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12V power Supplys not reading 12V can you help

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Enlightx

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Hi everyone

just signed up to this forum as iv been doing electronics now for over a year and have read various differnt answers about this question

i mainly use arudino stuff atm and as i work in a computer shop we normally have loads of spare 12V DC Power supplys from routers scanners ect which i can use for free.

now im having problems using these as when i read there voltage with a multi meter im getting a reading of 16V - 18v. which screws up the arduino voltage regulator thus not giving me a true 5v on the arudino.

iv read various differnt answers to this question from cheap power supplys just being rubbish to all power supplys are like this and will read 12v when there load on them (which with my arudino it drops to about 14v)

can some one give me a proper answer?
 
All the things you've read probably are right.

The problem is there are power supplies and then there are power supplies.


You can get
  • Power supply that outputs regulated DC on and off load
  • Power supply that outputs regulated DC but needs a load on it
  • Power supply that outputs un-regulated filtered (smoothed) DC
  • Power supply that outputs un-regulated and un-filtered DC
  • Power supply that outputs AC (no rectifier or filter)
A lot of kit like routers and scanners will have their own regulator within the equipment so they don't need a regulated power supply.

In fact if your Arduino board has a 5 volt regulator on it, it should work with an unregulated DC filtered power supply.
 
iv read various differnt answers to this question from cheap power supplys just being rubbish to all power supplys are like this and will read 12v when there load on them (which with my arudino it drops to about 14v)

can some one give me a proper answer?

I can try. The answer is all of the above. :)

Attached is a basic image of a wall wart power supply. Note that units like this do not afford any regulation at the output. Additionally if we look at the step down transformer it is just a simple ratio device. With an input of 120 VAC RMS the output is 12 VAC RMS or a 10:1 ratio. If the input increases, the output will follow it.

The transformer is coupled directly to a full wave bridge rectifier. The rectifier will have a forward voltage drop of about 1.2 volts for alternate cycles of the AC input, so with 120 VAC in the output of DC will be about 12 - 1.2 = 10.8 VDC.

This is fed to a filter capacitor. That cap unloaded or lightly loaded will charge to about Epk or the peak value of the AC waveform. Epk = Erms times 1.414 so we get:
10.8 * 1.414 = 15.27 or about 15.3 VDC at the output (unloaded).

now im having problems using these as when i read there voltage with a multi meter im getting a reading of 16V - 18v. which screws up the arduino voltage regulator thus not giving me a true 5v on the arudino.

Yes, you will because:

Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V

When the input voltage to your Arduino board starts exceeding 12 VDC there are regulation problems. What I use is a 12 volt DC wall wart and drive a LM7805 and power my PIC boards. This allows me to add additional filtering also.

So that is about how things work using a wall wart.

<EDIT> Beat by Geko..... :) </EDIT>

Ron
 

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Thanks guys you have said exactly what i was thinking just helps to have it all backed up.

taking this into account i think i need to look into make some voltage regulators that will take upto 20v and give me a smooth 12V to use in my projects

i have bought some 12v 1.5A (7812) voltage regulators but have read into things and it says you need capacitors to get to work. Can anyone explain how and why this works and a rough guide to know which capacitors to use and what type of capactiors as well.

Thanks .. think im gonna like this forum
 
In addition to the sticky post attached is a cartoon continuation of what I posted earlier.

The caps serve to filter and clean up the DC. Additionally they provide reliable operation of the regulator preventing oscillations. There are formulas for calculating their values but a simple rule of thumb is 1 uF per mA at the input side. The smaller .1 uF caps are in there to clean up any higher frequency noise.

The 3300 uF capacitors in my drawing do represent overkill but not knowing what the ripple looks like from the wall wart I went large. Actually 1500 uF would be more than adequate.

Matter of fact the little board I made for PIC projects is few by a 12 volt 1.5 amp wall wart and I used 100 uF in and out as well as .1 uF in and out. It works fine.

Hope that and the link help.

Ron
 

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Why not, just feed the uc +5 regulated from a 7805, off any "generic 12v power supply or wallwart"? This is how I am powering my Picaxe chips during development...

From what I had 'understood' in the past, a voltage regulator needs a few more volts in, that the desired volts out.
I am still new to electronics, so forgive me in advance if I am incorrect, but you cannot get a fixed 12v voltage regulator
such as the 7812 to output a proper 12v from a 12v input, it needs to be 14v or more...


Andrew
 
I am still new to electronics, so forgive me in advance if I am incorrect, but you cannot get a fixed 12v voltage regulator
such as the 7812 to output a proper 12v from a 12v input, it needs to be 14v or more...

That is correct and it is called the "dropout Voltage". A 78XX series regulator requires 2 volts above desired output at the input.

There are what is called Low Dropout regulators with less overhead but they are more exception than rule.

Ron
 
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