mA at 9 or 12 volts


Postby nitro23456 » Thu Dec 18, 2008 2:20 pm

Hi,

I have bought a wireless CCTV pinhole camera that runs off a 9v battery.

I want to buy a power supply for it. I have one which is 12v 900mA but it buzzes when it works (but it does work). I have also tried another supply with lower mA (12v 500mA I think) and it doesnt work at all.

It works off a 9v battery as well silently

What do I need to provide the power from mains silently? I dont understand how the mA system works or what it actually is.
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Postby ericmark » Thu Dec 18, 2008 10:24 pm

When one looks at these plug in boxes with a low voltage output they all look the same but inside there are huge differences.
1) There is the simple AC outlet type often used with modems often quite a high output for size but all the clever stuff is done inside the equipment it supplies.
2) The battery charging type are very simple similar to 1 but have 1 or 2 diodes to make output a very rough DC on audio equipment they are likely to produce a mains hum.
3) As above but also contains a capacitor still likely to give mains hum but not as bad and the voltage is very dependent on load OK to power Christmas tree LED’s but not too good at replacing a battery.
4) As above but also contains some electronics to keep the voltage stable these will work radios etc. But are now getting to the large and heavy side will very low output.
5) Switch mode these turn the incoming mains to DC which charges a capacitor then back to very high frequency AC which means a very small transformer is needed to transform down to required voltage then back to DC again where the voltage is sampled and the parameters of the part producing the HF is changed to ensure correct output voltage. Used for allsorts from mobile phone chargers upwards normally with a very high input voltage range typical 100 to 250 volt and run far cooler than all the others. The little one for my mobile phone is 1 amp at 5 volt output and will run off 100 to 240 volt input so rated at 5W double the output of one bigger but constructed as in Number 4.
So if you have type 2 or 3 you will have problems getting the voltage to within tolerances but with type 4 or 5 you should not have a problem. Of course they cost more. So look for either one which says regulated or which has a input voltage 100 to 240 volt marked on it which although the are often do not say regulated on them. The switch mode is also lighter and have a min as well as max output current so open circuit with voltmeter often read as 0 volts only when loaded do they start to work.
Does that help?
Eric
ericmark

Postby nitro23456 » Fri Dec 19, 2008 1:55 pm

yes thanks
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