Tongxin Current Limiter

[mkc1]

Following on from the discussion about Franks blown tongxin controllers i decided to make a current limiting device to have more control over battery consumption.

I bought a 0-50A LCD display (complete with Shunt) from ebay as the base for the project. The voltage drop is 0-75mv for the complete range 0-50amps.


The next step was to design a simple comparitor circuit with a variable threshold(current limit) which turns on a transistor to 'clamp' the throttle voltage. When i say simple, it really is...
Having tested the LCD meter on a bench i found it's calibration was wildly inaccurate. Simple solution was to add a 24swg copper wire to the shunt to make it good. (You can see this on the first picture)

Next step , monitor my Tongxin motor setup and see how much current is being drawn under certain conditions.
Amazing...when you think you are being careful and prudent with power, at the low speeds the meter was telling me different. On occasions i saw 18A being drawn (wide open throttle) and nothing unusual to see a constant 11-12 amps being unnecessarily drawn because of lack of throttle control. Experimenting on one hill, the controller and wiring was warming up nicely.........actually getting very hot indeed.

With my new accrued data and a few 'suck it and see' ideas. I came up with a circuit compromising of a few resistors, 1 transistor, a linear potentiometer and 1 dual op amp. All very simple.

It works like this. The voltage drop across the shunt resistor gets amplified by the first part of the op amp (a). The amplified the signal is then compared to threshold voltage (set by the Pot) which will send the output of the second op amp (b) either high or low. This will switch on (or off) a transistor which in turn, connects a resistor to 'clamp' the throttle voltage.
This in theory was the idea.
Does it work?
Well after bench testing and playing around with different resistors to set gain and voltage clamping. It appeared to work. However until it was on the bike i wouldn't know.

As usual with my bike.....liberal use of tape, cardboard and cable ties, it was installed......don't laugh! btw no throttle being applied in this picture


First time out everything appears to work very well, but i had to make a few changes to the output resistors. It would reduce the throttle voltage ok but i could still make the controller draw 10amps.
Its was tricky to work it out as the impedance of the controller was unknown to me
However, a few more trys, and i have now found the ideal value for throttle voltage clamping.

With the adjustment at the minimum, wide open throttle. 13.5 mph, level ground, no pedaling. 2.6A


I'll continue in a reply as i've reached my 4 pics limit

[mkc1]

In the next picture, i've increased the threshold to maximum and given it full throttle with no pedaling. All on level ground.
10.9Amps and 18.5 mph and increasing



As you can imagine trying to take pictures and ride safely was not really possible, so i abandoned that for the next test.

[mkc1]

The next test was riding the Tarka trail complete and back again. The real test for my current limiter was the gradual incline on the trail 449ft in 4 miles. With the minimum setting i could use very little power at wide open throttle to assist me. Most of the time when pedalling uphill only 0.9A was being drawn.
I could never have found that small amount of assist by using the throttle only.
So i declare my current limiter a success....
Now i have to build it into a neat waterproof box and make another for my wife's bike.

Mel

Any questions just ask

[Tiberius]

Mel,

Nice stuff; are you limiting motor current or battery current?

Nick

[mkc1]

Nick
The shunt is wired into the battery pack, so the battery current is monitored and 'throttle clamping' controls the output current to the motor. So i guess you could say the motor current is limited.
Whether this will prevent blown controllers is difficult to say, i hope it will. One of the added benefits should be less thrashing of the battery and more mileage between charges.

[RobinC]

Thanks for this, I've been toying with similar ideas but the basic kit is taking me long enough to finish, so have held off adding bells & whistles but it sounds like the meter can be instructive.
I think the Cycle-analyst has these features and more but also costs more.

From your experience do you think a 20A meter would be enough for a 36V Tongxin system?
How are you powering the digital meter?

Robin

p.s. I thought the impedance at the throttle control would be set by the throttle rather than the controller. In the Cycle-Analyst system they recommend using a series resistor between the throttle & controller so the current limit circuit will be pulling against a known load.

[mkc1]

Hi Robin
Firstly, in the case of the CA, i have no knowledge at all other than they are out of my budget for the needs i had. I expect they will have the bells and whistle's you speak of of. I thought of getting a 'wattsup' meter, very useful item for for the info they give you but not for current limiting.
I figured i would enjoy making this limiter anyway, and i have to have 2,(another for the wife's bike).
I see no reason why a 20A meter wouldn't be ok. If the current were to go over the max i guess it would show OL on the display. The volt drop will not be affected. I can't see it ever getting that high anyhow.
On the impedance of the controller/throttle. Surely the impedance would be a combination of throttle and controller? I expect the CA advise a series resistor for users to be able to connect to 'any' controller. My way works too....but i'm always open for listening to suggested improvements.
The display is powered by a single PP3 9volt battery and the limiter also has it's own battery. I ran into problems running them from the same battery. I'm going to make both switchable, one is not dependant on the other, they just share the shunt resistor.
What is the rpm and the wheel size of your tongxin hub?

[RobinC]

The wheel size is 700C & 175 rpm but I can't tell you how it runs since it is a work in progress.

Have you decided where to set the current limit or will you leave it as adjustable on the fly?

Robin

[frank9755]

Interesting, Mel
I hope it saves you from controller burn-out!

Frank

[RobinC]

Not sure what the exact problem you found using a single shared battery but using an opto-isolator instead of a simple transistor to clamp the throttle voltage might give the extra design freedom to use a single battery.
Maybe now you have it working it is too late to bother trying new experiments..

Robin