Re: motor speed controller for kids' electric car
- From: gearhead <nospam@xxxxxxxxxxxx>
- Date: Mon, 31 Aug 2009 20:41:15 -0700 (PDT)
On Aug 31, 11:37 pm, gearhead <nos...@xxxxxxxxxxxx> wrote:
On Aug 31, 2:43 pm, Michael <mrdarr...@xxxxxxxxx> wrote:
I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.
I saw this:http://zeva.com.au/speedy/
I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?
Thanks,
Michael
I tried to post this under my usenet account but it didn't work, so
I'll have to use google:
I read through most of the posts. I'll endorse those who made
statements
to the effect that the drive frequency for a motor is so low that a
555 will
do just fine. You can run it around 1kHz, ballpark, but anything from
100Hz
to 10kHz would probably work. I'd choose 400 Hz myself. Don't forget
the freewheel diode. A power Schottky is a good choice for this.
You can get the gearshift effect with a switch that has multiple taps
connected
to a resistor network. That ought to be really fun. Use 12 volts,
but have
the top speed something less than 100%. Experiment to find the best
"governor" frequency.
The usual way of hooking up a 555 astable gives duty cycles only
between 50%
and 100%, nothing less than 50%.
You can give it a full range of duty cycle by putting a diode across
pins 6 and 7
with the cathode at pin 6. Then it will get down to zero, or almost.
If you want continuously variable with a pot, tie one end of the pot
to pin 6,
the wiper to pin 7. Just a reminder, you still want a little signal
diode like a
1N4148 as I mentioned across 6 and 7. The other end gets connected to
a
resistor, and the other end of that resistor
gets connected to pin 8. The reason you don't tie the pot directly to
pin 8
is because if you turn the pot all the way to the end then it will
short Vcc to
ground when pin 7 discharges. The 1k resistor prevents that.
The pot should be an order of magnitude or greater than the protection
resistor, so you're looking at a 10k or 100k pot. Do your frequency
calculations and choose the timing cap accordingly.
Don't forget basics like a 0.1 bypass cap on the pins of the 555,
especially if you use the bipolar version.
I don't have a mosfet on the tip of my typing fingers to suggest, but
you can use digikey's parametric search. click on site map--part
search--
discretes--mosfets. Get N channel with I guess 100 or 150 volts Vds.
No need for a logic level device if you are using 12 volts, just get a
standard mosfet. Choose Rds by figuring out how much power it will
dissipate at 15 amps. Actually, if your kids stall it and keep the
throttle
down it could draw a lot more than that for a good ten seconds. Stuff
happens. So build in a big safety margin.
Make sure either that the heatsink is electrically insulated, or the
mosfet
is insulated from the heatsink.
To clarify my ambiguous wording
"Just a reminder, you still want a little signal
diode like a
1N4148 as I mentioned across 6 and 7. The other end gets connected to
a
resistor, and the other end of that resistor
gets connected to pin 8."
The protection resistor gets connected to the end of the pot opposite
pin 6. The protection resistor does not get connected to the diode.
.
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