velohead66

I want to run the above mentioned device off the car battery, when the car is locked, alarmed and immobilsed.

What does 50mA mean, and how do I get it to run off the car battery ??
Does it need some sort of resistor etc to run correctly ??
It is designed to run off 240v ac, but has a big combo plug/transformer as part of the power lead.

velohead (not an electrical engingeer, obviously)

alcazar

iTrader: (2)

Assuming it runs off 12v from it's mains adaptor, there should be no problem. It will only draw 50mA, so you won't need a resistor.
The 50mA bit is 50 milli Amps, or to put it another way 50 thousandths of an Amp,(0.05A).
Basically, with a good battery in the car, you should have no probs- you could leave it on for 20 hours b4 it had even used 1Ah (amp-hour), and most car batteries are above 400Ah
I wouldn't advise leaving it for 6 months though..........
Alcazar

velohead66

Sounds good to me.

I assume a car battery pushes out quite a few amps, and don't wont to blow potentially sensitive electronics.

matt.bowey

Alcazar is bang on, batteries are rated in Amp/Hours, so 50mA is an insignificant load to place on any car battery.

Sure batteries can deliver a lot of amps, a 400Ah battery could deliver thousands of amps for a fraction of a second!! Try dropping a spanner across the terminals and watch it melt!! (Seriously, don't try the spanner thing!!)

velohead66

Device number 2 runs off a 9v battery (the small square ones), not sure what power ampage this is though. Device is sensitive with regards to the circuitry inside.

I want this to also run off the car battery,
Can I assume all I need is a transformer that converts 12v dc -> 9v dc, and don't have to worry about any ampage issues ??
I thought that if volts is *stepped* down, then amps is *stepped* up ??

And yes, I've seen the spanner thing first hand, nasty !!

MartinM

Best way to think about is that devices DRAW current (amps) from the power source. You can't blow a device connected to a voltage source (at the right number of volts of course) by having too much 'ampage' in the source (unless it's a constant current source, a smart alec might point out, but they work by increasing the voltage to make sure that the specified current actually flows - and are fairly rare in practice)

Device 2 *could* be sorted with an inline resistor, *provided* it draws constant current, and not too much at that. With a bit of Ohm's Law you're sorted...




...OK, I'll do it for you

Say Device 2 draws 50ma (0.05A). You want an inline resistor that has 3V (12 volts from battery - 9 volts across Device 2) across it whilst carrying 0.05A. R = V/I = 3/0.05 = 60 ohms.

So, with a constant source of 12V, a 60ohm resistor in series with Device 2 will result in a voltage across Device 2 of 9 volts and Device 2 being able to draw its full 50 mA. Sorted ....

...BUT your battery isn't 12V, or constant. You can do the maths, but if the battery is 14V, the resistor would have to be 100ohms..and you have to have a different resistor for any given battery voltage

...AND you have to know how much current Device 2 actually draws, and whether it's a constant.

Best idea is actually the UG32K - Universal DC Adaptor from http://www.maplin.co.uk for £9.99 - and it doesn't make your head hurt thinking about electronics

alcazar

iTrader: (2)

Agree with MartinM about the universal DC adaptor.
You can't use a transformer, as transformers need AC to work, with DC they just get very hot!!!
Alcazar

velohead66

Mmmm. Looks like the UG32K plugs into the cigar lighter, which IIRC does not work when the car is immobilsed etc.

I was hoping for a direct connection straight from the battery, one to device 1 @ 12v, and one to device 2 @ 9v.

Well, food for thought, I may have to revisit my 'O' Level physics book thats gathered dust for countless years.

velohead66

PS - so the battery may have different voltages as well, of say 11v to 14v. Maybe this UG32K could be wired direct to the battery rather then the cigar lighter ???

MartinM

Wire it straight to battery - should be no probs, as long as you use a fuse - and as long as it's 'upstream' of any current detection device that the immobiliser may have i.e. connect it straight to the battery terminals

I never knew that cigar(rette) lighters were disabled when a car was immobilised....are you sure???

PS what are these devices anyway? Christmas tree lights?

matt.bowey

Transformers are no good with DC as DC does not see the volt drop caused by inductance! A transformer is almost a short circuit to DC (except for the low value of resistance!). It gets hot due to the huge amount of current flowing through the primary winding!!

Sure you could mess about with a voltage divider network to drop from 12V to 9V (connect a 10 ohm in series with a 30 ohm). From one end of one resistor to the middle you will either see 3V or 9V. Simply connect across the 9V leg.

I wouldn't bother though, I would expect most 9V devices to work at 12V.

Of course, I accept no responsibility for any explosion, fire, injury or fatality resultsing from the above!

velohead66

*devices* are additional security for when the car is in the car park at work.

Have a small camara & signal transmitter in the car (device 1 & 2), external pick up arial on the roof at work, and a black/white monitor on the desk.

Can't always see the car, or hear the car alarm, but I should be able to monitor it remotely, without leaving my desk every 5 mins.