Author Topic: Bike mounted USB Charger  (Read 63992 times)

Re: Bike mounted USB Charger
« Reply #25 on: August 12, 2008, 12:11:56 pm »
Another point. It is reccomended to fit a small heatsink to the LM2940. Although this would be uneccesary in normal use, this part can get quite hot if accidentally short-circuited (I wonder how I know that  ::-)  ). You could use the hole in the LM2940 to fix the circuit to the bike frame and that would suffice.

bobajobrob

Re: Bike mounted USB Charger
« Reply #26 on: August 12, 2008, 01:05:39 pm »
You can leave out C1, C2, C3 and R1 if you wish ... If you leave out the input tuning circuitry (C2, C3, R1) then connect the dynamo input between D2 and D4.

Like this? These are the bits I have at the moment:



D1 - D4     1N5818
C4             .47uF      Tantalum bead
C5             22uF       Tantalum bead

Re: Bike mounted USB Charger
« Reply #27 on: August 12, 2008, 01:16:49 pm »
Another point. It is recomended to fit a small heatsink to the LM2940. Although this would be unnecessary in normal use, this part can get quite hot if accidentally short-circuited (I wonder how I know that  ::-)  ). You could use the hole in the LM2940 to fix the circuit to the bike frame and that would suffice.

This assumes that you are using a TO-220 packaged LM2940, it can come in many other packages.  If you do this also remember that the tab is electrically connected to the Ground, so if you use the bike as one half of the connection to the lights, make sure you use the ground connection on that side.

I think I'd put a fuse inline with the output of the regulator, shorting it out will make it damned hot, and stands a good chance of frying it permanently, which would be annoying.
Actually, it is rocket science.
 

bobajobrob

Re: Bike mounted USB Charger
« Reply #28 on: August 12, 2008, 01:42:14 pm »
I think I'd put a fuse inline with the output of the regulator, shorting it out will make it damned hot, and stands a good chance of frying it permanently, which would be annoying.

What size fuse should I use? 1.5A?

Re: Bike mounted USB Charger
« Reply #29 on: August 12, 2008, 01:51:47 pm »
The Nat Semi datasheet says that the short circuit current is typically 1.9A, although it supposedly has short circuit protection.  I still think I'd fuse it, if nothing else since with 1.9A going through it, unless it's very well connected to a heat sink (ie frame!) it will get hot very fast, and you shouldn't have a short circuit, so it implies something else is wrong.  Blowing the fuse is probably a better idea than melting the light or somesuch.  I'd say that 1.5A sounds around the right ballpark.
Actually, it is rocket science.
 

Re: Bike mounted USB Charger
« Reply #30 on: August 12, 2008, 02:09:55 pm »
Don't bother with the fuse. The absolute max current that you will get out of the vReg is 500mA ('cos that's the max the dynamo will provide), so the internal protection will be fine (but it will get hot as mentioned previously). I have 'tested' this using a 7.2v LIon battery pack at considerably higher current.

 Good point about grounding the LM2940 - I assumed we all had Schmidt dynamos  ;)


Re: Bike mounted USB Charger
« Reply #31 on: August 12, 2008, 02:11:37 pm »
You can leave out C1, C2, C3 and R1 if you wish ... If you leave out the input tuning circuitry (C2, C3, R1) then connect the dynamo input between D2 and D4.

Like this? These are the bits I have at the moment:



D1 - D4     1N5818
C4             .47uF      Tantalum bead
C5             22uF       Tantalum bead

Yes, that's it.

JT

  • Howay the lads!
    • CTC Peterborough
Re: Bike mounted USB Charger
« Reply #32 on: August 12, 2008, 02:29:24 pm »
Any chance that someone could knock up an instructables style step-by-step guide for making one of these? I've never even picked up a soldering iron so while this sounds simple, for people like me, it's not.

Also, if it's so simple and cheap to make something like this, how come no one is making them commercially?

a great mind thinks alike

Re: Bike mounted USB Charger
« Reply #33 on: August 12, 2008, 02:45:20 pm »
Maybe one or two kind folks might consider contributing to a build your own project thread like this for instance?  Please?

bobajobrob

Re: Bike mounted USB Charger
« Reply #34 on: August 12, 2008, 03:05:07 pm »
I will gladly write up a report if I can use Simon's images (with credit given, of course). This is subject to me not blowing myself up :thumbsup:

Re: Bike mounted USB Charger
« Reply #35 on: August 12, 2008, 03:11:17 pm »
Don't bother with the fuse. The absolute max current that you will get out of the vReg is 500mA ('cos that's the max the dynamo will provide) ...

Doh. <fx: smacks head>
Actually, it is rocket science.
 

David Martin

  • Thats Dr Oi You thankyouverymuch
Re: Bike mounted USB Charger
« Reply #36 on: August 12, 2008, 03:11:50 pm »
I will gladly write up a report if I can use Simon's images (with credit given, of course). This is subject to me not blowing myself up :thumbsup:

Likewise..
My dynamo hub is not grounded. I don't suppose that will make much difference.

