eagle1109
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Just saw your post#18. Much better now.
But I miss some important informations:
* diode type
* SCR type
* transformer output voltage
Klaus
Circuit #18 will probably work in a simplified simulation circuit, less likely with real components.
Power SCR need a certain amount of trigger current that can't be provided by an analog opto coupler. For the intended phase angle control, you want to generate a trigger pulse with variable timing. Circuit #18 isn't particularly suited for this purpose.
A versatile method to provide isolated gate signals for multi phase SCR circuits are trigger transformers. Another popular means are opto couplers with triac output like MOC3020. They can be utilized for SCR triggering with an additional rectifier diode. There are also opto couplers with SCR output like Toshiba TLP748.
Post #1 is asking for variable AC voltage, but most of the presented circuits have DC output. You understand the difference?
Phase angle control isn't well suited to operate AC motors will variable speed, except for brushed "universal" motors.
The term "full-wave triggering" in the thread title is misleading. It's mainly used for controllers that switch full sine waves on and off in contrast to phase angle control which is cutting part of the wave.
This makes no sense to me. How can you know the trigger level, hold current and so on?The diode and thyristor types are generic,
This is quite expectable for me. Without knowing the parameters to control the SCR...it's still not working !
The fast diode D in the secondary circuit prevents negative gate current during the commutation swing of
the transformer secondary voltage and during dynamical VGK overshot. Purpose of the RC-element, RGK, CGK
is to filter unwanted glitches on the trigger line. A capacitance, CGK of 10 - 47nF is recommended to
achieve a discharge time constant τ = RGK · CGK ≈ 10 - 20μs (and RGK ≈ 220 - 2200Ω). The power loss PR of
RGK at maximum control angle via a half period of line frequency will be:
That means at VGK = 5V and extreme values (max. CGK = 47nF, min. RGK = 220Ω) and Τ ≈ 10μs, PR is
about 60mW. At τ ≈ 22μs with CGK = 10nF and RGK = 2200Ω, PR would be only about 6mW. The amount of
loss PR’ resulting from the discharge of CGK:
The schematics do make sense but as you point out, they are devoid of actual values. Treat them as conceptual, they show how it works without detailing the exact components used.
1. An SCR natively does not conduct at all - how do you start it conducting?
2. Once it is conducting, how do you stop it again?
3. What does 'phase control' mean and why do we use it?
I'll be back tomorrow to mark your homework :grin:
Brian.
...and the gate current need to be higher than I_GT to safely trigger the SCR.Put a positive pulse at its gate, connecting the positive voltage of the voltage at the gate and the ground reference to the cathod
No, it is not voltage related. It is current related. And no need to get reversed, it is sufficient that the anode(cathode) current goes below I_Hold.I can't stop it until the voltage between the anode - cathode, gets reversed biased that the cathode voltage is more than the anode voltage
Hi,
Why do you use R5? (It makes things more complicated than necessary)
What does tell you the SCR is not fully ON?
The trigger voltage has to be applied between gate and cathode. R5 causes a negative gate voltage, in a real circuit it might cause damage of the SCR and trigger source.
To reiterate: the trigger voltage is applied between the cathode and gate , that means if you lift the cathode voltage, the gate voltage has to be that much higher. Allowing the cathode to change is like trying to hit a moving target!
For simplicity, keep the cathode point your 0V reference, remove any components between it and 'ground'. Then all you have to do is provide the gate with enough voltage above ground to turn the SCR on. It is sensible to add a path to hold the gate voltage low when it isn't being triggered but your R3 value of 500 Ohms is unreasonably low, all it does is waste 24mA when the 12V is applied. It would work but a value of 10K would probably be more appropriate.
That is correct, and it is what the oscilloscope shows. The negative half cycles are when the SCR is NOT conducting, remember it only conducts in one direction, the tiny rise above zero on the positive cycles is the forward voltage drop, the rest of the half cycle is making the lamp light up.Why there's still a voltage drop on the SCR ? When it's fully on as I recall it should have a very low voltage drop like 1V or less.
That is correct, and it is what the oscilloscope shows. The negative half cycles are when the SCR is NOT conducting, remember it only conducts in one direction, the tiny rise above zero on the positive cycles is the forward voltage drop, the rest of the half cycle is making the lamp light up.
Note that Vgt is the trigger voltage to start it conducting, not the voltage drop across anode and cathode when it is already conducting.
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