# Running a 24v relay from mains voltage?

D

#### DaveC

Jan 1, 1970
0
I need to power a relay from 120 vac such that its action is delayed for a
few seconds after power-on. I realize that this means getting DC voltage (via
diode) and an R-C combination.

Ideally, a relay with DC-rated coil would be most appropriate, but I've got a
nice new relay with properly-rated contacts sitting on the shelf with a coil
rated at 24 vac, 330 ohms, 71 mA.

How complex would it be to run this relay with mains voltage? Is it as simple
as a diode followed by a resistor? Would this resistor be 120/0.071/2 = ~500
ohms and 2.5 watts?

And a fundamental question: can a relay with an AC coil function properly &
reliably on DC?

Since I need to delay the relay's turn-on, I'll be adding a capacitor,
anyway, whether I use this relay or a true DC-rated type. What should the
value of the capacitor be to give me a 1 or 2 second delay with the 500 ohm
resistor (or, if I've miscalculated that value, with the proper value
resistor)?

Any observations, suggestions, corrections (I'm getting used to these!!) are
welcome.

Thanks,

J

Jan 1, 1970
0
J

Jan 1, 1970
0
D

#### DaveC

Jan 1, 1970
0
How complex would it be to run this relay with mains voltage? Is it as
simple as a diode followed by a resistor? Would this resistor be
120/0.071/2 = ~500 ohms and 2.5 watts?

It occurred to me that I didn't calculate the *dropping* voltage of the
resistor, as I should have, and didn't use mains *peak* voltage, as I think I
also should have:

((170/2)-24)/0.071 = ~850 ohms and 5 watts. Since this resistor value is not
common, I'll have to use a more-common value. These more-common values will
result in an applied voltage to the relay's coil of:

750 ohms: 32 vac - too high
820 ohms: 27 vac - max voltage allowed by relay mfr.
910 ohms: 20 vac - too low
1K ohms: 14 vac - way too low

And yes, John, I learn from all my electronics work, so you can always assume
it's all for educational benefit

Observations?

Thanks,

R

#### Robert Monsen

Jan 1, 1970
0
Dave:

Here is a little circuit that will do what you want, delaying the relay from
latching for a few seconds. Its designed to pump up to 24V with a load of
330 ohms after about 2.5 seconds. It wastes a couple of watts in heat.

Don't use this in the bathtub...

http://home.comcast.net/~rcmonsen/relay/powersupply.jpg

E

#### [email protected]

Jan 1, 1970
0
DaveC said:
I need to power a relay from 120 vac such that its action is delayed for a
few seconds after power-on. I realize that this means getting DC voltage (via
diode) and an R-C combination.

Ideally, a relay with DC-rated coil would be most appropriate, but I've got a
nice new relay with properly-rated contacts sitting on the shelf with a coil
rated at 24 vac, 330 ohms, 71 mA.

How complex would it be to run this relay with mains voltage? Is it as simple
as a diode followed by a resistor? Would this resistor be 120/0.071/2 = ~500
ohms and 2.5 watts?

And a fundamental question: can a relay with an AC coil function properly &
reliably on DC?

Since I need to delay the relay's turn-on, I'll be adding a capacitor,
anyway, whether I use this relay or a true DC-rated type. What should the
value of the capacitor be to give me a 1 or 2 second delay with the 500 ohm
resistor (or, if I've miscalculated that value, with the proper value
resistor)?

Any observations, suggestions, corrections (I'm getting used to these!!) are
welcome.

Thanks,
--
DaveC
[email protected]
This is an invalid return address

Lets set aside the other issues, and just address the
delay time. In rough terms, a resistor in series with
a capacitor will charge the capacitor to about 63 % of
full voltage in a time equal to R times C. That figure
is called the RC time constant. When R is only 330 ohms,
you'd need a large C to get a few seconds delay - about
10000 uF to reach 63 percent of full voltage across
the cap at about 3.3 seconds.

The rest of this post goes into some light theory and
ends with a description of a nice circuit you can
experiment with and possibly use in your amp.

