Transistor calculation
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How does one calculate what the resistor value should be that goes between
the stamp and the base of a TIP42A PNP transistor? I would like to switch
8-12v at about 80mA.
Thanks, Dan
the stamp and the base of a TIP42A PNP transistor? I would like to switch
8-12v at about 80mA.
Thanks, Dan
Comments
and the transistor needs to be ON when the STAMP outputs a low (zero). So the
resitor will have 12 -.7-.3 ( Power supply minus base emitter drop minus Low
voltage from Stamp) or 11 volts across it. Now the current that is required
is dependant on the minimum value of Beta (hfe) of the transistor. Remember
that the approx. resistor (base) current is the collector current (80 mA.)
divided by minimum Beta. Most small signal transistors have at least a Beta
of 10 or more so you can safely assume a resistor current of 8 mA. so I would
use a 1K resistor.
[noparse][[/noparse]Non-text portions of this message have been removed]
handle 600 ma and is a much smaller package, not to mention cheaper. I use
this and a 2N4401 for most robot stuff. If you use a 2N2222, the 2N207 is
the PNP mate to it.
Original Message
> Since you are using a PNP you need to tie the emitter to the 12 volt
supply
> and the transistor needs to be ON when the STAMP outputs a low (zero). So
the
> resitor will have 12 -.7-.3 ( Power supply minus base emitter drop minus
Low
> voltage from Stamp) or 11 volts across it. Now the current that is
required
> is dependant on the minimum value of Beta (hfe) of the transistor.
Remember
> that the approx. resistor (base) current is the collector current (80
mA.)
> divided by minimum Beta. Most small signal transistors have at least a
Beta
> of 10 or more so you can safely assume a resistor current of 8 mA. so I
would
> use a 1K resistor.
the voltage drop but did not know how to calculate that either, bonus. You
guys/gals are great!
Dan
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| | 02/07/2003 11:27|
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| Subject: Re: [noparse][[/noparse]basicstamps] Transistor calculation
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Since you are using a PNP you need to tie the emitter to the 12 volt supply
and the transistor needs to be ON when the STAMP outputs a low (zero). So
the
resitor will have 12 -.7-.3 ( Power supply minus base emitter drop minus
Low
voltage from Stamp) or 11 volts across it. Now the current that is required
is dependant on the minimum value of Beta (hfe) of the transistor. Remember
that the approx. resistor (base) current is the collector current (80 mA.)
divided by minimum Beta. Most small signal transistors have at least a
Beta
of 10 or more so you can safely assume a resistor current of 8 mA. so I
would
use a 1K resistor.
[noparse][[/noparse]Non-text portions of this message have been removed]
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>Since you are using a PNP you need to tie the emitter to the 12 volt supply
>and the transistor needs to be ON when the STAMP outputs a low (zero). So the
>resitor will have 12 -.7-.3 ( Power supply minus base emitter drop minus Low
>voltage from Stamp) or 11 volts across it.
This won't work as described and is difficult to make work reliably without
adding a 2nd transistor. The problem is that a HI on the Stamp pin is only
5V, whereas the voltage on the base of the PNP is in the range of
8..12V. The ESD protect diodes on the Stamp pin will cause current to flow
even it is HI.
The best way is to use 2 transistors: a small NPN transistor (2n4401)
driven from the Stamp followed by the PNP transistor connected to the
8..12V supply.
NPN-e to gnd. Stamp pin to NPN-b via 1K resistor. Another 1K resistor
from NPN-b to NPN-e (gnd). PNP-e to +12V. NPN-c to PNP-b via 1K
resistor. Another 1K resistor from PNP-b to PNP-e (+12). Connect load
between PNP-c and gnd.
Note that simply changing the value of the resistor from NPN-c to PNP-b,
you can tailor this to almost any voltage.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
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dwayner@p... writes:
> This won't work as described and is difficult to make work reliably without
> adding a 2nd transistor. The problem is that a HI on the Stamp pin is only
>
> 5V, whereas the voltage on the base of the PNP is in the range of
> 8..12V. The ESD protect diodes on the Stamp pin will cause current to flow
>
> even it is HI.
>
> The best way is to use 2 transistors: a small NPN transistor (2n4401)
> driven from the Stamp followed by the PNP transistor connected to the
> 8..12V supply.
>
> NPN-e to gnd. Stamp pin to NPN-b via 1K resistor. Another 1K resistor
> from NPN-b to NPN-e (gnd). PNP-e to +12V. NPN-c to PNP-b via 1K
> resistor. Another 1K resistor from PNP-b to PNP-e (+12). Connect load
> between PNP-c and gnd.
>
> Note that simply changing the value of the resistor from NPN-c to PNP-b,
> you can tailor this to almost any voltage.
>
> dwayne
>
Or use a logic level mosfet www.glitchbuster.com
[noparse][[/noparse]Non-text portions of this message have been removed]
configuration to drive the resistor tied to the base of the PNP transistor.
But the calculations I gave you are still valid for both devices. I use 1K
resistors for both and use a 2N3904 for the NPN and a 2N3906 or MPSA56 for
the PNP. This works fine for switching up to several hundred mA through the
MPSA56. It has a slightly lower CE saturation voltage and can handle more
current than the 3906.
[noparse][[/noparse]Non-text portions of this message have been removed]
negative side of your device you can get away with a single NPN transistor
whose base conection can be driven through a resistor directly from the STAMP
output pin. The collector is then tied to the negative side of the device and
the positive terminal is connected directly to the 8-10 volt supply. This way
a HIGH output from the STAMP turns ON the device.
[noparse][[/noparse]Non-text portions of this message have been removed]
http://www.wd5gnr.com/basiccir.htm
http://www.wd5gnr.com/stampfaq.htm?article=13
http://www.wd5gnr.com/stampfaq.htm?article=12
Hope those help!
Al Williams
AWC
* Easy RS-232 Prototyping
http://www.al-williams.com/awce/rs1.htm