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Topic: Are all PNP BJT-s high side? (Read 1 time) previous topic - next topic

Batee

Hi guys,

i have recently bought a pair of BJT-s. BC635 - NPN and BC636 - PNP

the thing is that the BC635 behaves normally but the 636 has the lowest V(sat) when the load is at the high side.
here is the link to its datasheet http://www.produktinfo.conrad.com/datenblaetter/150000-174999/154806-da-01-en-TRANSISTOR_BC_636.pdf

I want to ask you that, are ALL PNP BJT-s high side? Because this one seems that it isn't and I'm pretty confused now :D

thanks

KeithRB

PNP's are PNP's, they are neither "high side" or "low side". We tend to use PNP's and NPN's in particular configurations for circuit reasons.

And you want low Vce(sat) when the device is on, just what is your problem?

Batee

I just want to build a new H-Bridge from BJT-s. I want to use the usual combination PNP-s as HIGH side switches and NPN-s as low side switches. My experience tells me that this is the right way how to use them. But this BC636 is not behaving as expected, when I have the motor below that transistor, its just not getting enough juice.
But when I shift the motor to the upper side, it suddenly starts working. Vce(sat) being only ~0.7V

KeithRB

There seems to be confusion. The PNP should be high side - between the voltage supply and the motor. The NPN should be low side - between the motor and ground...

fungus

#4
Jan 23, 2014, 10:13 pm Last Edit: Jan 23, 2014, 10:19 pm by fungus Reason: 1

this BC636 is not behaving as expected, when I have the motor below that transistor, its just not getting enough juice.
But when I shift the motor to the upper side, it suddenly starts working. Vce(sat) being only ~0.7V


It will work in either position so long as you can create a suitable current flow between emitter and base.

If the collector is connected to GND and your Vce(sat) is 0.7V then your emitter is at 0.7V.

An Arduino pin can only go as low as GND so this makes getting a good current flow through the base more difficult (you only have 0.7V to work with). This is the reason a PNP is usually placed "high side" although there's nothing intrinsically "high side" about them (same for NPNs...it's just easier to use them when the emitter is at GND).
Advanced Arduino

runaway_pancake

"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

Batee

#6
Jan 23, 2014, 11:07 pm Last Edit: Jan 23, 2014, 11:10 pm by Batee Reason: 1
thanks guys,

I have just noticed on the merchants site, that some of the PNP transistors are rated for positive currents, and some are for negative.

for example
Bipolar-Standard-Leistungstransistor BC636 PNP Gehäuseart TO-92 I(C) 1.5 A Emitter-Sperrspannung U(CEO) 45 V

and
Transistor bipolar Diotec BC557B PNP Gehäuseart TO-92 I(C) -100 mA Emitter-Sperrspannung U(CEO) -45 V

does that mean something?

KeithRB

It means there is no standard for reporting currents in a datasheet.

polymorph

No. It just means they are inconsistent.

The voltage rating could be seen as "how many volts between the collector and emitter", in which case 45 volts is correct. Or "how many volts can be on the collector with reference to the emitter", in which case -45 volts is more correct.

Current is normally given a direction, not a polarity. However, many people and datasheets give it a negative value when it is running in the opposite to the "normal" direction. Since most transistors are NPN, the current in a PNP is traveling in the opposite direction to this "normal" direction.

That's all it really means.
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