..d
"By creating we think. By living we learn" - Patrick Geddes

Re: Bike mounted USB Charger
« Reply #37 on: August 12, 2008, 03:15:52 pm »
I've never liked relying on the structure of the bike to provide the return path for the lighting circuit, but if it is wired up like that (even partially, ie the dynamo is bolted to the frame) then you've got to be careful.  In fact if you are using a traditional bottle dynamo, that is directly electrically connected to the frame, then you must not bolt the LM2940 tab to the frame, as this will cause problems (in effect you'll be partially shorting out the bridge rectifier).  The proper approach would be to use something like a small piece of mica insulator, and a small plastic insulating washer thingy to keep the metal tab electrically insulated from the frame (ie heat sink) and still provide a reasonably good thermal path.
Actually, it is rocket science.
 

bobajobrob

Re: Bike mounted USB Charger
« Reply #38 on: August 12, 2008, 08:50:36 pm »
Here's my effort:





Unfortunately it doesn't work. I've probably wired it up wrong. FAIL.

Re: Bike mounted USB Charger
« Reply #39 on: August 12, 2008, 09:18:28 pm »
Well, looking at the images, the layout looks correct, but I think you've got the diodes the wrong way around.  The stripe is normally on the cathode, and the current flow is from the anode to the cathode, so the cathode end should be connected to the input of the LM2940 (which doesn't seem to be the case).

Plonking an avo between the ground connection and the input to the LM2940 will soon tell which way around you've got things.  The data sheet says that the LM2940 has reverse battery protection, so with a bit of luck nothing will have been destroyed.

I really hope you didn't have anything plugged into the USB port when you tested it.  The reverse battery protection may stop things going seriously wrong, but I wouldn't rely on it.

(Edit, some amendments made after checking the LM2940 data sheet).
Actually, it is rocket science.
 

David Martin

  • Thats Dr Oi You thankyouverymuch
Re: Bike mounted USB Charger
« Reply #40 on: August 12, 2008, 09:32:50 pm »
I have taken note. I will learn from your mistakes. I'll be in Edinburgh next week so will pick the necessary bits up then.

..d
"By creating we think. By living we learn" - Patrick Geddes

bobajobrob

Re: Bike mounted USB Charger
« Reply #41 on: August 12, 2008, 09:47:23 pm »
I think you've got the diodes the wrong way around.

You spotted my deliberate mistake ;)

Quote
I really hope you didn't have anything plugged into the USB port when you tested it.

Only my phone, which is fine ;)

Anyway, I've reversed the diodes and it worked for a couple of seconds then stopped working. I think it might be b0rked. Will order some more bits and try again.

Re: Bike mounted USB Charger
« Reply #42 on: August 13, 2008, 02:14:58 pm »
Also check the polarity of the tantalum caps - they can't withstand a reverse voltage of over 5v. If they have popped then you may get away with just replacing them.

David Martin

  • Thats Dr Oi You thankyouverymuch
Re: Bike mounted USB Charger
« Reply #43 on: August 13, 2008, 04:16:48 pm »
Also check the polarity of the tantalum caps - they can't withstand a reverse voltage of over 5v. If they have popped then you may get away with just replacing them.

I can't find any 470uF electrolytic caps at Maplins (and RS want large quantities for the order which makes it quite pricey). Will tantulum or any other type of cap do instead?

..d
"By creating we think. By living we learn" - Patrick Geddes

bobajobrob

Re: Bike mounted USB Charger
« Reply #44 on: August 13, 2008, 04:25:56 pm »
Also check the polarity of the tantalum caps - they can't withstand a reverse voltage of over 5v. If they have popped then you may get away with just replacing them.

I'm guessing that if you're looking at the side with the writing on, then the left is negative and the right is positive. They have a + next to the RH bit anyway.

bobajobrob

Re: Bike mounted USB Charger
« Reply #45 on: August 13, 2008, 04:27:00 pm »
I can't find any 470uF electrolytic caps at Maplins (and RS want large quantities for the order which makes it quite pricey). Will tantulum or any other type of cap do instead?

You want .47uF not 470uF. They have them at maplins.co.uk.

David Martin

  • Thats Dr Oi You thankyouverymuch
Re: Bike mounted USB Charger
« Reply #46 on: August 13, 2008, 04:50:34 pm »
I can't find any 470uF electrolytic caps at Maplins (and RS want large quantities for the order which makes it quite pricey). Will tantulum or any other type of cap do instead?

You want .47uF not 470uF. They have them at maplins.co.uk.

C2,C3 on Simon's original circuit diagram.
470uF. I can find everything else at Maplin.

..d
"By creating we think. By living we learn" - Patrick Geddes

bobajobrob

Re: Bike mounted USB Charger
« Reply #47 on: August 13, 2008, 04:52:35 pm »
C2,C3 on Simon's original circuit diagram.
470uF. I can find everything else at Maplin.

..d

Ah yes I see now.

Re: Bike mounted USB Charger
« Reply #48 on: August 13, 2008, 04:58:20 pm »
Maplin part DT67X

Re: Bike mounted USB Charger
« Reply #49 on: August 14, 2008, 03:48:58 pm »
Having had a spare few minutes today, I started making another charger as above, but in a way that I could start with the really basic circuit and add components to gradually get to the full circuit.

The basic circuit works, but requires a fair turn of speed to get enough output. I think this is because there is hardly any load on the dynamo. The addition of C1 encourages the dynamo to output a little more and also significantly smooths the output ar slower speeds. I think I might go as far as to say C1 is required.

The addition of C2,C3 and R1 produces another significant improvement at lower to medium speeds and should be included if you have space.

If you have the inclination and requirement, the LM2940, C4, C5 can be replaced by (or switched to) a string of power LEDs (up to six LEDs) for a rather impressive headlight. I am currently using three SSC P4 (U bin) LEDs for a 500 lumen output.