USB Current limiting to 500mA

Charging super capacitor with USB - current limiter?

I have a torch that is powered by 2 50Farad Super Capacitors in Series. I use the USB port of a PC to charge the super capacitors using a 10ohm resistor to limit the possible maximum current (5V/10Ohms) to 500mA.

The torch charges up nice and quickly at first, but then as the voltage gets higher it drops according to Ohms law, so when the super capacitor voltage is at 4V, I am only charging with USB Voltage - Super Capacitor voltage / 10Ohm resistor = 100mA. As it charges more and more, the rate gets worse and worse and the last 0.5volts takes forever to charge.

I see a number of parts on the internet for limiting USB current to say 500mA or selectable. These parts are often difficult to obtain or they are almost impossible to solder by hand as they have no pins and the pads are underneath the part.

Is there a way of charging the super capacitors much quicker. A way of forcing the current into it perhaps? Or alternatively limiting the current to 500mA at all times in the charging - this would not help the charging rate when it gets to over 4V though, but the fast charging at the start of the charge would make up for this.

Any suggestions?:lol:

Re: Charging super capacitor with USB - current limiter?

Hi,

You could use a constant current source to charge the capacitors, there will be some points to think about, but there are quite some implementations of simple current sources with only a few parts.

Regards,
Andreas

Would you recommend an active current limiter using transistors? If so , what would you recommend? The main reason for the 500mA limiter is to stop the USB port from shutting itself down.

I'm not overly familiar with USB ports on a PC but I seem to remember
reading somewhere that the output can be allocated by software.
If your only concern is not overloading the port maybe that is worth
checking out.

The USB port provides 100mA. You can request up to 500mA in software which I have done. But most modern USB ports dont care about the requests and some even allow you 1Amp without shutting down. I find most allow up to about 500mA before shutting down.

All I need is something to keep it constant at 500mA so that it does not shut the USB port down. I was looking at using an active current limiter using transistors but not sure what the calculations are for 500mA and what transistors to choose.

For such a simple application (If I understand correctly that is is some DIY project) it will most likely be sufficient to use any current source you find via whatever search engine you prefer. E.g. like this one: A Constant Current Source for a Grounded Load using a Single Op-Amp , though I did not look very thoroughly, there are many other variants to implement the circuit. I would recommend searching the web for some, and maybe do some calculations based on the ones you find (or just simulate them).

Also: IIRC, the USB spec only permits you to draw 100mA unless the device attached to the bus has requested to draw up to 500mA. Most PC mainboard ignore this restriction and provide no protection for the USB ports, some do and might disable your device.

I'll be requesting the 500mA and most likely keep it limited to 400mA to be on the safe side. The problem is that these super capacitors act like a short circuit when empty therefore I need to limit the current to 400-500mA.

Thats clear. My reasoning was that you want to load as fast as possible anyhow, so a constant current source should be sufficient. At the beginning the voltage at the capacitor will be pretty low, but with increasing charge of the capacitor the current source should regulate the voltage to maintain an output of 500mA (or 400mA, if you design it that way).

Like I said, you could search the forum or the web for specific schematics.

Hi,
I know one thing the amout of current the USB can supply is negotiated during the enumeration process. Secondly one shouldn't put a caacitor greater than 10 uF on the USB power supply else its taken to be a short circuit(initial discharged state of the capacitor draws high current for charging). If you need a super capacitor that maintains the charging current you will need some sort of MPP charging algorithm.

Asimov