# Simplest electromechanical relay circuit

D

#### Don Kuenz

Jan 1, 1970
0
I construct electronics circuits in the same manner that I write
and then rinse and repeat until a full blown solution emerges.

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil and
a diode across the coil?

You might think that a simple Internet search ought to provide the
answer. It turns out that my question's too elementary or something.

Matter of fact, Internet searches provide less useful information with
each passing day. Search engines routinely ignore the very keywords
included in a query to winnow the results down. Search engines tend to
return the same useless information (sans keyword) repackaged by
websites that want to play the oracle and become everybody's universal
"go to" page it seems.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

R

#### RosemontCrest

Jan 1, 1970
0
[...]

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil and
a diode across the coil?

[...]

If the relay coil is rated for the voltage applied to it, there should
be no reason for a series resistor. If the relay coil is rated for less
than the voltage of your power supply, then choose a series resistor
such that the voltage applied to the relay coil is within its specified
rating.

P

#### Phil Hobbs

Jan 1, 1970
0
I construct electronics circuits in the same manner that I write
and then rinse and repeat until a full blown solution emerges.

Which will usually resemble a camel.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

D

#### Don Kuenz

Jan 1, 1970
0
RosemontCrest said:
[...]

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil and
a diode across the coil?

[...]

If the relay coil is rated for the voltage applied to it, there should
be no reason for a series resistor. If the relay coil is rated for less
than the voltage of your power supply, then choose a series resistor
such that the voltage applied to the relay coil is within its specified
rating.

suspected as much, given the length of the coil winding, I was unwilling
to gamble with one of my four relays (all of which are needed for my
project) to find out.

My bench power supply only goes up to 15 VDC. But, my intuition told me
that 15 VDC might just be enough to energize the coil to the point of
contact. This time my intuition was correct. 11 VDC is enough to close
the contacts, which then exhibit hysteresis and remain closed until the
voltage falls to 3.7 VDC.

My intuition also tells me that, for safety's sake, to place the relay
contacts into the (white) neutral side of the AC circuit instead of the
(black) hot side of the AC circuit.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

D

#### Don Y

Jan 1, 1970
0
Hi Don,

RosemontCrest said:
[...]

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil and
a diode across the coil?

If the relay coil is rated for the voltage applied to it, there should
be no reason for a series resistor. If the relay coil is rated for less
than the voltage of your power supply, then choose a series resistor
such that the voltage applied to the relay coil is within its specified
rating.

suspected as much, given the length of the coil winding, I was unwilling
to gamble with one of my four relays (all of which are needed for my
project) to find out.

My bench power supply only goes up to 15 VDC. But, my intuition told me
that 15 VDC might just be enough to energize the coil to the point of
contact. This time my intuition was correct. 11 VDC is enough to close
the contacts, which then exhibit hysteresis and remain closed until the
voltage falls to 3.7 VDC.

Chances are, you have a 12V coil.
My intuition also tells me that, for safety's sake, to place the relay
contacts into the (white) neutral side of the AC circuit instead of the
--^^^^^^^^

(black) hot side of the AC circuit.

Um, no. You want to switch the *hot* side, not the neutral.
Ditto with fuse placement, "power switch", etc. Always interrupt
the *hot*, not the neutral.

HTH,
--don

R

#### RosemontCrest

Jan 1, 1970
0
Hi Don,

RosemontCrest said:
On 12/22/2013 10:05 AM, Don Kuenz wrote:
[...]

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It
seems
intuitively wrong to simply connect 24VDC to the coil. It seems that
one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil
and
a diode across the coil?

If the relay coil is rated for the voltage applied to it, there should
be no reason for a series resistor. If the relay coil is rated for less
than the voltage of your power supply, then choose a series resistor
such that the voltage applied to the relay coil is within its specified
rating.

suspected as much, given the length of the coil winding, I was unwilling
to gamble with one of my four relays (all of which are needed for my
project) to find out.

My bench power supply only goes up to 15 VDC. But, my intuition told me
that 15 VDC might just be enough to energize the coil to the point of
contact. This time my intuition was correct. 11 VDC is enough to close
the contacts, which then exhibit hysteresis and remain closed until the
voltage falls to 3.7 VDC.

