January 2024
My dad recently bought our 2.5-year-old son an electric ATV off a Black Friday deal.
Always wanting one growing up, my wife and I (and our son) were thrilled, but being fairly cheap ($99) the throttle only had two modes: off and full throttle.
Thus, the starting jolt was so much that our son was scared to ride -- it was even a bit much for adults riding.
So with the help of Reddit, we modified it to have a smoother start -- documenting here because I didn't really see any other guides and so I can remember what I did later (note: I'm not an electrical engineer so some things here might be wrong)
Research
While Googling for solutions, I discovered the /r/PowerWheelsMods subreddit -- where parents soup up their kids' electric toy vehicles by doubling the voltage, adding gears, etc.
I’m generally more comfortable with bits, but determined to live vicariously I dove into hardware mode.
First problem was how to gradually increase the motor speed -- turns out this is typically handled via pulse-width modulation (PWM) [1]. Essentially this means turning "off" the digital signal pulse for a portion of the cycle so on average it achieves some desired percentage -- e.g. if we have a signal "on" 50% of the time and off the other 50%, our motor on average should move at 50% speed.
This is usually accomplished via a pre-made "soft-start module", like this $20 one (note: affiliate link) I picked up via a reddit comment suggestion.
Next was figuring out where to insert the module. It had 4 inputs:
- motor negative
- motor positive
- power negative
- power positive
The most intuitive to me was the motor circuit itself, this way the motor wires both only power the device when it's on (and thus doesn't drain battery unnecessarily), and can also modulate the power to the motors themselves.
After unscrewing the bottom panels, I found a yellow and green wire leading to the DC motors:
I cut the wire with wire cutters and using my Fluke multimeter, identified the green as negative and yellow as positive.
I then wired the PWM module in as follows:
- green (frame-side) - power negative
- yellow (frame-side) - power positive
- green (motor-side) - motor negative
- yellow (motor-side) - motor positive
First try, it worked -- I closed up the ATV, impressed with my own 15-minute handiwork and took the ATV for a spin, it was much smoother and easier to ride.
Then I tried the reverse setting... and blew the fuse.
What happened
The reverse setting operates by reversing the polarity of current in the motor circuit, which is what determines which way the motors spin.
So in reverse, suddenly my power + and - were connected to the opposite and caused the fuse to blow.
To remedy, I started by powering the soft-start module with my Korad power supply to make sure I hadn't fried the module itself -- that was all good.
Then I ordered an assortment of car fuses on Amazon, and started hatching a new wiring plan.
While browsing the Amazon reviews for the soft start module for wiring tips, I came across this diagram and explanation by a customer that I embarrassingly deemed as overkill:
The review text:
Worked on an off-brand jeep I upgraded to 18v. The acceleration was too much so this helped slow the start. To get slow stop, you need to add in a relay. I just let the tires skid to a stop.Follow this wiring diagram exactly and you’ll have reverse. When doing it the way it says, reverse blows the fuse. Also, programming is off by one. So on the instructions, slow start timing is number 1. Once I figured that out, programming is very easy. While you’re holding the pedal, change the arrow up and down and hit run/stop until back on that screen. (It’ll turn off then hit it again and it’ll run again.) That saves the setting.
Turns out there was actually quite a bit of wisdom in their approach in solving two important problems: motor polarity and battery conservation.
Problem #1 - Motor Polarity
If the motor polarity can change, then we need to insert our motor control BEFORE this shift. In the diagram, this was before the shifter.
Our ATV didn't have an explicit shifter, but just a simple double throw switch.
So I grabbed my extra long flat-head screwdriver (a grossly underrated tool imo) and popped the center module off.
Turns out the "shifter" was a double pole double throw switch with clever wiring to facilitate the polarity shift (the two pairs of green/yellow wires -- this will be important later).
Okay, so in theory we could just splice into the incoming power/ground to this double-throw switch for our motor power/ground. Problem one solved.
Problem #2 - Selectively-On Power
Since we can't power the soft-start module off the motor wires directly due to changing polarity, we need a new spot. An obvious solution is to just run it off the power wires near the battery, but there's a flaw here: our soft-start module will be powered even when we're not using it, draining battery.
