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adapting AC wall plug to battery operated gizmo

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fsam

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I have a drum machine which runs on 4 AA batteries but it runs through them like water. I'm thinking about taking the cover off and soldering in a plugin for an AC adapter. Does the AC adapter just need an output of 6 volts DC?

Also, what does the output amperage mean on the adapter? Is that a rating of how much current draw the adapter is designed to safely handle ?

Thanks!
 
Typically the output voltage rating on an adapter is at it's specified output current. So if it's a 6V 500mA supply then if the current is 500mA then the voltage will be 6 volts nominally. It will be higher bellow that current draw and lower above it. Something like the drum machine you're talking about will have a decent amount of allowance for varying voltages and it shouldn't be critical.

I would say a general good value for the current your AC adapter needs to supply should be between 300ma and 750ma. Typical wall warts are very safe, even if abused, but it's generally best to use an adapter capable of more supply than is required to keep heating down as long as cost isn't critical, except in bad fault conditions a power supply capable of more current than is required is not an issue.

Just to give you an example, the 12V power rail of a modern ATX (computer) supply is capable of anywhere between 5-20amps of current, yet you wouldn't think twice about plugging a 12V DC 300ma fan into it.
 
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Typically the output voltage rating on an adapter is at it's specified output current. So if it's a 6V 500mA supply then if the current is 500mA then the voltage will be 6 volts nominally. It will be higher bellow that current draw and lower above it.

Intriguing. Is there a formula or something that can be used to determine voltage change based on lower/higher current draw? Just wondering because this probably
won't be the last time I experiment with this.


it's generally best to use an adapter capable of more supply than is required to keep heating down as long as cost isn't critical...

Just to give you an example, the 12V power rail of a modern ATX (computer) supply is capable of anywhere between 5-20amps of current, yet you wouldn't think twice about plugging a 12V DC 300ma fan into it.

Ok, so having a higher amperage rating is good thing for heat build-up but also could be a bad thing because the voltage will be either lower or higher.

Does the 300mA fan just run at a faster RPM since it's not drawing anywhere close to 5 Amps (ie:voltage the fan sees would be more than the rated 12v @ 5A) ?

Thanks for your reply
 
You cant get more voltage but you can "use" less current. A 12 volt 300ma fan will only use 300ma off a 12 volt 5amp supply. The voltage will be the same at 12 volts.
 
There should be a label on any half-decent drum machine saying how much current (or power) it needs. What figures are on there?
 
Does the 300mA fan just run at a faster RPM since it's not drawing anywhere close to 5 Amps (ie:voltage the fan sees would be more than the rated 12v @ 5A) ?
No, because the condition I gave was with a PC supply, which is voltage regulated, in the case of an AC/DC adapter you'll have various voltage conditions depending on the load; virtually all AC/DC adapters are NOT voltage regulated.

There may be a formula but it isn't exact because construction and manufacturing methods and tolerances vary so widely.

I would recommend this. To test a power supply, determine it's rated voltage AT it's rated current. Test THIS value as point 0. Then test -1 -2 -3 which would be -10% of it's rated current for each step and measure the voltage, and then test +10% of it's rated current for +1 +2 +3 and measure the voltage. Plot the data. You'll see a distinctive knee curve (or approaching one) at both ends for AC/DC adapters and none for regulated. If no curve is noticed increasing/decreasing loads to -20% for each step will help characterize the output.

I know it sounds complicated, it's not, science is about measurement, determining values ranges thresholds and will invariably encompass some degree of statistical analsys. The 7 point data set is classic and provides a large amount of information if one is able to interpret the data.
 
There should be a label on any half-decent drum machine saying how much current (or power) it needs. What figures are on there?

Where have you seen this? If there is no wall plug input on this drum machine, it is battery-only device. I've never seen any device that operates strictly on batteries that has a label stating either current or power consumption...only voltage or battery type and number of cells.

Ken

Ken
 
No, because the condition I gave was with a PC supply, which is voltage regulated

Ahhhhh yes, I see. That makes all kinds of sense. All those electronics on a computer wouldn't last too long if voltage varied all over the place.

I would recommend this. To test a power supply, determine it's rated voltage AT it's rated current. Test THIS value as point 0.

Ok, so just plot the voltage output at (+/-)10,20,30% load from it's "labeled load" to determine what the voltage output would be at different current draw?

What type of gizmo would I use to get a variable load like that? Just add 1.5V LEDs in a series or something to raise the current draw?

Thanks
 
What type of gizmo would I use to get a variable load like that? Just add 1.5V LEDs in a series or something to raise the current draw?
Without something in series like a resistor to limit the current, LEDs would go POOF! The AC-DC adapter also might go POOF!
 
Also be aware that if you use a wall-wart, some are regulated and some are not. The unregulated ones will have the output voltage drop more as you pull more current from them; some I've measured also become quite noisy when you get near their rated current. I like to buy wall-warts for a buck or so at the second-hand stores. I look for ones like the HP ones with IEC connectors (typically used to power laptops) -- some of those that I've measured with a DC load have very good regulation and are conservatively rated. They're perfect for home projects.
 
fsam all you have to do is use multiple loads. Doesn't matter what the load is pretty much, could be small fans, little light bulbs, power resistors, anything that consumes a known amount of power. Add them in series or in parellel till you get your desired load current. Power resistors aren't that expensive.
 
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