Power ratings on battery

fulmenatrum

So I've been trying to work on this project that requires me to take a 12V AC to DC adapter (says 300mA on it) and supply power to a CPU fan (11.8V, 0.57A) and several LEDS (20mA forward current, 3.2V). My question is what does the 300mA exactly mean as I thought that there was no such thing as a current source.

Also, I connected my CPU fan to a 9V household battery and an ammeter in series and received a current measure of 10mA. This apparently wasn't strong enough to turn the fan. But then I connected my CPU fan to a 12V Universal Battery (model: UB1250ZH) and the ammeter, still in series with the circuit read 500mA, which started to spin the fan rapidly. Now I'm quite doubtful that a difference in 3V is enough to increase the current by that much so I'm pretty sure that the 12V battery has some internal resistors working but I have no idea how and why it works the way it does. Also, why do these batteries have ratings on them like "0.75A" Does that mean it's the maximum current the battery can preform under? Are there variable resistors involved?

Thank you, any help will be very much appreciated.

russ_watters

Welcome to PF.

Your devices will draw whatever amperage they want from the ac to dc adapter. 300mA is the amperage above which it is likely to overheat and burn-out.

fulmenatrum

Thanks! However, I'm still curious why the different batteries seem to be able to supply different currents.

Jiggy-Ninja

You will not be able to run your setup with that adapter. The fan alone draws more current than it is rated for. Using it will risk destroying the adapter.

BTW, there are such things as current sources. They are just much, much rarer than voltage sources because voltage sources are much easier to deal with.

Finally, motors are very different from resistors. I don't quite know much more than that yet, though.
Because they're made differently. Size, chemistry, and a few other things affect how big a load a battery can handle before it is destroyed.

fulmenatrum

So what you are saying is that I should use a power adapter with preferably a current greater than what the fan requires and then use resistors to drop the current down? Would it be alright if I were to use a adapter that actually had a higher voltage AND current rating and use a appropriate resistor to drop it down to the appropriate measures?

You also said that the fan draws more current than what the adapter is capable of supplying. However, I thought that the fan will draw a MAX of 0.57A...so can't it essentially draw anything under that depending on the voltage source? Please correct me if I'm totally incorrect to assume this. Thank you.

Jiggy-Ninja

The adapter is a voltage source, not a current source. Plug it in, and it will provide you with a constant voltage, with the current changing depending on what load is hooked up.

The current rating means that it can deliver up to 300mA without damaging the supply or current limiting circuitry kicking in. If you want to draw more current, you need a higher rated adapter. I'd go with 1A (1,000mA) rating for this use.

The fan is rated for 12V, so the current will also be rated for that voltage. It will draw less current with a lower voltage, more current with a higher voltage, but it will draw 570mA at 12V.

Bottom line, you need a 12V adapter rated for at least 1A.

fulmenatrum

I think I understand now...the AC to DC adapter acts as a voltage source that supplies the noted voltage and has a max current that it can handle without breaking. The current in the circuit is solely dependent on the electrical device(s) which basically draws the amount of current based on what it needs. Thus, the current that the adapter should be able to handle should be greater than what the device, such as the fan requires. So basically, the sticker that has all the current and voltage info on electrical appliances just indicate what the optimal conditions for it to run smoothly should be. (Hope this is all correct).

Now I know you previously mentioned that motors are different from resistors but motors do still require current which consequently results in lowering the overall current in a series circuit. I conducted an experiment where I hooked up the fan and ammeter in series to a Transformer from part of my old toy train set which allowed me to turn a dial to adjust the voltage I wanted (up to 16V). I graphed the different currents that resulted from different voltages in an I-V graph (more or less linear) and took the inverse of the slope in hope to find the resistance of the fan. It came out as 25ohms. Can I be confident that this is in fact the true resistance of the fan and do I account the fan as a 25ohm resistor when I calculate total resistance and such when I start adding other components?

Thank you so much for all you help!

russ_watters

Maybe I'm missing something, but you didn't say anything about a battery regarding current, just an ac/dc adapter?

The reason for an ac/dc adapter is the physical size of the components (and therefore cost) determines how much amperage they can handle.
[edit: sorry, missed the big part....] Well, fans are magnetic devices. Try pushing on the blades with your finger and you'll see how it pushes back against you and turns in steps. But you can still spin it and it will spin for a while. So you need a certain minimum torque to get it moving, but once it gets moving, it'll spin fast. That's why you sometimes find that just above a certain threshold voltage, it'll spin pretty fast. I think you must be misreading. Batteries sometimes carry their mah (mili-amp-hour) or ah (amp-hour) rating, which is a measure of capacity, not current. I've never seen a current rating except perhaps on a car battery CCA: cold cranking amps.

fulmenatrum

I just wanted to try and see if batteries were that different than the AC/DC adapters. So you mentioned that once you give it a push, the fan should pick up on it's own and start spinning. I did try this though and it just wasn't strong enough to spin on its own. The fan clearly calls for a much greater amount of current which the 12V Universal Battery was able to handle...the 9V must have not been able to handle anything past 10mA.

I do have a rating which says 5Ah but that wasn't what I was referring to earlier. There's a little chart on the battery that says...
Standby Use: 13.6V-13.8V; 0.75A
Cyclic Use: 14.5V-14.9V; 1.5A

Not too sure what those terms mean though.

And thank you for your help!