SCR and electrical stuff

Uhhh, a university Electrical Engineering program?

Seriously, to begin to understand an SCR and how it works is a process that starts off with electron theory and flowing holes etc. etc. Then you must understand power, power flow, RMS, etc. etc. etc. this is too much to find in a forum IMHO.

What I am trying to say is how can (to start from scratch) I learn about the SCR stuff installed in a heater attached to a duct....

Regards

Frank -

Think of the SCR as an odd kind of switch that only works on AC current. It's a 3-terminal device with an anode, a cathode, and a gate. If the anode voltage is higher than the cathode voltage, turning on the gate with a small current pulse will allow current to flow through the device from anode to cathode. Once the voltage reverses, the device shuts off until the gate is triggered again. Obviously, the device only works on AC current since, if the current were DC, once you turned it on it would never turn off.

There's a nice overview, along with some basic circuit diagrams, on the Fundamentals of Electrical Engineering web site.

Good luck, and start small. Unlike signal level or digital systems, power electronics goes BANG! when it fails.

An SCR is a thyristor(more in the family like triacs, but later on this). View it as a diode, allows current in one way only. But it requires a gate current. This current will switch the SCR on, letting it conduct. Once on, gate current can be removed and the SCR will still conduct current. The way an SCR is turned off is by reducing its anode current below the minimum holding current. So, thinking in these terms, you can see how in an AC circuit turn off will occur near the zero crossing point of each half cycle. Brief, but hope it makes a light buld go off.

Hello; Here is a very crude yet easy way to think of an SCR control. Please keep in mind you do not need to know how to build an SCR because you just buy a replacement off the shelf at whatever the Amp - Volt requirement is. Here goes. The circuit wants to give as much power to the resistor,(heater), as possible. but it can not unless something tells it to let a little bit more than nothing happen. Example...You have a dimmer switch in your house somewhere, the power is available to have that light on to the max but it takes your input in the form of twisting the knob to increase the amount you let through. You are the control. You represent the applied voltage to the SCR to 'twist' on more. In this electronic case however you are not twisting but you are increasing a control voltage to a circuit. The more V you apply, the more Amps are allowed to flow throught the SCR. Please don't take my example as a direct relation, it is meant to be just a simple example. You see if you look at it from the reverse side, all power is there and that resistor is allowed to go full on but you are the 'gate' voltage to take away applied voltage so the rectifier can't allow the max. cut short gotta go

Very good intention. Visit http://4qdtec.com/ to learn from basics to advanced. Wish you all the best.

Frank; Do you have any vocational schools near you? If so ,sign up for a night school class on electrical fundimentals. This will give you a stepping stone to go deeper into the electrical engineering field. This will also give you a feel for what that field is all about,so you can decide then if you want to go farther. Good luck

Just remember that without us mechnical engineers, all those electrons will have no home.

Even if your mechnical engineers exist everywhere, there are lots of them havent got a home

how many flats do you offer them every years?

where is the architects?

PetroPower,

I like your reaction, it 's really cool.

In your case, I assume you have a heating duct with an electric heating element that is controlled by an SCR (that's the only scenario I can think of where you would think you have an SCR on a heating duct).

Something is deciding that the air in your duct needs heating. That something, a sensor of some sort somewhere, is sending a signal to a temperature controller somewhere else. The temp. controller is deciding that it needs to turn on the heater in the duct, but not necessarily all the way on. So it in turn sends a proportional control (analog) signal to the SCR controller's firing board. The firing board then interpolates that analog signal into an SCR firing scheme that controls the amount of voltage that is allowed by the SCR to reach the heating element. Since the heating element has a (relatively) fixed resistance, by controlling the voltage fed to it, you can control the wattage that it puts out in heat.

The SCR controller's gate firing board does this voltage in one of two ways (you will not know which without knowing more about your SCR controller). Either is is doing what is called Variable Time Based Zero Sequence Firing of the SCR, or it is doing Phase Angle Control of the SCR.

In the Variable Time Base version, the firing circuit sends out a string of gate firing pulses to the SCR, telling it to turn on for a variable number of sine waves each second. For instance, if you wanted 50% output from the SCR, you would tell it to fire 10 cycles in a row, then wait 10 cycles, then 10 On, then 10 Off etc. etc. If you wanted 67% output, you would go 6 On and 3 Off, 6 On and 3 Off etc. etc. Each pulse by the way is gated On before the sine wave crosses the Zero Point, so there is no electrical "noise" created by the controller. This is the most common method used in simple heating systems.

In Phase Angle firing, the firing board fires the SCR at every sine wave, but delays the turn-on gate pulse for a proportional percentage of the sine wave phase angle. So if 1/2 of a sine wave in the forward direction is 180 degrees, and you wanted 50% output from the SCR, you would wait until the sine wave was at 90 degrees before turning the SCR on. Then it would conduct only until the voltage passed through the zero cross and self commutate, waiting for the next gate command. If you wanted 75% output from the SCR, you would only delay the firing angle to the 45 degree point, so that the SCR would conduct for the remaining 135 degrees. the advantage to this method is that it in more precise and you can slowly ramp the output voltage (which is what a Soft Starter does for motors), but it does create line noise because the SCRs are firing in the middle of the sine waves.

Is that enough for you?

Would you explain what you mean by the SCR being gated on before it crosses the zero point? The SCR anode has to have a positive polarity present before it can turn on, so are you saying the trigger is sent before and continues on just past crossing zero in a positive direction?

Yes, so that the SCR can begin conducting as soon as there is a forward bias on the supply. Technically you don't really turn the gate on with no power on the SCR, but you anticipate the zero cross because there is about a 3 degree delay from when you command the gate and when it can light the SCR. So by anticipating by commanding the gate pulse just before the zero cross, it is ready when the forward bias begins.

Thanks a lot fellas, I will try to join all the valuable pieces and let you know how it goes..

Regards