Ultra Light Electric Assist Current Limiter Circuit by John Tetz
Ultra Light Electric Assist Current Limiter Circuit
Aug 2008 - An article by John Tetz

In designing an Ultra Light E Assist system,  components such as the gear box and battery need to be small and lightweight. To prevent excessive forces on a small light weight gear box and to prevent excessive current draw from small batteries, a current limiter is needed.

This current limiter works with standard RC components. The first component is the Electronic Speed Controller (ESC), which controls the brushless RC motor. The second component "Servo Tester" is the throttle circuit. Because RC systems do not have a mechanical throttle like a system that was manufactured for electric bikes would, the Servo Tester is used to allow an RC system to be controlled with a variable resistance type throttle. In this low power application though, a push button is all that is required rather than a variable throttle. The system is either on or off.

The following is a circuit showing the addition of an Op Amp to the normal electronic circuitry (Servo Tester and Electronic Speed Controller – (ESC)). The Op Amp is used as a comparator looking at the voltage drop across a shunt resistance, and will lower the voltage at the servo tester depending on the current load. 

The shunt is 12 inches of .052 diameter wire. The drop across it is 24 mv with the motor running but not loaded, and  85 to 100 mv with around a 10 amp load. This drop is a slight loss in overall efficiency but is made up for by giving a choice of lighter components with protection of those components.

The range of voltage at the center of the pot on the servo tester is around 2.4 volts for ESC initialization and motor not running, to 3.5 volts motor running. (Higher for higher power).  As the current rises above a preset level, the Op Amp output voltage drops dragging the voltage at the servo tester down which  tells the ESC to limit motor current/power.  

I mounted the Op Amp circuit on the tiny Servo Tester board. It’s the black object to the right in the photo. The chip to the left is the Servo Testers, the white thing in the middle is the Servo Testers pot, and to the opposite corner is the 10 turn pot I added (trimmer on top). The back side has the hard wired circuitry. Three interconnecting leads are needed with appropriate connectors. The two wires across the Shunt, the 5 volt power from the ESC and the signal to the ESC (3 leads) and the two wires to the handlebar Push Button on/off switch.

The following is a bench test plot showing the kinds of current and wattage that could possibly be drawn under an inappropriate load such as engaging the motor at a low RPM. This kind of power would easily stress light weight components.

Plot 1

And here is a Plot showing limited current/voltage/watts.  

Plot 2

The current limit is set by the pot going to the + terminal of the Op Amp. Measure this  set voltage to ground as a reference. For around 10 amp limit its 2.25 volts.

I used a 497-4075-5-ND Op Amp from Digi-key. It has to have enough current capability to drive the voltage down on the Servo Tester – something around  10 ma or more should do it.  

The following is series of on road data showing how the system works over a variety of conditions.

Typical shifting sequence with very light HP help. For vehicle speed move decimal over two places to the left. Note how consistent the average watts are. The first blip at 7 seconds is 3rd gear.

Plot 3

This shows a more detailed current/voltage view of a standard shifting sequence.

Plot 4

This plot shows motor RPM with typical shifting sequence with a little HP help. Shifted two gears at 27 seconds.

Plot 5

Motor RPM when shifting to a 4 higher gear jump at 21 seconds. The power here would well be into the 300 watt range without the limiter, stressing a small gearbox and battery.

Plot 6

In normal operation this wouldn’t happen. The rider’s cadence would be too low to be comfortable so this is a strong influence to be in the correct gear. But if the handle bar push button was engaged at a very low vehicle speed the power can instantly climb to where a mechanical part may fail. This happened to me doing a very slow speed test and it sheared off the output shaft on the gear box. I had ground two flats on a 6 mm shaft which weakened it. However with the current limiter it would have survived.

With the addition of a tiny part the system can be idiot proof.

See Current Limit with Transient Spike Reduction for further development of this system.

More about John's project and others on the ULEA (Ultra Light Electric Assist) Yahoo Group

John Tetz

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