Interesting Transistor arrangements

[BartSimpson]

Can someone please explain how the resistor divider works in the circuit below ?
Why is the +12v rail connected to the Emitter of the transistor ?
What effect will Div1 and Div2 have on the output voltage ?

**broken link removed**

I would also like to know what the diodes in the circuit above do.
What type of diode would they be ?
Suggestions of typical part numbers ?

**broken link removed**
The diodes are in a SOT23 package marked "C3"
The "3" in the mark looks more like a sideways "M"

Thanks.

 

[Exo]

where did you find this? Looks like complete nonsense to me. especially the lower transistor...

 

[BartSimpson]

It was taken from a car engine management computer.

Most of the inputs to the CPU look like the lower circuit.

The inverters in the upper circuit are sink drivers.

 

[John Sorensen]

It makes my brain hurt just to look at it.

j.

 

[grrr_arrghh]

It was taken from a car engine management computer.

are you saing that you've drawn the diagram based on looking at a piece of hardware??? Because you could easily have mistaken NPN transistors for PNP, and got the legs the wrong way round, and hundreds of other things that would explain why they make no sense.

 

[Exo]

It was taken from a car engine management computer.

are you saing that you've drawn the diagram based on looking at a piece of hardware??? Because you could easily have mistaken NPN transistors for PNP, and got the legs the wrong way round, and hundreds of other things that would explain why they make no sense.

yes, the lower transistor can't do anything if it is a npn. if it was a pnp it would make a magic smoke puff without a base resistor to limit the BE current.

The upper part is equally troublesome...

 

[BartSimpson]

Yes I have drawn the circuit from a working board.
I have drawn the circuit which is 100% correct.
The transistors were confirmed as NPN out of circuit.

The lower circuit is used on every sensor input. I can't see how the transistor could ever do anything which is why I asked the question. I was hoping that someone who had just finished physics might be able to explain it. Could it be something to do with the reverse breakdown voltage across the junction ?

I have seen other circuits where transistors source current through the emitter as per the upper circuit. I want to know why.

In that circuit, the inverters are current sinks.
The Output pulse is controlled by the PWM line. That is the easy part.
What I can't get my head around is how Div1 and Div2 affect the voltage which would be output. Remember all the inverters are low-side sinks, so visualise a FET grounding the line.

 

[bogdanfirst]

have you cheked the datasheets of the transistors?
can you tell the name of the transistors? maybe you got the pinout wrong.

 

[zevon8]

Even using transistors that have built in resistors ( like 47K in series with base, and 47K from base to Emitter ) only helps the lower one...

 

[audioguru]

Hi Guys,
If the bottom transistor is silicon, then it would make a majic puff of smoke, since its reverse-biased base-emitter junction would avalanche at only 6 to 7V and doesn't have any current-limiting.

 

[Gaston]

i notice that the base of the collector does not go to a ground symbol but to an arrow. mabey it is supposed to go with the resistor below it to make a pot

 

[Exo]

Its a commonly used ground symbol

 

[grrr_arrghh]

I know you are confident that you have drawn the diag corectly, but just incase, maybe try this.

where you have put transistor symbols, write in the leg number, and the name of the transistor, incase you have misinterpreted the datasheet, and have confused which legs are which.

 

[BartSimpson]

I messed up. My bad.

The transistor was two diodes in a SOT23 package.
Please look at the circuit again. I have fixed it.
I want to understand what these do and how they work.

 

[zevon8]

Now that makes more sense. Easy mistake with SOT packages. They are likely just small signal diodes. 1n914 or 1n4148 or the like. The purpose is to prevent signals from exceeding the 12 volt supply or below ground by more than 0.6 volts ( the diode drop where the diode will conduct .) This is a common "clamping" design.

 

[grrr_arrghh]

lol, I feel sooo relieved.

I can sleep at night again!!

 

[John Sorensen]

I was hoping that someone who had just finished physics might be able to explain it.

If you have to wonder about the physics of a circuit, 99.9% of the time you're on the wrong track :shock: .

j.

 

[Exo]

There are still some strange things with that circuit.
The ouput of the 'div1' buffer is connected to the 12V rail trough a resistor - the only thing it does is waste current when the buffer output is low

 

[grrr_arrghh]

do we know if the 'div1' and 'div2' are inputs or outputs?

 

[Exo]

according to the drawing they're inputs