Could I convert a 9V battery to AC?

[ronvegas]

How easy and cheaply could I convert a 9v battery to AC, to power a 9v AC mains DSL modem for testing purposes only.
I don't need it to last long, but if this is possible, how long would it likely last and would the device stop working the moment the voltage dropped below the required 9V?

Thanks.

 

[audioguru]

It depends on the current. It will not be easy and cheap.
The battery will be 9V only when it is brand new then its voltage drops as it is used. It might be 9V only for a few seconds because a little 9V battery has a very low capacity.

9VAC has a peak-to-peak voltage of 25.5V so the 9V battery voltage must be stepped up to maybe 28V with an inverter circuit then a sine-wave oscillator and power amplifier can make the 9VAC. The amplifier will need as much extra power as the modem uses.

Or maybe the modem will operate perfectly from 12VDC.

 

[ronvegas]

Does this mean that it would work from a DC battery, just so long as it gets higher DC than is required in AC?
The current needed is 1A

 

[Willbe]

Does this mean that it would work from a DC battery, just so long as it gets higher DC than is required in AC?
The current needed is 1A

9v battery is 20 mAh?
1A @ 20 mAh = 1/50 hour ~ 1 minute.

 

[audioguru]

Energizer has datasheets on their website for all the batteries they make. The 9V alkaline battery has a capacity of 625mAh at a current of only 25mA and a capacity of 300mah at a current of 500mA. But its cutoff voltage is only 4.8V.

Its voltage drops quickly to 7.2V then slower as its voltage drops more. It is at 6V in one hour with a load of 300mA.

The modem must have a rectifier that changes the 9VAC to the 12V peak. So a 12V battery will probably work when connected to the rectifier.

 

[Hero999]

Sorry but I think it's a silly idea.

For 9VAC has a peak voltage of 12.7V so you'll need a boost converter. To convert the DC to AC you need an h-birdge circuit driven by an oscillator, a modified sinewave or even a square wave will probably do, which is good because they aren't hard to make.

Your best bet would be to use a 12V sealed lead acid battery, that way you only require an h-bridge (no boost converter required) and it'll last for a reasonable length of time too. The AC voltage will vary from 9.9V to 7.4V (from a fully charged battery to a flat battery) but the modem should work alright.

Another option would be to use 12 AA/NiMH cells but be aware that the voltage could be as high as 11.3VAC on a fully charged pack but it shouldn't be a problem.

 

[m2pc]

You could try connecting the 9V battery directly to the modem... as was mentioned, more than likely there's just a bridge rectifier and some caps past the AC in jack. Companies often do this to save money (AC wall adapters are cheaper than DC, and the cost of an AC adapter + bridge + caps is the same or less as well).

Unless the modem is using the 60Hz signal for something (unlikely) you should be fine.

 

[Hero999]

It's possible that it requires AC to derive a bipolar supply, i.e. -12V 0 +12V, using a voltage doubler.

Last time I cheacked an AC adaptor was a similar price to a DC adaptor and in any case, it's probably cheaper to have the capacitor and diodes in the adaptor rather than desinging them in to the modem.

 

[MrAl]

Hi,


I agree that using a small 9v battery is not a good idea. To get 9vac from a
9vdc source it has to be boosted as mentioned in the other posts, and the
average current after inefficiencies would be around 1.2 amps if you are lucky.
This would drain an 9v battery pretty fast, and might even cause a large
drop in voltage so the boost would have to be a higher percentage which
would cause a higher input current. For example, if the voltage drops to 7.5v
under load the battery drain would go up to at least 1.4 amps, probably even
higher like 1.5 amps. This would kill the little thing quite fast and that's a
good thing because the tiny 9v would get pretty darn hot.

Go with an AA battery pack or a Lead acid battery as mentioned in the other
post unless you only need 5 minutes of operation.

If you decide to stick with 9vdc to 9vac then use a pulse first positive
and then negative, where the pulse average has the same average as
the ac sine wave would have over 1/2 cycle. This means a positive
pulse quite shorter than 8.333ms, then at the next half cycle a negative
pulse the same width as the positive one, then repeat. A micro controller
would come in handy here.
If you want to get more sophisticated, go with a PWM sine pattern.
As someone else said, a MOSFET H bridge would be good here, while
bipolars would most likely mean another drop in efficiency and so more
load on the battery.