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[DPRG] PWM Controller
Subject: [DPRG] PWM Controller
From: Triffid Hunter
triffid_hunter at funkmunch.net
Date: Sun Mar 9 20:21:18 CDT 2008
On Sun, 9 Mar 2008, Andrew Lewis wrote:
> Hi,
>
> I just joined the list after finding your 555 based pwm tutorial on the web.
> I've made a PCB from the schematic, and put it online here:
>
> http://www.monkeysailor.co.uk/pcb-gallery.php
>
> I must admit it was nice to find a basic design that was described in a
> no-fuss way, so props to the author for that one :) The circuit fits on a
> postage stamp sized PCB, too which is handy for potting up.
>
> Just out of interest - does anyone know the maximum frequency that this
> circuit could be tuned to? I'm currently using it to control a pillar drill,
> and the whistle is just at the right frequency to make my ears bleed :p
That depends on two things: 1) the top speed of the 555 and 2) how fast
you can turn the mosfet on and off.
The 555 datasheet doesn't list a top speed, but the graphs only go up to
100khz. C2=1.5nF, R1=10k will give you 96Khz. Above 40khz should be
inaudible.
Also, I recommend connecting the mosfet gate to the 555's output (pin 3)
rather than the discharge. A mosfet's gate looks like a 2nF capacitor, so
when you charge it with a resistor it follows the standard RC time
constant curve. The current is controlled by the gate voltage, so it
spends a lot of time between on and off. Connecting its gate to the high
current totem pole on pin 3 will allow it to turn on and off much faster,
and produce significantly less heat. With a 10k resistor turning it on,
it'll never be fully switched on at any more than 40khz or so. Taking it
from on to off very quickly means that at all times, either V or I is
negligible, so P=VI is always fairly small.
Don't forget a flyback diode across the motor! Electrically, it's a big
inductor in series with a small resistor and a voltage source, and that
inductance will easily destroy your mosfet when it switches off without
something to limit the voltage to a safe level.
You should end up with something like this:
http://triffid.funkmunch.net/png/555-pwm.png
If you'd like to know exactly how much current your mosfet can handle, or
conversely how to make it handle a given amount, read
http://www.mcmanis.com/chuck/Robotics/projects/esc2/FET-power.html
An interesting next step for this project would be to hook up an encoder
of some description, feed its output to a frequency-to-voltage converter
then replace the 555 with a switchmode controller IC like the SG3525 or
TL494, to give you closed loop speed control. This will mean you can run
your drill at very low speeds, but still have full torque.
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