Hello, I am wanting to generate 24VAC and be able to pull a maximum of 1.5 amps off it (more typically 0.5 to 1.0 amp continuously). I have 36VDC or 42VDC available from a power adapter, such as one of these ( 2 examples ):
36VDC Power Adapter
42VDC Power Adapter

My original thought was to generate a 60Hz pulse with something like a 555 timer or a similar configuration, run it to a high current capable op-amp (provide the op amp with +36VDC and -36VDC), or a combination of op-amp and transistor configuration to boost current capabilities, then the output would be the 24VAC @ 60 hz that I could then pull up to 1.5 amps off it. I would need to generate the -36VDC for the op-amp, so was thinking somekind of charge pump or negative output configured voltage regulator, like a CS5172 for example.

Am I on the right track here?

You might ask why I can't just get a 24VAC power adapter / transformer, but this is not the rout I want to go for a number of reasons (24VAC @ 1.5 amps requires 40VA transformer. They are bulky and don't fit the look or form factor for this project. I need the DC voltage anyways for other parts of the circuit, so I would like to generate this 24VAC from the DC rails I have available if possible).

Thanks and any help or advice is greatly appreciated!

 

What you are looking for is an inverter. The problem becomes your intended application for the 24 VAC? Most lower end inverter designs will give you a square wave or modified sine wave output. Now if you want a 60 Hz true sine wave output things get a little more complicated. The MSW inverter outputs a sort of square wave output and a true sine wave a sine wave like standard mains power. Back to the application? Wanting 60 Hz the simple solution would be what you want to avoid and that being a transformer which is yes, large, bulky and heavy. Most common doorbell or thermostat transformers are 40 VA 24 VAC. A Google of "24 vac 60 hz inverter circuit diagram" will bring up some circuit examples. Keep in mind you want 60 Hz.

Ron

 

For that power level, you may be able to use an audio amplifier with a 60Hz signal generated from a sine wave oscillator.
A class D amp would be pretty small.
This is a stereo one..

2 x 50W or so,
Look for "TPA3116" on Ebay or similar.

Then there are these boards, designed to do just what you want.
Note. This is just the control board!

EG8010 is a digital, function very well bring their own dead-time control of pure sine wave inverter generator chip, used in two-stage DC-DC-AC power conversion structure or single-stage DC-AC power frequency transformer Boost architecture, an external 12MHz crystal oscillator, to achieve high precision, and harmonic distortion is very small, 50Hz or 60Hz pure sine wave inverter ASIC. The chip uses CMOS technology, the internal integration of SPWM sine generator, dead time control circuit, the multiplier factor range, soft start circuit, protection circuit, RS232 serial communication interface and a serial LCD driver module 1602 functions.
Features:
5V single power supply
Pin set up four kinds of pure sine wave output frequency: fixed frequency 50 hz pure sine wave; 60 hz fixed frequency pure sine wave; 0-100 hz pure sine wave frequency is adjustable; 0-400 hz pure sine wave frequency is adjustable.
Unipolar and bipolar modulation
Comes with deadband control, pin set up 4 dead time: 300nS dead time; 500nS dead time ; 1.0uS dead time ; 1.5uS dead time
External 12MHz crystal oscillator
PWM carrier frequency 23.4KHz
Voltage, current, real time temperature feedback
Overvoltage, undervoltage, overcurrent and overheating protection
Soft-start mode pin setting the response time of 1S
Serial communication to set the output voltage, frequency and other parameters External
External serial port 128 * 32 LCD module 1602 display the voltage, frequency, temperature and electric current of the inverter and other information;
Applications:
Single phase pure sine wave inverter
PV Inverter
Wind power inverter
Uninterruptible Power Supply UPS System
Digital generator system
IF power
Single-phase motor speed controller
Single-phase inverter
Sine wave dimmer
Sine wave regulator
Sine wave generator
LCD description:
In the use of the LCD,Connection EGS002 driver board and LCD cable must use shielded cable, otherwise.Inverter, high-voltage high-current environment will seriously interfere with the operation of the drive plate

 

What you are looking for is an inverter. The problem becomes your intended application for the 24 VAC? Most lower end inverter designs will give you a square wave or modified sine wave output. Now if you want a 60 Hz true sine wave output things get a little more complicated. The MSW inverter outputs a sort of square wave output and a true sine wave a sine wave like standard mains power. Back to the application? Wanting 60 Hz the simple solution would be what you want to avoid and that being a transformer which is yes, large, bulky and heavy. Most common doorbell or thermostat transformers are 40 VA 24 VAC. A Google of "24 vac 60 hz inverter circuit diagram" will bring up some circuit examples. Keep in mind you want 60 Hz.

