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choosing relay coil voltage rating specifications

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gary36

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Hi
Just searching for a relay and came up with lot of varieties for coil voltages (from 5V to 110VDC/AC). How to choose relay coil voltage based on applications?
 

Hi,

I really don´t understand the question.

If you want to run the coil in a car with 12V DC supply, then choose a 12V DC coil.
If you want to run the coil from mains 230V AC, then choose a 230AC coil.

Klaus
 

Suppose I have choice to choose between 5V, 12 V and 24V coil voltage, what should I select and why?
 

Hi,

Is the coil fixed connected to one of those supplies? or is the coil operation controlled somehow. If so, then why, how, what application...
Please give complete informations, not piece by piece.

Klaus
 

Ok. I have to operate the relay through micocontroller + relay driver. The choice has to be between 5V,12V or 24V relay. For 12 and 24V option I have to add a boost converter.
The question is specific and in general. Why not choose only 5V relay as most of the embedded applications run on 5V/3.3V supply?
 

Hi,

Why not choose only 5V relay as most of the embedded applications run on 5V/3.3V supply?
Click to expand...
Yes, I ask myself. Why don´t you choose it? It´s the straight forward solution.

Why bother?
Why do you consider extra effort for the boost converter?

***
It should be a quite simple answer.
Sorry for getting impatient.

Before I thought you have 5V, 12V and 24V available. (choose your relay about driver capability, price (including driver), availability...)
But now it seems you just have 5V available. (I see no benefit in stepping up the 5V to any other voltage and drive a high voltage coil. Maybe you hide informations)
I´m missing any relay specification (like coil power)
I´m missing any relay driver specification.
I´m missing your thoughts / doubts / ideas

Klaus
 

Hi KlaussT

Hmm, I have not hidden anything. The question was pretty straight forward. I have seen many embedded applications (on the web) which drive a 24V relay(coil) from a 5V microcontroller. I guess the power requirements are optimal. 5V relay seems to be power hungry. I don't see any other differences and if there are, I would greatly appreciate your wisdom on this aspect.
 

In general, a higher voltage coil has higher resistance and therefore draws less current. It depends very much on what the relay has to do though. One switching very high power may need considerable coil energy to move it's contacts so you could have a high voltage and high current requirement.

For most applications, you choose the most appropriate coil voltage according to what is available on the board. When you see 5V signals switching higher voltage relays, it is normally because a relay coil is relatively tolerant of the voltage you apply so in most cases the voltage doesn't need regulating. On the other hand, the microcontroller is critical of voltage so it DOES need regulating. Rather than increase the size of the regulator so it can also handle the relay current as well as the MCU, it is usually cheaper to feed the relay through a driver (maybe a transistor) from the UNregulated side of the supply where the voltage is higher.

Brian.
 
Last edited:

A relay coil needs a certain amount of watts so it can generate sufficient magnetic force to pull in the contacts. As you lower the voltage, you need more mA. So there are tradeoffs to be made. It is possible to wind the coil with more wire to increase sensitivity, but this also adds bulk and expense to the device.
 

Hi,

This is what I mean: Now you are concerened about power, but you never mentioned this before. Tell us your ideas.
--> Relay coil power: I don´t think a 5V coil power and a 24V coil power differs that much (with the same relay type). --> Read/Show a datasheet.
For sure it´s not a problem of the relay, it surely is a problem of your application, or price, availability....

I have seen many embedded applications (on the web) which drive a 24V relay(coil) from a 5V microcontroller.
Click to expand...
Reasons for driving a 24V from a 24V supply
* maybe 24V is already available
* maybe they have 24V relays on stock, but not 5V relays
* maybe they try to keep 5V power supply low power
* maybe they run sensible 5V analog circuit and try to keep 5V clean.

--> thus we need to know your concerns / requirements.

Klaus
 

As well as the current draw, there's also pull-in and drop-out voltages to consider. A 12 V relay will have enough margin to operate on '12 V rail minus driver drop' of a suitably rated and correctly driven Darlington or MOSFET.

A 5 Volt relay on a 5 V rail may struggle. I've seen ingenious circuits, kin to 'Joule Thieves', that charge an electrolytic capacitor and use that to boost the 'pull in' phase. IMHO, such are a last resort...

Hence driver chips which take logic levels and handle a dozen volts. Their outputs may even be paralleled with due regard to whole-chip dissipation.

Hence opto-isolators to separate your logic-level zone from the power zone, happily driving a duly rated Darlington or MOSFET to operate an eg 12 V relay / rail system. One gotcha is speed, as too-simple drivers may be too slow, dangerously increasing switching dissipation.

Um, don't forget an appropriate diode for catching the relay coil's Back EMF. 'Body diodes' have their limits...
 

Hi Nik

Should we seperate 24V ground and 5V ground? Why?
 

Hi ! If you use a logic-level-to-power driver chip such as a ULN2003 or related, it needs a common ground. Check their data sheets.

Opto-isolate a power Darlington or MOSFET for complete independence.
But watch switching speed of any real-simple circuit is not impaired due eg MOSFET's gate capacitance. You do not want it to linger in 'linear' region...
 

You were over thinking it. It’s quite straightforward for the relay manufacturers to provide multiple voltages so they do - to help you. There aren’t many differences in terms of relay performance. You’ll see the watts drawn is more or less constant across the different options.
 

Hello gary36,
For the life of me, I cannot understand why you're even contemplating using any other relay
(i.e. 12V, 24V, 110AC or 240AC) to operate what ever switching you need done.
If your question had to do with trying to decide whether to use a solid-state instead of a coiled
relay, then I could understand your concern.
In any circuit that anyone designs, MOST will opt to use a relay that gels with their main supply.
i.e. In your case: 5V
The real important thing in deciding what relay to use is the voltage/current that requires switching.
Whether you use a couple in a speaker protection circuit, switching mains or just switching in another
part of a circuit for its operation, just a basic one that covers all basic requirements is all you need:
Coil energizing voltage and current and switching voltage and current.
Lastly, provided you insert an appropriate diode to quench the back EMF of the coil, there should not be
any other concerns what-so-ever.
Regards,
Relayer
 

Hi,

The relay is built to safely isolate coil signal from switching signal.
There generally is no need for isolating 24V, there generally is no need to use 24V at all.
For sure you need to choose the proper relay for your application and you need to know safety regulations to design a correct PCB....
With correct clearance and creepage distances.

This all depends on your and your application's requirements.
For some applications it makes sense to use 24V (already answered before) ...
maybe one even can find a reason to isolate 24V...

Klaus
 

Hello Gary

just in simple terms if it comes we are using the voltage for switching purpose and if you are using high tech motors
then we required higher voltage and relay of good quality will be use, so as per me if you are using 5v relays then it will much helpful for the device which is easy to power up with 5v only and if you use the devices which uses 12 v or more than that will not be useful to power up with 5v relay or any damage will be caused

I am using these kind of relay https://store.ncd.io/product/2-channel-solid-state-relay-controller-6-gpio-with-i2c-interface/ to power up my water pump but if i think to use 5v relay with water pump will not successful. Not only that there are many relay which are like SPST, SPDT, DPDT for which should suggest to watch the video which will help you clear your doubts
 

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