Electronics > Projects, Designs, and Technical Stuff
Need some help building a circuit involving switchs and relays
Ian.M:
Transistors don't work on AC so any simple current monitor using one is not going to work.
To avoid further confusion, take a hilighter and on your schematic, hilight the sections of it that are purely internal in the device(s) and you have no access to, then re-post it.
Shaydzmi:
I've edited the picture above.
Renate:
Huh? I'm not soldering in 171 diodes in a machine with moving parts.
Just make sure you have the 24 VAC feed that is only going to the bulbs.
Put in this circuit (and the rest, an op amp to amplify and rectify and detect the 1.4 V P-P signal).
The diodes are beefy enough for any number of amperes.
The resistor is the right size that the diodes are still conducting when just a bulb bypass resistor is conducting.
The cap cleans up potential noise.
Shaydzmi:
Thank you, I didn't fully understand this circuit, can you explain to me how does it work, and the purpose of each component?
Tank you.
Renate:
Well, it's basically a shunt for measuring current.
You have to be able to measure 25 mA or so, but it has to be able to tolerate amperes going through it.
You could just use a single resistor, but then if 22 bulbs went on it would drop too much voltage, heat up too much and burn out.
So we have the two big diodes across it so that no matter what happens we only drop 0.7 V in either direction.
We just want to know if any bulb is on, not how many.
(If we wanted to count bulbs, we would use a very small value resistor and a much more sensitive/precise measuring circuit.)
What is the value of the bypass resistors across the light bulb?
Some rough numbers here:
If the bypass were 1k then the normal current would be 24 mA (RMS)
If the sense resistor was 30 ohms the voltage across it would be 0.7V (RMS without the diodes in place).
The diodes would flatten the peak to 0.7V instead of the 1.0V peak.
That's enough of a nice "to the wall" signal"
Why did they put in those bypass resistors?
Did they have their own measurement system for any of the bulbs going on?
If you wanted to make things easier and you could tolerate a bit more drop on the voltage to the bulbs:
You could use two (or 3?) diodes in series (and two (or 3?) diodes in series the other direction).
With that you could get enough voltage to run an LED on an optoisolator.
That simplies the whole problem of using an op amp with a separate floating supply.
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