Looking to generate 25 Hz square wave at about 90 V AC

I am looking to build a square signal generator tuned to ~25 Hz at a voltage of ~90 V AC that can output ~400 mA. I've searched around the internet and tried a few things but I've had no luck. My original idea was to have an Arduino and a LM741CN output a square AC wave and then attach that to a transformer to get it to 90 V AC. I achieved the square AC wave, but it only outputs about 2.5 V AC. Getting that far I was looking into transformers but low frequency transformers are not as easy to come by as I thought, I tried building one but the core I had on hand was not big enough to handle the amount of wounds needed.

My overall question here is: Was I heading in the right direction or is there a better way that I have not found yet?

• I would use an h-bridge running off a 90VDC supple produced from a transforner-rectifier-capacitor hooked to the AC mains. Similar to a brushed motor driver. Also, you mean a +/-90V bipolar square wave. AC is too vague here. I don't know if you mean RMS or peak and that is the problem when you use AC for things that aren't sinusoids and are not being specific. Jan 26, 2020 at 3:54
• You cannot design anything without detailed specs. What load current , capacitive load and rise time? Jan 26, 2020 at 4:02
• "...and a LM741CN" - read the LM741 datasheet (especially 'Output short circuit current) and you will see why there was no hope of it doing what you want. Jan 26, 2020 at 7:59
• In physical units, capitalization matters (small letters mean different things), so I'm fixing your Hz and V AC :) Jan 26, 2020 at 9:18
• Hey, Geeky, what's your application? Spehro's answer is very good, but it could be way more helpful to explain what you're planning on using that signal for :) Jan 26, 2020 at 9:20

2 Answers

Sounds similar to a telephone ring tone generator, for example like this one, based on the LT1684:

The LT1684 is a more complex chip that actually generates a PWM sinusoidal wave.

Generating a square wave is easier, you can just use four MOSFETs or IGBTs and bootstrapped MOSFET gate drivers (eg. from IR). A full H-bridge with ~ +95VDC supply can be used.

Depending on the MOSFETs you can drive 100mA or 20A with a similar circuit.

• This circuit will definitely work! I should've explained my original goal was to ring a phone (I had a long day yesterday). I'm going to order the parts and test this circuit. I think the only thing I still need to figure out is how to achieve +100v DC and -100v DC from a 12v DC power supply. I found some pre-made converters online but I will research some more to try to build my own. Appreciate your response! Jan 26, 2020 at 14:44
• analog.com/en/technical-articles/… I found this article on the lt1684. Putting it here for reference purposes in case someone needs it! Jan 26, 2020 at 16:39
• There are other types of chips, Supertex is one supplier, but I'm not sure they're still current. The POTS adapters ISPs supply must have ring generators. Jan 26, 2020 at 17:15

~90 V AC that can output ~400 mA

If these are RMS values then you need a device that can pump out circa 36 watts. This can be achieved with a regular AC transformer but you need a decent power amp to provide the primary power (around 45 watts).

Regular AC transformers can suffer from problems of core saturation at lower-than-normal operating frequencies so, if you pick a 230 volt primary transformer and drive the secondary such that the primary (now operating in reverse) produces 90 volts RMS you will be fine and saturation problems will be minimal.

My original idea was to have an Arduino and a LM741CN output a square AC wave and then attach that to a transformer

A 741 cannot produce anything like 45 watts; you need a push-pull power amplifier to drive the low voltage secondary (now the primary).

A transformer rated for 230 volts operation with a 12 volt secondary can be driven on the secondary with a 25 Hz peak-to-peak square wave of about 10 volts (5 volts RMS) and it will produce about 190 volts peak-to-peak on the primary.

Bear in mind that the power into the secondary will be about 45 watts under load and so the current required from the amplifier's output will be about 9 amps RMS. I recommend a 12 volt car battery and a couple of power MOSFETs with a MOSFET driver stage capable of taking a logic level input voltage.