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Consider a primary voltage measurement circuit and an optocoupler transmitting a digital "voltage good" signal.
Is there a better method I could use that is as low in cost or close?Hi,
the transformer is not suitable because:
* it is no 50/60Hz transformer. It is made for high frequency switching
* it is not made for safety isolation between mains voltage and low voltage.
Klaus
Hi,
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Instead of measuring the live/mains, what about a parallel transformer for the control circuitry, like 230VAC:12VAC or something, that measures the unrectified AC but at a level to suit the comparator supply voltage, etc., with the rectification for the control circuit after the measurement point (measured at the transformer secondary output pins)? Over or under voltages would be proportionately reflected in the transformer secondary, wouldn't they?
All this assuming a relay of some kind is going to be used to cut off the brown-outs to the pump, then everything (except phase and neutral to the pump via a DPDT relay) could be on the secondary, isolated, side.
I thought of this before. The concern I have about it is that the whole of my comparator circuit is going to be on the mains side but take power from my controls side, across the isolation.
You think I could just tap the rectified-and-filtered mains to power the comparator? This tapped voltage will be varying in magnitude though but I can size it to be within range at all times. Is the idea okay?
Is there a better method I could use that is as low in cost or close?
Here's the circuit with a pot replacing R1 to allow adjustment of the gain for calibration.
Note that R_Dummy is just to prevent a floating-node simulation error and would not be in the actual circuit.
(If it's not clear, Ix(U1:A) is the opto LED input current).
View attachment 135744
Here's the LTspice simulation of a circuit using a common 4N25 opto isolator, similar to what I mentioned in post #9.
I added a bridge at the input so the output is full-wave rectified instead of half-wave.
I used a resistor to generate the output voltage as I don't think a transimpedance amp has any significant advantage here, and it would have required a negative supply..
The output signal is filtered by R3C2 to give the average voltage of the rectified wave.
The simulation is shown for input voltages of 200Vac and 240Vac.
View attachment 135727
Hi,
CTR degradation is a known issue with optocouplers.
(I made the experience with a self designed circuit where the CTR dropped to less than 30% of the datasheet specification within about 5 years)
For CTR critical designs there are special optocouplers with monitoring diode.
The datasheet usually provides circuits to compensate for CTR degradation.
Klaus
I tend to think it may be easier to 'design out' the variations in CTR than calibrate for it. Though it depends on what your requirements are.
This circuit uses the TL431 for it's property as the cheapest reference/comparator that exists. The TL431 turns on when its ref pin exceeds 2.5V, as set by R3/R2. R1 should be set so there is roughly 1mA available at the turn-on threshold. More current is fine too it just burns up more watts. D2 is almost any zener between 5 and 30V.
I showed the secondary with 25V just so it would show up on the same scale as the AC in, obviously you can do what you need there.
This only has a single diode so it's 'half wave' but could have a full wave rectifier if you needed it to fire in both polarities. Though note that besides adding diodes that also effectively doubles the wattage of R1.
View attachment 135758
My circuit is analog and thus would need to be recalibrated if there is any CR degradation..........
Some things I want to get cleared with optocouplers as used in this circuit (where the transistor is driven in non-saturation mode, unlike saturation mode where you supply enough current from the start to still be adequate continue to keep the transistor in saturation even when the CTR has degraded significantly) centers around CTR degradation.
What's the life expectancy of this circuit? Will it require diode current tuning or collector current tuning or both anytime soon?
How's the issue of CTR degradation tackled in this case?
Could someone please explain the concept and state the calculations behind this design for completion's sake? It will be very helpful in subsequent designs and I'd like to just be able to design one by myself when I need to.
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