I'm working on a transistor switch to put on my raspberry pi through an arduino GPIO in.

The transistor setup works for lighting a led but there seems to be a big voltage drop over CE (5 volt CE volt becomes 4.4 volt when put a load on collector side)
The raspberry pi only starts when the input voltage is at least 4,7 volts.

If I measure between Collector ('Voltagehere' in schema) and ground, its 5 Volts, unless there is an actual load in between. Measuring at the same point ('Voltagehere' in schema) and ground the total voltage is only 4,4V. Its should still stay 5V I guess as I'm measuring before the load and ground?

Cant find a solution after days

 

hi svt,
Welcome to AAC.
Please post your LTS asc file, so that we can run it.
E

 

I never really worked with spice but I guess its a good scheme. I picked an similar PNP as the TIP125 is not in the standard lib.

It only works if I up the voltage to 5.5 volt but that's not the way it should work..

 

The R3 is too big, how much current of Load?
Assuming that the current of Load is 100 mA then the R3 should be like this --
TIP125 as Q2.
Ib2 = Ic2/10 = 100 mA/10 = 10 mA
R3 = (5V - 0.7V(Vbe_Q2) - 0.2V(Vce_Q1))/Ib2
= 4.1V/10mA
= 410 Ω

 

The raspberry pi (in the scheme replaced by the led) draws max 1 Amp

The transistor setup works but it's not delivering 5V over collector (only 4.4)
Even with the leds instead of raspberry pi in place as the load.

I replace the base resistor with 560 ohm one, still does the job but still with big voltagedrop.

 

hi svt,
There will be voltage drop across the transistor.
But it is quite small, also the LED in the asc file is passing 20mA.?
E

EDIT:
TIP125 and 4R load

 

The example for your application.
raspberry pi 1 mA → NPN 10 mA → PNP 100 mA
The R1 is too big.

 

If your load is a Raspberry Pi, then don't use a bipolar transistor to switch the power.
The Vce even in the best saturation is still going to eat a few 100 milivolts of your Vcc.

Use a MOSFET with a really low Rds-on, less than 20 milliohms.

Those computers are sensitive to supply voltage variations, don't make it a marginal situation.

 

k, switching to a mosfet.