| Electronics > Beginners |
| Using TIMM pot with 2 pins |
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| krish2487:
As I understand it, the entire section to the left, starting from the fuse is what is your power switch. A triac based static switch, driven by the VOM IC to connect and disconnect the primary of your transformer.. The optocoupler section to the right of the fuse is serving as a opto zero crossing detector, nothing more. when the line voltage, in either cycle goes beyond the diode forward drop voltage, the diodes conduct and the opto secondary transistor conducts. To answer your question, 1. Yes, a trim resistor can be used with only two terminals as shown. Yes, they do fail open. There are also fakes of bourns trim resistors that are spectacularly horrible And I don't understand the objective of your exercise in varying the trim resistor and expecting the voltage across the opto input to change. the resistor is for only limiting the current through the opto input diodes. Irrespective of what voltage you apply at the input of the resistor-opto combination, the voltage you see across the opto inputs will be the same ALWAYS. Why, you ask.. because the input of the opto is nothing but a pair of anti parallel diodes. The rest of the voltage is dropped across the trim resistor, which, depending on the wiper position decides the opto diodes current. Set the trim wiper to too low a resistance and you risk blowing out the diodes in the optocoupler and/or the trim resistor itself. Thats what JS meant by including a current limiting resistor. 2. The opto circuit in question cannot be used to detect a sag in the input voltage. its a zcd circuit. plain and simple. If you are seeing 32VAC at the input terminals of the opto, chances are high you set the wiper position to short and thus most likely blew out your opto input diodes. <rant> So, the question now is, what is it exactly that you want?? Please be coherent, simple and succinct in what your end objective is and how you plan to achieve it. Open ended questions do not help us, nor you. If you need someone to critique your work, then don't paste the entire work and ask the other members to spend time sieving through the rest of non important parts. highlight the area in question and be specific. </rant> |
| anishkgt:
What am trying to acquire is to detect a voltage drop at the primary when the secondary is shorted. The opocoupler is just an isolation for the microcontroller. A resistor in series with the optocoupler is all that i would need but not sure of what value. The resistance should be low enough for the optocoupler to light up and activate the transistor hence the microcontroller at pin 4 would read a "HIGH" and large enough to drop the voltage when the secondary is shorted which would read "LOW" continuously at pin 4. The circuit was in its simplest form in my first post. Thought i could give a clearer picture if i posted the remaining. |
| krish2487:
The opto data sheet is your best friend. Look under the absolute maximum ratings and try to decide a resistors values which does not violate the parameters for the diode current ratings. Voltage drop of what value?? do you want to detect a 5v drop or a 50v drop?? And after how long since the drop do you want to trigger an output?? As I mentioned, the circuit is a zero crossing detector. it will not be useful for anything otherwise without substantial changes.. Please go through this excellent link http://www.bristolwatch.com/ele2/zero_crossing.htm Your original circuit is the second image in that page. Now, do you understand how the circuit works?? and why it is not an appropriate solution for the problem what you are trying to solve?? Please look at comparator applications, specifically a threshold detector which is more appropriate in your case. And then see how you can isolate the mains from the low voltage circuits section and still achieve the same result. (Assuming you actually mean that you need to detect when the primary voltage sags during the spot welder on time..) |
| JS:
--- Quote from: anishkgt on July 11, 2018, 07:23:44 am ---connected the neutral to pin 2. So probably the pot may have blown or the optocoupler. i could see 32vac at the pins 1 and 2. But do these trimm pot fail short ? --- End quote --- No, it wouldn't fail short, check with your DMM for resistance... In this case your optocoupler's magic smoke went first. JS |
| anishkgt:
--- Quote ---Please look at comparator applications, specifically a threshold detector which is more appropriate in your case. And then see how you can isolate the mains from the low voltage circuits section and still achieve the same result. (Assuming you actually mean that you need to detect when the primary voltage sags during the spot welder on time..) --- End quote --- Yes i first thought of a peak detector then i was lost with no help on it. Second was the comparator again lost with no help. I guess i would need to give the peak detector a go again. |
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