12Vdc to 12Vac inverter

now before i buy components, i thought i would bring up the idea here.
working with a 12Vdc power source, i need to generate 12Vac for a alarm system (pre-bought, and can't change power needs).

now, i can't seem to remember, but can an H-bridge generate 12V AC (square-wave)? i think it would, as it would work like an inverter. for current, we are talking about 400ma.
i would use a large-ish H-bridge, in case of surge currents.
the reason i ask is, i have someone telling me it would generate 24V AC, not 12V AC. but i think they are thinking of a half-bridge, not a full H-Bridge.

i tried the alarm system on 12V DC, but i think it must have a sensor to check for AC power, before it will go off battery backup. i need the alarm to run off my solar battery bank (voltage fluctuations are within range for alarm power input, when charging batteries). worst case scenario, i can get a 12V DC - DC Buck-boost converter to feed the inverter circuit regulated 12V DC for stability.

i have tried using a 12V DC to 120V AC inverter, followed by a 120V AC to 12V AC transformer, but it is much too inefficient in the conversion process.

My plan is to generate the signal for the H-bridge using an atTiny. no monitoring needed, so the program i write will be very simple (startup, and generate signal forever)

i just need to confirm that as i figure, that it will result in 12V AC, not 24V AC as i have been told by someone local, that i don't think knows the difference between a half and full H-bridge.

Use a driver, actually a logic N channel MOSFET and analogWrite(512).
That gives You an AC swinging from 0 to 12 volt. If You want +- 12 volt swing other methods are needed.

How much current is needed?

no more than 400ma

Draw your signal on a piece of paper. You will have a ground line, then the signal goes to +12 volts on one wire. Now draw the signal for the other half of the signal. Where does it go? Also to +12 volts on the other wire. That is not AC. Run it through a center tapped transformer winding and the secondary will give you an AC signal.

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so ignore the primary, and use the 3 taps of a center tap secondary? or use a 1:1 transformer with a center-tap secondary? i think it is like the first, using the transformer on one side of the center-tap for input, and the other side of the center-tap as the output?

Use a 24 VAC center tapped transformer for the primary. You will be applying 12 volts to each side of the winding. The other winding will be whatever you happen to have.

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ok, i think i got it now. many thanks :slight_smile:

Your problem is crying for a two transistor multivibrator with the transformer driven by the transistors. So much simpler than what you are proposing. Look for circuit board mounted industrial transformers. Your problem is a common one solved by hundreds of designs. I think I have some transformers similar to what you need that were removed from circuit boards. No identification.

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I’m fascinated to see what alarm needs an AC supply.

If you can, please post a link or any information yo have on the problem device.

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it is an older (1997 - 1998) RadioShack 49-485 Home security/fire alarm control center.

If your 12vac goes to diode bridge you can just connect 12vdc to it

without destroying the protection around the control circuits, i don't know that. i just know it works with 12V AC, but it will not switch off of battery backup on 12V DC.

IF it goes directly to bridge rectifier THEN it is safe. The bridge rectifier will use 2 of its 4 diodes to conduct your DC, you will lose some voltage but it should be fine. I suppose you don’t have circuit diagram for the system?

i don't have a circuit diagram. for some reason companies don't publish schematics of their security hardware... :wink:

i have tested it to not work on 12V DC, as it will not return off of battery backup mode. but with 12V AC it returns fine. my guess is there is a AC sensing circuit... i wonder if simply chopped DC will work...

maybe there are other circuits that need that AC, like power failure sensing then it makes sense it won’t work properly

could simply be a DC-blocking capacitor before the bridge... either way, i will try chopped DC and see what happens. i have a un-filtered 12V power-supply that could work. if it does, i don't need a H-bridge, but just a single transistor (small MOSFET maybe) circuit to make it work off of DC power.

Just a wild thought. You might try connecting the 12Vdc to your alarm with the polarity reversed. Perhaps the circuit looks for a negative voltage to determine if AC is being provided.

Still thinking...

The simplest approach is two MosFets driving the ends of a 24VAC Ct transformer. However I doubt you will find any 12vac to 12Vac or 24 to 12 etc transformers.

But transformers are pretty efficient so you might get a 24Vac Ct to 120Vac connected to a 120 Vac to 12Vac transformers

Did you in fact try connecting the DC to the alarm system both ways round?

The problem - as always - is that you are not adequately describing the situation. :roll_eyes: I am going to presume the input connections are screw terminals, along with a (substantial) number of other connections including the backup battery which you mention.

Since you mention the backup battery, a logical approach is to connect your 12 V battery supply as the backup battery, since it is already - a backup battery. :grin: The only reason I can see that you would actually require 12 V AC is that its peak voltage is in fact, 17 V and that voltage may be required by the charging circuit for the lead-acid gel-cell backup battery. Otherwise there is no need for a transformer as a 12 V square wave would be sufficient (except for again - the 1.4 V loss in the bridge rectifier).

Frankly, the proper answer here is to figure out the internal details of the alarm unit and modify it appropriately. However simply connecting your 12 V battery supply as the backup battery and disabling any alarm setting which complains about "loss of power" would seem the simplest.