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24VAC relays on square wave OK? 12VDC to 24VAC inverter calculations ?

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Eventhorizon

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Can someone help me with a basic AC current formula?
And a formula for step-up transformer ratios/power?

In DC, based on Ohms Law, Volts divided Resistance = Amps

and Volts divided by Amps = Res(Ohms)

What is the same equivalent for AC?

I have a relay where the coil is designed to work on 24VAC. Res is about 12 ohms.

24/12=2Amps in DC But this relay uses only a couple hundred mA as per my AC amp meter.

In AC current, if I know the Voltage and the Resistance how do I get the Amps using an exact formula?

And is this same formula the same, if it is a perfect sine wave versus a square wave?

I am trying to built a 12VDC to 24VAC inverter that will give me at least 1.5Amps.
Will settle for 1Amp.

Is an AC square wave ok for these 24VAC coils? The inverter would be much easier to build.

I understand, that with even 12VDC they work but the coil gets hot after a few seconds….and it seems that if less is used, say about 8VDC, its not enough juice for the coil.

I found the attached schematic to make a simple square wave inverter. Based on the transistors it will provide 1 Amp at 24VAC

How do I calculate the step-up transformer I need.
1)Coil ratio
2)Watts (heat dissipation) VA rating? (how thick the coil windings need to be)

Size is important because I am using a project box of a specific size, with a self contained battery to be handheld unit.
The size of the step-up transformer I believe is what will be my biggest enemy for what I want to accomplish.

Can anyone help me?
Any thoughts?
 

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Any relay coil has inductance. Inductive reactance (impedance, measured in Ohms) = X = Ldi/dt or at 60HzAC , X = 2Πf*L, where L is the Inductance of the coil in Henries. Note that R (the resistance you measure with a DC Ohmmeter) is nowhere to be found in this formulae.

When you apply 60Hz to the relay coil at its rated voltage, the current that flows is determined by the inductance of the coil; not by its DC resistance.
 
Oh,...thanks that makes absolute sense.

Actually, I was just reading about impedance in an AC circuit a few days ago, but I never coupled the thought with my question on my AC relay.

I was perplexed that Ohm's Law seemed to cease when I measured and tested my AC relay.

Thanks...made my weekend!!
 
There is no need to build a complicated inverter, the relay will work off DC but you need to limit the current because the inductance won't limit the current at DC.

Try connecting it to 12VDC via a suitable series resistor to limit the current to the desired level.

For example, if it takes 200mA from a 24VAC supply and it has a series resistance of 12R.

Use Ohm's law.
12/0.2 = 60R

The coil's resistance is 12R.
R = 60 - 12 = 48R so use the nearest standard value: 47R.

The resistor will need to be rated for at least 2W.
 
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A 24V AC coil relay should work just fine on ~8 to 12VDC.
 
12V the coil gets, hot, really hot after a few seconds...I have to disconect

and 8-9v not enough to activate coil well

tried 3 different relays, same thing
 
12V the coil gets, hot, really hot after a few seconds...I have to disconect

and 8-9v not enough to activate coil well

tried 3 different relays, same thing

Here is how to get a higher pull-in current, followed by a much smaller (1/4 the power) holding current to your relay coil, starting from 12V.
 

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A 24V AC coil relay should work just fine on ~8 to 12VDC.

I calculated the currents for a couple of relays from RS whe powered from 24VAC and 12VDC and I agree: the current though a 24VAC relay coil will be roughly the same when operated at 12VDC.

Tyco Electronics | Relays, Switches and Indicators | Relays | General Purpose Relay | Interface Plug In Relay - XT Series |XT314R24

Coil power: 0.76VA
Coil resistance: 350R

@24VAC
I = 0.76/24 = 31.7mA

@12VDC
I = 12/350= 34.3mA

Tyco Electronics | Relays, Switches and Indicators | Relays | Electromechanical Relays | RPII Series |RP420524

Coil power = 500mW
Coil resistance = 280R

@24VAC
I = √(P/R) = √(0.5/280) = 42.3mA

@12VDC
I = 12/280 = 42.9mA

I think the original poster made a mistake measuring the current, 200mA sounds to large unless it's a large relay.
 
12V the coil gets, hot, really hot after a few seconds...I have to disconect

and 8-9v not enough to activate coil well

tried 3 different relays, same thing
This might sound like a silly question, but when you power the relay with 24VAC does it get hot?

Is it possible for you to measure the temperatures for the two cases under a matched set of conditions? (e.g. for 60 seconds, 12VDC vs 24VAC with each relay beginning at 25c.)
 
Actually, mneary, that is a very good question.

The assumtion is my 3 "contactors" (these are Air Cond. Contacters) coils are truly 24VAC coils and are working as designed with no mal-functions, if not our entire premise to obtain any calculation will be completly off even if we are mathematically correct.

I will test all 3 coils under a true 24VAC supply from my AC unit which is working perfectly to see if they get hot.

I will also test the 3 coils directly with a good AC amp meter to get the mA....of each coil while energized.

Now, when I was taking about 200mA for one coil, there are 3 I am using for the tests, one with 5R, one with 12R and one with 80R on the meter reading just the basic resistance of the coils. (Not taking in consideration impedence)

The 200mA is for the 12R which I was speaking about, but like I said, I will remeasure....200mA might be off....I was measuring a few coils all at the same time and might have made a mistake.

To have a foundation, I'll stick with one coil!
The one which has 12 ohms of resistance and is the most commonly used. They are used in the AC Condenser units. (feeds a compressor and a fan)

I will also be reviewing the formulas given above, specifically applying them to a 24VAC coil with 12R.

Will post the mA tests in an hour!
 
