12 November, 2015

ebay "0R - 9999999R Seven Decade Programmable Resistor Board, Step 1R, 1/4W" review

To satisfy my need for a decade resistor box I was looking for something cheep and I came upon this project. The idea was nice and simple, exactly what I was looking for but the price was not so attractive, 19$ + shipping for a little PCB with resistors and jumpers is way to much.

Next step was a little search on ebay and in no time I found the Chinese equivalent of the same idea at around 10$ and free shipping, now we're talking. You can find it on amazon also but the price is considerably higher. Fast forward a few weeks and the package was waiting for me at the post office.

And the ratings state:
- 7 decades coverage with 1ohm, 10, 100, 1k, 10k, 100k, 1M stages
- 1% resistors tolerance. Planning to replace resistors if the need comes with 0.1% tolerance
- 1/4W power ratting
- 250V max voltage, like that would be the case..

It came bulletproof packed. First time I've seen such a great packing, overkill in this case but a nice touch none the less. The PCB work was nice and clean, came with the male 4mm banana plugs and a bunch of extra jumpers. Banana plugs are the cheap ones but good enough and the extra jumpers beats me what they are for because I don't find a use in connecting more resistors in parallel.

With this occasion I had the opportunity to put my newly acquired DER DE-5000 to work. Got it with TL-21 and TL-22 accessories as the TL-23 is overkill for most cases and easy to home make anyway. Plugged the TL-21 alligator adapter, did an open/short calibration and clipped it on the Decade resistor board's jacks with all jumpers set to 0. Total disappointment! 0.8ohms!! That's almost 1ohm, not quite what I had in mind in terms of 0 resistance.

Couldn't find any problems other than, of course, the jumpers. Fiddling with them turned out values from 0.5ohms all the way up to 4ohms which is quite a tragedy. Took a closer look at the jumpers and couldn't find an actual fault with them. The mechanical contact is quite good, there was no sign of oxidation.. that's just the way it is, nothing to do about it.

So what did I end up with? The cheapest programmable resistor with 6 actually useful decades. Not the kind of equipment you can calibrate stuff with but good enough in situations where you need to test and tweak stuff.

22 April, 2015

Quick and dirty dynamo powered bike USB charger

One day a biker friend of mine asked me to give him a solution for charging his phone from the bike's dynamo while on longer trips. Sounded pretty simple and a nice challenge for a low cost solution to be found.

His bike has a Shimano HB-NX70 dynamo that's nothing more than a alternating current generator rated for 6V and 3W. That voltage rating is an estimative figure though as it all depends on the wheel's speed so you can have much less when cycling slow or much more when speeding.
That being said, the solution? Whatever I had laying around as left overs or in the junkbox. A small plastic case, small piece of prototype PCB, a PC bracket with two USB connectors, a DC/DC converter (a.k.a. BEC in RC hobby world), and some discrete components: a rectifier diode bridge, a big fat filtering capacitor and a little protection with a zenner diode.

On the bike's original electrical instalation, the dynamo powers a head light together with a tail light. I had to open the head light anyway because it wasn't working so I pulled the AC from there.

 The BEC starts operating at 6Vdc and goes all the way up to 23Vdc. I put it all together in a hurry and didn't give the time to pull up a schematic or do some multimeter work on the dynamo's output and how all things work together.

All I can say is that when the time came and we gave the charger a test spin, it worked, charging a phone with 600mA which is decent enough.