Based on my research so far, I've concluded that a relatively low-cost, yet reasonably reliable way to control operation (on/off) a motorized water pump, is a well spec'd EM-relay with flyback diode. I might also put in an opto-coupler for isolation, if that improves safety. The whole thing would be assembled on a general-purpose phenolic veroboard, with point-to-point connectors.

Given the pump motor's specs, I wanted help checking if the selected relay would fit the bill, or should I look for another.

The pump's specs (the relevant parts):

Self-priming 1.0HP mono-block pump
Operating voltage: 220VAC +/- 6%, single-phase 50Hz
kW / HP: 0.75 / 1.00
Current: 5A
Cap. run: 20uF 440V

The operating characteristics are about 100 on/off cycles per month, with each "on"-cycle being about 60mins long. AC power quality is not that great, with voltage fluctuating 10-12% around 220V, and line frequency fluctuating by 6-7% (as told by a local friend, who is knowledgeable in power electronics).

The EM-relay specs:

General-Purpose 1Pole Heavy Load SPST-NO Power Relay
Coil Voltage: 12V
Coil Resistance: 185 Ohm
Contact current: 25A
Switching voltage: 250VAC
  Resistive-Load: 100 x 10^3 operations
  Motor-Load: Min. 200 x 10^3 operations, given 
                     (250VAC inrush 80A, cosφ=0.7, cut off 20A, cosφ=0.9)

Does that sound good enough ? The above mentioned relay has 3 variants -- Standard, High Isolation, High Current. For the moment, only 'Standard' one is readily available. Would that suffice ? Just curious, what might be the other 2 variants good for.

A surprising observations was that the EM-relay has longer life with motor-load, than resistive-load. Is that normal ?

Also, the readily available part is a "PCB mount" relay. Is that acceptable, or should I really try to hunt for the socket-mount / tab-terminal kinds ?

Finally, is it fair to assume that I do not need any kind of "snubber circuitry". PS, I understand very little of "snubber" though, but it has appeared few times in the material I've read so far.

Edit: Signs of aging, and some carelessness. Had asked this question exactly a year back, when I had just began working with relays, and forgot all about Russell's excellent answer. It does cover many of the questions I re-asked here. Thought of linking this together.

  • 1
    \$\begingroup\$ Why unnecessary complicate a simple decision? Why do you need to develop a PCB for the relay unless you are not mass producing it? \$\endgroup\$ Commented Jan 23, 2013 at 7:46
  • \$\begingroup\$ Thanks for stopping by @Chetan. Like I wrote - "...whole thing would be assembled on a general-purpose phenolic veroboard". I am not designing a PCB of the relay. I am certainly not mass-producing, but a small cache for family/friends. \$\endgroup\$
    – bdutta74
    Commented Jan 23, 2013 at 7:49
  • 1
    \$\begingroup\$ @GustavoLitovsky, the 'Life' parameter in the specs share, says 200x10^3 operations. At 1200 cycle/year, at least theoretically, I infer that I could run this for 166years :-) ! Unless of course, I am not reading the specs right. \$\endgroup\$
    – bdutta74
    Commented Jan 23, 2013 at 8:04
  • 1
    \$\begingroup\$ +1 for a well-researched question. To answer your question: That part seems fine. I would add a snubber diode, it doesn't hurt to be safe. They are quite simple really - when you remove power from the coil, the magnetic field collapses, this has the same effect as waving a magnet near a coil - it generates electricity. In this case, the collapsing field generates a spike of reverse-voltage in the coil (like how a car ignition coil works to make sparks). The diode short-circuits this reversed voltage, protecting the rest of your circuit which may be damaged by spikes/reversed voltage. \$\endgroup\$
    – John U
    Commented Jan 23, 2013 at 9:10
  • 1
    \$\begingroup\$ Thanks @JohnU. I found that a snubber diode is probably same as (or quite similar to) a flyback diode, which I was indeed planning to add. However, while researching snubber vs flyback, I came accross this thread on edaboard, which suggests using a zener diode with a forward voltage that is 2x of the voltage used to drive the relay coil, if I understood is correctly. \$\endgroup\$
    – bdutta74
    Commented Jan 23, 2013 at 12:04

3 Answers 3


An electromechanical relay will work. Hey, they have been used for a looooong time. However, a solid state relay would be my choice for a quick solution. You don't have any contacts to corrode, arc, etc., and degrade. Using an electro-mechanical solution is not optimal... You seldom see an electro-mechanical relay in modern electronics.

  • 1
    \$\begingroup\$ This doesn't answer the question which is whether the specifications given will be suitable for a particular application. \$\endgroup\$
    – David
    Commented Aug 21, 2014 at 7:02
  • \$\begingroup\$ Sorry to disagree, but I see an awful lot of relays in modern appliances. \$\endgroup\$ Commented Feb 19, 2015 at 6:25

Your proposed relay looks fine to me.

The "high isolation" variants can be useful for applications where a specific level of galvanic isolation is required (normative requirements for example). That the "high current" relays are designed for higher currents should be obvious. From what i can tell, those are just slightly more expensive but have no drawbacks in regard to the "standard" variant.

As far as i know a motor load can cause a high inrush current, while the circuit breaking behavior is much the same as a resistive load. So i don't know why the relays should be able to perform more switching operations on motor load than on resistive. This was based on superficial knowledge, and after giving it some more thought it probably isn't correct. Google did not help me to understand it in detail, since there are many different scenarios. Maybe someone can improve the answer.

One should pay attention when breaking an highly inductive load, since that will generally produce heavy arcing.

I can't help you regarding the snubber circuit.

  • \$\begingroup\$ Why would a motor, i.e. an inductive load, have high inrush current, and have breaking behavior similar to resistive loads? That combination seems more apt for a capacitive load. \$\endgroup\$ Commented Jan 23, 2013 at 12:45
  • \$\begingroup\$ But isn't a motor winding seen as resistive at the instant its starting to turn? As the motors windings resistance is usually quite low the starting current is high. I have to think about the "breaking behavior" though. \$\endgroup\$
    – Rev
    Commented Jan 23, 2013 at 14:28
  • \$\begingroup\$ An inductor works against a change of current through it. At time of turn-on, it would therefore work against current increasing through the coil. \$\endgroup\$ Commented Jan 23, 2013 at 14:58
  • \$\begingroup\$ After some digging on Google i haven't found any conclusive information to correct/improve my answer, so I edited my post accordingly to point out that it maybe wrong. \$\endgroup\$
    – Rev
    Commented Jan 23, 2013 at 15:49
  • \$\begingroup\$ Induction motors require a great deal of startup current, partly because of their "start winding". Six times running current is pretty typical, with some going as high as fifty times running current if they're started at near-stall. \$\endgroup\$ Commented Aug 21, 2014 at 6:09

I am using EM relay ( 10 Amp. 250 ohm. SPST ) for the last 2 years. It is working well. I use it daily about 1 hour to run 1 HP Pump.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.