12v DC to 110VAC pure-sine converter

[astroboy907]

I'm working on a college project where we need to trigger a 110VAC solenoid (rated at about ~18W). We are running solely from a 65C LiPo cell at 11.1V. I screwed up and bought a standalone 12V DC to 110VAC converter module from amazon but it outputs a square-wave output at 20kHz. We need something that will boost the voltage to the correct level - I'd use a standard automotive inverter but our project is largely weight-sensitive (will be airborne), so any reduction in mass we can accomplish is hugely beneficial. Is there a commonly used chipset that we can utilize to do a very simple power inverter?

[Johncanfield]

Why the constraint of a 120vac solenoid? An engineering exercise?

[Zero999]

Silly question, can't you change the solenoid for a 12V one or re-wind it?

Try to avoid converting one voltage to another, especially DC to AC, whenever possible. Just use the correctly rated voltage component for the supply you have.

You didn't specify the frequency, presumably 60Hz?

There's no standard chipset to convert 12V to 110VAC at a mains frequency.. It's a very in-depth project and requires multiple steps.

1) DC:DC converter to boost the voltage to the peak voltage of the AC, which is 110*1.414 = 156V,.
2) H-bridge driven with a higher frequency PWM signal, modulated with a 60Hz sine wave.
3) LC filter to remove the high frequency element, leaving only 60Hz left.

Each of the above is a fairly decent project in itself. The correct solution is to use a 12V solenoid in the first place.

[Martin72]

The specs of the project will be needed….

First thoughts :

Converting 12Vdc to 60 (?) hz AC...

Create 60hz sinewave by e.g. DAC, feed it to a comparator for creating a modulated pwm signal, driving a power stage.
Filtering this one as mentioned before with LC filter and feed it into a transformer 1:12

[astroboy907]

The solenoids we are looking at are rated at 50/60 Hz so that's a major constraint.

We'd use a 12V solenoid but the manufacturer suggested that a 120V AC solenoid has a quicker actuation rate, and actuation rate is a driving factor for our design. I could likely swap it with a 24V DC one easily - a 12V one might be more difficult to find. At the end of the day we definitely would prefer lower actuation time over an over-complicated power system.

[james_s]

I think you answered your own question there then. You have a 12V system, it makes no sense to use a 120VAC solenoid and then have to convert the power. Just use a 12V solenoid and you will reduce the complexity considerably. If you need it to act quickly you could try using a small boost converter to charge a capacitor up to a higher voltage, the momentary higher voltage will pull in the solenoid faster and then the 12V will hold it in.

[Zero999]

The solenoids we are looking at are rated at 50/60 Hz so that's a major constraint.

We'd use a 12V solenoid but the manufacturer suggested that a 120V AC solenoid has a quicker actuation rate, and actuation rate is a driving factor for our design. I could likely swap it with a 24V DC one easily - a 12V one might be more difficult to find. At the end of the day we definitely would prefer lower actuation time over an over-complicated power system.

You'd be better off using a 12V solenoid and over-driving it with a brief, higher voltage pulse, to increase the actuation speed. It's much easier to convert 12VDC to a higher DC voltage, than to 110VAC.

A brief overvoltage will not damage the solenoid. If the manufacturer advises against it, it will be because they're covering their back. I've worked on a project for work which required some solenoid valves to be actuated as quickly as possible. The driver circuit powered 24V rated solenoids with a brief 120V pulse to actuate the solenoid very quickly, dropping the voltage down to 24V, once the valve was open.

The DC:DC converter which provides the higher voltage from 12V, doesn't have to be continuously rated for the solenoid's full current. It can charge a capacitor, which is discharged into the solenoid. Indeed the simplest solution is a capacitive voltage doubler, but it will need a large capacitor, which might be too bulky.

Attached is the simplest way to fire a 12V solenoid, with a brief 24V pulse, before dropping the voltage back to 12V. It uses a relay (L1) with two normally open and one normally closed contact. L1 is the solenoid. You didn't specify the series resistance and inductance, so I guessed. D2 and R1 is a snubber, which limits the back-EMF. A higher value resistor will make the solenoid switch off faster, but with a higher back-EMF voltage, no resistor will make it much slower, but with only 0.7V of back-EMF.

When the relay is off, C1 is charged from the power supply, via R2 which limits the current to protect the relay contact; not very efficient but it works. When the relay turns on, C1 is connected in series with the power supply, reverse biasing D1 and boosting the output voltage to double the input. C1 discharges through the load and the voltage falls back towards the supply voltage. When C1 has discharged, D1 conducts and powers the load directly from the power supply. The delay in the output being turned on and off is due to the relay. The model I used is for a small, high sensitivity relay, which is fast. A larger relay will be considerably slower.

Note that the models for the relay and solenoid are basic. In real life the inductance of the coil will change, depending on the position of the armature, which isn't modelled.