Getting 9Vac from 12Vdc ?

[Supertramp]

My alarm system for the boat runs off a 240v transformer/plug with an output of 9Vac.

Thats good whilst in the marina on shoreline, but I would like to wire it off the 12Vdc without needing a mains inverter.

 

Is there a gismo that will convert 12dc to 9ac?

[Paul C]

That's got to be the most unusual boat alarm system - is it especially good at it, or especially old, or especially unsuitable? I've not even heard of low voltage AC systems, but given that the difficulty is generating the waveform, then I'd be tempted to go 12v DC --> 240V AC using a pure sine wave inverter, then plugging in the "special" transformer.

[AndrewIC]

Chances are the 9v ac is rectified and fed into a dc voltage regulator inside the alarm. Adaptation to run on 12v dc might be possible, but not trivial.

[smileypete]

Maybe call the manufacturers and ask if it can run off 9VDC OK.

 

I suspect it would, but without looking at its gubbins, it's hard to be 100.0% sure.

 

cheers, Pete.

~smpt~

[Supertramp]

Thanks Paul & Andrew,

Its a wireless system with GPRS signalling. I work 'in the business' and its one of our systems we fit in homes.

Im sure it does convert to dc within the panel, but the input is ac.

I had hoped to avoid the 12v - 240v - 9v route, as it seems such a waste.

[dmr]

So if we assume that it rectifys the AC to DC internally then it is very likely that it will work directly from DC.

You could try it but there is a very small chance of destruction.

You could minimise, though not totally eliminate, the prospect of destruction by testing it on a bench power supply that allows you to set a low current limit, or maybe just use a low power wall-wart. The idea is to prevent the flow of destructive high currents.

 

It is also possible that although it rectifys the AC to DC that it also uses the AC as some sort of clock signal, in which case it either wont work or wont work right.

 

............Dave

[Alan de Enfield]

. I work 'in the business' and its one of our systems we fit in homes.

 

 

Maybe an 'off the wall' suggestion but why not ask one of the techies / designers in your own company ?

[Supertramp]

Good call Alan, Its a sad admission that our engineers seem to have lost the ability to think outside the box, If an alarm is faulty, its cheaper to swap the panel out rather than start looking for the problem. I fear many of them wouldn't know where to start. We buy the panels in, so no R&D in house.

 

I need to get a spare panel to open up and see what happens to the input.

 

Im guessing by the lack of 'gismo' answers, there isnt anything clever I can get from RS or Maplin to do the job

[John Williamson 1955]

If it takes 9V AC, then it may be rectifying and smoothing that to get 13.8 or so as an internal supply for keeping a sealed lead acid standby battery charged. If so, then a small adjustable SMPS module could replace the wall wart. Adjust it to give 13.8 volts at the battery terminals, and either do away with the internal battery or keep it, and monitor the voltages very closely.

 

If you can get hold of a circuit diagram or service manual, that should tell you what voltages are where on the circuit board. If you can't find anyone who can interpret a circuit diagram, post a picture of it here, and at least one person should be able to read it and advise of the picture's clear enough.

[MtB]

Good call Alan, Its a sad admission that our engineers seem to have lost the ability to think outside the box,

 

Try putting them into a really small, tight fitting box.

 

That might work...

 

 

 

MtB

[alan_fincher]

So if we assume that it rectifys the AC to DC internally then it is very likely that it will work directly from DC.

Surely the opposite is true?

 

If the thing takes an AC input, but the first thing that goes through is a bridge rectifier, then feeding DC into that bridge rectifier may well work? (Just that the current will pass permanently through one pair of diodes, and, unlike if it was rectifying AC, then the other set will never come into play).

 

It's my understanding that this is why you can for example buy LED lamp units to run on the "12 volts" DC of boats, and they work irrespective of which connector you attach to positive, and which to negative. They incorporate a bridge rectifier that sorts that out, and ensures polarity to the LED circuitry is always right.

 

I'm not claiming if you feed 9V DC into something marked at 9V AC input that it will work, but if the first component encountered is a bridge rectifier, then I think it might probably will.

 

(Unless someone can see the flaw in my thinking!).

[John Williamson 1955]

I'm not claiming if you feed 9V DC into something marked at 9V AC input that it will work, but if the first component encountered is a bridge rectifier, then I think it might probably will.

 

(Unless someone can see the flaw in my thinking!).

If the first thing a 9 volt AC input sees is a bridge rectifier and a smoothing capacitor, what comes out of that will be roughly 1.414 x 9 volts, or near enough 13 volts, which will give an internal DC supply of about 12 volts with a cheap regulator chip. To be sure it would (a) work and ( be safe, we need to see the circuit.

 

Or, if it specifies AC input, the first thing it might see is a step up transformer, as I've seen in one or two computer audio interfaces. It might even have a simple voltage doubler circuit as the first thing the power sees.

[Chalky]

I'd go with the comments about the 9v Ac being rectified inside to give about 13V. This would then be regulated down to the voltages needed to run the panel. The chances are that it uses 3 pin voltage regulators to get the 5V or 3.3 V to run the electronics, 12V for any sensors and the 13V ish current limited to charge the battery. Depending on the internals you might (and I repeat might) be able to connect it straight to the 12V supply and have it work. If you've got a circuit diagram this would be the starting point to find out. I would second the above comments about needing to check that it's safe and not operating in a marginal or dangerous condition.

 

A few words of warning. The 12V supply isn't 12V. Depending on the state of the batteries you may have anywhere between 10 and 16V available. With the engine running it will be somewhere between 12V (flat battery) and 14.5V ish (fully charged depending on regulator cutout limit) . If you run it off a battery that supplies the engine starter then you may be down to 3 to 4V under cranking (for a few ms). If you have dodgy connections between the alternator and the battery you may see spikes up to 60V (called load dump). There will also be some fast high energy transients into the kV region. Automotive spec regulators are designed to cope with these. If you know how then it may be worth upgrading any 3 terminal regulators - the Texas LM2940 is a typical device. Also have a look at the processor reset circuitry. Voltage drops on batteries can cause brownout issues that you don't tend to see in domestic circuitry. If you know your way round electronics these are fairly minor mods which will help make the installation more robust.

Edited March 28, 2014 by Chalky

[b0atman]

amazon do a 12v car plug adaptor might do the job for less than £12

[Supertramp]

Thanks for the replies guys, I willt ry and look inside a spare panel next week and see what can be done. Im guessing some of the spikes and troughs will be floated over the backup battery and therefore not as damaging as otherwise.

[John Williamson 1955]

It might even be worth looking for a caravan or car alarm system, which will already have the hard design work done and the protection circuits in place.