Chances are, you have a 12V coil.
My intuition also tells me that, for safety's sake, to place the relay
contacts into the (white) neutral side of the AC circuit instead of the
--^^^^^^^^

(black) hot side of the AC circuit.

Um, no. You want to switch the *hot* side, not the neutral.
Ditto with fuse placement, "power switch", etc. Always interrupt
the *hot*, not the neutral.

HTH,
--don

Don is correct. *Always* switch the hot leg; never the neutral leg.

P

#### piglet

Jan 1, 1970
0
burning up. Apparently a diode across the coil helps attenuate voltage
You don't really need a diode, either, if the switch is a mechanical pushbutton.

Or you could use a resistor instead of a diode across the coil. This would limit the transient peak voltage to the relay coil current (as it was just before turn off) multiplied by the resistor value plus of course the supplyvoltage. The disadvantage is the added power dissipation during the relay on time but the technique can be useful if polarity cannot be assured or for AC relay coils. For further refinement add a capacitor in series with theresistor, creating a classic RC snubber, then steady state relay on loss is reduced but the transient is still caught at the instant of turn-off.

Plenty of other solutions exist too.

D

#### Don Kuenz

Jan 1, 1970
0
The relay case clearly says 24V. It surprises me that a mere 11 VDC
energizes it.
Don is correct. *Always* switch the hot leg; never the neutral leg.

Of course! That rule of thumb is second nature whenever I replace wall
switches and outlets. AC power always spooks me. Something got lost (the
potential) when I downshifted back home to the low voltage electronics
domain where things are infinitely safer (for me).

Thank you for all of your great advice. It allows to start my project on
a sure foot without the nagging doubt of overlooking something basic.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

R

#### RosemontCrest

Jan 1, 1970
0
The relay case clearly says 24V. It surprises me that a mere 11 VDC
energizes it.

I should qualify that statement. Always switch at least the hot leg. If
you have a double-pole relay, switch both hot and neutral.

D

#### Don Kuenz

Jan 1, 1970
0
Jim Thompson said:
On Sun, 22 Dec 2013 18:05:15 +0000 (UTC), Don Kuenz

That's the simplest circuit _equivalent_ for analysis purposes. The
real-world relay winding has resistance that limits to current to its
nominal operating current.

The diode across the coil is to limit inductive voltage spikes when
the relay is switched _off_, needed to prevent switch pitting or
over-voltage with active-device switches. The diode orientation
should be such that it _doesn't_ conduct when the relay is
activated... it only functions to "capture" the inductive current when
that current can't flow thru the switch any longer.

Thank you for your practical advice. One or more of _The Art of Electronics_,
_The ARRL Handbook_, and _The Digital I/O Handbook_ (co-written by Jon
Titus if that name rings a bell) says as much. But, none of those good books
really get into the elementary aspects of relays.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

M

Jan 1, 1970
0

H

#### hamilton

Jan 1, 1970
0
I construct electronics circuits in the same manner that I write
and then rinse and repeat until a full blown solution emerges.

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.

So, is that the simplest circuit? A resistor in series with the coil and
a diode across the coil?

You might think that a simple Internet search ought to provide the
answer. It turns out that my question's too elementary or something.

Why are you re-engineering what has already been engineered ?

OK, I take it your new to electronics.

If you want to question the engineering of parts that have been on the
market for years, get an engineering degree and challenge those existing

IF your designs are better, you will take over the market.

It is good to verify those parts that may be questionable, but a relay
has been on the market for over 120 years.

You'd think they would have got it right by now.

On the other hand, people how don't read the specifications will have
problems.
Or those that do not have a reasonable engineering understanding, will
also have problems.
( does not necessary mean a degree, just a good/reasonable understanding )

Good Luck on your future projects.

hamilton

H

#### hamilton

Jan 1, 1970
0
The relay case clearly says 24V. It surprises me that a mere 11 VDC
energizes it.

This is where a basic/reasonable understand of electronic components
would be nice.

Then you would not have to ask such a simple question.

A relay do not just have contact parameters. ( Contact Voltage/Current )

There are also timing parameters.

The speed of contact closure is a parameter that is based on the voltage
of the coil.

