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More About Lights
On 2017-03-16 15:07, Jeff Liebermann wrote:
On Thu, 16 Mar 2017 10:20:43 -0700, Joerg wrote: There is no need for 27.5W but there is for 8-10W. That should be reasonably possible, else the dynamo would have smoked out during their testing. If all you're powering are head and tail lights, 10 watts should be sufficient. However, if you're powering other devices (GPS, smartphone, rear view camera, electric shifter, fog horn, etc), then more power is better. I also have to power an MP3 player. No smart phone since I won't likely buy one until they can function as good as a GPS devise even while off-grid. Still, 10W suffices because I can ratchet down or turn off the light on many trails and MUP. It's mostly needed on roads which I try to avoid where I can. http://www.ebay.com/itm/122373782338 (Yet another project that I'll never finish). That is a neat little instrument. Yep. I don't have one yet, but it's a real temptation to install one on all my various unmetered power supplies and battery packs. However, there's a catch. The common ground is positive (+), not negative. You can see that in the schematic: https://img.alicdn.com/imgextra/i3/121163002/TB2NoBogpXXXXahXpXXXXXXXXXX_!!121163002.jpg where the + leads of both the "DC in" and the load are connected together. The only ways I could make it work in a negative ground system was either an isolated power source, isolated load, separate isolated power supply to run the meter, or a DC-DC inverter. That would be a show stopper in most applications. Got to have high-side current sensing. Impediment, not show stopper. What most users have done after turning the + wire into an impromptu fuse, is to install a small isolated DC to DC inverter. The problem is that the current measuring part of the circuit needs to have some source of power to run the devices used. If one part of that power source happens to be grounded to either the + or - input of the voltage measuring part, it won't work. So, the vendor recommends either floating the input or output grounds, using independent volt and amp meters, or using an isolated power supply to power the conglomerated meter. I know that one can get such DC to DC inverters quite cheaply, but I haven't bothered to search for them. I'll post something if I find one as I think it might be useful for your battery pack and dynamo. Digikey has lots of those. But when such extra measures are needed this all gets old. Why can't they do high-side sensing? It ain't rocket science. From a dynamo it's still feasible after things have been turned to DC because the hub versions usually have two ground-free terminals. Bottle dynamos unfortunately not. However, when doing MPPT there already is a micro controller which can then also perform the job of energy metering. Yep. I've setup some cheap MPPT solar charge controllers (from eBay). The better one's have extensive monitoring capabilities, usually on an LCD or OLED display. If you're counting and budgeting coulombs, it's a good way to do it. I would not need that though, maybe just an indicator of whether the dynamo is able to feed enough for a given light setting. Easy enough. The power output of the dynamo is directly proportional to the RPM and unaffected by the load. No really. Keep in mind the various losses. ... The load might change radically, but the dynamo will only deliver XX watts at XX RPM. You could just measure the RPM, build a lookup table in some kind of NVRAM, and display the output power. All I need is a sensitive voltmeter. That tells me whether my battery juice is being consumed or juice is being added. I have seen waterproof LED meters with two digits after the decimal point for less than $5. This is what I am going to add some day. It will also let me see the charge state before heading out on either bike. Or, you could get fancy, measure the load voltage and current, and calculate the power needed to run the lights or whatever. Then compare the input power available, with the average output power needed. If there's not enough input power available, you need to shed some load, or pedal harder. Nah, too fancy :-) Even that is expendable if you have a voltmeter riding along with the Li-Ion battery. In my case I'd just have to make sure it won't get close to 7V where the electronics will eventually shut things off. If you do go over, you might want to shed the load slowly or you might lurch forward as pedaling suddenly becomes easier. With a 10-15W load? Nothing beats trying it on the bench, using a variable speed drill or similar. I haven't observed a sharp knee effect on any of them so far. Which probably explains the large number of bulbs I blew out in the 80's when I had to get somewhere fast. Yeah, I should do that. However, as I previously mumbled, I'm overloaded, busy, lazy, and uninspired right now. Same here. The MTB still needs to be fixed, taxes to be done, then there is work, and right now I am busy with a much higher priority. I brewed a Belgian Tripel this morning and this afternoon a Superior Strong Ale is in the process. -- Regards, Joerg http://www.analogconsultants.com/ |
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