What type switching relay do I need?

[Bill C]

Does anyone know the kind of switching relay that I should be looking for?
I need a 24vdc current from an irrigation controller to open a US standard
120vac circuit for a small bird bath water pump.

I don't know if I need single throw or double throw (or what that means), if
a 12dcv coil will accept a 24vdc current or if a relay designed to manage
220vac will open a circuit for 110ac. I also think that there are various
terminal configurations to relays and don't know what I should be asking
for.

Thank you for helping me with this. It's frustrating to recognize that
there's a simple and economical off the shelf solution to this problem, but
the people able to identify it are relatively rare.

Thanks,
Bill

 

[William P.N. Smith]

"Bill C" <newsgroups@REMOVETHISnobsys.net> wrote:

Does anyone know the kind of switching relay that I should be looking for?
I need a 24vdc current from an irrigation controller to open a US standard
120vac circuit for a small bird bath water pump.

Are you certain it's 12VDC and not 12VAC? Most irrigation stuff I've
seen has been 12VAC...

I don't know if I need single throw or double throw (or what that means), if
a 12dcv coil will accept a 24vdc current or if a relay designed to manage
220vac will open a circuit for 110ac. I also think that there are various
terminal configurations to relays and don't know what I should be asking
for.

You need a coil rated for very nearly your supply (24V{A,D}C), SPDT
contacts rated for 110VAC minimum with a minimum current rating of
whatever your birdbath pump is rated for. So I woudn't use a DC relay
coil on AC (or vice versa), don't use a 12V coil on 24V, but 220, 600,
anything above 120VAC for the relay contacts is fine.

You can goto http://www.digikey.com and type "relay" into the search
box to get a relay selector...

--
William Smith
ComputerSmiths Consulting, Inc. www.compusmiths.com

 

[Henry Kolesnik]

You need a normally open set of contacts. What most relays have is a
normally closed and normally open, so select the NO. On energizing the NO
contact will close and simultaneously open the NC contact. You could use a
12 volt DC rlay on 24VDC but you'll need a resistor in series to drop the
voltage. How big will depend on coil current. If the relay contacts are
rated for 220VAC it'll do just fine on 220VAC. Many home gas fired furnaces
use 24 volt relays but these are generally 24 VAC.
73
hank wd5jfr
"Bill C" <newsgroups@REMOVETHISnobsys.net> wrote in message
news:OzbRb.49513$nQ3.13963@bignews1.bellsouth.net...

Does anyone know the kind of switching relay that I should be looking for?
I need a 24vdc current from an irrigation controller to open a US standard
120vac circuit for a small bird bath water pump.

I don't know if I need single throw or double throw (or what that means),
if
a 12dcv coil will accept a 24vdc current or if a relay designed to manage
220vac will open a circuit for 110ac. I also think that there are various
terminal configurations to relays and don't know what I should be asking
for.

Thank you for helping me with this. It's frustrating to recognize that
there's a simple and economical off the shelf solution to this problem,
but
the people able to identify it are relatively rare.

Thanks,
Bill

 

[Tim Wescott]

I hesitated to answer this, because switch types really are fairly well
known, and 110VAC can be lethal. I'm trusting you to not kill yourself or
your children, or to burn your house down. Please make _sure_ that you are
comfortable with what you are doing, both for personal and fire safety. Ask
among your friends, neighbors, coworkers and relatives to see if there's an
electronics whiz to help you.

If you really want to do this you may want to get your hands on a Popular
Electronics magazine or something similar, and look for books on the
subject.

You may also consider that if you have homeowners insurance and the system
causes any problem resulting in a claim -- even if it is unrelated to your
modifications -- _you_ will be on trial for _your_ design decisions, and the
burden of proof that your modifications didn't cause the problem will be
_yours_. You will be grilled for your knowledge of electric circuits, and
made to justify everything you did. This will be done by a person who has
the expertise in legal examination to make _me_ look bad, and I'm almost an
expert. Your lawyer will want to hire a professional engineer to analyze
what you did, and said PE will charge enough to buy a new, shrink-wrapped,
UL approved, sprinkler-company-supplied relay every hour or less. Your
lawyer will want more.

If there is a little box on the market already that's UL approved for 24V in
and switched 110VAC out it may be cheap insurance to just buy it and be
happy.

