Measuring PVM signal from ATmega 8A

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Formatting went crazy, please see below. Sorry.

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Last Edited: Mon. Jan 29, 2018 - 03:30 PM
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In my kitchen I have a cooker hood where the motor speed is controlled by an ATmega 8A (the keyboard controller is a Quantum QT60168). An SPI bus for communication.

 

 

 

I want to integrate the suction into my ventilation system and remove the motor from the cooker head. For the ventilation system to know when to change suction in the kitchen it need a 1-10 VDC signal. My plan was to find the PVM control from the ATmega and change this to 1-10 VDC.

 

So using the keyboard on the cooker hood to turn the suction up and dawn, instead of using the extra 1-10 VDC keyboard that came with the ventilation system. But when measuring the PVM I was expecting pulse modulation but I am unable to see any change.

 

The cooker head panel have 10 steps, but the first 9 steps make no difference and the 10th step just gives full DC (3.745 VDC).

 

 

 

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So is this an AC motor?

Maybe they use some sort of phase control.

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"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

 

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Do you mean, "PWM" ?

 

Ducatimatrix wrote:
The cooker head panel have 10 steps, but the first 9 steps make no difference

Sorry, I'm a bit confused: are you saying that was the behaviour of the unmodified hood?

ie, the "control" never worked in the first place?

 

 

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Hi,

 

It is a AC motor. wink

 

Everything works fine, I can with the 10 steps change the motor speed. It is the measured "PWM" output from the ATmega that does not change in the first 9 steps, same frequency and same pulses. As shown in the pictures.

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Ducatimatrix wrote:
same pulses.
I know this is probably obvious but in PWM what you expect to see change are the duty cycle of the pulses.

 

Image result for pwm duty cycle

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clawson wrote:

Ducatimatrix wrote:
same pulses.
I know this is probably obvious but in PWM what you expect to see change are the duty cycle of the pulses.

 

Image result for pwm duty cycle

Exactly what I expected, but there are not changes to the pulse length sad

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Ducatimatrix wrote:

Exactly what I expected, but there are not changes to the pulse length sad

 

But images 6, 7 and 8 show three different pulse widths.

"This forum helps those that help themselves."

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

 

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but there are not changes to the pulse length 

The point of PWM is that the "length" (ie the frequency) does not change - in PWM all that changes (for fixed length pulses) is the ratio of high to low in the cycle

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PWM into a single phase AC signal can be - messy.  The result could phase in and out, providing more and less power.

Can you provide a bit of the control part of the circuit?
The component that does the high current switching is of particular interest.
Also, maybe an image of the waveform into the motor?

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But images 6, 7 and 8 show three different pulse widths.

6 & 7 show the same pulse width but at two different time scales (5ms, 100us).  8 shows (I presume) the 10th step where the output is 100% as stated.

 

The point of PWM

Seems the OP understands this.  The query is why does he >>not<< see this on the m8a's PWM output.

 

I'd guess the PWM output has little to do with the motor control...?

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It can be a pulse cascade controller where it's hole periods that are missing, (depending on what you trigger on you could miss that :) )
 

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What the OP, if they still have any interest in this topic, needs to do is give us a photo of the component side of the board.

 

And if they have lost interest then it'd be nice if they told us.

"This forum helps those that help themselves."

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

 

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Sorry guys, I have been traveling - so not being able to respond.

 

I uploaded a video of the measurement, this should answer most of the questions. And a picture of the component side of the board.

 

https://www.youtube.com/watch?v=...

 

Last Edited: Fri. Feb 16, 2018 - 08:13 PM
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Looking at the photo, the opto is a moc3052 so it most likely is doing phase control. Thus the pulse is positioned relative to the zero crossing of the mains. The pulse width itself need not change - it only has to be wide enough to allow the triac to latch.

Last Edited: Fri. Feb 16, 2018 - 09:12 PM