Ron

I'm going to be using the output to drive water solenoid valves. It doesn't have to be exactly 60Hz, just close. E.g. 60Hz +-1Hz would be fine. It also doesn't have to be a perfect sine wave, just resemble a sine wave. I think I can do better than just a square wave though.

TPA3116

Thanks for the suggestion. Your response did not quote properly for some reason. I am looking to build the circuit onto my own PCB, I don't want to rely on sourcing sub assemblies from other manufacturers.

 

Water valves can be run on DC, provided that the current is limited. (On AC, the inductance limits the current).
They could also be run on a squarewave by driving them with an H-bridge, using something like an Allegro A4953

 

I'm going to be using the output to drive water solenoid valves. It doesn't have to be exactly 60Hz, just close. E.g. 60Hz +-1Hz would be fine. It also doesn't have to be a perfect sine wave, just resemble a sine wave. I think I can do better than just a square wave though.

OK, got it. If possible I would look to replace what solenoid valves you have with DC valves. AC solenoid valves can run on DC as mentioned but at a lower voltage and even at that I do not see it as a good idea. In one of my designs I used quite a few DC solenoid valves for air, gas, steam and water. One evening in my absence 3rd shift decided a valve needed replaced. This was the result.


The above was a valve having a 24 VAC 60 Hz coil installed in a 24 VDC system. It worked just fine for about 10 min, right till .... I used that image in safety lectures of the importance of using the drawings and parts listings when replacing parts. In a home or hobby application I may try and run an AC coil at a lower voltage and make sure it did not get hot. In any commercial application I would not consider it. So while lowering the coil voltage possibly placing a resistor in series with the coil will reduce coil current I just do not see it as a good idea.

In many cases a good quality valve will have a removable coil as pictured above only the valve coil is easily replaced. Depends on manufacturer and valve design. Coils alone are not that expensive. In your case I would consider the valve before trying to develop 24 VAC.

Ron

 

A PMW module could reduce the DC voltage to operate the valves on a lower voltage DC.

 

"10-Watts" is clearly stated on the sticker.

At 24-VDC that's ~0.4-Amps, and ~60-Ohms.

Measure the DC-Resistance of the Coil,
then add a Power-Resistor to bring the total Circuit-Resistance to ~60-Ohms.
It will be trouble-free.
Better-yet, just buy a DC rated Coil.
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Guys, let's keep in mind the 10 Watts on the label is just my example of a fried coil. The thread starter posted his requirements.

Ron

 

Water valves can be run on DC, provided that the current is limited. (On AC, the inductance limits the current).
They could also be run on a squarewave by driving them with an H-bridge, using something like an Allegro A4953

OK, got it. If possible I would look to replace what solenoid valves you have with DC valves. AC solenoid valves can run on DC as mentioned but at a lower voltage and even at that I do not see it as a good idea. In one of my designs I used quite a few DC solenoid valves for air, gas, steam and water. One evening in my absence 3rd shift decided a valve needed replaced. This was the result.
View attachment 264822

The above was a valve having a 24 VAC 60 Hz coil installed in a 24 VDC system. It worked just fine for about 10 min, right till .... I used that image in safety lectures of the importance of using the drawings and parts listings when replacing parts. In a home or hobby application I may try and run an AC coil at a lower voltage and make sure it did not get hot. In any commercial application I would not consider it. So while lowering the coil voltage possibly placing a resistor in series with the coil will reduce coil current I just do not see it as a good idea.

In many cases a good quality valve will have a removable coil as pictured above only the valve coil is easily replaced. Depends on manufacturer and valve design. Coils alone are not that expensive. In your case I would consider the valve before trying to develop 24 VAC.

Ron

A PMW module could reduce the DC voltage to operate the valves on a lower voltage DC.

"10-Watts" is clearly stated on the sticker.

At 24-VDC that's ~0.4-Amps, and ~60-Ohms.