You could get the same time constant with a much
bigger resistor, and a much smaller capacitor.
Make the resistor 33000 ohms, and the cap 100 uf
(.00010 farads): 33000 * .0001 = 3.3.
The trouble with that occurs when you connect the
relay across the cap. All the time the 33000 ohm
resistor is trying to charge the cap, the 330 ohm
relay is discharging it. This will keep the cap at
a very low voltage.

However, suppose instead of placing the relay coil
across the cap, we place a very high impedance (many
times higher than 33K ohms) across it? It won't have
much of an effect on the time constant - but it will be
able to "see" the voltage rising on the cap. Now,
suppose we prevent that high impedance from seeing
the voltage at all, until it reaches (or exceeds)
some specific level? Well, we can do that by putting
a zener diode between the high impedance circuit
and the cap. Lets select a 9 volt zener, put it
in series with the 33K ohm/ 100 uf cap junction,
and stuff the other end of the zener into the base
of a darlington transistor. Complete the darlington
(NPN) circuit by connecting the emitter to ground and the
collector to the relay coil, while the other side of the
relay coil goes to + 12 volts.

What happens? When power (12 volts DC) is applied across
the series RC made of the 33K resistor and the 100 uF cap,
the cap charges to ~7.5 volts in ~ 3.3 seconds. In
another second or so, it will reach 9 volts, and the zener
will conduct. In turn this will cause the darlington to
conduct and the relay will transfer. If you replaced
the 33K ohm resistor with a 50K ohm pot, and added a 4.7K
resistor in series, you would have a nice adjustable
time delay.

This is a nice experiment, and the parts are cheap. You need
to select (or make) a darlington that can easily handle the
power that the relay coil needs and an appropriate zener and
relay voltage. For a turn on delay for your amp, precision in
the timing is not important, so even if the timing varies with
temperature that should not matter. There are other alternatives
to the circuit, but for initial experimentation with RC timing,
it is hard to beat and it could also be a nice fit for your amp.

D

#### DaveC

Jan 1, 1970
0
Here is a little circuit that will do what you want, delaying the relay from
latching for a few seconds. Its designed to pump up to 24V with a load of
330 ohms after about 2.5 seconds. It wastes a couple of watts in heat.

What kind of caps are these? Electrolytic? Poly? Please specify each so I can
know what to acquire.

Thanks for the circuit!

R

#### Robert Monsen

Jan 1, 1970
0
DaveC said:
[snip]
What kind of caps are these? Electrolytic? Poly? Please specify each so I can
know what to acquire.

The 4.7uF cap is a big fat non-polar 250V cap. A mylar cap should work fine.
The voltage rating is marked on the cap, or you specify it when you buy it.
Don't use anything with a lower voltage rating!

The 4700uF cap is a big electrolytic cap (you probably can't get anything
else that big.) The minus on the cap (it'll be marked clearly) goes to the
ground (which is the bottom line) in the circuit. Since you are going 24V,
you need something that'll handle a high enough voltage, like a 25V cap. You
should go 50V just to be safe and to extend the life of the cap.

You can get both of these, and a sandstone 10 ohm 5W resistor mailorder from
www.goldmine-elec.com.

If you are going to build this, mark HIGH VOLTAGE clearly somewhere on
inside the case, make sure NOTHING, including the relay, can be touched by
users, and encase the wires and leads in heatshrink tubing (you can also get
that at goldmine, I believe.) You could also use a cheapo hot glue gun to
cover all the wires in goop. That should insulate them. Just make sure the
goop is actually non-conductive.

One last thing, I'd put a 1MEG 1/4 W resistor in parallel to the 4.7uF cap
just to be safe. This will keep you from shocking yourself when the circuit
has been off for 2 hours and you pick it up. If you put the switch right
before the cap, it could conceivably be charged up to 165V, which would mean
64mJ of energy stored across the cap, enough to wake you up.

