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Basic Electronics » Generic 120VAC adapters (transformers)

May 25, 2010
by DavidinLA
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I have been looking at several 120VAC adapters that I have for powering different devices (shaver, cell phone, etc.) They all say:

CLASS II transformer

INPUT: 120VAC 60 60Hz 30W

Then they have an output which includes both volts and amps, for example:

OUTPUT: 12VDC 1500mA -or- OUTPUT: 6VDC 150mA

What is the amp rating for? Does it has something to do with current fluctuation as the AC is being rectified?

May 25, 2010
by Ralphxyz
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Hi David, the simplest answer is it has to do with the maximum amperage output of the device.

6VDC @ 150mA will not light up the NerdKit LCD (324mA) backlight.

Ralph

May 26, 2010
by bretm
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It generally implies two things: 1) if your circuit tries to more than the rated amount of current, the supplied voltage is no longer guaranteed, 2) if your circuit tries to draw more than the rated amount of current, the device may even be damaged.

May 26, 2010
by DavidinLA
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Thanks for your replies!

So just to make sure, it has nothing to do with any fluxiations in current. The power supply is assume to produce a steady current.

It has more to do with what the maximum amps in order to not damage the device it is supposed to power?

This next question is going to sound REALLY dumb, but I re-read Ohms law several times and can't seem to understand something. Common sense indicates both these transforms output at fewer or equal amps than what is printed otherwise they would cause damage. But 12V @ 1500 mA seems like LESS current flow than 6V @ 150mA and in my feeble brain this does not compute.

I like to use the analogy of water when looking at the realationship of volts, amps, ohms. I have come to view Volts as pressure. So 12V would be a bigger tank of water than 6V. Current (amps) would be the speed the water flows and resistance (ohms) is created by a valve inside the pipe used to slow the water down. If you are trying to achive a smaller flow of water (1500 mA) Why build a big tank of water (12V) and then slow the flow down clamping it with strong resistance. Why not build a smaller tank of water (6V) and then not have to apply as much resistance? Or am I just way of in my basic understanding of electricity?

May 26, 2010
by mongo
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Look at in watts. A product of volts and amps. One mA is .001 Amp. 1000 mA is 1 Amp

12 Volts at 1500 mA would be 12 X 1.5, comes out to 18 watts.

6 volts at 150 mA would then be 6 X 0.15, which would equal .9 watts.

To use the water analogy, the voltage would more be like pressure than tank size. 1500 mA compared to 150 mA is 10 to 1. Then the pressure behind that at a 2 to 1 ratio would net an increase of 20 to 1. The tank size would be more analogous to cell size, AA as opposed to D size batteries.

Voltage is one way to overcome certain obstacles and internal losses in electronics. Just to get across a diode junction, you need at least .7 volt. If you have a small supply, say 3 volts, it leaves you with only 2.3 volts to do any work. The more control you have over an output, the less effective the work potential.

Newer circuits and micro-power stuff is designed for the tiny voltages but at a cost. That's one of the reasons the 9V batteries are smaller than say a AA cell. They both have similar wattage capacity but at different voltage and current levels.

May 27, 2010
by bretm
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My comment about "damaging the device" was actually referring to the power adapter, not the device it powers. If the adapter is rated at 150mA and your circuit tries to draw 2 amps, it could theoretically damage the adapter. It depends on the design of the adapter--some have protection circuitry.

The amperage listed on the power adapter doesn't indicate that it "supplies steady current". Usually the goal is to supply steady voltage. The current then depends on the load of the circuit, measured in ohms. The amperage listed is the maximum that it is rated to supply and still maintain the rated voltage.

May 27, 2010
by DavidinLA
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Thank you all for comments. This helps.

Anyone recommend a good Web tutorial or beginners book on topics like volts, watts, loads, etc?

I am hoping to use my nerdkit to power a variety of devices (motors, lights, mps players, etc.) I am well versed in C programming but not in electronics. I am finding that many of the devices I want to power I can find pre-made (no soldering). Turning lignts on and off are simple enough as long as you supply the correct power. Even mp3 players I am finding eval boards that don't require any design or soldering. Just connect the correct power to the correct terminal. Worst case scenario is you have to learn UART or SPI which I think I can handle.

So at this point I am less interested in learning specific electronic parts (what a capacitor is or how it works) which a lot of beginner books focus on. More interested, at least right now, in knowing what the correct voltage, amp, etc. for a particular device is and then selecting proper transistor or relay to step up to correct voltage, amps, etc. Seems to be a bit more involved than I initially thought and rather than fry a $100 eval board I think I should learn this stuff.

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