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Substituting a 9vAC @ 3.3A transformer with 9vAC @ 700Ma

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New Member
Hello, I am an shade-tree mechanic in this field (if even that), who has painted himself into a corner so to speak, and is hoping for help.

I recently installed a Simon XT alarm system for my father. The Simon XT is the fourth model of the Simon series and has a 9vAC @ 3.3A transformer. All previous Simons have used 9vAC @ 700Ma transformers. I was told the new Simon had the higher Amperage capability due to the new GSM cell phone module that can be installed, which uses 1.7A at peak transmitting power.

I believed this, and believed the guy who told me he has used the old Simon 700Ma transformers on the newer Simon XT for years. This is advantageous because the new higher amp transformer has yet to come equipped with an X10 line carrier as previous 700Ma Simon transformers did. Apparently, no one has been able to make a 3.2A transformer and line carrier that can pass UL testing.

So, my Simon XT worked fine for weeks on the smaller transformer, until I physically unwired the unit and moved it to a better location at my parents request. It worked for a minute at this location, but as soon as i closed the unit's case, it went berserk (lcd screen and speaker putting out gibberish). It never came back to normal. I think it was a physical break due to circumstances, but father is convinced that it was my transformer substitution.

So... I need help convincing my father (and myself) that i was right and that we can still use the 700Ma transformer with X10 on the new alarm panel that will be arriving shortly.

1) Can you tell, from the details below, whether a 700Ma transformer will be adequate for my alarm.
2) If you cannot tell from these details, is there any test that I can do with my Multimeter to ensure that the unit doesn't draw more than 700Ma (without the GSM option that we did not install)?

*All Simons require the same voltage (9vAC).
*All Simons (old 700Ma and new 3.3A) use the same rechargeable battery. (6v @ 1.2AH).
*All Simons have the same maximum output for hardwired sirens (250Ma)
*The new Simon (3.2A) has a backlit LCD screen and a GSM transmitter that draws 1.7A peak, unlike the older Simons which have neither. I have the LCD screen but not the GSM module.

My thoughts - if both panels can operate on the same backup battery, isn't that a key point, proving that they should be able to operate on the same transformer?

Thanks in advance for any replies.


Does the transformer still work?

It might be that the mains voltage in the new location is slightly lower and the secondary voltage on the smaller transformer is too low.

Perhaps the smaller transformer has overheated and developed a shorted turn which would reduce the secondary voltage.


New Member
Thank you for the reply.

Yes, the smaller transformer still works (I tested the voltage on everything I could find, and all was in spec), the unit is likely the problem, confirmed by the fact that the alarm still did not operate when wired to the other transformer. I also felt the transformer after the incident and several times during it's week of operation... it never seemed too warm.

If I read your reply correctly, you are thinking that the new location required the transformer to be moved as well. It did not, since the move was simply to the other side of the same window, and the outlet supplying the transformer is centered underneath that window. Same circuit, same transformer, same alarm.

I just noticed that my Multimeter does not have a setting for AC current testing. Darn. May have to go bum a better meter of one of my uncles.
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Yes you're right, from what you've said, it sounds like the the unit is broken.


New Member
Hmmm.... not may replies. Maybe if I rephrase the question to be more concise:

I have a product/circuit board that used to require a 700ma transformer, then this product was given a new "option" which required 3.3A transformers. I do not have this new "option", so could I run the 700ma transformer or would doing so be harmful in some way? (exe: would the new circuit have been redesigned around the 3.3A maximum or something?)



New Member
Thank you!

I am getting ready to purchase a new multimeter for just that purpose, since my current meter doesn't measure AC current for some reason. (It is a 9vAC transformer).

So... if the unit never pulls more than 700ma, can I be confident that everything will work fine? (actually, it shouldn't pull more than 2/3 of 700ma, right?)

Thanks again for the reply.


Well-Known Member
Most Helpful Member
It certainly sounds likely that a GSM module would need a larger transformer. If you don't have the GSM module, you don't need the larger transformer. Also, it seems likely that the manufacturers use just one type of transformer, as even a 3.3A one is quite cheap, and probably cheaper than messing around with two transformer options.

GSM modules take up to 1.9 A at 4 V. They only take that current for 570 µs at a time (1140 µs when using GPRS) so your multimeter won't show the peak current correctly.

I can't quite see why the alarm manufacturers have specified such a large transformer. GSM modules only average 0.5A max. The transformer is AC out, so there must be a smoothing capacitor to hold the supply up for just less than half a mains cycle. That capacitor would have to be about 4700 - 10000 µF, and so it would dip very little during the GSM pulses.


New Member
Diver - thank you for your input. Also, I too have noticed the discrepancy between the Amps of the components and the amps of the transformer.

The only guess I had was that the 1.9A max of the GSM + the load of the original components (somewhere below 700ma) would come out in the mid 2A range... then accounting for a cushion room, maybe the 3.3A transformer was the best/cheapest/available option. No idea, since I am NO expert on this stuff, just learning as I go.

Blueroom - Most people I have talked to in my search for the perfect alarm for my parents would NEVER use x10 as the main component of the alarm. Mainly because it would not work when the power is out.

But most, if not all, high end alarms do support x10 in addition to the proprietary alarm components. Just like my alarm... i can set it to blink the house lights when the alarm goes off, my parents can turn on the house lights from their key fob, I can add wireless sirens set off by x10 signals, or I can have the basement lights come on when I pass the motion sensor located on the basement stairs... it is pretty fun stuff :)


Well-Known Member
Is there something better?

A wired home alarm system professionally installed. You may even qualify for a break on your homeowners insurance.

The 1970s X10 standard is awful, made worse by almost anything you add to your mains. Halogen lamps are very troublesome for X10. It's been around for almost 40 years, even Radio Shack cleared it out several years ago.
X10 (industry standard) - Wikipedia, the free encyclopedia

If X10 worked it would be commonly used. I'd love to control my home with it.
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