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How do I reduce the speed of a 12v children's car?

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surreyman

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how do i reduce the speed of a 12v childrens car

Hi all , i would be so grateful for any help- i have bought a half scale electric willys jeep for my 5 year old son. It currently runs on three car batteries wired in series which are connected via a progressive accelator unit to an 1100 Wat motor. The problem is that it does over 20mph and he could badly hurt himself as the car is heavy. I would like some advise as to how to put in some speed regulator. preferably something that i can turn up/down depending on who is using it. Will happily post pics if someone tells me how on this site! Any help appreciated
 

Re: how do i reduce the speed of a 12v childrens car

Hi,

Disconnect one battery and run the motor from 2 batteries in series.

Other solutions:
* gearbox
* duty cycle controlled halfbridge (it gives better speed control than simple one-Mosfet solution)

I assume there are suitable circuits on ebay.

Klaus
 

Re: how do i reduce the speed of a 12v childrens car

Hi,

Disconnect one battery and run the motor from 2 batteries in series.

Other solutions:
* gearbox
* duty cycle controlled halfbridge (it gives better speed control than simple one-Mosfet solution)

I assume there are suitable circuits on ebay.

Thanks but you will have to treat me like an electrical idiot ! i can obviously work out how to disconnect one battery but as for a duty controlled halfbridge i am lost! can you point me towards what i need and where i wire it in - presumably just after the batteries
 

Re: how do i reduce the speed of a 12v childrens car

Hi,

There will be different circuits with different connection schemes.
Look for a circuit that seems to fit your needs.
Then go through the datasheet/ manual for details.

Klaus
 

Re: how do i reduce the speed of a 12v childrens car

sorry i am lost
 

Re: how do i reduce the speed of a 12v childrens car

Hi,

Does this have an accelerator pedal? We could skip the electronics tampering and affix a rubber stop under the accelerator pedal in an appropriate place so as to limit the maximum pressure your son can apply. Even one of those ~3cm high rubber cylinders with a screw used as door stops might suffice.
 
Re: how do i reduce the speed of a 12v childrens car

Hi,

Does this have an accelerator pedal? We could skip the electronics tampering and affix a rubber stop under the accelerator pedal in an appropriate place so as to limit the maximum pressure your son can apply. Even one of those ~3cm high rubber cylinders with a screw used as door stops might suffice.

thanks , i tried this but this leaves hardly any travel in the pedal if i set it to a level that keeps it circa 7 mph - it feels like its either on or off rater than progressive as it is now. i also tried eliminating one of the 4 batteries from the series and the thing just doesnt work
 

Re: how do i reduce the speed of a 12v childrens car

Just remove batteries as Klauss said and you'll probably get the desired effect of slower speed. Add in switches to make this selectable although good high current switches are expensive ('battery combiner' switches is a keyword)

On a separate note what car is this? It sounds pretty fun.
 

Re: how do i reduce the speed of a 12v childrens car

An alternative that doesn't need too much technical knowledge is to electrically (as opposed to the door stop being mechanical) limit the accelerator. I would assume it is a variable resistor (works a bit like a rotary volume control) with a return spring. If you can find out the value of the control you can add a second one to limit it's range of adjustment. A second control could be adjustable from 'no effect' to 'crawl along' so you can pre-set the top speed by just turning it (but don't tell your son that!). A pre-set control will cost very little (about 1 pound UK).

See if you can find out what is actually operated when the pedal is pressed. You will probably find a small rotary control with some numbers on it. Let us know how many wires are attached to it and what those numbers are. A photo would be useful.

Brian. ( who's car doesn't go much faster than your son's :cry: )
 

Re: how do i reduce the speed of a 12v childrens car

An alternative that doesn't need too much technical knowledge is to electrically (as opposed to the door stop being mechanical) limit the accelerator. I would assume it is a variable resistor (works a bit like a rotary volume control) with a return spring. If you can find out the value of the control you can add a second one to limit it's range of adjustment. A second control could be adjustable from 'no effect' to 'crawl along' so you can pre-set the top speed by just turning it (but don't tell your son that!). A pre-set control will cost very little (about 1 pound UK).

