Darlington Transistor
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Posts: 46,084
Hi, thanks to Al Williams for the help ob the debug command.
Now another question:
I'm using a Darlington Transistor to close a relay (2 amps at 12 volts).
How do I calulate the size resistor to put in series with the base to fire
the transistor ??
I'm using a 1k now and it works but I'm not sure if this is right. The
transistor is a TIP120 from Radio Shack.
Bob
Now another question:
I'm using a Darlington Transistor to close a relay (2 amps at 12 volts).
How do I calulate the size resistor to put in series with the base to fire
the transistor ??
I'm using a 1k now and it works but I'm not sure if this is right. The
transistor is a TIP120 from Radio Shack.
Bob
Comments
writes:
> I'm using a 1k now and it works but I'm not sure if this is right. The
> transistor is a TIP120 from Radio Shack.
>
You probably don't need one. Most Darlingtons have a built-in series base
resistor.
Sid Weaver
W4EKQ
Port Richey, FL
[noparse][[/noparse]Non-text portions of this message have been removed]
robertp@v... writes:
> Hi, thanks to Al Williams for the help ob the debug command.
>
> Now another question:
>
> I'm using a Darlington Transistor to close a relay (2 amps at 12 volts).
> How do I calulate the size resistor to put in series with the base to fire
> the transistor ??
>
> I'm using a 1k now and it works but I'm not sure if this is right. The
> transistor is a TIP120 from Radio Shack.
>
>
>
> Bob
>
1k is fine. However you can use a much larger value resistor. The way to
calcluate it is as follows.
1. Know what current the relay coil uses. Say 100 ma for example
2. Find out what the hfe rating of the darlington is. At 100 ma its hfe is
about 400.
3. Divide the coil current by hfe rating and that will give you the value of
base current needed. 0.1 / 400 = 250 uA
4. Now determine what size resistor is needed to give you 250 uA. 5volts
(from stamp) - 1.2 volts (two base emitters of the TIP120) / base current.
3.8 / 250 uA = 15.2k ohms
Normaly to give some saftey margin, the value of the base resistor is lowered
a little to make sure you drive your transistor into saturation (make it act
like a closed switch).
Hope this helps.
Ken
[noparse][[/noparse]Non-text portions of this message have been removed]
writes:
> In a message dated 02/21/2003 07:44:24 Eastern Standard Time,
> robertp@v...
> writes:
>
>
> >I'm using a 1k now and it works but I'm not sure if this is right. The
> >transistor is a TIP120 from Radio Shack.
> >
>
> You probably don't need one. Most Darlingtons have a built-in series base
> resistor.
>
> Sid Weaver
> W4EKQ
> Port Richey, FL
>
The TIP120 does not have a series base resistor. You MUST use a base resistor
or risk damage to your stamp.
Jon W, agree?
[noparse][[/noparse]Non-text portions of this message have been removed]
2nd question, I beed to switch on the relay several time very quickly .25
second on, .25 second off, etc for about 4 cycles, my problem is that the
relay doesn't bleed off quick enough I think ?
I have tried a pulsout command with little success.
Bob
writes:
> 2nd question, I beed to switch on the relay several time very quickly .25
> second on, .25 second off, etc for about 4 cycles, my problem is that the
> relay doesn't bleed off quick enough I think ?
>
That is only 4 cycles/sec. I am operating the 13 relays for my model train
setup much much faster than that - over 200 cycles/sec. I am using TQ-2
miniature PCB relays from Digikey. Did you look at the data sheet for your
relay to see what the closure and release times are?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
Relay contacts have a lot of bounce. In other words, when the contacts
close, the bounce open and then closed several times before stabilizing.
Typically, relays are slower to release than to activate also. If you need
precise control without bounce, you should instead just switch your device
by using a transistor. The transistor will operate fine using the pulsout
command. Don't forget to consider the actual times needed to run the
program and switch on/off the transistor for best accuracy. Turn on/off
times using a transistor is much less than activating a power relay and vary
on the type number.
Jim
Original Message
From: Bob Phillips [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=707DbWLc9-iYaKfhhAtCnosjsGqBTRgzf5YQ3booxFettlvg7IKZMAlMQlFxzXg5wg_rTPwbSg]robertp@v...[/url
Sent: Friday, February 21, 2003 12:20 PM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Darlington Transistor
Thanks Sid for the info on the Darlington
2nd question, I beed to switch on the relay several time very quickly .25
second on, .25 second off, etc for about 4 cycles, my problem is that the
relay doesn't bleed off quick enough I think ?
I have tried a pulsout command with little success.
Bob
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> > I'm using a 1k now and it works but I'm not sure if this is right. The
> > transistor is a TIP120 from Radio Shack.
>
>You probably don't need one. Most Darlingtons have a built-in series base
>resistor.
Most darlington transistors do NOT have a built-in series base
resistor! The TIP120 is one of these: if you drive it directly from a
Stamp pin, you WILL damage the Stamp.
You are using 1K - this is about (5-1.2)/1000 ~= 3.8 mA. The TIP120 has a
minimum Hfe of about 500..1000 at low (2A) currents, depending upon
manufacturer. I'd say you are in the ballpark - I'd be more comfortable
with 470R myself.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
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