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PWM Ports and Relay Switching Question
#1
Posted 29 May 2012 - 02:45 PM
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#2
Posted 29 May 2012 - 03:00 PM
Ohhh, I'm not sure, I wouldn't do that... But have you looked at mosfets? I'm no electronical engineer, but I think my first weapon of choice would be a mosfet.Also, is varying PWM duty cycles with a relay a really good idea? I've read where it's not an issue with solid state relays but I'm uncertain about normal relays.
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#3
Posted 29 May 2012 - 03:45 PM
The multimeter shows you the average voltage: 50% duty cycle PWM = 50% of 3.3V = ~1.6V. If your meter has min/max recording mode, you will see min = 0 and max = 3.3V. You'd need a logic analyzer or an oscilloscope to see the waveform.... at 50% Duty cycle I was only seeing 1.6 volts instead of 3.3 volts. Maybe this is because I am using a multi-meter instead of looking at it with an actual scope. Does anyone know what I should be seeing?
PWM is not good for mechanical relays, because they have moving contacts that wear out due to arcing. Switching the relay once a while does not hurt that much, but for higher frequencies solid state relay is better. It is basically a power transistor (usually with an opto-isolator), so there are no moving parts. Also, if your heating element is powered directly from mains, you can use a TRIAC-based control circuit (= AC light dimmer, Phase Shift Power Control).Also, is varying PWM duty cycles with a relay a really good idea? I've read where it's not an issue with solid state relays but I'm uncertain about normal relays.
#4
Posted 30 May 2012 - 04:27 AM
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#5
Posted 30 May 2012 - 05:30 AM
#6
Posted 30 May 2012 - 01:09 PM
Mario, could you please explain a little bit more how the energy is wasted? My understanding is that when the triac is off, the circuit is virtually disconnected (there is only very little leakage current flowing), so the triac-based circuit in fact does not waste any energy (?)That's because you'd "load" all the required energy on a certain part of the sine-wave, leaving unloaded the remaining part. Believe it or not, that introduces a *LOT* of energy waste on the power line.
#7
Posted 30 May 2012 - 02:46 PM
Mario, could you please explain a little bit more how the energy is wasted? My understanding is that when the triac is off, the circuit is virtually disconnected (there is only very little leakage current flowing), so the triac-based circuit in fact does not waste any energy (?)
It's very simple if you know that any periodic signal can be expressed as a composition of harmonics (Fourier series).
The behavior of the triac is exactly as you say. The waste is *not* on the triac, which is wasting (theoretically) always zero Watts.
The problem is on the value of the current flowing, with is a sliced sine-wave. This wave is actually composed by a HUGE quantity of harmonics, even to a relatively high frequency (compared to the mains: 60 or 50Hz). Now, the power wiring (I mean the Electric Co's) are running for several km, and that length could be important for frequencies much higher than the fundamental one. That leads to bad impedance coupling, power losses, in a short: power waste.
This is also one of the reasons because the "old" diode rectifiers were replaced by switching PSUs.
I must go to the dentist!
Cheers
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