See if you can find out what is actually operated when the pedal is pressed. You will probably find a small rotary control with some numbers on it. Let us know how many wires are attached to it and what those numbers are. A photo would be useful.

Brian. ( who's car doesn't go much faster than your son's :cry: )
hi i am trying to post some pics but this site just refers me to something called igmur! thats exactly the sort of thing i am looking for - something like a second speed control- it looks like its kind of a bespoke unit that slides in and out on the end of the accelerator pedal- i will go see if it has any specifications on it
 

Re: how do i reduce the speed of a 12v childrens car

thanks- thats exactly what i need- it has a variable type resister that slides in and out when you depress the accelerator- i will go see if it has any numbers on it but failing that how would i know ? i am also trying to post images but the site just sends me to something called igmur!
 

Re: how do i reduce the speed of a 12v childrens car

Try this - I'm an administrator on here so my screen may be a little different to yours so forgive me if I'm writing about things you can't see.

Above the message window see if there is an icon that looks like a square with a dot in each corner, the mouse-over text would be "Insert Image".
Click "From Computer" then "Basic Uploader" (bottom right of the pop-up window)
Click 'Browse' and select the file from your computer.
Click "Upload files".
Give it a moment to finish the upload then it should close the window and add some 'attach' number to the message.
You probably wont see the image in the text until it is actually posted, the forum insert the image into the message as it prepares it for viewing.

Most variable resistor have the value marked on them. It will probably have something like 10KA and a manufacturing date code. From the numbers we can probably decode the value and from the number of wires connected to it was can tell how it is connected into the circuit.

Test image is me trying the method out. Unless you want a picture of my hand and an extrusion, ignore it!

Brian.
 

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Re: how do i reduce the speed of a 12v childrens car

Try this - I'm an administrator on here so my screen may be a little different to yours so forgive me if I'm writing about things you can't see.

Above the message window see if there is an icon that looks like a square with a dot in each corner, the mouse-over text would be "Insert Image".
Click "From Computer" then "Basic Uploader" (bottom right of the pop-up window)
Click 'Browse' and select the file from your computer.
Click "Upload files".
Give it a moment to finish the upload then it should close the window and add some 'attach' number to the message.
You probably wont see the image in the text until it is actually posted, the forum insert the image into the message as it prepares it for viewing.

Most variable resistor have the value marked on them. It will probably have something like 10KA and a manufacturing date code. From the numbers we can probably decode the value and from the number of wires connected to it was can tell how it is connected into the circuit.

Test image is me trying the method out. Unless you want a picture of my hand and an extrusion, ignore it!

Brian.

image1.JPGimage2.JPGimage3.JPG
 

Re: how do i reduce the speed of a 12v childrens car

one of the pics shows the end of the throttle pedal- the black plastic wrectangular thing is the resistor that has two parts that slide in and out of each other- two wires coming out but no numbers on it- i have sent an email to the manufactures to ask its spec
 

Re: how do i reduce the speed of a 12v childrens car

This is similar to a golf cart. It might use a similar method of speed control. It must be compatible with your system's voltage and Ampere draw.

Internet articles tell how golf carts run on the pulse width modulation concept (similar to duty cycle control per post #2). The further you press the pedal, the faster the motor goes.
 

Re: how do i reduce the speed of a 12v childrens car

Surreyman, thank you for posting the pictures. I will try to explain how the motor speed is controlled without being too technical, it might help you understand how it works:

There are two ways to control the amount of power a motor produces, analog control continuously varies the power made available to the motor depending on the accelerator position and digit control that pulses full power on and off rapidly. The amount of power made available to the motor has a direct influence on the amount of mechanical power it uses to move the car and hence it's speed.

Analog control.
Basically, a DC motor as used in the car runs at a speed proportional to the voltage you apply to it. Ignoring the effects of loading, such as going uphill or dragging something, the speed is faster when more voltage is applied. Controlling the voltage using analog circuits isn't very efficient because if the battery voltage is steady at (presuming 3 batteries of 12V) 36V but the required speed only needs say 20V, the 16V difference has to be lost somewhere. Unfortunately, you can't just magically make it vanish, it has to be converted to heat which is wasted and potentially creates a safety hazard. The efficiency calculation is complicated but I would guess for normal 'around the garden' use, it wouldn't go higher than 50% so you waste half the battery power into the air.

