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 18:02:51 GMT
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Original-From:
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Jeff Jackowski <JEFFJ@spamlessRO.COM>
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Viewed:
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674 times
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On Mon, 27 Sep 1999, Jeffrey Hazen wrote:
> 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>
>
> 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.
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 think a 7805 is the way to go for this application.
A nice 5.1v zener diode should do quite well. I use one to bring down the
voltage of a 12v tach pulse in my car to 5v. Just connect up the zener
with one end to ground and the other to RCX out, pointed to block current
from flowing to ground. Connect the relay in series with the zener diode.
I think that should work. Someone mentioned a resistor, too. Voltage
regulators are happiest when the input voltage doesn't quickly change. A
capacitor could smooth out the RCX's PWM, but I still don't think a
voltage regulator is the right part for the job. A diode should handle the
volatge changes without any trouble.
Jeff Jackowski
http://ro.com/~jeffj/
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Message is in Reply To:
 | | RE: 5VDC Voltage Regulation
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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 (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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