--- Enough disclaimer, already ---

Single throw means that it's just an on-off switch, double-throw means that
there's a center contact that will connect to one "end contact" or the other
depending on whether the relay is energized or not. You'll also run into
the term "pole", as in "single pole" or "double pole" switch; this is just
the number of identical switches that are moved by the same lever (or relay
coil). These will also be abbrieviated: SPST (single-pole, single-throw),
DPST, etc.

A switch that turns on the lights in your house is a single-pole,
single-throw switch. A "three-way" switch like you may have in a long hall
or a basement stair, where either switch will turn on the light is a DPDT
switch in a clever arrangement.

Putting 24VDC on a 12VDC relay coil will run a serious risk of burning out
the relay. You'll be asking it to dissipate 4 times the power that it's
used to. You can do one of three things: Get a 24V relay, wire the coils
of two _identical_ 12V relays in series so they'll share the voltage, or
find the resistance of the 12V relay coil (by measurement or by checking
it's specs) and wire a resistor of the same resistance and adiquate power
dissipation in series. The power in the resistor will be 12 volts squared
(144) divided by the resistance, so a 150-ohm coil will require a one-watt
resistor.

Make sure that the relay coil current (voltage divided by resistance) isn't
more than the irrigation controller can supply.

A relay designed for 220VAC will switch 110VAC safely -- switch contacts of
any kind are usually rated for their maximum working voltage, and it's only
when they get enormous that you need to worry about minimum voltages.
Normally I'd tell you to check the current rating as well, but for a bird
bath you're OK (unless you view the Niagra Falls as a mere bird bath, in
which case you need professional help of an entirely different kind than I
offer). To at least pretend to meet wiring standards, though, the relay
contacts should probably be rated to carry at least the current that the
circuit is fused for (you could put your own fuse in the box, however).

Depending on whether you need to have the relay contacts open or closed when
your 24V is on you either need NO (normally open) or NC (normally closed)
contacts. You probably want to get a SPDT relay, which will have both, and
you can choose when you try it out.

Put it all in a metal box that's well grounded to your wiring system. Make
sure to switch the power side of your wiring and not the neutral. Make sure
the relay contacts are rated for the current that the line is _fused_ for,
_not_ what the pump draws. Think about all the ways that the system could
go wrong, and make no one will get hurt as a result (being irritated is OK,
having to call 911 is not).

"Bill C" <newsgroups@REMOVETHISnobsys.net> wrote in message
news:OzbRb.49513$nQ3.13963@bignews1.bellsouth.net...

Does anyone know the kind of switching relay that I should be looking for?
I need a 24vdc current from an irrigation controller to open a US standard
120vac circuit for a small bird bath water pump.

I don't know if I need single throw or double throw (or what that means),
if
a 12dcv coil will accept a 24vdc current or if a relay designed to manage
220vac will open a circuit for 110ac. I also think that there are various
terminal configurations to relays and don't know what I should be asking
for.

Thank you for helping me with this. It's frustrating to recognize that
there's a simple and economical off the shelf solution to this problem,
but
the people able to identify it are relatively rare.

Thanks,
Bill

 

[Davis Redding]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:101al1353i10qb1@corp.supernews.com...

I hesitated to answer this, because switch types really are fairly well
known, and 110VAC can be lethal. I'm trusting you to not kill yourself or
your children, or to burn your house down. Please make _sure_ that you are
comfortable with what you are doing, both for personal and fire safety. Ask
among your friends, neighbors, coworkers and relatives to see if there's an
electronics whiz to help you.
go wrong, and make no one will get hurt as a result (being irritated is OK,
having to call 911 is not).

I happen to agree with Tim, since the devices are UL approved, your modification will not
be. You must also make certain that you switch the HOT side of the line, not the neutral
(on the HV AC)

You can buy a premaid device just like that from an electrical store.

Instead of a relay, ask for a contactor, have them put it into a weatherproof box with
appropriate SJ cable in and out with a plug end, and apply your low voltage to the
contactor to switch it.

Voila.

 

[William P.N. Smith]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote:

The power in the resistor will be 12 volts squared
(144) divided by the resistance, so a 150-ohm coil will require a one-watt
resistor.

