# Amplify low voltage signal to 120V AC

#### Harry Muscle

Joined May 16, 2017
8
Hi, I was wondering if a ready made kit or device exists that can amplify a 2V signal to 120V AC levels. The current requirements are fairly low at less than 1A. Basically I'm looking for an audio amplifier but one that puts out more voltage and less current. Does something like this exist?

Thanks,
Harry

#### KMoffett

Joined Dec 19, 2007
2,918
Harry,

What are you trying to create? "Color organ"?

Ken

#### Harry Muscle

Joined May 16, 2017
8
Harry,

What are you trying to create? "Color organ"?

Ken
I'm trying to figure out a way to amplify a variable frequency sine wave generated by an Arduino to 120V AC levels in order to vary the speed of a shaded pole synchronous motor. Generating the sine wave seems to be the easy part. Getting it up to 120V AC levels is proving more difficult. I'm trying to find a ready made solution of some sorts, either a pre-made board or an actual device.

Thanks,
Harry

#### #12

Joined Nov 30, 2010
18,216
There is one called a variable frequency drive, but it creates its own frequency and then drives the motor. That might be the easy way to get there, but they don't cost $5 to$10.

I could whip together an analog amplifier. Other people could whip together a Class D digital PWM amplifier, but I don't think you can buy one in a store.

Now, watch the next guy point to exactly what you need on eBay.

#### crutschow

Joined Mar 14, 2008
26,035
1A @ 120V is 120W so that's not a trivial amount of amplifier power.
You could use a 120W audio amp, which puts out about 30V into an 8Ω load, and use that to drive the 28V secondary of a 150W. 120V-28V transformer.
That will give you 120V on the primary to drive the motor.

If you can measure the actual current of the motor when it's operating under its normal load, that will tell us more precisely what you need.

#12

#### Harry Muscle

Joined May 16, 2017
8
There is one called a variable frequency drive, but it creates its own frequency and then drives the motor. That might be the easy way to get there, but they don't cost $5 to$10.

I could whip together an analog amplifier. Other people could whip together a Class D digital PWM amplifier, but I don't think you can buy one in a store.

Now, watch the next guy point to exactly what you need on eBay.
I looked into variable frequency drives, but like you said they aren't exactly cheap. Also they don't actually produce a true sine wave and generally seem to cause lots of EMI.

An analog amplifier would seem to be what I'm looking for ... but having to create one from scratch is a little beyond my skills.

Thanks,
Harry

#### #12

Joined Nov 30, 2010
18,216
An analog amplifier would seem to be what I'm looking for ... but having to create one from scratch is a little beyond my skills.
That's what crutschow is talking about...how to get an audio amplifier built for speakers to calm down on the current, jack up the voltage, and do what you want.

Actually, I do that in my work shop, but my amplifier is only 70 watts.

#### Harry Muscle

Joined May 16, 2017
8
1A @ 120V is 120W so that's not a trivial amount of amplifier power.
You could use a 120W audio amp, which puts out about 30V into an 8Ω load, and use that to drive the 28V secondary of a 150W. 120V-28V transformer.
That will give you 120V on the primary to drive the motor.

If you can measure the actual current of the motor when it's operating under its normal load, that will tell us more precisely what you need.
Interesting idea.

The motor is rated at 120V and 20W, so it should be drawing about 166mA. However, if this works I'm hoping to control three such motors, so total current would be 0.5A.

Thanks,
Harry

#### Harry Muscle

Joined May 16, 2017
8
1A @ 120V is 120W so that's not a trivial amount of amplifier power.
You could use a 120W audio amp, which puts out about 30V into an 8Ω load, and use that to drive the 28V secondary of a 150W. 120V-28V transformer.
That will give you 120V on the primary to drive the motor.

If you can measure the actual current of the motor when it's operating under its normal load, that will tell us more precisely what you need.
That's what crutschow is talking about...how to get an audio amplifier built for speakers to calm down on the current, jack up the voltage, and do what you want.

Actually, I do that in my work shop, but my amplifier is only 70 watts.

If I were to use a 100W audio amplifier (lots of these on eBay) instead of 120W that should give 28 volts I believe. Would that be better since it better matches a 28V to 120V transformer?

Also, I know there's a difference between RMS and peak voltages when dealing with AC and I just want to make sure I understand this correctly. A 100W audio amp would put out 28V RMS or about 40V peak or 80V peak to peak. Using the transformer would convert this to 120V RMS or about 170V peak or 340V peak to peak which are standard 120V AC voltages, correct?

Lastly, most of these audio amplifiers have a volume knob on them. In order to get the 28V would I simply crank the volume all the way up to full? If I wanted to use a 24V to 120V transformer instead would I simply lower the volume until I measure 24V RMS?

Thanks,
Harry

P.S. One more question, what would be the lowest frequency that I could expect to have this setup work? I know transformers require AC so I'm assuming there is a minimum frequency they will work at.

#### #12

Joined Nov 30, 2010
18,216
Wow. Lotsa questions. Random stabbing to follow.
Most American power transformers are designed for 60 Hz. Go below that and the magnetic core starts heating up.
Some power transformers won't make it to 20 KHz. I don't know which or why.

You want to operate near the rated output because that's where the volts and amps (impedance) are correct for the load.
The numbers on the knob don't mean nuthin'. That part depends on what voltage you insert from the Arduino.

For this math, RMS works just fine because you can assume an audio amp can do sine waves properly.

Yeah, at first glance 100W into 8 ohms = 28.28 VRMS

#### crutschow

Joined Mar 14, 2008
26,035
How low in frequency do you want to go?
If you go below the transformer rated frequency then you need to reduce the voltage in the same proportion (.i.e. operating at 40Hz with a 60Hz transformer means the maximum output voltage should be 40/60 * 120Vrms = 80Vrms.
That may not be a problem because the motor load is likely less at a slower speed so won't need the full 120V.
So you could program the Arduino to lower the voltage in proportion to the frequency.

Using a 50-60Hz transformer will give you more margin for the lower frequencies.
If you want to go to a still lower frequency, then you may need to look for an appropriate audio transformer which is typically designed to go down to 20Hz.

Another option for lower frequency operation is to use a 48V to 220V transformer but operate it at 1/2 its voltage rating (28V in for 120V out).
That would allow you to go down to 1/2 the transformer's lower frequency rating before you would need to reduce the voltage.

#12

#### Harry Muscle

Joined May 16, 2017
8
Wow. Lotsa questions. Random stabbing to follow.
Most American power transformers are designed for 60 Hz. Go below that and the magnetic core starts heating up.
Some power transformers won't make it to 20 KHz. I don't know which or why.

You want to operate near the rated output because that's where the volts and amps (impedance) are correct for the load.
The numbers on the knob don't mean nuthin'. That part depends on what voltage you insert from the Arduino.

For this math, RMS works just fine because you can assume an audio amp can do sine waves properly.

Yeah, at first glance 100W into 8 ohms = 28.28 VRMS
What does the volume knob control if it doesn't affect the output voltage? Allows more current?

Thanks,
Harry

#### #12

Joined Nov 30, 2010
18,216
The volume knob controls the gain of the amplifier. If you input 1 millivolt RMS and you want 28 VRMS out, you turn the knob until the gain of the amplifier is 28/0.001
If you input 3 volts RMS from an Arduino, you turn the knob until the gain of the amplifier is 28/3

The numbers on the knob are only relevant for that amplifier. Any other amplifier will have a different meaning for the numbers on the knob.