Measure the DC-Resistance of the Coil,
then add a Power-Resistor to bring the total Circuit-Resistance to ~60-Ohms.
It will be trouble-free.
Better-yet, just buy a DC rated Coil.
.
.
.

I have to be able to control 24VAC solenoids specifically. I cannot do DC here unfortunately.

 

"" I am looking to build the circuit onto my own PCB, ""

This will probably turn-out to be larger and more expensive than a proper Transformer.
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Was my original plan of using a negative voltage charge pump, along with high current op-amp/transistor not a good way of accomplishing this?

 

Was my original plan of using a negative voltage charge pump, along with high current op-amp/transistor not a good way of accomplishing this?

It was not a good way to accomplish this.
Firstly, if you really do need AC, for something as inductive as a solenoid, a squarewave will be perfectly acceptable. The current waveform will be the integral of the voltage, so it will be a triangle.
If you really think you needed a sinewave, a Class-D amplifier in a bridge-tied-load configuration would be good.
For a linear amplifier, two amplifiers in a bridge-tied-load configuration would be easier than making a negative supply.
If you really wanted a negative supply, a Cuk cpnverter would be far superior to a charge pump.

 

I have to be able to control 24VAC solenoids specifically. I cannot do DC here unfortunately.

Then all things considered a transformer will be the simplest option. There is no simple solution.

Ron

 

Can you determine the inductance and resistance of the solenoids you are using?

 

If you can't measure the inductance directly, could you measure the running current when operated from a transformer? That might be more useful, because the "open" and "closed" inductance values may be different.

 

Alternately the coil current and the power it consumes would also be useful.

 

This whole project doesn't make any sense.
I'm assuming that this project is a "one-off" design, not a system intended for commercial sales.
In that case there is no reason for having 3 different Voltage-Supplies,
unless you're just trying to "cobble-up" a system out of parts/equipment that is already on-hand.

Standard-Valve-Solenoids are readily available, relatively cheap,
with a large variety of operating-Voltages, in AC-ratings and DC-ratings.

Let's start from the beginning ...........
What do You have ?, (part-numbers and schematics please, at the very least a block diagram),
What does it not do satisfactorily ?,
What parts, Voltages, devices, do You absolutely have to keep, and why ?,
and what are You trying to accomplish as an end result of all this ????
.
.
.

 

This whole project doesn't make any sense.
I'm assuming that this project is a "one-off" design, not a system intended for commercial sales.
In that case there is no reason for having 3 different Voltage-Supplies,
unless you're just trying to "cobble-up" a system out of parts/equipment that is already on-hand.

Standard-Valve-Solenoids are readily available, relatively cheap,
with a large variety of operating-Voltages, in AC-ratings and DC-ratings.

Let's start from the beginning ...........
What do You have ?, (part-numbers and schematics please, at the very least a block diagram),
What does it not do satisfactorily ?,
What parts, Voltages, devices, do You absolutely have to keep, and why ?,
and what are You trying to accomplish as an end result of all this ????
.
.
.

No I intend to make a PCB with everything on it for a prototype.
In the design, I do need a 5VDC power rail for the microcontroller. This microcontroller will activate the valves via a relay. There are 8 valves, and I have to support every single one being on at the same time.
Yes, I agree solenoids are readily available and cheap. I must supply them with 24VAC, not DC.
I can source whatever parts are necessary, but like I mentioned in my first original post, it needs to be small, and 24VAC (40VA) transformers are large and bulky (40VA is needed to power all 8 solenoids). Unless its better to do 8x small transformers... which are expensive.... which made me wonder if there was a better way of accomplishing this via using a negative voltage charge pump, along with high current op-amp/transistor, or something similar.

 

You have a project in the prototype design stages and you must use 24 VAC solenoids. Driving solenoids with DC is not a problem but driving AC solenoids is not going to be a simple easy task. I can see no suggestion beyond what has been suggested. Allowing for overkill let's just say each solenoid is a standard 10 watt 24 VDC coil with 8 of them on at the same time you have a 80 watt load and you are looking at a 5.0 amp load. Everything in the BOM (Bill Of Materials) is considered at design time. With your design plan there is no simple solution. What you see here and the suggestions here are as good as it will get. I don't know what else to suggest? This is your design and prototype.

Ron