Re: Monitor mains voltage with transformer


fssg wrote:
Hi,

I made a microcontroller circuit to control speed of a small fan with TRIAC
phase control. It works fairly well, but the fan never completely stops,
among other things, so I am wondering if the TRIAC is being triggered the
way I expect.

I don't have a CRO, so I want to sample the output voltage from the TRIAC
using the microcontroller's ADC.

I could do this using a simple resistor divider, but then I have to connect
circuit ground to Neutral, and circuit ground is my computer ground.
Neutral is supposedly connected to earth, which in theory would make this
safe (I would test first), but it seems like bad practice to do this...

I have a transformer, 240V->24V 20VA 50/60Hz. If I put it's primary on the
TRIAC output and a resistor divider on the secondary, that would probably be
safe, but will it work? Will I be able to see the sharp edges where the
TRIAC turns on in a half cycle?


Another question about whether the transformer will pass the signal: To
detect zero crossing, I am using an optocoupler, but other circuits show
using the AC output of the power supply transformer. Because the
transformer is inductive, wouldn't there be a delay between the actual zero
crossing and when the zero crossing occurs on the secondary?


Thanks,
David

Hi, David. Did you expect a good answer without providing much of any
pertinent information? Maybe you could start by answering a few
questions:

* What type of motor do we have here? Is it an AC induction motor?

* What's the current rating (or watts rating) on the nameplate?

* Are you using an optocoupler to drive your triac? (Hope so!) If
so, which one? Is it a zero-crossing triac trigger? (If it is, throw
it out and get one that isn't.)

* Are you timing a delay from the zero crossing of the AC line?

* Have you provided a snubber (R-C or other) across the triac? Could
you sketch out your circuit, using an ASCII Circuit program? (possibly
use Andy's ASCII Circuit v1.28 http://www.tech-chat.de )

To start with, there are a few things here. First, you should know
that phase control of AC induction motors is difficult, and sometimes
it just can't be reliably done. Problems do tend to occur at the low
end, and can actually be more difficult with smaller motors.

However, there are several things you can try here.

* If you haven't already, put an R-C snubber across the triac (start
with 220 ohms 1/2 watt in series with 0.1uF line service self-healing
cap).

* Obviously the biggest problem here is the inductance. You're going
to have difficulty turning the triac off if the curent is going great
guns while the voltage crosses zero. One good solution for this is
using back-to-back SCRs, with separate optocouplers for each SCR.
Obviously the reverse-biased one won't trigger, and the other one will
stop when the voltage reverses. There are many solid-state relays
which are made like this, and all you have to do is provide the 3mA or
so to drive the opto input (make sure you don't get an SSR with
zero-crossing triggering). One cheaper solution, which *might* help is
to get a Teccor Alternistor, which has many of the characteristics of
back-to-back SCRs. It provides better turn-off behavior than a
standard triac. I've found I had better luck driving inductive loads
with these:

http://www.littelfuse.com/data/en/Data_Sheets/E4AlternistorTriac.pdf

* If you're desperate, and the motor's small enough, you can start
trying to change the power factor of the load by adding a power
resistor in parallel with the motor. Adding a capacitor across the
motor is a spectacularly bad idea, because you'll cause di/dt failure
in the thyristor (too fast a change in current causes the triac to
smoke). Of course, you're just burning power this way, but it can
help.

* If worse comes to worst, you can always do it like the dimmer
switches used for ceiling fan controls -- they reduce the speed as you
dial down, and then use a switch to turn off. You can always just use
a relay in addition to your phase control.

Having said all of this, I'd guess that, especially without a scope,
you're going to have an easier time just working on solving your
problem than trying to diagnose it by reading the voltage across the
thyristor or the load with your uC. You're going down a rabbit hole
here -- you just want to control the motor load. Take the steps you
need to do that, and worry about the other stuff later.

Actually, there are many applications where metering the voltage (or
more frequently, the current) on an AC load *is* a good idea. It can
be a good part of the control circuit, and there are many ways to do
this -- but that's actually another problem. In fact, as long as you
don't load the transformer secondary, you can get a pretty good idea
where the zero crossing is, and what's going on with the load voltage.
And there's no lag to speak of between transformer primary and
secondary. You are also adding inductance, and changing the circuit
you're measuring, too. But let's work on the fan first here.

Feel free to take the time to express your problem a little better --
the few minutes it might take to actually describe your problem well is
a lot easier to spend than hours thrashing around in the lab.

Good luck
Chris

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