Cheap dynamo USB power

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edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 24 Oct 2013, 11:06pm

[XAP]Bob wrote:
"load" is provided by a rear dynamo light, it doesn't need to be alot.



Does the load have to be 'upstream' of the regulator? Or can I just permanently solder a light (LED and resistor) parallel to the USB output?

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[XAP]Bob
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Re: Cheap dynamo USB power

Postby [XAP]Bob » 25 Oct 2013, 10:44am

edocaster wrote:
[XAP]Bob wrote:
"load" is provided by a rear dynamo light, it doesn't need to be alot.



Does the load have to be 'upstream' of the regulator? Or can I just permanently solder a light (LED and resistor) parallel to the USB output?

It needs to be either upstream of the regulator or connected to the regulator outputs.
The issue with connection to the USB is that it could become disconnected.

A dynamo rear light isn't expensive, and provides a very nice way of providing load (and a useful function as well)
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edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 30 Oct 2013, 6:42pm

Belatedly, thanks. I've got a tail light, but connect that straight to the front light as I want to keep the whole USB thing separate.

What I'm working on now is taking two red LEDs in serial with a resistor (had a 47 ohm one lying around), and will solder those permanently in parallel to the USB output. I'll mount them at the end of the tube, so they'll light up when the regulator is working. Quick test indicates that at 5v they use 14mA, which is a little on the low side compared to a proper tail lamp, but should bring the voltage figures (judging by that German link upthread) much closer to the simulated tail light load than the open circuit load. This is all conjecture, but if I'm never going above, say, 60 km/h hopefully the components will survive.

I'm even tempted to make another one of the regulator circuits to try on a light, as I've used 3aaa Cree LEDs which do seem to output better than my Axa Pico (30 lux). Would it be possible to replace the 3aaa batteries with a wired in 5v DC input without frying anything?

edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 2 May 2014, 8:22pm

I thought I'd update this, in case anyone was to attempt the same circuit (which seems to be by far the most popular DIY USB-bike dynamo circuit on the web).

In short... it doesn't quite seem to work. At least not in a perfect, fit-and-forget way.

I only really had a chance to test this recently in a controlled fashion with the bike upside-down, monitoring voltage and wheel speed. I couldn't measure input voltage (would require an extra lead which I didn't have), but I measured output voltage.

Unloaded, the correct 5V was achieved from very slow speeds (5 or 6 km/h), but the voltage suddenly dropped to 0V shortly after 20km/h. Normal output recommenced after speed dropped below 14km/h.

Loaded, the story was almost the same, but I seemed to get to about 24km/h before voltage dropped to zero. OR, if I increased speed gradually, there was no cutout (tested to about 40km/h), and the charger worked as it should.

My guess is that the LM2940 regulator I used has overvoltage protection, and cuts out safely. That's good news, as there should be no risk to anything, and makes the question of building a protection circuit moot. It's just that the range of speeds is too low to be useful. I guess I could load the system down a bit more by running lights too (or building parallel lights into the circuit), but that seems inefficient.

How other builders on the web got the system to work, I don't know. Could be their regulators were different, or that touring speeds and acceleration were always within the permitted range.

And I have no idea about the 'slow acceleration not tripping the cutout' behaviour. Monitoring input voltage would be the only way to get more data.

Brucey
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Re: Cheap dynamo USB power

Postby Brucey » 3 May 2014, 8:40am

How did you do the test exactly? Apologies if this is b.obvious (or I have missed it) but I think you probably need to have a proper load on the USB output when testing the circuit. A typical (unprotected) generator will go overvoltage at quite modest speed in the absence of a suitable load.

A voltage regulating circuit (with no output load) + a tiny load won't be enough to hold the output voltage down as speed increases.

A suitable load would be of the order of 500mA at the generator (or a similar one at the output of the regulator). Lower load = higher voltage at the generator. I'd just use something like an old dynamo bulb as a load on the output for test purposes.

If you are using the USB to charge batteries then the load will reduce as the charging cycle progresses. If this effect is strong enough the charging cycle may not complete, since the input may go overvoltage as the output load reduces.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 3 May 2014, 7:27pm

The point of the test was precisely to see how the circuit behaved without and with a load.

All I did was attach the DIY charger to the dynamo, and (for unloaded) measure the USB voltage at the output, and then (for loaded) measure the USB voltage while charging a phone.

The problem isn't the output voltage going overvoltage per se. That stays at 5V unless the cutout occurs (at which point it drops to 0V). It must be the input voltage which is causing the regulator to shut down (although I can't guarantee this as I couldn't measure it).

With no load, this behaviour is a blessing. But when charging the phone, it's certainly not welcome.

In short, the behaviour I've observed is almost exactly the same as here: http://www.crazyguyonabike.com/reviews/ ... 301&page=1

Perhaps the regulator's protection is over-sensitive? The LM2940 is rated for up to 26V input, and apparently was built for 'automotive applications' to survive voltage spikes: 'During line transients, such as load dump when the input voltage can momentarily exceed the specified maximum operating voltage, the regulator will automatically shut down to protect both the internal circuits and the load.'

