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Old 12-08-2010, 05:09 PM
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Default On power supply specifications & ratings...

Lots of users have questions about selecting a DC power supply for use with a MIDI decoder or encoder, or any electronics project.

Voltage and current are very fundamental concepts in electronics...if you don't have a really solid understanding of what these words mean, you will save yourself all kinds of headaches by taking a few minutes to learn.

Google & Wikipedia can help a lot. Plus, here's a thread with some links to beginner's electronics info:

http://forum.highlyliquid.com/showthread.php?t=14

Voltage is a measure of the "eagerness" of electrons to flow from one point to another. Current is a measure of how many electrons are flowing through a wire or other object.

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Now for the power supply discussion:

A DC power supply has 2 or 3 basic output specs:

Output voltage: This is the voltage across the output terminals when the supply is "on". In theory, this number is constant at all times, but in "real life" there will be some variation from the specification based on manufacturing tolerances and what exactly is connected to the output. "Wall wart" supplies in particular will vary a lot from the "nominal" output voltage. It never hurts to get a voltmeter and measure the output of your supply in various operating states (open circuit, various loads).

Output current: This spec is the maximum current that should be allowed to flow from one output terminal to the other.

How much current flows from the supply (current draw) depends on what load is connected. If a load draws 100mA of current, and the power supply is rated for a maximum current of 500mA, there should be no problem.

If a load draws 1A (1000mA) of current from a supply that is rated for a maximum of 500mA, what happens? If it is a poorly designed supply, it might melt or catch on fire. A better supply will blow a fuse or shut down before something catastrophic happens. Also, the output voltage of a supply might drop as a result of overloading.

What if we connect two 100mA loads in parallel to the same 500mA supply? 100mA + 100mA = 200mA. 200mA is still less than 500mA, so it's ok. But what if we connect six 100mA loads in parallel to the supply? 600mA > 500mA, so there's potential for an overload condition.

Lots of power supplies have some sort of overload protection...but it's a bad idea to blindly connect a load to a supply without calculating the current draw and comparing it to the supply current rating. Don't rely on the power supply safety features to cover for your carelessness.

Output power: Sometimes a supply manufacturer does not provide a current rating, but instead gives an output power rating. Another basic electronics concept is that power = voltage * current. So, if a supply has an output voltage of 12VDC and a power rating of 6W, we can calculate the maximum current draw from the supply: 6W / 12V = 0.5A or 500mA.

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Current ratings for loads: Let's consider the example of powering the MIDI CPU. If you take a look at the MIDI CPU Rev K hardware manual, you'll see a power supply spec of 150mA. This means that the MIDI CPU is not expected to draw more than 150mA during normal operation. If you have a 500mA power supply, you can power 3 MIDI CPU units (150mA * 3 = 450mA, which is less than 500mA).

It should be noted that the MIDI CPU might also draw much less than 150mA, depending on the operating conditions. When the board is completely idle, it will only draw 10mA or so. The current draw will fluctuate as the MIDI CPU powers an LED, sends a MIDI message, etc. To find the exact current draw of the MIDI CPU in your application, you must measure it.

Last edited by John; 12-08-2010 at 05:13 PM.
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current, dc power supply, power, power supply selection, voltage, wall adapters, wall warts

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