12 volt power supply reads 20 volts, 9 volt reads 12???

Question

I need a power supply to use for a project I'm building. The power supply needs to supply 12 volts (power supply as in 120 volt AC to 12 volt DC). But here's the thing. I found one rated for 12 volts. I checked it's output with a voltmeter and it's reading 20 volts almost perfectly. I jokingly thought to myself "Well, if the 12 volt reads 20 volts, then a 9 volt should read 12 volts right?". Well, I got a 9 volt power supply and sure enough it's reading 12 volts :P . I don't think my voltmeter is bad, but you can never really be sure unless you have something to compare it to. The max AC volts it can take is 15, so hooking it up to the wall is out of the question. So which power supply should I use? Since I'm pretty much finished and really want to try it out, I'll try it using just the 9 volt one. If it helps any, I'm building an alkaline battery recharger.

Answer 1

Unregulated power supplies always deliver more than the rated output when no load is on them.  When you connect the load the voltage will drop down somewhere near what it is rated at.

Circuits which are very sensitive to the initial voltage should be powered by a regulated power supply, which stays at it's correct voltage with or without a load.

You are talking about charging batteries, I am sure the voltage will drop down to near the rated amount the instant you connect it to the batteries.  No harm will be done.

However, it is also very easy to cook batteries by over charging them.  Good battery chargers have regulators to prevent this.

Answer 2

thats ''normal'' because the power supply you are using is no regulated. if you want to regulate the output to get exactly 12Volts out of the PS use a 7812 integrated circuit, if you don't know how, a simple search in google will get you a lot website where you can find information, try typing ''12v 7812 power supply''

good luck