12v ac adapter reads 18v?

[Scruit 0]

While diagnosing a problem withg one of my cameras I noticed the power supply (no load) showed 18v on my multimeter.

 

Then I tried a different one and it also had the same problem.

 

I then tested every power supply in the house, and all showed abou 5 or 6 volts above their rating.

 

I tried this with two different multimeters, both gave the same result. The same two mutimeters show correct voltage for household batteries and my car battery (12.34v while the car is off)

 

Next I looked at the battery in my RC plane - it's supposed to be 9.6v and the charger was spitting out 19v... When I checked the battery it was 12.5v with no load, and about 9v under load.

 

 

 

So, what is it? Either these 12v ac adapters are supposed to be 18v while not under load, or somethign has caused every power supply in my house to fail in the same way at the same time...?

 

The wall outlets show 119v AC.

[survtech 0]

The adaptor is supposed to deliver 12VDC? If it is "regulated" DC it should show 12 volts but many are not well-regulated or require at least a minimum load to regulate properly. Test it with a camera hooked up.

 

Disregard the voltage on battery chargers, they will normally deliver higher voltage than the battery - that is how they charge.

[Kiwi 0]

Measuring power supply output voltage without a load is meaningless and you have simply now discovered why.

[mr.surveillance 0]

Sounds like you have an un-regulated power supply.

(DON'T USE IT ON YOUR CAMERAS!)

 

A regulated 12 volt CCTV power supply will read 11.95 to 12.8 volts no load.

 

A 12 volt supply that is un-regulated will typically put out 15 to 22 vdc unloaded and are used for chargers and items that have internal regulation.

 

I have had customers fry cameras using Radio Shack un-regulated supplies.

 

Remember: All electronic devices contain smoke and if you accidentally let it out there is no way to put it back in. If you let the smoke out you will have to replace the device.

[rory 0]

if the camera says 12VDC then dont leave 18VDC on them .. they will die quick.

 

Always test voltage before connecting it to a camera to make sure you have the correct volts and the polarity is correct. Some more expensive cameras can handle varying levels but the average bullet camera will fry on the spot.

[Scruit 0]

if the camera says 12VDC then dont leave 18VDC on them .. they will die quick.

 

Always test voltage before connecting it to a camera to make sure you have the correct volts and the polarity is correct. Some more expensive cameras can handle varying levels but the average bullet camera will fry on the spot.

 

If I have an AC adapter that is unregulated and spits out 18v, then can I just solder a 1A 12v regulator inline with the power cable? I did this in the car/motorbike camera installations to stop the cameras seeing 14.5v whent he car is running.

 

I learned the hard way that some cameras protect against reverse polarity by shorting the power input with a diode. There goes the magic smoke from the power controller.

[survtech 0]

Yes, you could use a 12 volt regulator. The only problem is that voltage regulators typically require the input voltage be at least a couple of volts above the desired output voltage to work properly. That works OK with 14.5 volts but may not work at 13.2 volts. It also means it can be dependent on your power cable length and size (gauge). The voltage drop in the power cable could affect the regulator's ability to regulate.

 

Manufacturers cheap out if they use "crowbar" reverse polarity protection without providing a simple-to-replace fuse. The better method uses a full-wave rectifier bridge. That allows the end user to hook up DC power without regard to polarity, but it also drops the voltage by 1.2V and many camera circuits won't run at 10.8 volts.

 

Some cameras require exactly 12VDC. I smoked a Dallmeier camera by hooking it up to a power supply that put out 13.2VDC. I later learned that the camera required 12VDC +/- 5% (11.4-12.6 volts). Stupid on my part, but also stupid on the manufacturer's part since it is very difficult to maintain that tight a voltage tolerance in real-world conditions.

 

 

It just proves the point - always read the directions!

[Scruit 0]

 

Manufacturers cheap out if they use "crowbar" reverse polarity protection without providing a simple-to-replace fuse.

 

Cheaper than a fuse would simply be replace the diode with an LED and put a resistor in there. If the polarity is wrong ther a light comes on on the camera, and the resistor keeps the current low enough to not burn out the device supplying the power.

[mr.surveillance 0]

Scruit:

 

If your are referring to a LM7812 or the equivalent added to a cheapo 12v adaptor they seem to work pretty good. I usually add a small filter cap for good luck.

 

The MacGyver emergency power supply:

 

Another dirty trick for an emergency, take your 18 vdc supply and put 7 diodes in series with the power lead and put a small filter cap on it. It wont be regulated but you will wind up with just a little over 12 volts.

Yes I've actually done this for an emergency power supply.

[vin2install 0]

Make it easy on yourself. Buy a regulated 12 volt power supply

[scorpion 0]

WHAT??

 

Give up the bubble gum, and duck tape too!!

[Rebco 0]

Give up the bubble gum, and duck tape too!!

I have graduated to play doh