Subject:
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RE: 5VDC Voltage Regulation
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 27 Sep 1999 17:11:43 GMT
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Viewed:
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381 times
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-----Original Message-----
From: Wes Matchett [SMTP:wes@augustschell.com]
Sent: Monday, September 27, 1999 11:02 AM
To: lugnet.robotics@lugnet.com
Subject: 5VDC Voltage Regulation
I'm in over my head with a project and need some help. I'm trying to use
an RCX
output port to control a 5VDC SPDT Relay from Radio Shack (part #275-240A).
I
need to limit the voltage of the port to 5 volts. How can I accomplish
this?
Is a voltage regulator IC correct? Alternatively, does anyone know if
putting
excessive voltage across this relay (max. VDC is listed as 6.5) for long
periods
of time will damage it?
<snip>
The usual caveats apply, don't try this at home, professional driver on
closed course, not my fault when it smokes..... BTW, you will damage the
relay eventually, with the resulting smell of burnt plastic filling your
room......
A voltage regulator is easy to use, only three pins. Use a 7805 (available
anywhere about 75 cents), attach pin 1 (+VDC in) to the positive vdc out of
the rcx. Attach pin 2 (ground) to both the negative rcx out, and the (-) of
the relay coil. Attach pin 3 (regulated VDC out) to the (+) of the relay
coil. For good housekeeping, and because the rcx output is PWM, put a
0.33uF 35V capacitor across pins 1 and 2 (note polarity if you have an
electrolitic cap). Also for good measure, put a diode #1N4005 across the
relay coil with the cathode (the neg (-) side with the stripe) attached to
the (+) of the relay. This will protect the regulator when the relay is
turned off.
Some things to be sure of: DON"T draw too much current!! The relay coil *
IS * rated at less than (say) 100ma right?? Some Radio Shack general
purpose relays can draw that and more. I saw some motor notes from Dave
Baum (I think) that suggest the gear motors only draw 10ma while running
and 110ma stalled, so you'll also chew up batteries (and your rcx) pretty
quickly if the current is too high. Also, set power to high(7) to maximize
the PWM time on. I don't have a scope, so I don't know the % of "on time"
at power 7, but the capacitor should filter most of that.
If you're into this kind of thing, I suggest you take a look at the Aromat
relays: www.aromat.com. Look around for part number TF2-9V... 10 pin DIP
package, DPDT, 9 VDC coil, 8.9mA current (thats tiny!), and contacts rated
at 30W (62VA). Available at Newark: www.newark.com, about $6.50 ea. You
could drive these directly off the rcx without a blink...Diode still
required.
I agree with Ralph that the simple Radio Shack books are a great place to
start....
HTH
Jeff
jeffrey.hazen@northmill.net
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Message has 1 Reply:
 | | RE: 5VDC Voltage Regulation
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| (...) I've used 7805 voltage regulators with a faulty power supply (12v) that put out an unsteady voltage. It caused the 7805 to quickly overheat, even with a heat sink and a large capacitor across the supposed 12v and ground inputs. I really don't (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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