# Safe AC for exercises.

#### hp1729

Joined Nov 23, 2015
2,304
Some exercises just can't be done without AC. Since my breadboards are battery powered I need a way to supply AC that is safe and available. The attached circuit was what I came up with. 60 Hz, variable voltage, modest output current.

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#### Alec_t

Joined Sep 17, 2013
13,781
Nice, but I'm curious as to what exercises you'd be doing, given that the battery supply and push-pull output stage will obviously limit the current you can draw?

#### DickCappels

Joined Aug 21, 2008
10,067
Looks like a good circuit for modest amplitudes and power levels. By the way,

Incidentally, you can do many AC experiments using a capacitively couple square wave too, and you might find that easier to build..

#### hp1729

Joined Nov 23, 2015
2,304
Nice, but I'm curious as to what exercises you'd be doing, given that the battery supply and push-pull output stage will obviously limit the current you can draw?
Diode, rectifier, basic power supply design, triacs. Peak vs RMS AC voltages, amplifiers, transformers ... Yes it is intentionally low and safe for those in first year electronics.

#### hp1729

Joined Nov 23, 2015
2,304
Looks like a good circuit for modest amplitudes and power levels. By the way,

Incidentally, you can do many AC experiments using a capacitively couple square wave too, and you might find that easier to build..
Yes, just for early basic classes. Sometimes you just gotta have a sine wave to talk about sine waves.

#### Alec_t

Joined Sep 17, 2013
13,781
safe for those in first year electronics.
Ah, in that case I suggest you need some sort of current-limiting arrangement to prevent your AC supply frying when the students short-circuit something (which they will ).

#### hp1729

Joined Nov 23, 2015
2,304
Ah, in that case I suggest you need some sort of current-limiting arrangement to prevent your AC supply frying when the students short-circuit something (which they will ).
good suggestion. Depending on the op amp used, parts are about $1. I keep it build up on a small breadboard like most of my "doodad" circuits. Voltage regulators, adjustable voltage references, and such. All possible to be built in under$1. The dual op amp used is a TL062 but most any dual op amp designed for + and - 15 V will do. Or you could even use two LM741s. What ever you have on hand.
Buy prime parts? Only by exception. For the most part I buy "grab bag" parts, in bulk or the cheapest possible. Seldom I will buy prime stuff. I gotta have a good reason.

#### hp1729

Joined Nov 23, 2015
2,304
current-limiting arrangement
Something like this?
It makes the cheap, disposable circuit more complicated. If I were selling a marketable product. maybe. There are single chip solutions on the market but I designed this for parts on hand.
This brings down the output voltage a bit, too.

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Joined Jul 18, 2013
27,624
Unless I am missing something, what is wrong to use one of the older mains transformer Wall Warts, the DC portion can be removed if necessary.
Max.

#### Wendy

Joined Mar 24, 2008
23,273
You can buy AC only wall warts.

#### hp1729

Joined Nov 23, 2015
2,304
You can buy AC only wall warts.
Very true, but battery powered was a requirement.

#### hp1729

Joined Nov 23, 2015
2,304
Unless I am missing something, what is wrong to use one of the older mains transformer Wall Warts, the DC portion can be removed if necessary.
Max.
Very true. I suggest using what you have available. Battery powered was one requirement.

#### Wendy

Joined Mar 24, 2008
23,273
Do you want 60 Hz? A power audio amp (DIY) will meet most of these requirements.

I found a circuit in the ebook that is similar to yours, but will have less crossover distortion. Give me a bit and I will post it.

#### Wendy

Joined Mar 24, 2008
23,273
BTW, I have a similar project in work, different emphasis. A AC signal from a single power supply.

I have some peanut bulbs, 1.5V at 20ma.

Be back with the other circuit.

#### hp1729

Joined Nov 23, 2015
2,304
Do you want 60 Hz? A power audio amp (DIY) will meet most of these requirements.

I found a circuit in the ebook that is similar to yours, but will have less crossover distortion. Give me a bit and I will post it.
Excellent idea! Just not anything I have on hand.

#### hp1729

Joined Nov 23, 2015
2,304
BTW, I have a similar project in work, different emphasis. A AC signal from a single power supply.

View attachment 100707

I have some peanut bulbs, 1.5V at 20ma.

Be back with the other circuit.
Also excellent. 440 Hz? A above middle C?

#### Wendy

Joined Mar 24, 2008
23,273
A = 440 Hz

What most tuners use.

This is the amp, I designed it to dovetail with the Wien Bridge Oscillator. My thought is you can take whatever works for you out of the design.

It should be a really clean AC signal, keep the battery isolated from the rest. If the input signal is near the battery voltage the P-P will be nearly twice the battery voltage.

#### hp1729

Joined Nov 23, 2015
2,304
A = 440 Hz

What most tuners use.

This is the amp, I designed it to dovetail with the Wien Bridge Oscillator. My thought is you can take whatever works for you out of the design.

View attachment 100710

It should be a really clean AC signal, keep the battery isolated from the rest. If the input signal is near the battery voltage the P-P will be nearly twice the battery voltage.
Super!

#### Wendy

Joined Mar 24, 2008
23,273
Just merged my schematics. I'll be posting this on a separate thread sometime in the future, after I test it.

#### hp1729

Joined Nov 23, 2015
2,304
Just merged my schematics. I'll be posting this on a separate thread sometime in the future, after I test it.

View attachment 100713
Using my previous oscillator and the suggested output stages the transistors in the Q1 and Q2 positions kept self destructing. I couldn't find what I did wrong after many attempts I modified the outputs to something I knew. Yes the output was larger, as expected.

The light bulb in the Wein oscillator ... is it the inductance of it that matters or the changing resistance, or what?

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