Re: Transistor as a relay vs mechanical relay
- From: "Larry Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx>
- Date: Sun, 29 May 2005 20:17:32 -0700
"Watson A.Name - "Watt Sun, the Dark Remover""
<NOSPAM@xxxxxxxxxxxxxx> wrote in message
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>
> "Chris" <cfoley1064@xxxxxxxxx> wrote in message
> news:1116715409.256914.69340@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> * A transistor will typically require much less power to operate than
>> a relay coil, especially if you use a darlington transistor or a
>> MOSFET.
>
> A Darlington transistor requires a V drop of twice that of the single
> junction transistor.
That is not a useful statement of the facts.
> The C-E V drop of a single junction can be a tenth of a volt,
True (assuming you mean a single BJT). Or it can be more,
or less. A BJT in hard saturation can have 50 mV C-E drop.
> but a darlington has to be at least 2 diode drops or about
> 1.2V to function.
Actually, the input BJT can and often does saturate to the
kind of drop stated above. It keeps the output BJT out
of saturation since the input C-E drop is in series with the
base of the output BJT. This results in typical darlington
C-E drops of 0.8 to 1.0 V. There is nothing about this
situation that makes "2 diode drops" significant.
> So the power wasted by a darlington is much greater
> than with a single junction transistor.
Assuming you mean a single bipolar junction transistor,
the truth of your claim depends largely on what supply
the base current is taken from. Where the input BJT
forced Beta is X, then for a bias supply greater than
X times the additional drop of darlington, your claim
is strictly false, (meaning "much greater" is no greater).
> If you want to minimize this, use a regular power transistor, and drive
> it with another transistor connected common collector or emitter
> follower. This basically means do _not_ connect the collectors together
> in a darlington config.
That may be a good strategy if a low voltage bias
supply is used and if the extra parts count is worth
the power savings. But the OP should be aware
that Darlington transistors have been used in many
places by people familiar with the alternatives.
These days, stringing together BJTs as you suggest
is rare. If the output drop is important, a single
MOSFET is generally favored.
--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.
.
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- Re: Transistor as a relay vs mechanical relay
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- Re: Transistor as a relay vs mechanical relay
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- Re: Transistor as a relay vs mechanical relay
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