Transistor as a relay vs mechanical relay

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Just a very basic question:

Can anyone share with me what's the benefit of using transistor as an
electrical relay compared to mechanical relay when the incoming signal
is from a computer?

Is it related to pricing, input voltage/current, power rating and etc?

Pls help.

Thanks.

Albert

 

[Rich Grise]

Just a very basic question:

Can anyone share with me what's the benefit of using transistor as an
electrical relay compared to mechanical relay when the incoming signal
is from a computer?

Is it related to pricing, input voltage/current, power rating and etc?

No moving parts.

Cheers!
Rich

 

[Chris]

etc?


No moving parts.

Cheers!
Rich

Yup.

A basic question, indeed. Let's count the ways.

* Tranistors switch in microseconds, relays in milliseconds.

* Transistors switch directly on without contact bounce, relay
contacts smack and bounce together for a millisecond or so, causing
intermittent on/off for that time until the contacts settle in.

* Relays are mechanical devices with moving parts -- they will wear
out. A typical relay is rated for somewhere between 10 thousand and 10
million operations before failure. And relay contacts usually wear out
first, if they're swicthing at anywhere near rated load. Actually,
they're supposed to.

* Relay contact bounce can cause EMI which can cause problems.
Although you generally have to be more careful with transistors when
switching inductive loads, there isn't a spark being created with
switching like with relays. That EMI can cause the computer to spit
up.

* A typical high current transistor will usually cost much less than a
relay rated to switch equivalent current.

* A transistor will typically require much less power to operate than
a relay coil, especially if you use a darlington transistor or a
MOSFET.

I guess there are more reasons, but these are the ones that come to
mind first.

Good luck
Chris

 

[Harold Ryan]

It's none of the items mentioned. The signals from the PC may be on a
separate ground than the other electronic circuitry that is receiving the
signal. In this case, isolation is required by either by a mechanical relay
or an opto coupler. Just a transistor is not adequate.

Harold

 

[Nikolas Britton]

Just a very basic question:

Can anyone share with me what's the benefit of using transistor as an
electrical relay compared to mechanical relay when the incoming signal
is from a computer?

Is it related to pricing, input voltage/current, power rating and etc?

Pls help.

Thanks.

Albert

Read the the intro, pages 1-6, for this book:
http://www.amazon.com/gp/reader/0521478790/ref=sib_dp_pt/103-2146462-2957458#reader-link

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

[snip]

* 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. The C-E V drop of a single junction can be a tenth
of a volt, but a darlington has to be at least 2 diode drops or about
1.2V to function. So the power wasted by a darlington is much greater
than with a single junction transistor.

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.

 

[Larry Brasfield]

"Watson A.Name - "Watt Sun, the Dark Remover""

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.