Monster USB charger

[Justinicus]

Alright, folks. I'm a beginner electronicist, and I'm hoping you Elmers out there can give me some advice.

I'm well and truly sick of paying ~$20 for USB chargers claiming to supply 2A @ 5VDC to my myriad phones, tablets, cameras, etc., but delivering 700mA on a good day. I'm sick of not being able to find something that will charge two phones and three tablets simultaneously. I'm sick of USB power meters cutting the current flow in half when I put them into the circuit. In short... MOAR POWWWAAAAH! So I decided to take matters into my own hands.

I want to build my own USB charger. I know about the the various data pin resistor schemes for the USB charging standard, Samsung, and Apple (at least I think I do). I bought a 5VDC 20A switching power supply from ChinaBay, and have some perfboard, USB-A PCB-mount jacks, and a volt/ammeter on the way. My intention is to keep it pretty simple... volt meter across the DC + & - terminals, ammeter in line before the DC - return, and about 4-ea android- and apple-spec voltage divider/resistor getups between the data pins on the USB jacks, all fed in parallel by the 20A power supply.

I've had a heck of a time finding other people who've done this. I can only assume other people aren't as obsessed with fast charging as I am. Regardless, I don't have any examples to follow! I've only found one (poorly documented) project remotely similar to mine and he didn't make it high-powered. So my question is this: what am I doing wrong? I'm sure there's something I could do better. The project linked above has what I assume are filtering capacitors on each port. How do you select capacitors for filtering (type and capacitance)? Are they a good idea? Do they go on the + or GND side of the jack? Will perfboard be able to carry 2A for a centimeter or two without any problems?

Am I completely nuts?

 

[Justinicus]

Just had another thought... fuses. Not using polyfuses like thelifeofkenneth blogger did. Is it a good idea to put something like that on each port (even just a simple 2A glass fuse), or is it safe enough to rely on the power supply's fuse? Ooops, just realized those polyfuses are what I mistook for filtering caps. Well, anyway, are filtering caps a good idea?

 

[Anon_LG]

Could you post a schematic of what you are doing, this helps. If you have no way to draw schematics either get LTSpice IV, Yenka or draw it. And if you are trying to feed more current into devices than they are specified for it will most likely break the battery/overheat something inside the device.

 

[Gryd3]

There is also http://www.digikey.ca/schemeit if you want to use an online service.
I personally would use a filter cap, but am concerned with how you plan to wire in the required resistances on the data lines to provide more than 500mA to the devices you want to charge.

 

[Justinicus]

I puzzled my way through LTspice IV enough to make an ugly schematic (attached). Gryd3, I got the Android (USB-standard?) spec from various places around the XDA-Developer's forum which claim the standard for everyone but Apple is >200Ω across the data pins (Anandtech, below, shows it AT 200Ω). Other Samsung (tablet) specs seem to have varying voltage requirements on the data pins, in the same vein as Apple.

Got the Apple design from Anandtech

Lavaguava, don't all modern phones/tablets have intelligent, current limiting charge cycles? I've safely and successfully charged Li-Ion batteries manually, with just a lab power supply by following the charge guidelines listed on BatteryUniversity. The phone should take care of the constant-current part of the charge cycle anyway -- if it didn't, it seems like we'd have a lot more battery fires. And I've always been told that as long as you regulate voltage properly, devices will take care of the current themselves; e.g. you can plug a laptop into a 5A or 20A mains circuit without a problem, or plug a 12VDC fan into 9 AAs or a car battery. I know I've plugged 500mA USB devices into my 2.1A chargers and never noticed a problem. Same for plugging a 1A iPhone into a 2.1A iPad charger. Am I looking at this wrong?

 

[KrisBlueNZ]

if you are trying to feed more current into devices than they are specified for it will most likely break the battery/overheat something inside the device.

That's not even wrong! The device will draw whatever current it decides to draw. The only way to force too much current into a device is to apply too much voltage to it (generally).

Lavaguava, don't all modern phones/tablets have intelligent, current limiting charge cycles? [...] The phone should take care of the constant-current part of the charge cycle anyway -- if it didn't, it seems like we'd have a lot more battery fires. And I've always been told that as long as you regulate voltage properly, devices will take care of the current themselves; e.g. you can plug a laptop into a 5A or 20A mains circuit without a problem, or plug a 12VDC fan into 9 AAs or a car battery.