Regards,
Bob Monsen

W

#### Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
Dave:

Here is a little circuit that will do what you want, delaying the relay from
latching for a few seconds. Its designed to pump up to 24V with a load of
330 ohms after about 2.5 seconds. It wastes a couple of watts in heat.

Don't use this in the bathtub...

http://home.comcast.net/~rcmonsen/relay/powersupply.jpg

Rather than use that monster 4.7 uF capacitor, it might be cheaper to
use a small power transformer. About the only place I can remember
seeing a cap that size is for a fan motor, and it would have to be
paralleled to get that much capacitance. And it was about the same
price as a power transformer, maybe $5 or so. Among this list, the 7841 looks like it's a good choice; it's cheaper than that monster capacitor, and safer because it isolates you from the AC line voltage. When it is rectified and filtered, it will be about 24VDC. But your relay will take less than 24V if it's DC instead of AC. http://www.mpja.com/listitems.asp?dept=45 -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/electronics/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 <at> hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@ F #### Fred Bloggs Jan 1, 1970 0 Ideally, a relay with DC-rated coil would be most appropriate, but I've got a nice new relay with properly-rated contacts sitting on the shelf with a coil rated at 24 vac, 330 ohms, 71 mA. Where are you getting these numbers? Did you measure 71mA or the 330 DC ohms? In either case the AC relay reactance is usually 10x the resistance if the relay is of any quality, and therefore all of your component planning will not come close. Measure the DC ohms Rdc and the total ac-RMS current, Irms. Then total reactance X=24VAC/Irms and inductive reactance XL= sqrt(X^2-Rdc^2). Then you solve for series R required at 120VAC operation, Rs, as |(Rdc+jXL)/(Rs+Rdc+jXL)|=24/120 etc... R #### Robert Monsen Jan 1, 1970 0 Watson A.Name - Watt Sun said: Rather than use that monster 4.7 uF capacitor, it might be cheaper to use a small power transformer. About the only place I can remember seeing a cap that size is for a fan motor, and it would have to be paralleled to get that much capacitance. And it was about the same price as a power transformer, maybe$5 or so.

Among this list, the 7841 looks like it's a good choice; it's cheaper
than that monster capacitor, and safer because it isolates you from
the AC line voltage. When it is rectified and filtered, it will be
about 24VDC. But your relay will take less than 24V if it's DC

http://www.mpja.com/listitems.asp?dept=45

A transformer would also work here. However, you can get two 2.2uF 250 caps
for a buck at www.goldmine-elec.com and put them in parallel. That would
also work (albeit without the isolation)

Actually, I was thinking this was a DC relay, but the OP clearly stated its
a 24V AC relay, so... I'm not sure about the delays now. Aren't AC relays
just DC relays with a diode? If so, the circuit should still work as
advertised, since he measured DC current/resistance, and the circuit will
supply DC current. If they are something else, all bets are off.

Regards,
Bob Monsen

D

#### Da Man

Jan 1, 1970
0
Rather than use that monster 4.7 uF capacitor, it might be cheaper to
use a small power transformer. About the only place I can remember
seeing a cap that size is for a fan motor, and it would have to be
paralleled to get that much capacitance. And it was about the same
price as a power transformer, maybe \$5 or so.

Many film caps in that range are not that big. I have several 3.3 uF caps
rated for 250 or 350V, which are only about 1.5" by 0.5" by 1", and I saw
2.2 uF 250V caps that were about 1" by 1/4" by 1/2".

D

#### Da Man

Jan 1, 1970
0
A transformer would also work here. However, you can get two 2.2uF 250 caps
for a buck at www.goldmine-elec.com and put them in parallel. That would
also work (albeit without the isolation)

Actually, I was thinking this was a DC relay, but the OP clearly stated its
a 24V AC relay, so... I'm not sure about the delays now. Aren't AC relays
just DC relays with a diode? If so, the circuit should still work as
advertised, since he measured DC current/resistance, and the circuit will
supply DC current. If they are something else, all bets are off.