Or will require a resistor that will dissipate one watt, so use a 2W
resistor to keep it from overheating.

--
William Smith
ComputerSmiths Consulting, Inc. www.compusmiths.com

 

[Tim Wescott]

Yes, of course. I _thought_ this real hard.

Unless you like your resistors with that nice gradiated tan-to-brown color
scheme that they get from living on the edge. Then go ahead and run it
right at it's rated dissipation.

<William P.N. Smith> wrote in message
news:k12b10h687i8u7kfu3adfsqijomap320cl@4ax.com...

"Tim Wescott" <tim@wescottnospamdesign.com> wrote:
The power in the resistor will be 12 volts squared
(144) divided by the resistance, so a 150-ohm coil will require a
one-watt
resistor.

Or will require a resistor that will dissipate one watt, so use a 2W
resistor to keep it from overheating.

--
William Smith
ComputerSmiths Consulting, Inc. www.compusmiths.com

 

[Bill C]

You're right, the irrigation controller puts out 24v AC, not DC. I might
have made a real mess if you didn't catch that.

I called the http://www.digikey.com people, real nice. They recommended
this:

http://oeiwcs.omron.com/webapp/commerce/command/ExecMacro/Omron/macros/itemdisp.d2w/report?prmenbr=316&prrfnbr=4161

Thanks for the help!

Bill


<William P.N. Smith> wrote in message
news:htia10traiafp0dofjraeaka1oe3t112l3@4ax.com...

"Bill C" <newsgroups@REMOVETHISnobsys.net> wrote:
Does anyone know the kind of switching relay that I should be looking
for?
I need a 24vdc current from an irrigation controller to open a US
standard
120vac circuit for a small bird bath water pump.

Are you certain it's 12VDC and not 12VAC? Most irrigation stuff I've
seen has been 12VAC...

I don't know if I need single throw or double throw (or what that means),
if
a 12dcv coil will accept a 24vdc current or if a relay designed to manage
220vac will open a circuit for 110ac. I also think that there are
various
terminal configurations to relays and don't know what I should be asking
for.

You need a coil rated for very nearly your supply (24V{A,D}C), SPDT
contacts rated for 110VAC minimum with a minimum current rating of
whatever your birdbath pump is rated for. So I woudn't use a DC relay
coil on AC (or vice versa), don't use a 12V coil on 24V, but 220, 600,
anything above 120VAC for the relay contacts is fine.

You can goto http://www.digikey.com and type "relay" into the search
box to get a relay selector...

--
William Smith
ComputerSmiths Consulting, Inc. www.compusmiths.com

 

[Bill C]

You're right, the irrigation controller puts out 24v AC, not DC. Thanks! I
got DC off some component specs on the net somewhere and ran with it

Thanks for your help!
Bill

Many home gas fired furnaces
use 24 volt relays but these are generally 24 VAC.

 

[Bill C]

Thanks for the warnings Tim, and all the advice.

17 years ago I helped install custom lighting for a couple of summers, and
have done a lot of rewiring on my house since so I'm fairly comfortable,
excluding all the shocks. (Turning off power was not always practical on
complex client remodels.)

And thanks for the reminder not to switch the neutral. I'll be safe. Having
a concrete block house with concrete roof helps. But like you say, burning
down the
house is not the only danger.

I was wrong about control voltage. Its 24v ac, not dc

I'm ordering this:

http://oeiwcs.omron.com/webapp/commerce/command/ExecMacro/Omron/macros/itemdisp.d2w/report?prmenbr=316&prrfnbr=4161

Thanks for all your help!

Bill



"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:101al1353i10qb1@corp.supernews.com...

I hesitated to answer this, because switch types really are fairly well
known, and 110VAC can be lethal. I'm trusting you to not kill yourself or
your children, or to burn your house down. Please make _sure_ that you
are
comfortable with what you are doing, both for personal and fire safety.
Ask
among your friends, neighbors, coworkers and relatives to see if there's
an
electronics whiz to help you.

If you really want to do this you may want to get your hands on a Popular
Electronics magazine or something similar, and look for books on the
subject.

