"PNP transistor as a SELECT SWITCH" is it possible?

Thread Starter


Joined May 3, 2020
Hello, "PNP transistor as a SELECT SWITCH" is it possible?

I want control with MCU (atmega328p)
24v -> LM7805(regulator) -> MCU -> a. 24v Relay (700mW)
-> b. 24v motor (3A)
-> c. 24v motor (1A)

PNP (bc327)


a. 700mW -> 70mW


c. 24V ->>>> 15vc.jpg

so selecting 'a' or 'b' or 'c'

'a' worked. but 'b', 'c' I think i have to use more high Armps PNP TR.

so i want to ask like

1. " 'is these 'b', 'c' are possible? not using relay "

2. If i using high Armps PNP TR, for low temp. Do i have to use maximum current over 10A TR?

is this cal, if i want change pnp tr
it would be
Id2 = 9 Armps
Rds under 0.055
Rthj-a Under 50
is it right?
Last edited:


Joined Jan 29, 2010
You cannot control a PNP transistor with a 0v/5V Base signal, that has 24V on its Emitter, it will never be switched Off.

The last two circuits require a high power transistor , not a BC327.

Audioguru again

Joined Oct 21, 2019
Use a small NPN transistor (BC337?) with its emitter grounded so it can turn off. The 47k and 47k voltage divider on its base should be 2.7k and 2.7k. The relay coil should have a reverse biased diode in parallel to clamp the high voltage flyback produced by the coil when the transistor turns off.


Joined May 11, 2020
Regardless of resistor values, none of the circuits can switch the load. It will always be active. Why? Because the base potential will always be less than the emitter

Audioguru again

Joined Oct 21, 2019
If a relay coil uses only 70mW from 24V then its resistance is 8229 ohms which is way too high for a 24V relay coil.
Previously the relay coil's power was 10 times higher at 700mW then its resistance is 823 ohms and its current is 29mA.
The transistor saturates well when its base current is 1/10th the collector current (1/20th for European BCxxx transistors) so its base current should be 1.5mA to 2.9mA. The base voltage is about 0.7V so your 10k resistor has a current of 70uA and the 1k resistor has 4.3V across it producing a current of 4.3mA. Then your base current is 4.23mA which is higher than is needed but it will work fine.