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Generating 24VAC Output (2 Amp Capability) from 36-40VDC

Mahonroy

Oct 21, 2014
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Hello, I am wanting to generate a 24VAC output, and be able to put up to a 2 amp load on it. The available power is 36VDC or 40VDC (3 or 4 amp). No transformer - this has to be generated off the main DC voltage.

As we know, a 24VAC output consists of a sinewave with a peak voltage of +34VDC and -34VDC.

My initial thought was to use 2x LM2576 regulators, to create a +34VDC rail, and a -34VDC rail.
Very similar to this example below - instead of +-12v I would do +-34V.
1651177435823.png

I would then run each +34VDC and -34VDC to an op-amp, and could use 2x transistors to boost the amperage capabilities of the output. Like this:
upload_2022-4-28_15-57-5.png

Would I then be able to use a microcontroller to privide a square wave going into the op-amp, to generate the 24VAC @ 60 HZ? Or I could make a sinewave generator out of a 555 timer for this?

It also got me thinking that I could just drive the gates directly to produce a square wave right? At one point I thought I saw someone do this with a gate driver, but did not understand how it worked.

Am I on the right track here for generating the 24VAC @ 60 HZ from a higher DC voltage? How should I go about doing this?

Thanks and any hep or advice is greatly appreciated!
 

crutschow

May 7, 2021
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You mention sinewave but then talk about generating a square-wave.
So does the output need to be a sinewave or not?

If you need a sinewave, then one way would be to generate that with phase-shift oscillator, and use that to drive an audio amp.

Does the load for the AC need to share a ground with the DC supply?
 

Mahonroy

Oct 21, 2014
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You mention sinewave but then talk about generating a square-wave.
So does the output need to be a sinewave or not?

If you need a sinewave, then one way would be to generate that with phase-shift oscillator, and use that to drive an audio amp.

Does the load for the AC need to share a ground with the DC supply?

Why do you need the 24VAC supply? Is this to run a CCTV camera by any chance?

I want to use this to drive rainbird sprinkler valves. Ideally the output would be sinewave if I can pull that off - but technically they run ok with a 24VA squarewave as well. Either will work. I want to incorporate it into a PCB to just use raw components.
 

kellys_eye

Jun 25, 2010
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How is the original DC source (36/40V DC) derived? It's an unusual voltage.

It's possible to run AC solenoids on a DC source (12V DC for a 24V AC solenoid) when used intermittently. Seems like an awful lot of trouble to have to go to.....
 

Mahonroy

Oct 21, 2014
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How is the original DC source (36/40V DC) derived? It's an unusual voltage.

It's possible to run AC solenoids on a DC source (12V DC for a 24V AC solenoid) when used intermittently. Seems like an awful lot of trouble to have to go to.....
There is a power module on the circuit board that receives 120VAC and converts it to 36VDC. You need 34VDC to create the peaks of 24VAC. I have to create a 24VAC output. DC output is not an option.
 

crutschow

May 7, 2021
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If you generate a square-wave. then it should be 24Vpk or 48vpp.
This could be generated from a 24V supply with a bridge output as long as the output doesn't need to be grounded to the DC supply voltage.
 

Martaine2005

May 12, 2015
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There is a power module on the circuit board that receives 120VAC and converts it to 36VDC. You need 34VDC to create the peaks of 24VAC. I have to create a 24VAC output. DC output is not an option.
May I ask why a transformer can’t be used if the module has AC?

Martin
 

kellys_eye

Jun 25, 2010
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Is there a really good reason why size matters? (note to Mrs k_e, not a comment directed at you dear....).

There's a good reason for the KISS principle (Keep It Simple Stupid) - less cost, less trouble etc.
 

Mahonroy

Oct 21, 2014
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Is there a really good reason why size matters? (note to Mrs k_e, not a comment directed at you dear....).

There's a good reason for the KISS principle (Keep It Simple Stupid) - less cost, less trouble etc.
I've already suggested this to everyone that this is simpler - everyone hates the look of a massive transformer and is absolutely not going to work for this. It has to be a 120V plug (C8 plug receptacle), and it has to be small.
 

kellys_eye

Jun 25, 2010
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Have you tried to drive the solenoids with DC?

Not totally against your idea but it seems an awful lot of effort for something that's so 'basic' (powering a solenoid). AC (unless it's 'pure') is awfully troublesome with inductive loads.

I'd even look at changing the solenoids for DC versions!
 

Mahonroy

Oct 21, 2014
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Have you tried to drive the solenoids with DC?

Not totally against your idea but it seems an awful lot of effort for something that's so 'basic' (powering a solenoid). AC (unless it's 'pure') is awfully troublesome with inductive loads.

I'd even look at changing the solenoids for DC versions!
Solenoids are not the only thing that its going to power. 90% of the time it will be solenoids. It has to be 24VAC output. Yes it is a lot of effort but it has to be done, there is no way around this requirement.
 

Martaine2005

May 12, 2015
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Yes it is a lot of effort but it has to be done, there is no way around this requirement.
Sorry, but you sound like a robot! Hal’s niece or nephew?.
People are trying to help you but you give no real world response. I suggest a thank you for previous efforts and a reason why those work for you!.

Martin
 

kellys_eye

Jun 25, 2010
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Solenoids are not the only thing that its going to power.
So what else is it powering? This can mean the difference between a 'simple' AC source and a complex one. Any answers may be wasted effort unless we are fully informed.

I feel that there is a lot of information we aren't getting here and it doesn't help.
 

Mahonroy

Oct 21, 2014
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So what else is it powering? This can mean the difference between a 'simple' AC source and a complex one. Any answers may be wasted effort unless we are fully informed.

I feel that there is a lot of information we aren't getting here and it doesn't help.

24VAC Valves only - no DC valves. On the more rare occurance it will power 24VAC relays, and control HVAC devices that require 24VAC signals to operate. Sure, some of the things "might" work with 24VDC. Technically they can put whatever they want on the power source, so long as it doesn't violate the max current draw. But If I am advertising that its outputting 24VAC, it should probably actually output 24VAC. If someone puts a volt meter on it, or an oscilliscope on it, it should probably be 24VAC.
 

Mahonroy

Oct 21, 2014
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Can I use an H-bridge chip to accomplish this? If I wanted to do a square wave to approximate a 24VAC output, would I use 34V and negative 34V? Or would I use 24V and negative 24V?

These are the H-bridge chips I was looking at:
DRV8251DDAR
https://www.ti.com/lit/ds/slvsfz6/slvsfz6.pdf?ts=1651531694969

TB67H451FNG,EL
https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TB67H451FNG-EL/11568781

Could I just alternate between forward and reverse on this chips? Then either enter the state of "Coast" or "Brake" for turning off?
 

Harald Kapp

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Nov 17, 2011
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17V positive and negative.
No.
+-24 V.
The 24 V stated by the op are RMS, not peak. AC 24 V RMS is equivalent to 24 V DC or a square wave with 24 V peak voltage or 34 V peak for a sine wave, which is why the +-24 V appear in the original post.
 

AnalogKid

Jun 10, 2015
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To be clear, a 24 Vac sine wave power source is 24 Vrms, or 34 Vp, or 68 Vp-p. An equivalent square wave would be +/-24 V, or 48 Vp-p.

- - -

I am absolutely *not* picking on the TS, just observing:

Once again, a young power system designer has independently discovered that a true sine wave power source is hard, and a square wave alternative is . . . attractive. Not exactly a lure toward the dark side, but still . . .

Now, about all of those pesky harmonics ...

ak
 
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