DC output of 12v down to 5v (for usb)

[stormrazer121]

Curiosity kills the cat they say, in this case it would kill me if i did this wrong

Long story short i have my hands on a pre built 2.1 amp which has a dc transformer inside of it, it has a output of 12v at 1.6A (1600mAh correct?) An i want to be able to use this to "steal" or should i say bridge? or some sort of way of moving the current over to provide power to a USB device, is this possible? do i need a bigger transformer or would this possibly destroy the amp itself? i am confused big time with this

 

[MikeMl]

El cheapo consumer electronics tends to have power suppplies rated just slightly bigger than the device actually needs to run on, so "stealing" a few hundred extra mA is likely not a good idea. OTOH, if the 2.1 amp is plugged-in, turned on and is not being used for audio, or is being used at a very low level, then you will likely get away with it...

I go to flea markets and garage sales and pick up assorted wall-warts (plug-in power supplies) so always have one on-hand that will work for a project. I have several that will put out 5Vdc (well-regulated) that would be ideal for powering a USB device.

 

[stormrazer121]

So, correct me if im wrong but could i not bridge one of the mains lead? for arguments sake i swear I've seen it done before, where by you would have say parallel transformers instead of one large one, but in my case i would have a smaller transformer connected to the mains lead directly (as in the blue and brown lead) before it goes to each so essentially "splitting" it as if i was using an extension lead. Am i right in thinking this would in theory work?

 

[MikeMl]

I don't understand what you want to do??? Are you asking if it is ok to mount a second Mains-to-low voltage transformer inside the same cabinet as the existing amplifier? I'm assuming you are planning to build a regulated 5V DC supply inside of the audio amp cabinet? Question is: Why?

5V to power USB must be well filtered and well regulated. Since 5Vdc power supplies are easy to come by, why go to all the trouble of scratch building one?

 

[stormrazer121]

So i can say i have built it myself but i do have a usb charger im very much eyeing up!

Im going to try and explain this the best i can,

i already have my 12v transformer which gives power to the amp, this runs naturally from the blue and brown wire so that wont change and i wont mess about with it power to amp = sorted.

I want to add another transformer into the mix to provide USB power to a separate device in my project, in this case to charge a phone, so to achieve this i want to connect the second (USB power) transformer to the blue and brown wire i did draw up something (rather silly looking XD) in paint to kind of give you a better idea of what i wanted to do, heres a picture,

**broken link removed**

Please excuse the bad drawing i am unable to understand diagrams 100% fully and dont have the ability to draw it out as I've only just started with all this interesting world

 

[unclejed613]

yes you can do it that way. USB ports have a maximum available current of 500mA, so in addition to using a good regulator, some current limiting might be called for as well. a constant current source set to 500mA would provide foldback current limiting, and keep things like a shorted battery in the phone from damaging the supply. it's late now, but tomorrow evening i will try to remember to draw something along those lines.

 

[stormrazer121]

Thanks for your reply, so from what your saying i would be able to use the 12v transformer for my amp, and use a second transformer for my usb power. These would then both be connected to a mains lead (in this case it is a thin 2 cord lead so i have every intention of changing the connect to a kettle lead.

This way they wont be contacting each other at all, it would be two different circuits in 1 box just connected to the same mains supply.

 

[unclejed613]

the additional current for the second transformer won't be much more than a few milliamps, so you can keep the same line cord.

here's what i came up with for a regulated and current limited supply for the USB. just pay attention to the stuff inside the dotted box. the "idealized" supply to the left can be anywhere from about 12-20VDC. which is what you'll get from a transformer, bridge rectifier and caps, similar to the amplifier supply. inside the dotted box is a constant current source feeding a 3 terminal regulator. below the set current (520mA, 500mA for the load, and the rest for the regulator itself), the input voltage to the regulator is well above it's dropout voltage. when you hit the current limit, the current remains constant at 520mA, but the input voltage to the 7805 begins dropping rapidly, until the dropout voltage is reached and the output shuts off. it's probably a good idea to add an LED and resistor to the output as a status indicator.

 

[alec_t]

@stormrazer
Your drawing shows + and - connections. Are you aware that the mains wires (brown, blue) and the transformers provide/use AC, not DC voltages? USB requires 5V DC.

 

[stormrazer121]

Thanks and yes im well aware, the idea is to pull apart a usb charger....waste nothing me this way the hard work is technically done for me hence the way i drew the diagram