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

[Hero999]

I was continuing for educational purposes and had no intention of recommending an inverter.

Here's the cheapest pure sine inverter I could find.
**broken link removed**

You then have to add on the cost of a transformer to convert the 230V back down to 24VAC - you're not going to find a 12VDC to 24VAC inverter and if you do it'll be more expensive than a 230V inverter and a transformer.
**broken link removed**

You're looking at £103.28 or $157.40 (US).

How much do the relays cost?

 

[Eventhorizon]

The relays won't cost me anything because I don't need them.
I don't need to buy relays, the relays will exist at whatever device I am trouble shooting with my tool. The relays I mentioned are only target test relays.

I answers I seek, and to go through all this to feed a AC relay might sound "silly" but thats because the final purpose of this circuit is not known to anyone but me.

I am creating a tool to achieve certain dignostic functions and this tool HAS TO BE Handheld, Portable, as small as possible...to hold with one hand.

My greatest enemy is size for what I am trying to do.

Of course there are dozens of ways of doing this if size is not a condsideration, I can buy store inverters, modify them, or... I have seen an inverter with pure sine wave with a 24VAC output for 100 bucks, but bulky and would defeat the my purpose.

Thats why I love the world of electronics, you can make your own customized..... invent, change, expand, use your imagination until your vision is created.

I know a small inverter can be built, I have seen the different possible circuits blocks, I will just have to figure how to put it all together for my purpose.


The transformer is what is the killer, but doable for I only need 20-25Watts of output on 24VAC......if I go that route...but I believe I will try to go transformerless first.

I have heard that there are ways to go transformerless.....feed the whole thing with 38DCV (Which is the peak of 24VAC RMS, isolate this high voltage from whatever I will use as an oscillator, and not need to step it up at the end. (light and powerful battery power is not a problem to me)

I just have to figure out a good circuit to shape the wave to a nice sine wave...and everything thing else the circuit needs in between.

No, its not easy I know, and one can't just whip up a circuit like this.....but I will set up my mind to research until I can put all this circuit blocks together in perfect harmony!!!!

And when done...I will post the final, tested circuit on this thread and it will be up for grabs.

Thanks for everyones input, and bearing with me. I truly have learned a lot.

 

[Hero999]

A pure sine inverter will be much harder to make than a modified sinewave.

You need a motor controller h-bridge IC.

It's possible to generate the modified sine shape using a couple of logic ICs.
https://www.aaroncake.net/forum/uploaded/Audioguru/200885121135_modified Sine-wave Inverter.PNG

A pure sine inverter needs PWM.

You shouldn't need a transformer, if the output doesn't need to be isolated from the input.

https://www.electro-tech-online.com/custompdfs/2010/04/LMD18200.pdf

As I said before, a boost converter is needed to get a higher voltage than 34V, to make up for the losses in the switching transistors, 35V or 36V will probably do.

Do you want different voltages?

The voltage booster could be used to power other relays by adjusting the output votlage i.e. 24VC, 48VDC as well as for the AC supply.

You could use a SEPIC converter so you can have lower voltages too.

 

[Eventhorizon]

I just need one output voltage for my application..24VAC

Input / Output does not need to be isolated.

Great resource, thank you for those links...now I think I am cooking with oil.

It is still somewhat difficult for me to put it all together...but I'll keep on studing and researching.
That documention of the IC from National has some great application samples.

What are the 2 specific Mosfets used in that circuit? All other component values are described in detail.

Also, excuse my ignorance, I don't recognize the schematic symbol located between the two Mosfets. One terminal connects to both mosfets, arrow coming out of the center of the circle pointing down is to ground.....what is the actual "small circle"?

 

[Hero999]

The MOSFETs are part of the IC.

You need an LC filter on the output of the bridge to filter out the harmonics leaving a pure sine wave.

What you are building is a class D amplifier. High quality class D amplifiers are uses for audio but yours doesn't have to be so good. Lots of information on class D amplifiers can be found on Google, Wikipedia etc.

 

[Eventhorizon]

I see, the Mosfets are the LMD18200 IC.....different than the ones used on the schematic, though, based on the pins available.

I'll have to chew on this for a while....

 

[Ubergeek63]

relays are current driven... forget what the voltages say...

there is a pull in current, a holding current and a coil resistance. lowest power consumption would be achieved chopping your 24V such that the holding current is maintained after a short full power pulse to pull in the armature.

 

[Eventhorizon]

A circuit such as post #9 of this thread?

By the way, I don't know what was meant just above that "the Mosfet's are part of the IC" IC's are IC's and Mosfets are fancy, very effecient amplifiers like transistors.

I found what those mosfets are in the schematic, they are IRFZ44 available at mouser.com for around 2 bucks each.....in case anyone has been monitoring this thread and wants to give
a shot at that inverter circuit.

I think I might.

 

[Ubergeek63]

A circuit such as post #9 of this thread?

By the way, I don't know what was meant just above that "the Mosfet's are part of the IC" IC's are IC's and Mosfets are fancy, very effecient amplifiers like transistors.

I found what those mosfets are in the schematic, they are IRFZ44 available at mouser.com for around 2 bucks each.....in case anyone has been monitoring this thread and wants to give
a shot at that inverter circuit.

I think I might.

yes ... a simple 30% duty cycle should work with an additional capacitor to hold the output on steady while the relay pulls in.

depends on the ic ... opamps are very good amplifiers and mosfets are a different type of transistor.

dan

 

[tcmtech]

Have you considered just using the correct resistor to get the correct DC current through the relay coil with a larger capacitor in parallel to the resistor to get the nessisary higher pulse of current to first get the relay to activate?

A 2200 uf capacitor in parallel to the resistor should be more than enough to get it to pull in and the steady current at a lower voltage will keep it pulled in.

I have done this many times and it works quite well once you get the right resistor and capacitor values matched to the relay.

 

[Eventhorizon]

I tried with resistors, the coil would not even click...even down to 5 ohms or less, but I never tried with the addition of a cap.

Maybe the res was enough to carry the "stay on" current but not to initially bring the contactor in. I have never though about it that way. I'll try with Res + Cap, thanks

It a good fix for now, to work with DC....but I am still going to venture to build a "nice wave" DC/AC inverter.