CA showing negative Watts with headlight

[ort5]

Hi all,

I just added a DC to DC converter on my bike to power my 12V headlight with my 56V batteries. I have it wired up and it works, but the CA shows -38W instead of 38W being used. Any idea what's up?

I have it wired after the CA shunt, just like my motor controller (which shows the correct Wattage power draw instead of power generation).

Thanks,
Dave

 

[lawsonuw]

This seems to be an issue that can crop up with any Watt meter when hooked to loads that draw current in short bursts. (like DC-DC converters or Controllers) Specifically, if the load doesn't have a big enough input capacitor these short bursts of current cause a lot of noise on the power wires. This noise can fool a watt meter into seeing a negative current.

To fix this, either add an appropriate capacitor to the DC-DC converter's input or wire the DC-DC converter very close to the controller. (This allows the DC-DC converter to borrow the controller's input capacitors)

Marty

 

[justin_le]

Hi all,

I just added a DC to DC converter on my bike to power my 12V headlight with my 56V batteries. I have it wired up and it works, but the CA shows -38W instead of 38W being used. Any idea what's up?

Hi Dave, this is something that can happen when the Cycle Analyst with a remote shunt is used with noisy or poorly filtered switching power sources (which unfortunately includes a lot of battery chargers too). To be honest, I am not entirely sure why this happens but it is something that we'll certainly be investigating and trying to sort out with a future design change. In the meantime though, if you use a quality DC-DC converter like the Vicor units then it behaves just fine, or you could try adding capacitive and/or inductive filtering to the input of the DC-DC which you have now.

One final question, does the -38W show up regardless of whether the headlights are on or not? And if so, does the wattage then change by the expected amount once they do? That is, if you have a 20 watt headlight, does it go from -38W to -18W when DC-DC is on all the time but the light goes from off to on? If so, you can also re-zero the amps offset of the CycleAnalyst with the DC-DC unit connected and have it display correctly after that.

Justin

 

[marty]

lawsonuw and justin_le,
You both talk about "noise on the power wires" and "noisy or poorly filtered switching power sources" Think that is why I hear static on radio every time I turn on DC/DC Converter? Radio is about 10 feet from electric bicycle experiments.

Using 48V in 5V out DC/DC Converter Datel UWR-5/8-D48A-C
Go here to see in great detail http://www.cd4power.com/data/power/uwr26-40w.pdf

I copy:
All UWR 26-40 Watt DC/DC Converters achieve their rated ripple and noise specifications using the external output capacitor specified in the Performance/Functional Specifications table. In critical applications, input/output noise may be further reduced by installing additional external I/O caps. Input capacitors should be selected for bulk capacitance, low ESR and high rms-ripple-current ratings. Input capacitors serve as energy-storage devices to minimize variations in line voltage caused by transient IR drops in PCB conductors from backplane to the DC/DC. Output capacitors should be selected for low ESR and appropriate frequency response. All caps should have appropriate voltage ratings and be mounted as close to the converters as possible.

Input Filter Type Pi (0.01μF-2.2μH-2μF)

Would someone please explain Pi (0.01μF-2.2μH-2μF)
There are 42,366 capacitors here http://www.newark.com/capacitors
WHICH CAPACITOR SHOULD I BUY? Radio Shack? Where do I put capacitor?
Battery is 48V 20Ah Li-ion. Go here to see http://endless-sphere.com/forums/viewtopic.php?f=14&t=1170

PS: justin_le - Good to see you test the products you sell by riding across Canada

 

[ort5]

One final question, does the -38W show up regardless of whether the headlights are on or not? And if so, does the wattage then change by the expected amount once they do? That is, if you have a 20 watt headlight, does it go from -38W to -18W when DC-DC is on all the time but the light goes from off to on? If so, you can also re-zero the amps offset of the CycleAnalyst with the DC-DC unit connected and have it display correctly after that.

Justin

Hi Justin,

Thanks for the input. The -38W shows up only when there is a load on the converter. Otherwise it's 0W. I played with it a bit and found that the Wattage figure is actually quite variable, depending on any number of strange things (if I'm touching the converter, if it's on a heatsink, if it's rotated...etc.). I put the ESC back into the loop and the behavior is much better now. I think Marty's suggestion of "borrowing" the ESCs input caps worked. The CA shows a steady 45W now. Incidentally, this is a pretty high quality DC to DC converter. The data sheet is here: http://www.part-datasheets.com/data/qw050b1.pdf I'm surprised how noisy the switching is, and how hot it gets. My headlight is a 35W bulb, so it's within the 50W limits. The datasheet says it's 85% efficient at full load so I guess that means it has to dissipate around 8W of heat.

Dave