Transistor / Voltage Question

[casdsys]

Hello All.
The bottom diagram shows what were currently using, minus the diode circled in RED. The profet is actually a mosfet. When we short out the contacts ( say a light bulb )the transistor turns the LED on which indicates continuity. We then turn the mosfet on to light up the light bulb. The current system runs on a 12v Dc supply. The new system runs of a 9 volt battery. We have a DC to DC step up converter, 9 volts in - 30 volts out. Were a bit concerned that the 30 volts may go back up through the transistor to the 9 volt supply line and damage the rest of the electronics. A blocking diode with the RED circle placed in the circuit will help solve this problem. I have actually made a mock up of this circuit without the blocking diode and it works but someone told me that eventually the transistor will die over time. Is this true......The circuit is only turned ON for one second and used no more than once a week. So the question is, would you use the diode or not.

[pebe]

I have 3 questions to get a better understanding of your posting.

1. If you only need to turn on the PNP transistor, why do you need a 30V supply?
2. What is the purpose of the circuit?
3. What is this PROFET that has 4 terminals?

[casdsys]

[quote="pebe"]I have 3 questions to get a better understanding of your posting.

1. If you only need to turn on the PNP transistor, why do you need a 30V supply?
2. What is the purpose of the circuit?
3. What is this PROFET that has 4 terminals?[/quote]


1. See number 2

2. This circuit is used not only to check the continuity of an " ematch " ( electric match ) but also to fire it. The 30vdc allows me to connect more ematches in series than if I were to fire it of just 12vdc

3.The Profet is also known as a Smart FET. It's simply a mosfet with certain features, such as for my application, the outputs are short circuit proof.
The 4 connections are, Gate, GND, VCC and Output.

An ematch is a device used in the pyrotechnic industry. It is used to fire, fireworks electrically.

[pebe]

Assuming your Smart FET has a built-in resistive drain load, then when switched off its output will be at -30V. That means that the voltage on the base and emitter of the PNP are the same and it will stay switched off. So no harm will result in leaving out the diode.

But it seems quite an elaborate circuit for the job. Why did you not just switch an N channel FET with the micro and make your 'ematch' its drain load?