Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.
I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.
Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).
I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.
I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.
Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?
Thanks in advance.
--HC
Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.
I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.
First, John Popelish suggested running the relays with 12VDC and that
the line-loss would be negligible. I did the math on the line-loss,
which I had not conceptualized or understood correctly before his post
and it seemed like a good idea. I had some crappy wire already run
out (I took a headphone jack to RCA Y-cable and cut it in two and
spliced some thin wire between the pieces to make a long-distance
auxiliary input for the stereo in my shop to run off the stereo in my
house). The cable is probably thinner than what John and I had
bounced back and forth about and the RCA audio stuff is pitifully
thin. The overall cable length is about 80 feet. I ran 12VDC across
the cable to a Shrack RP510012 relay with a coil resistance of 327
ohms. The coil snapped briskly in and out when the voltage was
applied. John was absolutely correct, the line-loss was insignificant
for the resistance of the coil. As I told John I would, I checked the
cut-in voltage of the relay. I started with it hooked up and engaged
at 12VDC and then turned the voltage down until it dropped out. I
then removed the relay and checked the voltage. Cut-out voltage was
2.79 VDC. I then hooked the relay back up and turned the voltage up
until it engaged and then disconnected and checked the voltage. Cut-
in was very soft at 6.5 VDC. I upped the voltage until I had crisp
cut-in which I got at 6.9 VDC. There seems to be no problem with a
very long-distance run to control these relays. Thank you, John
Popelish.
Next was John Larkin who suggested running the relays directly off 24
VAC with a couple of diodes. I used his diagram and two 1N4004
diodes. One diode came from the AC input to the relay, one went
across the relay coil. On the Shrack RP 50012 with a 327 ohm coil and
my 24 VAC source (RMS meter says it's 26.9 VAC) the relay chattered
constantly. I tried a Zettler AZ8-1CH-12D with a 312 ohm coil. It
also chattered. There seems to be some discussion as to why this
would or would not work and, honestly, it's beyond my current skill
level with electronics. But, FWIW, that's what I tried and that's
what I got. Thank you, John Larkin.
Next, Ed suggested I try something involving a Zener and a
transistor. I formulated my own idea based on this and then, in
response to that, Ed suggested a schematic. It tried his way and
mine. His way was to take the 24 VAC (which I full-wave rectified and
then buffered with a 2,200 uF 63V electrolytic) and make a voltage
divider with two 1k ohm resistors to drive a transistor. I hooked up
a LM78L12 exactly as (and the app notes) said, and the voltage divider
exactly as he said. I ran the voltage divider output to the base of a
2N4401 transistor. Applying 24VAC (same power supply as used above)
gave me a crisp turn-on of the relay and a crisp turn-off. However,
the turn-off came about 1.5 seconds after I disconnected the power
from the circuit indicating that the capacitor was still carrying the
circuit for a little bit, but at least it was a crisp turn-off. I
only tried this with the Schrack relay. I then tried the circuit the
way I had postulated in response to Ed's suggestion before he gave me
the schematic. I did it exactly as he suggested except that I ran the
rectified and buffered 24 VAC (about 35 VDC) trough a 30V Zener to the
base of the 2N4401. I got the same crisp on and off but the off came
much more quickly after I disconnected the power from the circuit. I
did the math on the current through the relay coil (12 / 327) and got
that it should be 36.7 mA. I measured it in actual use and got about
35 mA. In either case, the current should be well below the maximum
current capability of the LM78L12 (100 mA, 140 mA peak), however, the
voltage regulator was getting very, very hot (enough it was painful to
touch and I could smell the chip). I checked the actual current
through the voltage regulator to the coil of the relay (which is where
I got the number above; 35 mA). At 12 volts that should be about 0.4
watts, which I thought the LM78L12 could handle but the heat seems
excessive. I may need to put a heat sink on it. Regardless, the idea
works. Thank you, Ed.
Thank you all for your help.
The best, I think, is going to be to use the 12 volts DC from the
beginning as it is simpler and seems to work very well. With what I
think I understand now, I should be able to easily calculate the
maximum run of wire for the 12 VDC so it should be easy to know if
whatever run I choose will work or not. If, for some reason, that
doesn't work for the distance I might want to run, then I can switch
to the 24 VAC with the voltage regulator/transistor/zener.
Thanks again.
--HC