unclejemima said:
the controller reads throttle input as 1.2 to 3.5V.
...
So, if I'm understanding this correctly, I need a resistor to bring up the voltage from 0 to 1.2, then reduce the top end of the throttle from 5.0 to 3.5.
Exactly - you need both resistors to properly bracket the Magura range - a single resistor will leave you with a 'dead zone' at the other end.
Here's the rub: Variations in components in the controller and throttle can lead to different throttle resistance and different controller input range. As far as top and bottom 'dead zones' are concerned you can detect a difference of only .1v in throttle rotation, so to get the best possible throttle range, the resistors need to be adjusted for the particular throttle and controller - not just the general brand and model. This is why many folks use trimpots instead of fixed resistors - as in the little PCB earlier in the thread.
That said, you *can* use fixed resistors, but you will do better if you make a few measurements:
- the actual Magura resistance
--Just measure this with a meter (mine have been around 5.5K)
- the actual min/max controller voltages
--Put the bike on a stand and hook up the Magura without resistors. Advance the throttle and note the throttle voltages at which
- the wheel just begins to turn (min) and
- spins no faster with added throttle (max).
You can get these by probing the back of the throttle connector with a meter.
You will typically want to increase the throttle min/max about 0.1v beyond what the controller expects to ensure there is no creep at ZERO throttle and that you can actually achieve WOT - even with a little drift in parts values. For instance, in your case of (min, max) = (1.2v, 3.5v) you would want to use a thorttle range of (1.1v, 3.6v).
Here's a little spreadsheet that will do the calculation for you - it works in Excel or Apache Open Office (free). Plug in the stuff in the green cells and it will give you the resistances. Plug in the actual standard value resistors you want to use and it will calculate back the actual voltage range you will get.
View attachment MaguraThrottleResistorCalculator2.xls
I don't know what your Magura resistance is or the actual controller range, but...
Here are two runs assuming 5K and 5.5K for the Magura. I used the 'claimed' controller voltage range and jacked it open a bit as mentioned above. Looks like 2.2K and 2.7K resistors will work fine if your Magura is 5K and okay even if it's 5.5K.
View attachment 1
Here's a last run using resistors of 2.4K and 3.0K if your Magura is 5.5K - somewhat better fit to the range.
So - if you don't want to measure things, you might want to buy all four values and tinker a bit....
