# Pull down resistor on PNP transistor switches

Discussion in 'General Electronics Chat' started by vinnywilliams, Oct 3, 2014.

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1. ### vinnywilliams Thread Starter New Member

Oct 3, 2014
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What are these pull down resistor on the PNP transistor switches used for?

The pull down resistor goes to a Negative supply -12 volts, if there is a +5 volts on the base or a +12 volts on the base of the transistor, wouldn't the -12 volts and +12 volts cancel eachother or voltage divide?

I don't get how this can work

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Apr 28, 2012
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3. ### wayneh Expert

Sep 9, 2010
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In each case the pull-down ensures that the transistor is on and conducting unless the signal at the base exceeds the pull-down and turns off the transistor.

4. ### vinnywilliams Thread Starter New Member

Oct 3, 2014
5
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Without using the pull down resistor what would happen?

If you have -12 volts on the base and exceed the base with +12 volts, i would think it would cancel or short out

5. ### vinnywilliams Thread Starter New Member

Oct 3, 2014
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I don't understand how this series pass transistor works with this pull down resistor to a negative supply

The pull down resistor is on the collector, but it's going to the input of a Gate.
why a series pass transistor going to a input to gate?

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6. ### crutschow Expert

Mar 14, 2008
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It's not entirely clear what all these circuits are doing since you only show part of the schematic but it each case it would appear the transistor is being used as a switch.

In general you just calculate the transistor state based upon the transistor bias voltages. If the voltages are such that the base-emitter junction is forward biased (base more negative than the emitter) with a collector resistor going to a negative voltage then the transistor will be ON and the collector voltage will be close to the emitter voltage (transistor is saturated). If the base emitter voltage is such that the base-emitter junction is near 0V or reverse biased (base more positive than the emitter) then the transistor is OFF and the collector voltage will be near the negative voltage source of the collector resistor.

7. ### shortbus AAC Fanatic!

Sep 30, 2009
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Hi BillyMayo, long time no see. Still working on avionics?

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8. ### takao21203 AAC Fanatic!

Apr 28, 2012
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its of course too much effort to resize a picture. Dont view 2 mbyte black/white schematics.

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9. ### takao21203 AAC Fanatic!

Apr 28, 2012
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What sense does it make without presenting the whole circuit- at least what it is good for.
It is either "not knowing how to ask" or intentional trolling.