diode biasing
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Leroy -
Can you expand on the biasing of the diode connected to the relay
coil? What markings on the diode indicate it's bias? Thanks!
Scott
Can you expand on the biasing of the diode connected to the relay
coil? What markings on the diode indicate it's bias? Thanks!
Scott
Comments
answer:
Short: Banded end goes to +V, non-banded end goes to the collector (or
drain) of the transistor doing the switching.
Long:
A relay is nothing more than a solenoid coil. When current is steady
through the coil it generates a magnetic field and the coil passes
current easily. When the current changes in any way, this causes the
magnetic field to change also.
A changing magnetic field and a coil of wire is a prescription for an
electric generator. You think of a generator as being a shaft that
rotates, but that shaft is really just changing the magnetic field by
moving magnets or moving a coil through a stationary field. Anything can
change the field and it still generates electricity. A transformer, for
example, is a generator where the "shaft" is a coil carrying AC current.
So when you change the current through the relay coil, you get a change
in magnetic field. The changing magnetic field generates a voltage.
Here's the important part: The size of the voltage is almost completely
related to the rate of change in the current. If you could change the
current instantly (which you can't) and the coil were perfect (which it
isn't) you'd get an infinite voltage. That would take infinite power, so
you can't do it. The sign of the voltage will depend on the sign of the
change in current. So you can get a + or - voltage.
However, switching in the nano or microsecond ranges still can generate
a whopping big voltage. Say you are using a 2N2222. The Collector/Base
junction looks like a diode. If the voltage on the collector were to
rise to, say, 100V the diode would be reverse biased and probably won't
survive the voltage. In forward bias, the diode is more worried about
current and there isn't much current so that's OK. It is the reverse
bias that gets you.
So the diode added across the relay will serve to shunt the voltage once
it raises above the supply voltage (by a diode drop; usually .6 to .7V).
So the voltage can't rise above that level and this protects the
transistor.
By the way, I've seen the same problem in power transistor circuits
where a wire wound emitter resistor causes big voltages when the emitter
current changes. The answer there is to use non-inductive wire wound
resistors that are wound 1/2 one way and 1/2 the other way to cancel the
magnetic field.
Hope that helps.
Al Williams
AWC
Al Williams
AWC
* Control 8 servos at once
http://www.al-williams.com/awce/pak8.htm
>
Original Message
> From: southernpost [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=FQ5XlAgAjykkl7MHFJfyUWM3YGbJ8JTgXyRF2KjMatbyD-OWsmm3nOO_DDiLDhqa7gDNdZy-oQeRmSpGPoTf9bS6]Scott@b...[/url
> Sent: Saturday, June 08, 2002 2:46 PM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] diode biasing
>
>
> Leroy -
>
> Can you expand on the biasing of the diode connected to the relay
> coil? What markings on the diode indicate it's bias? Thanks!
>
> Scott
>
>
>
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The diodes are correct as shown on the drawing you posted.
HTH,
Leroy
southernpost wrote:
>
> Leroy -
>
> Can you expand on the biasing of the diode connected to the relay
> coil? What markings on the diode indicate it's bias? Thanks!
>
> Scott
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and Body
of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/