How do I get acurate power?
Interactives
Posts: 83
Hello, My name is Jennifer, and I have just made a sobering realization about "wall wart" power supplies- or AC to DC wall mount power transformers. I work in a shop that has many many projects running on Stamps (BS2). Usually, to power these projects, I use a run of the mill, 500mA Universal AC adaptor. They have adjustable output between 1.5 to 12 VDC. I noticed right away that when set on 12VDC the voltage read 19.26, when set on 9vdc = 14.11v, 7.5 VDC = 11.80 at so forth and so on. It kind of made me scratch my head, but usually I would just wire up my VIN with the transformer set to 7.5 VDC, which was 11.80 in reality. Some time passed, and I was shocked to find that one of my projects, which I am making for a museum to have on 24/7- mysteriously died. I replaced the STAMP, and it came back to life. It has occured to me, that I might very well be up a creek with this- I have several projects wired up with these power supplies. It has only recently come to my attention, when I ordered 45 illuminated push buttons with 6volt LEDs, that when I actually use a 6V power supply, things tend to burn up. What is a girl to do? If it really doenst output 6volt, why in the world would it say 6vdc output? How am I supposed to actually get these voltages? Am I up a creek, or what?
Comments
A voltage regulator will take care of this. The Stamp has one on-chip, but it has limited current capability. You can get an inexpensive +5V regulator at Radio Shack which will turn that unregulated power into nice clean regulated +5VDC, which the Stamp (and your LEDs, etc) can then "eat".
You might also consider filtering both the input and output sides of the regulator with an appropriate filter capacitor - this will eliminate any noise that could otherwise cause things to misbehave.
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·1+1=10
As stated above *most* wall warts are unregulated. They have a higher than rated "float" (meaning no load) voltage that is considerably higher than the rated output. This is so when a load is connected, the wall wart will not drop below the rated voltage until the rated current is exceeded.
But no, you are not up that dark brown river without a certain instrument. As said above, you can regulate the power yourself. Or, you can buy regulated wall warts. They are of course more expensive, but you could just plug them in and go.
As to the LEDs, what burns up? Do you have current limitng resistors on the LEDs? Tell us more.
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
Also when you are testing the supply's voltage, are you doing it with the board powered or just measuring the voltage with nothing hooked up? To get a better measurement, you should test it with the board powered, because the voltage of an unregulated supply will vary according to load. The voltage value provided on an unregulated wall wart is obtained by using a certain load (resistance). If the load has a higher resistange, the voltage will be higher than listed; if the load has a lower resistance, the voltage will be lower than listed. You can trick an unregulated power supply into seeing the proper load by placing a power resistor in series with the load (if the load's resistance is too low), or by placing a power resistor in parallel with the load (if the load's resistance is too high). But you have to be sure the resistor can handle the current it will see (especially if you place it in parallel).
The resistor solution may be the simplest method for your already built displays.
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If you want to test this, get a nice high-wattage load resistor of 15 ohms (15 == 7.5 / 500 mA). Power dissapation is: P = I * V, so P = 500 mA * 7.5, which is 3.75 watts. A 5 watt resistor of 15 ohms should handle this (it WILL grow warm though).
So, put the load resistor across the wall-wart output, and THEN measure the voltage. It should be 7.5 volts.
In theory, you could design a small 'adapter' board with a regulator and a couple of capacitors on it. The 'adapter' would accept the unregulated voltage from the wall-wart, and put out a regulated voltage you could depend on.
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If your looking to save some cash on your power supplies, then check out the following links:
www.allelectronics.com
and
www.mpja.com
Mike
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Of course, you'd have to find a place in the wire where you could splice in the resistor. And you'd have to find out exactly what the worst-case voltage on your supply was in order to select your resistor. The good news is that LED's tend to light mostly the same brightness over quite a range of currents (8 to 15 mA).
The other good news is that if you've purchased 45 identical un-regulated supplies, their open-circuit voltages are probably all pretty similar to each other.
Post Edited (PJ Allen) : 12/13/2005 9:59:16 PM GMT
This guy is, and always be your best friend....
http://www.national.com/ds/LM/LM340.pdf
Voltage input 5-12V....output: 5.0V, up to 1 Amp.
i am not shouting. I have been able to read and type all caps much easier for over 55 years.
One thing i may have missed but don't think so. Are these devices all in seperate locations where several devices can not share a single power supply?. If not and they are withing a resonable distance , then with carefull wireing maybe you could cut down on overall number of power supplys.
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73
spence
k4kep