BEGINNER HELP- Is it ok to draw a lower amperage output than your Voltage regulator is rated?

Thread Starter

Ashar Rahman

Joined Apr 25, 2018
2
I am working on a first-project to convert my hand crank flashlight into a phone charger, i need a voltage regulator because i don't want to kill my phone, the one i'm looking at seems to fit my specifications however it says it is rated for 2 A, while my phone only needs 500 mA, i want to know if i can use it to charge my phone or if there is something else i should buy. When i crank the flashlight it should output only around 3 V and i need 5V 500 mA. I don't wanna risk damaging anything.

Voltage Regulator(buck DC/DC because its more efficient):
https://amzn.to/2I0Dl2m (concerned because it says booster, if i were to stop cranking and the voltage were to drop the phone would stop accepting charge, this would mean that the voltage would jump to an insanely high voltage and possibly burn/short my phone charging port.)
vs.
https://amzn.to/2HsHUBh (seems to have both capabilities but it is rated too high.)

HandCrank:
https://amzn.to/2JqbI1X
 

wayneh

Joined Sep 9, 2010
17,504
When i crank the flashlight it should output only around 3 V and i need 5V 500 mA.
How do you know what the crank will output? The voltage and power produced will depend on how fast you spin it.

Overall I think you're going to be disappointed with your project, though. Charging a smartphone can require up to 1A and maybe even more for some models. That's 5W. Those old-fashioned bottle dynamos for bicycle lighting were 6W. That was about as much drag as a rider could tolerate. It would be very difficult to achieve this output with a hand crank, if the generator was even capable of it, which it likely is not.

And, phones have smart chargers that negotiate for higher current charging and otherwise will charge very slowly or not at all. You may be able to overcome this without too much effort, but it should be part of your plan.

To answer your question, no, the amperage rating of the regulator is of no concern as long as the rating is more than the load requires. If your load draws 0.5A, a 1A rating would be fine. I'd be nervous about running at 1A with a device rated to only 1A. It might be OK, but it might not.
 
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Thread Starter

Ashar Rahman

Joined Apr 25, 2018
2
Alright, should i ditch the project entirely then? i want to learn to apply the novice understanding of electronics i have, any ideas?
 

wayneh

Joined Sep 9, 2010
17,504
Alright, should i ditch the project entirely then? i want to learn to apply the novice understanding of electronics i have, any ideas?
Well maybe stick to the plan but exchange the phone charging idea for something more modest, like a battery and LEDs.
 

MisterBill2

Joined Jan 23, 2018
19,424
Alright, should i ditch the project entirely then? i want to learn to apply the novice understanding of electronics i have, any ideas?
It will be an interesting and educational project, and while charging a phone from that generator will take a while, it ought to work.Delivwering 5 voltS at 500Ma equals 2 1/2 watts, so you will feel the load turning the crank.
Now as for the regulator ratings, that is a MAXimum, a limit not to be exceeded. So you will do fine with it. What is important when reading specifications is to read the whole thing, not just the numbers. There are different numbers, some for "typical operation and that is quite different from MAXIMUM, which usually means that exceeding that limit leads to damage, or at least degraded operation.
 

Tonyr1084

Joined Sep 24, 2015
7,996
Amperage capabilities and amperage demand are two different things. If your phone (or whatever) draws a certain amperage then the source of that power must be at least capable of delivering that much. However, if your source can deliver 100 times what the device may use - your device will still use only what it will draw. It's like the fuel pump in your car. It's rated to be able to deliver the amount of fuel needed for top speed of the car. But when you're driving at minimal speeds, even though the fuel pump can push that much fuel, the engine will only use what it will use. The rest is ignored. Actually, in my analogy it is looped back to the fuel tank, but the point is that the engine or phone or whatever, will only use the power it requires.

NOW! If you have a phone that charges at 500 mA (0.5A) and you have a 12 volt charger - you'll burn up your phone because you're not following the proper voltage. But I think you already know that you'd need a 5 volt regulator to prevent going over voltage. The thing with a hand cranked generator is that it converts energy of one form into another. In this case, muscle energy into electric energy. @wayneh commented on those bicycle generators. I had one when I was a teen. Thought I was smart. Didn't need to carry batteries. But one night after a long day of riding I needed to flip on the light. That thing drug me down and stopped me dead in my tracks. I just didn't have the strength to run it for even one mile. That day we rode our bikes well over 100 miles. Not bragging, not exaggerating. It was 50 miles by freeway from my house to Laguna Beach. Surface streets and bike trails were much much longer. Not to mention the knocking around we did in Huntington and Newport beaches. Once I flipped that generator on my legs just could not sustain the ride any longer. Generating power takes power. And just 6 watts - yeah, that's a lot. Now, on fresh muscles, you can probably generate the power you need to charge a phone, but since it takes anywhere from a few hours to overnight - I don't think you're going to be able to muster that kind of muscle energy for that long a period.

Should you change your project goals? Well, that depends on what you want to learn and do. If you want to see if you can do it - I'm sure you can. If you want to make something practical - well, I think this is where you'll learn something of real value. Realize that it takes a Horse to generate One Horse Power. True, 1 HP is = to 745 watts, and you're no where as powerful as a horse; but you should be capable of producing sufficient power to generate the needed current to charge your phone. Just how long you can do that and how much charge you can generate is another question.

There's a video out there, and I'm not going to link it up. It shows an olympic bike rider running a generator and toasting a piece of toast. He could BARELY do it. And his leg muscles are as huge as a teen age girl's waist. So build your project and learn from it. I don't think there's a lose situation here. I think you'll learn something valuable that you can take through life with you. Who knows. Perhaps you'll be the next great inventor and invent the flux capacitor.
 

mcgyvr

Joined Oct 15, 2009
5,394
The answer to your question is yes..
In general a voltage regulator or power supply rated 5V @30 Amps is going to deliver 5V constantly and UP TO 30A of current..
Those are called "constant voltage" power supplies..

The opposite of that is a "constant current" power supply which is typically used in LED drivers as LEDs themselves do not behave like typical resistive loads and need to be "fed" a proper current level to avoid letting out the magic smoke..
A constant current power supply will carry a rating like 12-48VDC @700mA.. That means it will constantly deliver 700mA to the load and it will vary the output voltage to achieve that..
 

MisterBill2

Joined Jan 23, 2018
19,424
The one exception to needing to draw some minimum level of current is the internal power supply for desktop computers, where there needs to be some fairly large amount of current from the 5 volt supply. That is because it is cheaper to make them that way. It does make testing them a bit more complex because of needing a fairly high powered load of some kind.
 
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