Hey guys....I was posting to you a while ago and my computer got a really bad virus like I never had before.
McAfee did not catch it. None of my applications open now...but I have everything backed up and to-date...I do it almost daily!


Anyway I am posting from my laptop.

All tests under room temperature, and activating the coil for 1 minute.
Used a good muti-meter AC for AC and DC for DC measurement.


Under 24VAC

Coil R80 - 140mA
Coil R12 - 310mA
Coil R5 - 490mA

Coils energized perfectly for 1 min...Non got hot except R80 got a little warm


Under 12VDC

Coil R80 - 149mA
Coil R12 - 990mA
Coil R5 - 2670mA

Coils energized until they got to hot...R80 got hotter than in AC, R12 got very hot in seconds in AC, R5 I can't get around the thick plastic encapsulation of the coil to check temp.

Very strange...R80 doesn't obey the pattern....almost same Amp draw on AC and DC (Its a small relay used mainly for handler fan motor)
Maybe its by coincidence that a coil around 80 ohms intersects, for lack of a better word, the numeric balance of both a DC and AC formula coefficients. Is the only thing I can think of!




**********************************************************

I will use the 12 ohm coil only for all other tests and formulas which is about the ball-park in amp draw for a contactor/coil used to turn on a residential AC Condenser (Relay controls compressor and fan)
So now, building on the same foundation analyzing one component with fixed values.


Relay designed for 24VAC with a 12 ohm coil

Again,
Under 24VAC

Coil R12 - 310mA
Energized for 1 min. - practically no noticable heat dissipation.



Under 12VDC (with a good battery, plenty of Amps)

Coil R12 - 990mA
Could not energize for more than 10-15 seconds. Coil was getting very hot.....and rising!

The amps, though, were exactly where it should be under Ohms Law 24V/12R = 1Amp (1000mA)

I wish the calculation was this easy for AC!!!



Under 12VDC tried a 180R 10W resistor to reduce the current.....coil doesn't even click.

Tried a 10R for the hack of it, to my surprise, not even with 10 ohms the coil clicks.

I like the posted circuit, safety diode, cap to take the inrush of current and 5 ohm res to limit the current.
I don't have a 5R resistor but I am afraid that if with 10R my relay didn't even click...5R will not work very well either!

I guess there is a reason why entire industry segments have their relays working on AC and not DC.
 
10R is probably too high.

Coil R80 should be able to work find straight off a battery with no modification.

The other coils will need a series resistor which can be worked out using Ohm's law.

For example coil R5.

AC: 490mA
DC: 2670mA

We need a resistor to limit the current to the AC level.

Calculate the total resistance required for a current of 990mA with a voltage of 12V
R = 12/0.99 = 12.1R

Calculate the coil's DC resistance:
R = V/I = 12/2.67 = 4.5R

You need a series resistor equal to the difference between the above.
12.1 - 4.5 = 7.6R

So use a 7.5R resistor or two 15R resistors in parallel.

The actual current drawn is 12/(4.5+7.5) = 1A which shouldn't be enough to damage the coil.

Calculate the minimum power rating for the resistor:
P = I²R = 1²7.5 = 7.5W

Repeat the above procedure to calculate the required series resistor for the other relays.
 
Thank you for all your input on "direct 12VDC to relay"...I will try these suggestions!!

In the meantime I will continue my quest to build a small DC/AC inverter to 24VAC.

I'll start by trying the one I attached the schematic in the beggining of this thread.

I am trying to figure out the transformer needed. Need center tapped 12-0-12VAC in/24VAC out.


I believe the ratio is 1/2 but what is the VA or Amp rating (thickness of the wires) Need trans at 24VAC at 24Watts.

Does anyone know the caculation.....and/or what transformer?
 
It's not likely that the relay will like square waves any better than it liked DC. Half of the energy of a square wave is in the odd harmonics, and an AC relay is only expecting the frequency for which it was designed.

The transformer ratio has to take losses into account, such as the TIP120's which will take about 1.2V. The transformer will have its own losses which also must be paid up front.

If you really must try an inverter, look into "modified sine wave" (really modified square wave).. it uses simple logic to reduce the harmonics dramatically.
 
A better option is to use a switch mode power supply to boost 12V to 34V (the peak voltage of a 24V sine wave) and use an h-bridge to convert it to a modified sine wave.

That way, you only need a transformer, if the AC needs to be isolated from the DC supply, which in this case it doesn't.
 
Actually what both of you posted confirmed what I have been reading all over, I just didn't take to heart and was stuck on wanting to build a square wave inverter.

I understand the sq wave is not one freq. such as 60hz, but several frequencies bunched up together around a 60. I can see how the coil will not like that! To me that translates into more heat dissipation the coil has to handle.

I saw also in many texts, the mention of going from H-brigde to convert the wave.

I don't much about the h-bridge design but will study it.

I am going to go for what I really need and not settle.....a modified sine wave inverter.

The modified sine wave is not the one which needs a PWM circuit (Pulse Width Modulation) is it?

I believe only to get a pure sine wave that circuit is needed correct?
 
I am going to go for what I really need and not settle.....a modified sine wave inverter.

The modified sine wave is not the one which needs a PWM circuit (Pulse Width Modulation) is it?
No, that's a pure sine wave inverter.


I believe only to get a pure sine wave that circuit is needed correct?

No, DC will work like we've being saying all along.

And even if it doesn't, building/buying an inverter just to power a relay coil is silly because it will be cheaper just to buy a relay with the correct voltage rating in the first place.
 
...

No, DC will work like we've being saying all along.

And even if it doesn't, building/buying an inverter just to power a relay coil is silly because it will be cheaper just to buy a relay with the correct voltage rating in the first place.

Bingo! This is why I stopped posting to this thread about 20 posts ago ;)
 
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