12V - 6V circuit to drive motor

[Sadlercomfort]

Hi guys,

I'm planning to repair my nephews electric-powered quad bike. I measured the '6V 5AH' battery voltage after a full charge cycle with a reading of just 3.6V, so i'm going to replace the battery.

Having three spare 12V batteries I decided it would be best to create a small circuit to drop the voltage to 6V to drive the 6VDC motor. I only have 7805 Voltage Regulators so want to try use this. I modified an online circuit and managed to get a 6V output when I simulated it.

My worry is the current in these batteries, the old battery is rated at 6V at 5AH. However 12V batteries are normally rated at around 10AH. I can't remove the motor to check its current rating but I'm hoping its no more than 1.5A. Note: the 6V motor in the circuit diagram is automatically drawing 745mA.

Do you guys think this circuit will function properly at these currents, with a stable 6V output?


Sorry for the essay,

Ash

 

[(*steve*)]

Whilst in theory this might work, in practice it almost certainly won't.

Firstly, the motor probably draws far more than 1A, and secondly you are wasting as much energy in heat (from the regulator) as you are sensing to the motor.

A far better option is to apply 12V to the motor via PWM with a duty cycle of 50%. This will be very similar to applying 6V to the motor. You may want to be able to vary the duty cycle a little to fine tune it so it goes almost the same as on a 6V battery (don't be tempted to make it go faster).

PWM will waste far less power and the 12V 10Ah battery will last 4 times as long as the 6V 5Ah battery. (In theory it will be about 4 times the weight too)

AH is a measure of capacity, not of current.

 

[Sadlercomfort]

Okay, so how about a 555 timer set-up in PWM with a trim-pot to preset the duty cycle to 50%. Then a high rated transistor or fet to switch the motor on the collector?

AH is capacity - I really should of known that.

 

[(*steve*)]

I would recommend you use a circuit with 2 steering diodes to allow the duty cycle to be changed from <50% to >50%. No need for a very high frequency -- 1kHz would be fine.

If the motor makes a 1kHz whine, change the frequency up or down a bit to find a frequency that doesn't resonate with anything.

* Use a small gate resistor (say 10 ohms)
* Ensure that the motor is between the supply rain and drain
* Place a diode across the motor
* Place a high value resistor (say 10k) between gate and source
* Ensure the mosfet is generously rated (stall current may be quite high)
* Remember to insulate the mosfet from the heatsink so you don't short stuff out.

 

[Sadlercomfort]

I'm comfortable with PWM and those conventional good practices you mentioned.

So hopefully all goes smoothly. One simple question, what is the 10k resistor between the gate and source for?

 

[(*steve*)]

One simple question, what is the 10k resistor between the gate and source for?

When power is disconnected it provides a path to turn the mosfet off. It ensures the gate won't be left floating. It also means that you can switch power OFF to the 555 and know the motor will stop.

 

[Sadlercomfort]

Thanks Steve, appreciate your help.

I'll post a circuit diagram before I make a prototype on a board,

Ash,

 

[JonBoy470]

This sounds like a Power Wheels Mini Quad you're working on. The running current is not what you need to be worried about... It's when the kid drives into a wall and keeps on the gas. a stalled DC motor is a very good short circuit. The stock batteries for these things have a self-reseting thermal fuse to prevent damage to the toy from such abuse. Such protection (such as from an automotive fuse) is a good start, but be aware your circuit could be called upon to supply better than 20 amps for longer than instantaneous amounts of time in a stalled motor scenario. Unless you have literally all the parts lying around to build your circuit, it may well be more cost effective to procure a 6V battery. You could also build a battery pack from Ni-CDs or NiMH rechargeable cells.

Http://www.modifiedpowerwheels.com has some great info for people tricking out their kids' toy cars.