An AC relay is a DC relay with a shorted turn so that the current induced in
the shorted turn (remember, this iron core is like a transformer when
operated on AC), which keeps the magnetic flux around for a while while the
AC cycle changes direction. The plunger also weighs more in most cases, so
that it takes more time to accerate ftom the rest of the iron, helping
prevent the relay from buzzing. When operated from DC, they often turn on at
much lower voltages.

D

#### DaveC

Jan 1, 1970
0
Actually, I was thinking this was a DC relay, but the OP clearly stated its
a 24V AC relay, so... I'm not sure about the delays now. Aren't AC relays
just DC relays with a diode? If so, the circuit should still work as
advertised, since he measured DC current/resistance, and the circuit will
supply DC current. If they are something else, all bets are off.

None of these numbers were measured; they come from the mfr's spec sheet:

(BTW, I goofed on the coil resistance; it's 303, not 330 ohms.)

According to Fred (posted later in this thread), an AC coil has one coil
shorted.

Bob, how does this change your circuit values, if at all?

Thanks,

W

#### Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
None of these numbers were measured; they come from the mfr's spec sheet:

(BTW, I goofed on the coil resistance; it's 303, not 330 ohms.)

According to Fred (posted later in this thread), an AC coil has one coil
shorted.

Actually, more like a half turn. That's that D shaped piece of copper
that covers half of the end of the relay's pole.
Bob, how does this change your circuit values, if at all?

Thanks,

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

R

#### Robert Monsen

Jan 1, 1970
0
DaveC said:
None of these numbers were measured; they come from the mfr's spec sheet:

(BTW, I goofed on the coil resistance; it's 303, not 330 ohms.)

According to Fred (posted later in this thread), an AC coil has one coil
shorted.

Bob, how does this change your circuit values, if at all?

According to your OMRON tiff, the thing 'must operate' at 75% of the rated
voltage of 24V, so thats 18VAC. According to a different poster (Da Man) AC
relays will operate at even lower values when operated with DC voltage.

Post the DC voltage that operates the thing, and I'll tell you what values
to use to get it to operate after 2 seconds.

Regards,
Bob Monsen

D

#### DaveC

Jan 1, 1970
0
According to your OMRON tiff, the thing 'must operate' at 75% of the rated
voltage of 24V, so thats 18VAC. According to a different poster (Da Man) AC
relays will operate at even lower values when operated with DC voltage.

Post the DC voltage that operates the thing, and I'll tell you what values
to use to get it to operate after 2 seconds.

Bob,
Since it will work within a range of voltages, shouldn't we aim for one that
minimizes the size of components? Which would be max voltages, allowing for
smallest dropping components. (But which, I am reminded, means larger
components -- ie, working voltages -- for the others.)

Thanks,

R

#### Robert Monsen

Jan 1, 1970
0
DaveC said:
Bob,
Since it will work within a range of voltages, shouldn't we aim for one that
minimizes the size of components? Which would be max voltages, allowing for
smallest dropping components. (But which, I am reminded, means larger
components -- ie, working voltages -- for the others.)

If you really want a 2 second pause, you'll have to experiment with the
values. Since I don't know what voltage the relay will close at, I can't
choose the values for you.

If you are just looking for a delay near 2 seconds, then I'd guess (assuming
it'll close at 15VDC) 3.3uF, 10R, 4700uF, which are standard values. I'd
also put in a fuse for fire safety in case the 3.3uF cap fails.

I've updated the web page to reflect this and your new component values.

http://home.comcast.net/~rcmonsen/relay/relay.jpg

Regards,
Bob Monsen

D

#### DaveC

Jan 1, 1970
0

Thanks, Bob.

What's R1 (303 ohm) for? Is that the relay's coil? There was no R1 in
previous schematic...

Thanks,

R

#### Robert Monsen

Jan 1, 1970
0
DaveC said:
Thanks, Bob.

What's R1 (303 ohm) for? Is that the relay's coil? There was no R1 in
previous schematic...

Yes, its the relay coil's resistance. I used it to model the time delay.

Regards,
Bob Monsen

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