Think old time relay dialing circuits.

hamilton

R

#### Robert Baer

Jan 1, 1970
0
Don said:
I construct electronics circuits in the same manner that I write
and then rinse and repeat until a full blown solution emerges.

For my purposes, the simplest electromechanical relay circuit uses a
push button switch to energize the coil and close the contacts. It seems
intuitively wrong to simply connect 24VDC to the coil. It seems that one
needs to insert at least a resistor in series to keep the coil from
burning up. Apparently a diode across the coil helps attenuate voltage
spikes.
* One CANNOT "burnup" a relay unless one applies at least twice voltage
rating for a goodly period of time.
24V across the coil of a 24V relay can be applied "forever".

U

#### [email protected]

Jan 1, 1970
0
My bench power supply only goes up to 15 VDC. But, my intuition told me
that 15 VDC might just be enough to energize the coil to the point of
contact. This time my intuition was correct. 11 VDC is enough to close
the contacts, which then exhibit hysteresis and remain closed until the
voltage falls to 3.7 VDC.

Good observation, most relay circuits consume far too much power in

With a suitable circuit, 24 V industrial relays can be reliably
operated from a 12 V car battery, provided that the switching cycle is
not too frequent to allow the capacitors in the circuit to recharge.

D

#### Don Kuenz

Jan 1, 1970
0
hamilton said:
Why are you re-engineering what has already been engineered ?

You lost me.
OK, I take it your new to electronics.

You might want to re-think your feelings. I earned an electronics
merit badge in Boy Scouts over fourty-five years ago. I started
designing my own electronic circuits in junior high school. I started
taking EE classes after high school and my first job was with a
company that manufactured electronic medical devices (brain scanners).

Now, don't you feel silly? (You ought to even if you don't.)

The only reason I'm troubling myself to school you is that you offered
up a good suggestion in another followup. You more or less said told me
to look at the datasheet. Excellent advice!

Too bad that this datasheet lacks an example circuit, ergo my post.
http://datasheet.octopart.com/RP710024-Tyco-Electronics-datasheet-12624042.pdf

I'm not afraid to ask questions. The only stupid question is the one
that remains unasked out of fear.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

R

#### RosemontCrest

Jan 1, 1970
0
You lost me.

You might want to re-think your feelings. I earned an electronics
merit badge in Boy Scouts over fourty-five years ago. I started
designing my own electronic circuits in junior high school. I started
taking EE classes after high school and my first job was with a
company that manufactured electronic medical devices (brain scanners).

Now, don't you feel silly? (You ought to even if you don't.)

A reply like that may likely result in no future help from this group.
The only reason I'm troubling myself to school you is that you offered
up a good suggestion in another followup. You more or less said told me
to look at the datasheet. Excellent advice!

Too bad that this datasheet lacks an example circuit, ergo my post.
http://datasheet.octopart.com/RP710024-Tyco-Electronics-datasheet-12624042.pdf

An example circuit for an electro-mechanical relay? Shirley, you jest.

D

#### Don Kuenz

Jan 1, 1970
0
RosemontCrest said:
On 12/22/2013 5:49 PM, Don Kuenz wrote:

A reply like that may likely result in no future help from this group.

Well, that'd be a pity because your earlier answer proved most useful.
Allow me to apologize for my hasty words. What can I say? hamilton
"got my goat." It happens.

--
__
__/ \
/ \__/
\__/ Don Kuenz
/ \__
\__/ \
\__/

R

#### RosemontCrest

Jan 1, 1970
0
Allow me to apologize for my hasty words.

Smart move. Good luck with your project.

D

#### Don Y

Jan 1, 1970
0
Hi Tim,

If getting the most speed out of the relay opening is important, there's
a whole science to designing the snubber circuit -- the diode is the
simplest, and also the slowest.

Even if you're controlling one relay from another, it's not a bad idea to
have some sort of a snubber, to prevent the inductive kick back from the
controlled coil causing sparking at the contacts of the controlling relay
and wearing them out.

Find an old electromechanical pinball machine and have a look at
what switching coils via relay contacts does to the contacts over
time! You'll be amazed at how much material migrates! And,
what a PITA it is to service! :<

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