lot more current from collector-to-emitter. In this scenario, you are using the transistor as a switch by putting it into the saturation region of operation.

According to the linked datasheet, U(CEO) and U(CBO) is max. 45V, U(EBO) is max. 5V. These are absolute maximum ratings for the device, voltages beyond will result in insulation breakthrough of the semiconductor.
But this is not relevant as U(CE) will only be 12V in Off-State. And U(BE) is below 5V.

Another hint: add a Pulldown resistor of ~1K5 between Base and Emitter (GND). This will clamp the Transistor to Off-State during Reset of the PIC.

That's really great guys, thanks a lot for all the info.

If u need more current sourcing, better to use a ULN chip(darlington Amplifier) it is cheaper and simple.
ULN2003 is a better choice.

sreepss said:

If u need more current sourcing, better to use a ULN chip(darlington Amplifier) it is cheaper and simple.
ULN2003 is a better choice.

Click to expand...

If you want to steer away from a current-based switch (BJT), you could always pick out a MOSFET that would do the same job, using only the voltage to saturate the device (turn the switch "on")... minimal output current needed (a few mA). You should look for an N-channel, enhancement mode (not depletion-mode) part with Vdss > 15V, and continuous Ids > your maximum load current.

As above only I'll suggest one, an IRF540, they are popular at the mo.
Coonect the output pin of the pic to the gate through a 10 ohm resistor, also connect the gate to ground with a 220k resistor.
Then connect the source to ground, connect the drain to the - of the strobe, and the pos of the strobe to the strobe supply, presumably 12v.

If you want to use a bipolar transistor which are a bit more hardy, use a power darlington they are higher gain, I suggest a TIP112, connect the output of the pic to the base via a 1k resistor, the emitter to ground and the collector to the strobe - as before.

Despite the mosfet only having a 10 ohm resistor feeding it, it will pull on average a lot less current from the pic.

I agree with sreepss , use ULN2003 or ULN2803 , you don't need external resistors since the inputs are TTL compatible, each gate can drive 500mA and you can put two or three in parallel to reach the current needed.

Yes, there are many solutions: Bipolar, Darlington-Bipolar, FET etc.
Another PnP-solution may be the **broken link removed** from **broken link removed**. Advantages are CurrentLinitation and no other external components. There are also LowSide-SmartFETs available.

Things get tricky if you want to switch the + if you wanted to do that.
Seen it done you'd have to use a pulse transformer on the gate of a mosfet, then you'd have to keep pulsing the output to keep the gate capacitance charged, pretty similar to refreshing dram memory in software (the good thing though the micro and the stobe circuit would be non connected to each other).

Better stick to low side switching.

The BTS4141 is no usual N-FET, it is a SmartFET HighSideSwitch with integrated ChargePump for Gate control and a CMOS compatible input. Therefore it is a PnP-solution.
The corresponding SmartFET for LowSide switching is **broken link removed**