Digital control.
A far more efficient way to control the speed is using PWM (Pulse Width Modulation) which is a digital technique. Thinking of the speed controller, imagine two extremes, firstly you disconnect the battery completely - the motor obviously doesn't turn but no heat is produced. Then imagine wiring the motor directly across the battery, it runs flat out at maximum speed but again, no heat is produced. So it is possible to go from zero to maximum speed with no losses, what is needed is a way to do that but with reduced speed. PWM works by using the efficiency of the two extremes but adding a time element to the control. Imagine you could turn the FULL motor voltage on and off, say on for one second, off for one second. You get a very shaken up child but the average speed they achieve is only half of maximum because power was only there half the time. Now picture the same scenario but instead of once per second, it was pulsed maybe 1,000 times a second. The momentum of the motor and car smooths out the jerkiness but still only half speed power is present. So by using efficient "fully on / fully off" switching the speed has been controlled without having to lose the voltage as heat anywhere. So how is the speed varied? Well, it's very simple, you change the proportion of time the power is on to the time it is off. In most cases the rate (we call it PWM frequency) stays constant but within each time period the part the power is on compared to off is adjusted to control the average. On for a short period then off for a longer period gives reduced average power, the opposite, keeping the power on for longer in the period gives increased power. The accelerator control just 'tunes' the electronic control to alter the pulse length. Stop is when there is no pulse width at all and maximum speed is when the pulse fills the entire time period so the power is applied continuously.

What we need to find out is the way pedal control is connected to change the pulse width. I would guess the box attached to the pedal contains a variable resistor. It's value will change as the pedal is pushed, what we don't know is whether the value reduces or increases as the pedal is depressed. If all my assumptions are correct, a second 'hidden' control can be wired to limit the change in resistance and therefore the maximum speed it can reach.

Brian.
 
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Re: how do i reduce the speed of a 12v childrens car

image1.JPG

Thankyou so much for that reply- the attached is a pic of the variable resistor thats connected to the end of the throttle pedal. the other end is connected yp the chassis- one side slides into the other as the throttle is pressed. the makers of the car in vietnam dont seem to know much about the electrics as they just buy a package from china. the suppliers in china make engines/axles for town cars and arent interested in me.
 

Re: how do i reduce the speed of a 12v childrens car

This is the chinese company that makes the motor/controlbox and throttle resistor. This jeep has a 1200w motor so far too powerful for a child- it can power 2 adults in it at 20mph! a second adjustable hidden resistor is the way forward. but i dont know how to get the spec of what we have currently!
 

Re: how do i reduce the speed of a 12v childrens car

is there anyway i could call and speak with you directly as you seem to have the best understanding of my problem? i would be most grateful and happy to pay for anytime spent on this

- - - Updated - - -

is there anyway i could call and speak with you directly as you seem to have the best understanding of my problem? i would be most grateful and happy to pay for anytime spent on this

The variable resistor on the throttle then connects to the control box in my most recent pic. Its all like a mobility scooter except with a 1200 watt motor!image1.JPG
 

Re: how do i reduce the speed of a 12v childrens car

I'm on the move much of the time so a phone call would be difficult. However, your last photograph sheds some light on the control mechanism, it uses a three-phase drive system rather than controlling the voltage as I previously assumed. The kind of motor it drives has three power wires and the power is switched sequentially between them to 'pull' the rotor to a different angle. The rate and sequence of switching is what controls the motor speed and direction. The motor may also have feedback wires that sense the difference between actual and required speed so it can adjust the power to compensate for real-World loads.

I can't find any reference to that actual controller but bear in mind that the same manufacturer may change company name every day so finding who they really are is nearly impossible. However, some research shows that all the similar units on the market use a simple control voltage to set the speed. Your photograph of the variable resistor in post #17 confirms that yours uses the same method. The control has three wires, one will be 0V, one will be 5V and the third picks off a voltage between them according to the accelerator position.

You need to identify which wire is which. Do you have a voltmeter available?

Brian.
 

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