eagle1109
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From post#55:1. Measuring the signal at the gate, is negative with one scope probe. Why ?
But you inverted both!Thus we need to invert ONE input.
Yes, luck. Sadly luck, because it seemed you forgot all we tried to teach you.I guess it was a hit of luck
Hi,
Yes, luck. Sadly luck, because it seemed you forgot all we tried to teach you.
Designing electronics has nothing to do with luck. It´s learning, understanding, reading and calculating.
* negative voltage on an electrolytics capacitor
* too high voltage on a 40V capacitor
* too much power dissipation on a resistor
* ...
In my eyes the current discussion is about "SCR triggering". I´ve tried to tell you that the gate triggers with current, not voltage.
You still insist in voltage measurement... you never showed a valid gate current measurement.
I´m sad that I didn´t find the right words to teach you. Sorry for this. I don´t know how to go on..
I clearly talked about the "resistor to measure the current" and wrote it should be not higher than 10 Ohms.Of course not, I actually learned a lot. Like I thought about the last information you told me that the resistance has to be very low, so I removed the unnecessary resistors and kept the overall resistance lower than before.
I don´t understand how you want to understand the "gate current" when you never measure the "gate current"... (At least I can not see this in your posts)I know the goal you're trying to tell me, but I hope you appreciate that it takes me time to realize this point
Here is my problem. In my eyes learning is step by step.I reached the level now to know in which case I can control the phase of the output, and since I got to some point.
Back to basics eagle1109 - if you charge up a capacitor, it doesn't matter what value or what the voltage is, then short it out, how much current flows out of it?
Try this simulation: use nothing but a 10K resistor, a 100nF capacitor and a Diac. Connect the diac across the capacitor and ground one end of them. Wire the resistor to their other end and apply a DC voltage of say 100V. Connect the oscilloscope across the capacitor and show what you see.
You are not reading what Klaus explained - it is current that triggers the SCR but it does come from the discharging capacitor.So you mean with this analogy, is that when the scr triggers, it's actually taking up the voltage of the discharging capacitor.
You are not reading what Klaus explained - it is current that triggers the SCR but it does come from the discharging capacitor.
Your schematic is correct but the scope settings are not. Switch off the A+B and show channel B instead of C, at the moment we have no idea where zero volts is on the scope display. Try increasing the resistor to 100K and see what happens.
100V and 10k ohms gives 10mA.Δ V Dynamic breakover voltage (1) VBO and VF at 10 mA
Hi,
You still repeat not to strictly follow the recommendations.
You modify the circuit, you modify the part values...
1) May I ask why you used 1k instead of the recommended 10k?
You say you "know" .... so please tell us what you know.
There is Ohm's law. You know?
The datasheet shows an operation point at 10mA.
100V and 10k ohms gives 10mA.
I'm sure it was not "luck" that makes Brian choose the values. He calculated.
2) the switch is of no help. It makes things more difficult.
3) the scope time should start at zero...the voltage at the capacitor should be zero when starting the simulation...
Press the "one shot" button at the scope.Yes, I mostly don't catch the 0 time because the reading of the oscilloscope is moving constantly and I even can't goes back to the 0 point.
Hi,
Press the "one shot" button at the scope.
Klaus
One shot and try a slower timebase speed.
What you should see, if the simulation is working, is a repeating sawtooth waveform. The capacitor charges through the resistor until enough voltage is across the Diac for it to start conducting (VBO in the data sheet), the current it draws from the capacitor makes it discharge until there isn't enough for the Diac to stay conducting. Then as the Diac 'turns off', the voltage across the capacitor starts to rise again and the cycle repeats.
I'm trying to demonstrate that the current to operate your SCR gate doesn't only flow through the resistors, most of it comes from the capacitor discharging. If the simulation works, I was going to suggest adding a small resistor (about 10 Ohms) in the ground side of the Diac and using the oscilloscope to look at the voltage dropped across it. Ohms law will let you convert the voltage across the resistor into the current through it. I suggested 10 Ohms because it is small enough not to limit the current too much while still letting the circuit work. A smaller value will be better but at the same time it will make it harder to monitor the voltage across it.
Again, if the simulation works, you should see short spikes of voltage across the resistor as the capacitor discharges, these would be the trigger pulses to the SCR in a real phase control circuit.
Brian.
I fear we are going backwards....
A 100nF capacitor has a reactance (Xc) of 26.5K Ohms. So in your bottom circuit the voltage across the Diac will only reach about 68V peak-to peak even if the diac is removed. Your scope shows 68.5V so it is pretty much exactly as expected.
However, I didn't say to use AC, for the experiment you should be using 100V DC! Did you try 100K in the simulation?
NO! Diacs do not 'clip' the voltage, that's what Zener diodes might do. There is a special characteristic of Diacs that once they start to conduct, the voltage across them drops suddenly and they continue to conduct until the current through them is too small. VBO is the voltage that starts them conducting but then the voltage across the ends will typically drop to just a few volts.So the diac would break any time the voltage across it is from 35 - 45 V. Any voltage more than the max break over voltage would be clipped; for example
When the switch is open, then node may be seen as "floating". Floating means "undefined" .... anything can happen.The capacitor continues to charge up to 22V as the switch is open ! That's not logical.
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