You may also consider that if you have homeowners insurance and the system
causes any problem resulting in a claim -- even if it is unrelated to your
modifications -- _you_ will be on trial for _your_ design decisions, and
the
burden of proof that your modifications didn't cause the problem will be
_yours_. You will be grilled for your knowledge of electric circuits, and
made to justify everything you did. This will be done by a person who has
the expertise in legal examination to make _me_ look bad, and I'm almost
an
expert. Your lawyer will want to hire a professional engineer to analyze
what you did, and said PE will charge enough to buy a new, shrink-wrapped,
UL approved, sprinkler-company-supplied relay every hour or less. Your
lawyer will want more.

If there is a little box on the market already that's UL approved for 24V
in
and switched 110VAC out it may be cheap insurance to just buy it and be
happy.

--- Enough disclaimer, already ---

Single throw means that it's just an on-off switch, double-throw means
that
there's a center contact that will connect to one "end contact" or the
other
depending on whether the relay is energized or not. You'll also run into
the term "pole", as in "single pole" or "double pole" switch; this is just
the number of identical switches that are moved by the same lever (or
relay
coil). These will also be abbrieviated: SPST (single-pole, single-throw),
DPST, etc.

A switch that turns on the lights in your house is a single-pole,
single-throw switch. A "three-way" switch like you may have in a long
hall
or a basement stair, where either switch will turn on the light is a DPDT
switch in a clever arrangement.

Putting 24VDC on a 12VDC relay coil will run a serious risk of burning out
the relay. You'll be asking it to dissipate 4 times the power that it's
used to. You can do one of three things: Get a 24V relay, wire the coils
of two _identical_ 12V relays in series so they'll share the voltage, or
find the resistance of the 12V relay coil (by measurement or by checking
it's specs) and wire a resistor of the same resistance and adiquate power
dissipation in series. The power in the resistor will be 12 volts squared
(144) divided by the resistance, so a 150-ohm coil will require a one-watt
resistor.

Make sure that the relay coil current (voltage divided by resistance)
isn't
more than the irrigation controller can supply.

A relay designed for 220VAC will switch 110VAC safely -- switch contacts
of
any kind are usually rated for their maximum working voltage, and it's
only
when they get enormous that you need to worry about minimum voltages.
Normally I'd tell you to check the current rating as well, but for a bird
bath you're OK (unless you view the Niagra Falls as a mere bird bath, in
which case you need professional help of an entirely different kind than I
offer). To at least pretend to meet wiring standards, though, the relay
contacts should probably be rated to carry at least the current that the
circuit is fused for (you could put your own fuse in the box, however).

Depending on whether you need to have the relay contacts open or closed
when
your 24V is on you either need NO (normally open) or NC (normally closed)
contacts. You probably want to get a SPDT relay, which will have both,
and
you can choose when you try it out.

Put it all in a metal box that's well grounded to your wiring system.
Make
sure to switch the power side of your wiring and not the neutral. Make
sure
the relay contacts are rated for the current that the line is _fused_ for,
_not_ what the pump draws. Think about all the ways that the system could
go wrong, and make no one will get hurt as a result (being irritated is
OK,
having to call 911 is not).

"Bill C" <newsgroups@REMOVETHISnobsys.net> wrote in message
news:OzbRb.49513$nQ3.13963@bignews1.bellsouth.net...
Does anyone know the kind of switching relay that I should be looking
for?
I need a 24vdc current from an irrigation controller to open a US
standard
120vac circuit for a small bird bath water pump.

I don't know if I need single throw or double throw (or what that
means),
if
a 12dcv coil will accept a 24vdc current or if a relay designed to
manage
220vac will open a circuit for 110ac. I also think that there are
various
terminal configurations to relays and don't know what I should be asking
for.

Thank you for helping me with this. It's frustrating to recognize that
there's a simple and economical off the shelf solution to this problem,
but
the people able to identify it are relatively rare.

Thanks,
Bill

 

[Bill C]

I think a consumer product like this where homeowners could simply plug
appliances into it and have them controlled by an irrigation controller
would sell. There must be tens of thousands of people who'd like to run
lighting,
fountains, fans or whatever from theirs.

I haven't checked with an electrical store yet, but tech support at three
electronic component places and 3 irrigation suppliers weren't aware of it.

Any entrepreneurs out there?

Thanks for your help!
Bill

> You can buy a premaid device just like that from an electrical store.