Controller shunt mods

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fechter   100 GW

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Re: Controller shunt mods

Post by fechter » Dec 27 2018 12:48pm

newbiehere wrote:
Dec 27 2018 12:59am
oh ok cool got it about the shunt resistor, but I still need to break the trace for the wiper connect, correct?

Ive seen complicated work arounds for measuring current with an arduino, seems this type of shunt could do the same thing?
Yes, you need to cut the trace to insert the divider.

An Arduino A/D input would not have very good resolution at the voltage level on the shunt. It would work better if the signal was first amplified about 10x. Typical circuits I've seen use an LM358 or similar op amp to boost the signal ahead of the Arduino.

Another option is to get an inexpensive power meter. There are several out there. I use a TK15 coulomb meter on one of my bikes and like it other than the screen is pretty small. I use it mainly to keep track of my battery level.
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newbiehere   100 mW

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Re: Controller shunt mods

Post by newbiehere » Dec 30 2018 3:20am

for the pot in parallel, how does it not take on the a large current, like the shunt is doing?

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fechter   100 GW

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Re: Controller shunt mods

Post by fechter » Dec 30 2018 12:43pm

The shunt resistance is about 1milliohm. The pot can be somewhere between 1 ohm and 100 ohms and it will take about 1/1000 of the current if it's 1 ohm. Another way to look at it is the voltage across the shunt at 100 Amps would be around 100mV. This is nothing for the pot. I used a 10 ohm pot.
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newbiehere   100 mW

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Re: Controller shunt mods

Post by newbiehere » Dec 30 2018 6:27pm

oh ok cool, got it

newbiehere   100 mW

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Re: Controller shunt mods

Post by newbiehere » Jan 04 2019 7:27am

so if the voltage is very low, then a pot can be used? in the other post, it was mentioned that I would need a huge pot or rheostat to control the current. here we are not controlling the current (fixed) but instead the voltage so a large current can still move across the pot?

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fechter   100 GW

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Re: Controller shunt mods

Post by fechter » Jan 04 2019 9:26am

The big shunt resistor on the controller board has to carry the full current. But at the rated current, the voltage across it is still very small. The pot is only carrying a tiny amount of current and is just changing the voltage that the MCU "sees".

If you wanted to make the shunt resistor on the board variable, that's where it would get difficult.
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newbiehere   100 mW

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Re: Controller shunt mods

Post by newbiehere » Jan 07 2019 2:36am

Oh I see, thanks

Yungyakz   10 mW

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Re: Controller shunt mods

Post by Yungyakz » May 07 2019 8:47pm

fechter wrote:
Dec 26 2018 11:43am
newbiehere wrote:
Dec 25 2018 5:31pm
so i dremel tool off some of the trace, then put a pot in between? if its a 3 terminal pot, how do I connect the terminals?
Sorry for the delay, holidays are interfering with the advancement of technology again.

The way you did it will work but you if you turn the pot too far, you can totally defeat the current limiting and possibly blow something up. I've used that method before and it will be OK if you just dial it in carefully and leave it alone after that.

There are many other ways to do this. Below is just one. By using a pot and a fixed resistor, you can limit the maximum increase. In the circuit below, the pot will adjust the current limit from stock level (100%) to 2x (200%). The pot and the fixed resistor are the same value, which gives the 2:1 range. With a different fixed resistor, you could make the range whatever you want.

2x variable shunt divider.jpg
I'd like to increase a controller from 25A to ~34A.
I'll use the shunt modification, but like to be a bit precise.
The controller, currently use 2x 5 mohm shunts.

So, please tel me if I'm right :

2x5mohms= 2.5 mohms

2.5 mohms -> 25A
??? mohms -> 34A

From 25A to 34A, it's +36%
So, 2.5 mohms - 36% = 1.6 mohms

3x 5 mohms =~1.67 mohms

I finally just need to add a 3rd 5 milliohms shunt on my controller to reach my goal ?
To get 34A, the shunt would be about 1.84 milliohm. (2.5 x 25/34)

If you add one 5 milliohm to the existing, your limit would be about 37.5A (that's probably close enough)
This is great mod to do on chineese controllers and i have a 15 fet 1kw controller and done few mods like ebs and adding solder to the shunt etc but really want to try it but I dont want to cut the trace and then not be able to solder wire wire to the cut trace. Is there any pictures on how it is done and how the trace was cut and wire soldered to it. I have a backup controller theese controllers are cheap but I dont want to mess things up.

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