Re: choosing a suitable transistor
- From: Jonathan Kirwan <jkirwan@xxxxxxxxxxxxxx>
- Date: Sat, 07 Oct 2006 17:36:50 GMT
On 6 Oct 2006 18:26:27 -0700, "randomname" <randomname12345@xxxxxxxxx>
wrote:
I need a transistor (used for switching, from 6v supply) that will
power a maximum of 400mA. For now I'm using a TIP31 but this is
blatant overkill.
Will this transistor work instead: MPS3704
Also, which resister do I need to use?
So I don't ever have to ask again, what's an easy way to compute
transistor specs, and resistor values?
Your last paragraph begs something else, entirely. At least, it
appears to ask for you to be "taught to fish, rather than be given a
fish to eat," so to speak.
Understanding how to set part values and otherwise using transistors
for switching (and just staying within the domain of bipolar
transistors [BJT's] here), involves knowing more than what you suggest
above. It means understanding a little about how they work and
knowing at least some of the many possible topologies -- hopefully, in
fact, being able to arrive at some of them on your own imagination, in
fact.
So rather than answer your more immediate questions, I'm going to
point out some directions to look.
BJT's are usually "looked at" as inverting. When you use one BJT as a
switch and actively drive it, the low-voltage drop of its emitter to
collector is usually a condition caused by creating a significant (in
the neighborhood of 0.8V) voltage difference between its base and
emitter. So "pulling down" the base of a PNP tends to "pull up" its
collector; or "pulling up" the base of an NPN tends to "pull down" its
collector. However, there are other cases -- such as "pulling down"
on its emitter (with base tied at some V) also pulling down the
collector.
Here are some non-inverting voltage translation circuits (which amount
to switches):
: Vcc2
: Vcc1 |
: | |
: | |
: | \
: \ Vcc1 R5 / Vcc2
: / R2 | \ |
: \ | / |
: / | | |
: | | | |
: | R3 |<e Q2 | |<e Q4
: +----/\/\------| PNP +-------| PNP
: | |\c | |\c
: | | | |
: | '--, | '-,
: | | |/c Q3 |
: R1 |/c Q1 | CONTROL >----| NPN |
: CONTROL >---/\/\-----| NPN \ |>e \
: |>e / LOAD | / LOAD
: | \ | \
: | / | /
: | | \ |
: | | R4 / |
: gnd gnd \ gnd
: Vcc4 /
: | |
: | |
: \ gnd
: / LOAD
: \
: / Vcc3
: | |
: | |
: Vbase | \ Vcc3
: | | / R9 |
: | R12 |/c Q7 \ |
: '---/\/\-----| NPN / |
: |>e | |
: | | |<e Q6
: | +--------| PNP
: | | |\c
: CONTROL >--' | |
: | '--,
: \ |
: R10 / |
: \ |
: / \
: | / LOAD
: | \
: | /
: | |
: R8 |/c Q5 |
: CONTROL >---/\/\-----| NPN gnd
: |>e
: |
: |
: gnd
If you learn to understand these reasonably well, you will pretty much
be able to do your own calculations of resistor values.
BJT's also include some effective capacitance between base and
collector and base and emitter. These values are often roughly in the
area of 10pF, or so, for general purpose BJTs. How these affect speed
depends on the resistor values (you can often use lower values to help
out on speed, at the expense of operating current), but in most cases
where one millisecond is considered "fast enough," you won't need to
worry about that aspect. If very short switching times are required,
you will need to worry about this capacitance and provide a means to
overwhelm it.
Jon
.
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