Alt-BEAM Archive
Message #02190
To: Ben Hitchcock beh01@uow.edu.au
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 10 Apr 1999 14:37:24 +0200 (CEST)
Subject: [alt-beam] Re: H-Bridge
On Sat, 10 Apr 1999, Ben Hitchcock wrote:
> You need to use transistors that can handle the current (obviously!), and
> you also need to give them enough base current for them to work properly.
That's correct, but...
> For example, if I were to use a transistor with a gain of 100, I would need
> to give it at least 2 mA of base current to get 200 mA flowing in the motor.
...The gain (hFE) you see in a table is often the max. in ideal
circumstances, and is not equal to what you get if you need to
saturate (fully switch on) a transistor against a sizeable
collector-emitter current. What you need to know for H-bridge use
is the base current you must supply to get minimal voltage drop
across collector and emitter (VCE). For instance:
Ib Ic VCE gain
BC337-25 1uA 100uA < 0.1 100
10uA 1mA 0.1 100
100uA 10mA 0.1 100
5mA 100mA 0.1 20
10mA 300mA 0.3 30
Which compares with:
Ib Ic VCE gain
2N3904 10uA 100uA 0.1 10
50uA 1mA 0.1 20
500uA 10mA 0.1 20
5mA 100mA 0.24 20
10mA 240mA 1.2 --
And:
Ib Ic VCE gain
BC549C 1uA 100uA < 0.1 100
5uA 1mA < 0.1 200
50uA 10mA < 0.1 200
5mA 100mA 0.2 20
10mA 240mA 1.2 --
The best hFE of the actual transistors used in this test was:
BC337-25: 250
2n3904: 200
BC549C: 570
> Since most 74HC chips don't give out more than a few mA,
This depends on Vcc and temperature. At 4.5V supply, a normal high
output will drop about 0.2V when sourcing 4mA, and a normal low
output will rise about 0.2V when sinking 4mA. But if Vcc comes down
to 3V or the temperature goes up, you're soon looking at about half
that performance.
> None of my robots need 200 mA, so I'm pretty safe with my BC337
> and BC327's.
Perhaps the best you can find, especially the BC337-40 and BC327-40.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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