That's right.

Sounds like a good project to me. It's a good idea to have a separate polyswitch on each connector, in series with the feed to the +V pin. A decoupling capacitor (e.g. 1 µF 50V X7R ceramic) from +V to 0V on each connector won't hurt, but not likely to help either.

 

[Gryd3]

That's not even wrong! The device...

Typo? Not wrong = correct?

 

[(*steve*)]

I think Kris was trying to tell you that it was worse than just wrong.

 

[KrisBlueNZ]

"Not even wrong" = doesn't make enough sense to be wrong, strictly speaking.

 

[Gryd3]

Gotcha

 

[chopnhack]

Perfboard can carry the 2a just fine, its the size of wire you choose to put across the perfboard that matters A piece of insulated 22-20 ga should be ok for that load (23g will get it done but not much of a safety margin). I can't offer you any further advice, but I did come across some interesting reading material - see post three in the attached thread. Also see some links left by Kris. Happy developing!

https://www.electronicspoint.com/threads/usb-chargers.269299/

 

[Justinicus]

Excellent information, everyone, thanks!

Polyswitch... I can use that as a current limiter, right? Just did a little research, and I'm confused by holding, tripping, and carrying current.

Holding current, according to wikipedia, is the minimum current required for operation. That means if I'm not drawing this current through the polyswitch, it's "off," right? They won't pass less than the holding current?

Tripping current is when it starts limiting the flow?

Having trouble finding a definition of carrying current. Is that perhaps an overload limit? Above this, the polyswitch will just short? Seems like more of a voltage thing than a current thing.

In my case, then, it seems I would need a holding current of at most the minimum my devices draw for slow-charging. I don't mind if the circuit opens when they reach 100%, but it would be a failure if it cuts out at like 90% charge. Tripping current would be just above my max intended draw of 2.1A, yes? And then carrying current as high as I can get? Just searched Digikey and Newark, and I can't seem to find anything with a holding current <400mA AND a tripping current between 1A and 2.2A. Newark has one with holding at 1.85mA and tripping at 3.7A, but 3.7A could fry anything I'd plug into it, defeating the purpose, right? Edit: oops, Newark has an error. That one (60R185XF) has a 1.85A hold current.

 

[(*steve*)]

I'd recommend looking up a datasheet. It should define these terms.

A polyswitch is a type of positive temperature coefficient resistor. In many cases a thermistor used in a circuit is there to detect ambient temperature, and we want to minimize the amount of self-heating caused by current flowing through the device.

A polyswitch is a member of a fairly common group of these devices where self-heating is a notable and typical feature of their operation.

A polyswitch has a low, but definitely non zero resistance at ambient (and even high ambient) temperatures. However at some temperature the resistance starts to increase rapidly. This, combined with higher dissipation will force the temperature to a point where the resistance is high (but not infinite, and sometimes not very high at all). When the current drops significantly the device cools and returns to its original (low) resistance state.

So for a given device there are several specified currents.

• Carrying current is the current that can pass through the device in the low temperature state without causing sufficient heating to get the device to the point where the resistance starts rising rapidly.
• Tripping current is the current that will cause the un- tripped (cold) device to heat to the point at which resistance rises rapidly, and where self heating will increase faster than heat can escape, causing a rapid climb in temperature and resistance.
• Holding current is the current required to maintain sufficient heating in the high temperature and high resistance state to maintain the device in this state

Choosing a polyswitch is a non-trivial process as you need to take into account the effect of the untripped resistance on the supply rail, loads between your normal max (which should be at or below the carrying current) and the tripping point of the polyswitch, and possibly loads with the polyswitch tripped.

Because a polyswitch takes a loooooong time to trip, you may need to consider the effect of currents well beyond the tripping current. Practically you might employ a regulator with a current limit, or at least one capable of operating briefly under short circuit conditions.

One thing you may have noted is that the voltage you measure across USB ports is almost always near the top end of the specified range (4.5V to 5.5 V). There are a number of reasons for this, but one is that the untripped polyswitch's resistance is going to cause a voltage drop as the load current rises. The poly switch must be chosen in this case to drop an absolute maximum of 1V at the maximum rated current in order for the USB voltage to be in spec. Practically you want it to drop less as you need to allow for contact and cable resistance.