Volts, amps etc.

[Kittyhawk]

Need some help with this one please, electricity is not my forte.
I have a small heater dehumidifier box thing used for drying hearing aids overnight. The box is fed power from a 240v household socket via a little transformer with a standard USB outlet on it and a USB cable from it (the transformer) to the drying box.
The transformer output is 5V, 0.3A. The box is 5V, 0.5A.
So, using the water in a pipe analogy, volts = pressure and amps = flow then you can't muck around with the volts but I don't understand the amps side of it. Is it so that a device will only consume the amps that it requires to run whatever amps are actually available to it?
The reason I ask is because I have a motorhome fitted with USB outlets that put out 5V, 3 amps.
Can I plug the USB cable from the drying box into these 3amp plugs without frying the box?

 

[guineafowl21]

Is it so that a device will only consume the amps that it requires to run whatever amps are actually available to it?

Pretty much. Your motorhome sockets could be read as ‘5V, up to 3A output’. Should be fine.

 

[imageel]

Hi - the current taken is dependent on the drying box (the load) and not the capability of the power source (potentially up to 3A).
So if the box is rated at 5V @0.5A then that equates to 2.5W which sounds about right for the intended use, so if you plug it into a USB socket capoablae of delivering 3A @ 5V then the box will only take its rated 0.5A - so it will be fine!

 

[Kittyhawk]

Thanks for clarification.
The box emits UV light into its interior - presumably for antibacterial purposes and there is also a small computer type fan that circulates warm air around the chamber.
I imagine that the fan is a real cheapie.
I had plugged the box into the USB 3 amp outlet and the fan stopped working after a few days but the UV light and heater continued to do so, hence my original question - did I damage the fan by doing so or was its failure just a coincidence.

 

[pe2dave]

V=IR as Mr Ohms had it.
R is the resistance (narrow pipe in your water analogy). Give me a huge pipe (low resistance) and the flow will increase to ....
either the value which satisfies the law above, or the max the power supply can provide... then what happens?
Again in line with the law, if the power supply (water pump, battery, electronics) can't provide sufficient pressure to push that level of current, the voltage drops / the fuse blows / the battery gets warm / the pump overheats.
It's all a balancing act, the balance between V, I and R!

 

[imageel]

Thanks for clarification.
The box emits UV light into its interior - presumably for antibacterial purposes and there is also a small computer type fan that circulates warm air around the chamber.
I imagine that the fan is a real cheapie.
I had plugged the box into the USB 3 amp outlet and the fan stopped working after a few days but the UV light and heater continued to do so, hence my original question - did I damage the fan by doing so or was its failure just a coincidence.

Almost certainly the fan failing is a coincidence, albeit the quality of wall-wart style usb power adaptors is usually pretty poor so there is a possibility that the adapter over-volted the connected device. Unfortunately as with a lot of stuff these days, they are manufactured to a price point with little regard for longevity or perhaps tolerances especially with regard potential and likely mains-born transients and in many instances if you have a PC/Laptop device that can supply USB power they are a safer bet since they usually have quite sophisticated control over the voltage and current supplied,
/Ed

 

[HamsterJam]

In simple applications such as this, Volts x Amps = power (in Watts).
Provided the voltage matches and the rating of the supply is adequate for the item being supplied, then you should be OK.
In your case USB is standardised at 5v so voltages match. The supply is capable of delivering up to 5v x 3A=15watts whereas your device only requires 5v x 0.5A = 2.5watts so there is plenty of capacity available.

As others said, I think the fan failure was likely a Co-incidence. Possibly vibration being a contributing factor especially if it was running while you were driving?

 

[MorrisWoodman12]

@Kittyhawk I'm a bit worried about your statement:

The transformer output is 5V, 0.3A. The box is 5V, 0.5A.

It looks as though you're overloading the PSU/transformer. I would expect it to get hot, well at least warmer than it should, but doubt it caused the fan to fail. If designed properly the PSU will include a thermal fuse that catastrophically opens if the PSU gets too hot to prevent fire but who knows. Please try to use a PSU/transformer rated at 0.5A or above. As many have said before your box will only take what current (amps) it needs. It will be fine in your motor home.
Martin

 

[Spectric]

One way to think is that volts are provided and amps are taken. From a power supply design perspective you set the output voltage and then design for it to be capable of supplying upto so many amps and the protection circuits ensure this is not exceeded.

 

[Kittyhawk]

Thanks for the replies.
Attached a pic of the drying box, cable and transformer. It came as a complete kit so a bit unusual that the amps on the charger and box are different. It's made in France and retailed through a local hearing aid supplier who when approached for a bit of technical info on the product responded with a non comprehending 'huh?'.
But one thing I've just realized - I installed the two USB ports in the back of the truck, hooking them into an existing 12v circuit. They were rated 12 - 24v in, 5v/3amp out. They were made in China and rather cheap. I think it would pay me to check the actual output voltage to ensure it is as stated on the label and not a little bit more which if so could be why the fan failed.
And as soon as I get the truck back I will do this. It's in the repair shop just now. Clipped the neighbour's low concrete boundary wall and tore a chunk of fibreglass out of one of the panels.