Car USB charger in parallel with switchable dyno hub light?

wqlava1

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i have a few bikes with dyno hubs - SON, SP and Shimano, and a variety of lights- Trelock 885, E3 Pto and Edelux 2. I wanted to charge USB things, and was wondering to myself if I can save some coin by using a $1.88 double USB car/truck charger off eBay in parallel with one of my lights. Especially if it was with a switchable light and I used a socket so it was only plugged in when needed. The truck chargers can take at least up to 24v so that should help a bit. Tell me where I'm not thinking this through fully, and please comment on what would happen if both light and charger were used at once.

thanks, Andrew
 

wqlava1

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To reply to myself with an initial (obvious) correction, it'd also need a diode bridge to rectify the AC. But my budget still hasn't hit $5 - AUD.
 

Steve K

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if your USB devices being charged can be charged with 500mA, then it seems like it might work out.

If both the light and the USB charger are being used at the same time, you'll get less power to both the headlight and the USB charger (which was probably expected).

The only other concern might be the voltage out of the dynamo when it is lightly loaded. i.e. if using it to charge a USB device, and the battery reaches full charge, then there won't be any power drawn from the dynamo. In that condition, the dynamo voltage will rise above the nominal 6V. I've measured 28VAC rms from my SON when unloaded, so that means about 40V peak. That might present a risk to the USB charger, and as a result, the device being charged.
 

wqlava1

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if your USB devices being charged can be charged with 500mA, then it seems like it might work out.

If both the light and the USB charger are being used at the same time, you'll get less power to both the headlight and the USB charger (which was probably expected).

The only other concern might be the voltage out of the dynamo when it is lightly loaded. i.e. if using it to charge a USB device, and the battery reaches full charge, then there won't be any power drawn from the dynamo. In that condition, the dynamo voltage will rise above the nominal 6V. I've measured 28VAC rms from my SON when unloaded, so that means about 40V peak. That might present a risk to the USB charger, and as a result, the device being charged.

Steve,

your response is quite positive really. The high voltage problem is largely covered by use of a USB charger suitable for a truck as well as a car. There must be some design margin in them (although I have let the smoke out of two that were rated only for 12V cars when used in a truck I drive sometimes), and if that's a worry, there's another one on eBay titled" 22mm 12V-80V Motorcycle ATV Bike Waterproof USB Power Charger Socket " for AUD5 that's even got a handlebar clamp built in. Still needs the 4 diodes added in a bridge, but this is something I should try, because it is so much cheaper than the gear that's built just for bicycles.

Andrew
 

Steve K

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There are standards for vehicle battery buses that require equipment in cars to withstand some brief high voltage spikes, so something designed for a 24V battery should be good.

I've designed electronics for earthmoving equipment that used 24V batteries, and the expectation was that a charging battery might be as high as 32V, so handing 40V peak should be easy (and that only happens at 20mph, which seems unlikely to be the cruising speed on a bike with a dynamo).

My bigger concern is trusting a cheap piece of ebay electronics to charge my expensive electronics. If it fails, there is a chance that the battery (or dynamo) voltage would be applied directly to my electronic device being charged. It might be worth testing with a resistive load before you connect something you care about.
 

WesleyfuhPP

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Car USB charger in parallel with switchable dyno hub light

please do ,and good luck ill be waiting to hear from you wats the price tag on something like that?
 

Steve K

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Re: Car USB charger in parallel with switchable dyno hub light

practically speaking, if you added a 18V, 5 watt zener at the output of the bridge rectifier, and just before the automotive USB charger, you should be in pretty good shape.
The 1N5355B zener would be a good choice...
http://www.digikey.com/product-detail/en/micro-commercial-co/1N5355B-TP/1N5355BTPMSCT-ND/1114297

edit: just in case it's not obvious, the anode of the zener would be connected to the negative (ground) input of the USB charger, and the cathode of the zener would be connected to the positive input of the charger.
 
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tspoon

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Re: Car USB charger in parallel with switchable dyno hub light

If only a dyno hub is connected, the electronic module possibly isn't necessary, as the hub is limited to around 500mA naturally and the voltage will adapt to that necessary (within reason) to push 500mA through the circuit. I used such a module on a recent tour and I am certain that at points the supply voltage was below 5V, but the battery in question (3.7-4.2V) continued to charge until complete. This suggests that many USB charge ports don't need the voltage to be 5V, only a sufficient voltage to allow current flow. Having said that, it's probable that this isn't the case for all USB chargeable equipment.
You may be able to get by with only a rectifier and zener limiting diode sized to prevent burnout of the lithium battery protection circuit.
 
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