# Transistor question

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I need to make a circuit and only have PNP transistors available at the smd
size I am working with.
I have 12V power and a input signal that is either 5V or 0V. I want to use
the transister to make an output that is 12V when the input is 5V and 0V
when 0V (or 12V when 0V and 0V when 5V is acceptable). How do I bias it to
do this? I put resistor at base to 5V and resistor at collector to gnd, and
emitter to 12V. But base is at 12V still. How do I calculate the resistor
size values?? Output current not important (<1mA).

Re: Transistor question

Thats actually quite difficult - at least 2 transistors and 3 resistors.
Are you *absolutely* sure you cant source an NPN transistor ?  Even in
a bigger size, would still be smaller overall than doing it with PNP only.

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Regards,

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Re: Transistor question

view with a fixed pitch font (eg paste into notepad)

-----------------+------+--
|      |
1M      |
in             |    |/
----[4.7K]-->Z--+----!~
6.8V      |\
zener       |
+-----
1K
|
------------------------+--

will give 0V for 5 and 12 for 0

the input may need a pull down resistor (eg 10K)

Bye.
Jasen

Re: Transistor question

except:
- 1M isnt a great choice for Rbe, a much smaller value (eg 4k7) works
fine, it only has 700mV across it (150uA into 4k7). 1M will mean the pnp
turns off pretty slowly, and Icbo might turn it back on at high
temperature, it only takes 700nA....

- depending on how sloppy the 12V and 5V supplies are, a 6V8 zener might
not be quite high enough. pick:

Vzmin + Vbemin + 5V_min > +12V_max

- for an smt process, 1k is not a very good choice for the collector
resistor, it will dissipate almost 144mW when the output is high. A 1206
can do this, just.

- you could reverse-bias the b-e junction of another pnp transistor for
the zener diode. connect c to b, then use e as anode, b+c as cathode.
depending on the actual breakdown voltage (5-10V), you might want to
adjust the ratio of Rbe and [4k7] to ensure the pnp is off when Vin = 5V

I do a lot of work with BC847bpn dual npn & pnp transistors, I use them
as diodes, zeners and transistors.

- if it doesnt switch off as fast as you want, place a cap across the
zener. choose C : (0.5CVz^2) >= Qstored and the pnp will turn off quick
smart. in practice 100p - 1n will do. that way when the logic input
rises very quickly to 5V, the zener cathode will rise by the same 5V,
sweeping charge out of the pnp base.

Cheers
Terry

Re: Transistor question

The device ended up to accept 12V for logic high and about upto  9V for
logic low. So I was able to use 1 transistor as a zener to swing the 12V
with 5V no enough for the zener and full 12V enough.

Re: Transistor question

:)

Cheers
Terry