# Transistor / Voltage question

#### casdsys

Joined Mar 15, 2009
10
Hello All.
The bottom diagram shows what were currently using. 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 top diagram is what we would like to do. The systems 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 to the 9 volt supply line and damage the rest of the electronics. Would a blocking diode place at "X" help solve this problem??? Will the 30 volts pass through the transistor and blow the LED.

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#### ecka333

Joined Oct 1, 2009
76
I think 30 volts can go to your 9V circuit through 4,7k and 10k resistor chain, but the current will be small 30/(4700+10000)=2mA. But maybe better remove 10k resistor and problem will be solved. Another way is to place low power diode in series with 4,7k resistor.

#### crutschow

Joined Mar 14, 2008
34,394
I think 30 volts can go to your 9V circuit through 4,7k and 10k resistor chain, but the current will be small 30/(4700+10000)=2mA. But maybe better remove 10k resistor and problem will be solved. Another way is to place low power diode in series with 4,7k resistor.
Even if the 10k resistor is removed, there will still be current through the 4.7k resistor from the PNP base-emitter junction.

The diode will work but it must be placed between the output of the profet and the contacts.

Another way to protect the circuit. that won't generate a diode drop in the profet output, is to use a zener diode of the appropriate voltage (higher than the maximum normal output voltage) between the profet output (cathode) and ground.

#### Ron H

Joined Apr 14, 2005
7,063
Even if the 10k resistor is removed, there will still be current through the 4.7k resistor from the PNP base-emitter junction.

The diode will work but it must be placed between the output of the profet and the contacts.

Another way to protect the circuit. that won't generate a diode drop in the profet output, is to use a zener diode of the appropriate voltage (higher than the maximum normal output voltage) between the profet output (cathode) and ground.
Are we looking at the same schematic? I don't see how either of those methods would protect the 9V supply.
I would put the diode in series with the 4.7k resistor.

#### crutschow

Joined Mar 14, 2008
34,394
Are we looking at the same schematic? I don't see how either of those methods would protect the 9V supply.
I would put the diode in series with the 4.7k resistor.
Why would it not protect? What is the path for the current that has you concerned?

If you put the diode directly in series with the 4.7k resistor, then the circuit will not operate correctly.

#### Ron H

Joined Apr 14, 2005
7,063
Why would it not protect? What is the path for the current that has you concerned?

If you put the diode directly in series with the 4.7k resistor, then the circuit will not operate correctly.
Attached is how I propose to block 30V from getting onto the 9V rail when the PROFET is turned on. Why won't it work? What do you propose?

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#### crutschow

Joined Mar 14, 2008
34,394
I see the problem with the problem. You are concerned about the voltage from the PROFET and I was concerned about the voltage from the PNP base going through the 4.7k resistor...

So the question is, what is the real concern?

#### Ron H

Joined Apr 14, 2005
7,063
I see the problem with the problem. You are concerned about the voltage from the PROFET and I was concerned about the voltage from the PNP base going through the 4.7k resistor...

So the question is, what is the real concern?
From post #1:
Were a bit concerned that the 30 volts may go back up to the 9 volt supply line and damage the rest of the electronics.

#### crutschow

Joined Mar 14, 2008
34,394

#### Ron H

Joined Apr 14, 2005
7,063
So what is the path for that to occur?
As I understand it, the PROFET is a switch which connects (in the OP's top schematic) +30V (on the VCC pin) to the OUT pin when the IN pin is raised to a logic 1.
While the PROFET is off, the contacts in the schematic will be bridged by a low resistance load. This continuity will be sensed by the PNP and indicated by the LED in the collector circuit. When this is seen, the input to the PROFET will be raised high, applying 30V to the load which is across the contacts. The concern is that this high voltage will get back onto the 9V supply, possibly damaging other circuitry which is powered by that same 9V supply.
Hence the diode that I added. The OP's suggestion of putting the diode in the emitter circuit should work as well.

#### casdsys

Joined Mar 15, 2009
10
I think I like the idea of the diode being placed in series of the 4k7 resistor. This way will it not only prevent a higher voltage reaching the 9v section of the circuit but protect the LED incase something goes wrong with the transistor.
I will try a mock up and see how we go. Many thanks.

#### casdsys

Joined Mar 15, 2009
10
Well I tried the placement of a diode in the 2 different locations and the circuit works. In fact I was surprised that it still worked fine without the diode being used and just going with the original circuit. I guess the max
voltage between the base and emitter-collector junction is 35volts and Iam running the unit at 30vdc.

#### Ron H

Joined Apr 14, 2005
7,063
Well I tried the placement of a diode in the 2 different locations and the circuit works. In fact I was surprised that it still worked fine without the diode being used and just going with the original circuit. I guess the max
voltage between the base and emitter-collector junction is 35volts and Iam running the unit at 30vdc.

The circuit without the diode will break down the base-emitter junction of the PNP when the profet is on. It will work, as you say, but the transistor may eventually degrade and die.

#### casdsys

Joined Mar 15, 2009
10
The circuit without the diode will break down the base-emitter junction of the PNP when the profet is on. It will work, as you say, but the transistor may eventually degrade and die.
Ron, if this circuit, without the diode was turned ON for only 1 second, and at the most, once a week. Would you be happy to use it this way....

#### Ron H

Joined Apr 14, 2005
7,063
Ron, if this circuit, without the diode was turned ON for only 1 second, and at the most, once a week. Would you be happy to use it this way....
IMHO, failure would be unlikely in this case, but my decision would be based on the consequences of failure. You hopefully know what that is. I don't.