An extra cent I forgot in the last message:
The DC to the motor, in addition to being rectified right there at the
motor, is unfiltered -- it just looks like an AC waveform with the
negative half flipped over. PWMing on that will not have the expected
result, depending on how the PWM cycle lines up (or doesn't line up)
with the DC pulses. Consider the difference of a 25% duty-cycle PWM
waveform lining up perfectly with the peaks of the DC waveform, versus
lining up perfectly with the zeroes. Now consider if the PWM waveform
is ever-so-slightly out of sync with the DC pulses: the PWM "on" window
will slide along the DC waveform, and the output power will vary over
time. It's definitely not the same as applying PWM to a constant DC
supply.
The earlier discussion regarding using a PWM period equal to the AC
half-cycle time along with an AC switching element that has a zero-cross
detector (e.g. many, if not most, SSRs) is probably the most reliable
way of modulating the fan power.
Cheers
-B
On 1/2/20 4:55 PM, Shane Trent via TriEmbed wrote:
Craig,
I was actually talking about the voltage rating of the optoisolator used
to drive the FET gate. The image below more fully describes what I
meant. I expect your final circuit for the fan would include most of the
items shown below (I am not sure that you would need D2).
image.png
https://electronics.stackexchange.com/questions/67663/criteria-behind-selecting-pwm-frequency-for-speed-control-of-a-dc-motor
Shane
On Thu, Jan 2, 2020 at 8:10 AM Craig Cook via TriEmbed
<triembed@triembed.org <mailto:triembed@triembed.org>> wrote:
> The 4N25 is a common choice but is rated for use up to only 30
volts while the HCPL-181 looks to be around the same price and is
rated for up to 80 volts.
Oh, I am actually using one of these
https://cdn.sparkfun.com/datasheets/Components/General/FQP30N06L.pdf
(32A, 60V). I was lazy doing to schema diagram. It's been updated
again now. I think I have the diode in the correct place and direction.
Thanks
Craig
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