Brucey
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Re: Cheap dynamo USB power

Postby Brucey » 3 May 2014, 7:41pm

a phone may not take current in the way a simple load might; it may take a few moments for the phone to start to charge. During those moments there is no load on the generator and the input voltage to the circuit will rise to the no load voltage of the generator which is too high. If the charger was drawing current it would not allow the generator voltage to rise to the same extent.

If you do some very simple tests on your generator you will see how loading it affects the voltage it produces.

Is it not the case that use of a suitably configured cache battery could largely overcome this whole issue?

A very simple way of protecting the charger would be to install some 24V (net) Zener diodes. If the circuit is switched, the zeners would only be conducting to protect the charger circuit when it is connected to the generator but doesn't have a load connected to it. Briefly the Zeners might have to dissipate about 8W or so.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

fatboy
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Re: Cheap dynamo USB power

Postby fatboy » 3 May 2014, 8:36pm

edocaster wrote:One question for the sparkies here though: Some of the parts (e.g. the 2200uF capacitor and the bridge rectifier) I seem to be able to find components with higher voltage ratings for not much more in cost (possibly a little larger in size). Is it worthwhile using those higher rated components? Or does it screw up something else in the circuit?


Electrolytic capacitors don't like being over voltages and can go bang. In my early days of my electronics career I made a very big bang with one. Not pretty!

Make sure the capacitor voltages are greater than the zenots that you should add for the input capacitor and more than 5 for the output.
"Marriage is a wonderful invention; but then again so is the bicycle puncture repair kit." - Billy Connolly

edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 4 May 2014, 12:00am

Brucey: I think you may have hit the nail on the head with the phone not taking full available current immediately. That would completely explain why slow acceleration to even high speeds did not exhibit any cutout behaviour. It may be that the smartphone needs time to analyse and 'handshake' before it goes into full charge mode.

What this really needs is some extra adapted cables to test the input voltage, and perhaps output current.

Regarding the zeners, I'm a little confused about the correct configuration. The circuits for most voltage sources seem to show a current limiting resistor in-line with the zener. But would I be right in thinking that wouldn't work with a dynamo's current-ish source, and the zener would have to deal with the 500mA come what may? Where does the 8W figure come in? And does it matter whether back-to-back zeners are used before the rectifier, or just a standard DC configuration after the rectifier (given that the rectifier is rated to 140V)?


Fatboy: Good point. The main smoothing cap is rated to 25V, so has probably seen some stress if the regulator is shutting things down shortly past that point. I guess if the regulator didn't have overvoltage protection, it'd be toast by now.


The title of this thread should probably be changed...

edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 14 Aug 2014, 11:58pm

This is just a quick note to say I built a mk2 of this charger, largely based on the original circuit here, with the overvoltage protection (TVS/zener triggered thyristor on the DC side) suggested by DoctorRad here (and also the schottky diode on the ground pin of the regulator, to send it slightly past 5V): viewtopic.php?f=5&t=86041&p=802691#p802691

I packaged the whole thing in a plastic tube (I also reused the USB socket from a car USB charger which I gutted). Was almost going to fit a switch too, but decided to keep it clean.

And it seems to work. I can get my LG G2 charging at roughly 1% per km. It's a big 3000mAh battery, so that's not bad at all. All the actual testing of the overvoltage aspect was indoors, so I can't vouch for it in all circumstances, but nothing has gone wrong.

I also bought a Tecknet iEP360 6000mAh power bank and tried charging that (largely as I want to avoid charging the phone directly). It seemed stuck on two lights out of four for ages before I worked out that if I start charging from a complete stop, the power bank shows one light, which either means it's not happy with the initial voltage or current (too low), or is otherwise only accepting a limited charge rate. But if I unplug the charger and replug it at 8km/h or more, such that the correct voltage and decent current is immediately available, as would be the case for a mains charger, the normal charging flashing sequence lights up. A little inconvenient (I should have fitted that switch!) but knowing this I have now been able to charge at roughly one light (20-25%) every 60km. I believe quirks like this on the receiving device are responsible for many people having issues charging certain devices. I know the Bright Bike (now Sinewave) Revolution charger has a programmed 10 second delay before it delivers power, possibly to counter this very problem.

edocaster
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Re: Cheap dynamo USB power

Postby edocaster » 23 Nov 2014, 11:58pm

A video that may be of interest to those contemplating buying a dynamo-to-USB converter: http://www.youtube.com/watch?v=NhWvmrcNEnY

The Lightcharge seems to be stuck at a permanently low voltage. Draws power when unloaded, but the tester didn't say if the button was left switched to the charge position. It may be a zener or something to prevent it going overvoltage. The real killer is the low output.

The Plug III has a lot of output, but the input voltage goes very high. Dynamo current seems to be half of the usual saturation current, so one could speculate that the Plug III has a voltage doubling circuit. It's close to 40V loaded, so hopefully all components are spec'd for that. The raw figures don't show great efficiency at speed though - at one point (2:52) the reading for input is 0.28A and 37.4V = 10.47W in? For USB output (5V) at 0.74A = 3.7W out. It's a lot better at lower speeds though.