How to turn on/off system using transistor or mosfet?

[Danesh_SA]

Hello!

I started building a system and this system should always power on ,but when it connected  to 5v DC power supply it should be turn off.the system is using 3.7v Lithium Polymer battery.

im using simple PNP circuit and connect the Base of PNP to 5v input of charger,but it wasn't work!

how should i design this circuit?
is there any schematic?

[Zero999]

Connect a pull-down resistor between R1 and 0V?

How much current does the load draw?

[Danesh_SA]

Hello Hero999!

Thank you for your reply.
the current is about 300mA at peak.
you mean i should have pull up and pull down resistor in this circuit?!

[danadak]

Something like this, you will have to adjust to your load
requirements and power in PNP. Also drive R to sat the
PNP, and the base to V+ to absorb the Zener leakage
when supply has dropped to insure PNP fully off.

Attached. Blue curve load current, the others Voltages.


Regards, Dana.

[danadak]

If you want switch to function as a switch, either on or off,
then a comparator might be more appropriate, one with a
reference in it in order to set accurately trip point.


Regards, Dana.