Instead, as this diagram shows, we want it to only turn on when the pedal is pressed.
So we need to find the wires that deliver power after the pedal and only when it's pressed.
So I unscrewed the pedal casing and pried it open.
So we have three wires, but what did they do?
When I traced the power from the main battery compartment, there was a white wire going from the main power switch to the pedal.
The blue and black wires lead to the double-throw switch we found earlier (you can just make them out in the photos).
To figure out exactly what they did, I ran a continuity test on different combos of the black, white, and blue wires to see what effect the pedal had.
Results from test:
- while the pedal is NOT pressed, blue and black have continuity, nothing else does
- while the pedal IS PRESSED, blue and white have continuity, nothing else does
Feeding this into ChatGPT yielded the following:
Pedal Functionality:
• The pedal acts as a signal relay. When pressed, it sends the “go” signal (via the blue wire) to the 3-setting switch, which then routes power to the motor depending on the selected direction.
• When not pressed, the pedal keeps the circuit inactive (via the black wire).
• **Blue Wire**: Acts as the primary signal carrier to the 3-setting switch, toggling between “off” (connected to black) and “on” (connected to white).
• **Black Wire**: Likely sets the 3-setting switch to a default “neutral” or inactive state when the pedal is not pressed.
Okay so we know how to power the module after the pedal, and we know we need to splice into the power into the double-throw switch for the motors, but there was one problem -- I couldn't find the power into the double-throw switch.
If we revisit our photo:
The only potential power and ground wires could be the brown, black, or blue wires. But the brown wire when traced led to the charging port, which suggested it was to disable powering the motors when plugged in. This left only the blue and black wires -- suggesting our blue wire from the pedal was not just a signal wire, but also power delivery.
Wanting to confirm this, I grabbed my multimeter, but encountered another problem. The circuit was fused at 35 amps, my multimeter had a 10 amp fuse. I also didn't want to splice a wire just to check it's amperage.
That's when I discovered clamp meters that could measure amperage without needing to be inserted into the connection, for only $30. However, the costs were adding up and the ATV itself was only $99, so I thought about my options.
If power does NOT come from the pedal, our circuit gets complicated* — because we somehow need to incorporate the non-power-delivery signal loop while also ensuring we have power delivery, and we only have two terminals.
If power DOES come from the pedals, now our circuit is:
- power in comes from blue (cut, pedal side), power out = goes directly to white’s ground instead of the double throw switch
- motor in = blue (cut, double throw switch side)
- Motor out = black ground wire coming out of switch
This was getting complicated, involving significant additional wires, wiring work, etc.
With a lot on my plate with a new job starting and potential move, I thought about the success I had with the original, simple direct-to-motor wiring and wondered, what if I just removed “reverse”?
Remember the two pairs of green and yellow wires from earlier? While one green and yellow pair ran to the motors, the other two connected the double-pole double-throw switch to itself via diagonal connections.
My guess (and ChatGPT corroborated) was that this is what enabled the polarity switch.
So a simple solution would be to simply snip the diagonal yellow and green wires and just wire the module directly to the motors like I did originally.
Figured I’d test it, so I cut the diagonal wires and tested — only to see one of my fears materialize.
While I was correct in guessing the diagonal wires handled the polarity switch, I didn’t actually know which polarity was default and which the diagonal enabled.
As it turns out, the reverse was wired directly and the diagonal wires enabled the forwards, which id just cut 🥲.
Thankfully there was an easy remedy, I just switched the motor wires when I connected them, reversing the polarity yet again, and all was well.
So I terminated the diagonal wires, wired in the module to the motors directly like I’d originally had it, closed everything up, and took a test spin.
Me taking a test spin:
Success — but there was one final check, I called in the main stakeholder.
Acceptance tests passed — huge dad win.
[1] I originally wondered why resistance wasn't used, according to this StackExchange answer, it's for superior power efficiency, higher torque, and less power dissipation.
[2] while i originally thought we could just splice the motor in/out on a single wire like a resistor, I learned that this is not how motor PWM speed controllers typically work, and that they tend to function via an H Bridge, which requires both power and ground connected to the module, with power in/out providing the power then passed through the module to motor in/out. However if I’m wrong about this someone please let me know.
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