Question on Evaluating a Power Supply

[jeffamm]

I'm evaluating a wall-transformer type power supply used to power guitar effect pedals as a replacement for a 9V battery. It's rated at 9V and 2000 mA. I put resistive loads on it of (nominally) 16, 8 and 4.5 ohms, and made measurements with a Klein Tools MM1000 multi-meter. Here's what I got:

Load DC V DC I AC mV

None 9.14V 0A 280-305 varying
16 ohms 9.01V .56A same
8 ohms 8.87V 1.11A same
4.5 ohms 8.67V 1.88A same

I measured the AC millivolts in hopes of characterizing the ripple voltage but I found the readings varied from 280mV to 305mV after about 5 minutes, and didn't change when the loads where changed. I thought I might just be measuring noise, and unplugged the transformer to see if the AC reading went to zero (it did) but I don't know if that proves anything. I don't have access to an oscilloscope. Is there a better way to measure ripple and can I conclude anything from my measurement?

The DC output voltage varied by about 5% with loads. Is that good, bad or typical for battery replacement power supplies? Any other observations about the data?

thanks in advance for any help

 

[Fish4Fun]

I assume you are talking about a standard 9V battery. The rated capacity for an Alkaline PPP3 => 565mAh. A 16 ohm load on a nominal 9V source should draw:

9 = 16 * I = 0.5625A

and should last about 1 hour. An 8 ohm load would drain the battery in half that time etc, etc.

Assuming the 9V battery has an internal resistance of 2 ohms, the apparent battery voltage with an 8 ohm load would be:

9V = (8 + 2) * I = .9A

The voltage drop across the load would be:

E = .9 * 8 => E = 7.2V

This means that IF your circuit uses a standard 9V battery AND it presents an 8 ohm load to it AND the circuit performs as it should, then the circuit can perform satisfactorily @ 7.2V input, suggesting that your wal-wort would be well within tolerance @ 8.87V.

Actually I think the power supply you are evaluating is considerably larger than required; however, it should work just fine.

Fish

 

[jeffamm]

Thanks Fish. I should have been clearer that this unit would replace the batteries in many pedals at the same time, not simply a single battery. A typical guitar pedal board might have 5 to 10 pedals, and the battery connectors would get daisy chained to this power supply, up to its maximum capacity.

 

[schip666!]

Because the voltage doesn't vary _that much_ with load it looks like you have a regulated wall-wart -- which is a good thing. Your usual garden variety wart is un-regulated and thus the unloaded voltage can be up to twice the marked value, approaching to the speced value at the speced current draw.

I don't know why the apparent ripple changes over time, but it might not matter. The best course of action is to power up a couple of your effects boxes with the supply and see if you can hear any objectionable hum. If it's too bad, adding a big capacitor across the supply output could help event out the ripple.

You can probably guestimate the total current you need for all your boxes based on the time it takes for all your batteries to run down and Fish4Fun's numbers above. Or you can measure the current by putting a small (maybe 1 ohm) resistor in line to a battery terminal and measuring the voltage across it when the box is humping it's mostest.