Buck Converters in Series

Why not just use a separate battery?

I suppose that would be an option. I could use nimh batteries because they can be kept fully charged all the time unlike the lithium polymer batteries powering the motor.

No.

The input impedance of a buck converter is highly non-linear and time variant. So putting two in series will not predictably distribute the voltage across each input.

As usual you're design is absolutely correct and will work for the conditions proposed. However, the OP will have to locate a different switching supply than what he has in hand since the input voltage maximum for these switchers is less than the lowest source voltage. The OP cannot safely just series the inputs of two of these 26V maximum input switching supplies to obtain a 52V maximum input voltage that now exceeds the source supply.

On the other hand I'm certain one very familiar with switching supply design (like you Mr. Hill) could safely put the inputs of two switching supplies in series to make the point that it can be done. Maybe not the specific models the OP has but I'm certain a clever person could do this with some switchers. IMHO this will take much more design and loading analysis than it is worth. Locating a switcher that handles the source voltage range to generate the 6V and then using a LDO linear regulator is exactly the approach I'd take if this task was mine.

Yes, provided the first voltage output gives sufficient headroom for the second converter and has sufficient spare current capacity for the current drawn by the second converter.

However depending on the efficiency of the converters, max input voltage they can handle and total loading it may be better to run them in parallel - you need to look at all the numbers to decide the best configuration.

Do these devices have voltage ranges that overlap? Can they all work on 5 or 5.5V? I would expect the GOpro to work down to a lower voltage to allow for battery drain. (Apologies if you have already stated the lowest voltage in the range)

Regards

Craig

[+36V]-[U.ext.6V.Switchin'Gizmo=8V.@t=t0]--[U.ext.5V.Switchin'Gizmo=3V.@t=t0]--[Ground]
say your lights eat 120mA@6V=720mW say 6V SwG is 88% efficient e.g. 720/.88=818mW required = I=P/U=818/8=102mA required
say your Cam eat 1200mA@5V=6W say 5V SwG is 93% efficient e.g. 6/.93=6.45W required = I=6.45/3=2.15A required

so if you have a special storage battery to collect milliamps entering from lamp while you don't use it and it's switched off - it'd take several hundred years to load it from gigaohms pass - the worst case scenario - so to boost things up you'd have to bypass some current for Cam and voila ! you became parallel with your devices

thee are chips available that have multiple voltage outputs - perhaps even custom set by user ...

(i understand i do 72h workdays very rear but at some points it becomes a struggle answering the questions like who am i? what i'm stearing at? why dont those numbers just go home and let me have a nap? )

a good night sleep clears your sences - (well 24+20:25 muhahahahaa)

http://www.infineon.com/cms/en/product/automotive-ics/dc/dc-converter-automotive/channel.html?channel=db3a304320d39d59012153bf683e056c

I am needing 6v for powering the lights, a voltmeter and a Gopro camera.

Current demands?

The current demand is free engineering without comprehension.

actually it's a problem and with grids connected in parallel (if you're familiar with the topick ofcourse - 1 never gets there by sealing itself off the by some stupid glishee)

(Hi!)

it's like a current demand (from where i see it _{(WTSh cares)})

Why not take an output from 3 of the batteries in the pack and regulate them down to your required voltages ? Your loads are small, aside from the lights, are you using incandescent lighting or LED ? If you are using LED, you can series the LED's to use the 36V directly at a good efficiency. If you are running more than one incandescent light, you should series them up as well, you might be able to build an 18 or 36 volt load there even if you have to add more lights ( who doesnt need more light ?).

3 batteries gives you 18 volts, you can use one set of 3 regulated to supply the camera and the other set of 3 regulated to supply the meter. Splitting the pack will lessen the chance of unbalanced discharge, BUT you will have to be aware that you will need to isolate both of these low voltage supplies from the frame, and each other as well.

Rather than supply everything from the bike battery, its best to have each unit with its own batteries.

Its like NOT putting all your eggs in one basket!!!

Forgetting for a moment the building of a proper unit will cost more that 3 years supply of rechargeable batteries for the units!!!

Plus a failure in something could "kill" everything.....36 volts on the camera for example.....