[TriEmbed] Coffee roaster sanity check

Brian triembed at undecidedgames.net
Tue Jan 7 11:01:41 CST 2020


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 at triembed.org <mailto:triembed at 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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> 
> -- 
> photo 		
> *Shane D Trent*
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