Back to Basics(With a Twist): SIDAC

Posted under Back to Basics, Educational, Technology on December 11th, 2022 by

SIDAC is a specific type of thyristor that is designed to switch on when the voltage applied to it reaches a certain threshold. It is often used in high-voltage applications, such as in power distribution systems. To understand how a SIDAC works, it’s helpful to think of it as a switch that has three states: off, on, and holding. When the voltage applied to the SIDAC’s gate terminal is below its threshold, the SIDAC is in the off state and does not allow current to flow through it. When the voltage applied to the gate reaches the threshold, the SIDAC switches on and allows current to flow through it. Finally, once the SIDAC is on, it remains in the holding state until the current flowing through it drops below a certain level, at which point it switches off again.

It is similar to a DIAC but with higher voltage breakdown regions and current-carrying capabilities(SIDAC = Silicon Diode for Alternating Current). Its a 5-layer PN junction device as shown in pics. Once breakdown voltage(VBO) is reached, it goes to a negative resistance region when current conduction happens. SIDACs are used in relaxation oscillators which are used to create non-sinusoidal waveforms like triangular waveforms. Here a SIDAC is connected in parallel to a capacitor. Initially, the capacitor charges up and when the voltage reaches the threshold of the SIDAC, it makes the SIDAC discharge to obtain your sawtooth waveform whose timing can be adjusted via RC time constants.

Now for the twist, the first paragraph wasn’t written by me. It was generated automatically by the Chat-GPT3 AI service(built on 175 billion parameters). I just gave it an input to explain what a SIDAC is and it gave me the first paragraph as is. An AI model gave me an answer instantly which would have taken me a couple of hours to read up from different papers and books. I am totally mind-blown by what can be done with the tool. It’s going to be used as a companion tool to improve your efficiency drastically(Think GitHub CoPilot). It’s not correct always, it usually gives plausible-sounding but sometimes incorrect answers. If I ran it a few times, I can clearly see it getting some stuff wrong. But possibilities are endless with this tool and I can’t even imagine what it will become 3-4 generations down the lane. All the low-hanging jobs in future will be taken over by AI, in a similar way to how automation killed a large part of manual labour. Start upskilling yourselves in whatever field you are in or you will struggle in a decade or so.

If you liked the post, Share it with your friends!

Back to Basics: Crystals and Damping Resistors

Posted under Back to Basics, Educational, Hacks, Technology on November 20th, 2022 by

Continuing from the last post, one key element that one of my readers pointed out was the addition of an extra resistor at the output of the Pierce oscillator driver circuit in a uC. It’s a damping resistor and this is something which is not explained in a few newer datasheets and is usually ignored.

A damping resistor is needed primarily to adjust the Drive power of a crystal. Every good crystal manufacturer would add the maximum drive level in its datasheet. This is the maximum power(usually in microwatts) at which the crystal can be driven without damaging it permanently. To measure the driven level, you need to measure the current waveform to a crystal but Measuring this is a hard thing to do in the circuit. One method is to connect another know test resistance and measure the voltage difference with a differential probe to measure current. After the current is measured, test resistance is shorted out from the circuit. This is useful in SMD parts with no space. Another way is to measure it with a current probe in one of the traces. It’s expensive and hard to do on SMD parts so it’s preferred for thru-hole crystals with a probe connected around one of the legs of the crystal.

Once the RMS current to the crystal is measured, power is the squared multiplication of this current and the ESR(mentioned in the crystal’s datasheet). This power should not exceed the max value mentioned. If it does, you can try reducing the drive voltage from the uC(By adjusting the gain of the oscillator) or you add a resistor in series. The usual starting value of this damping resistor is equal to the impedance of external load capacitance(at the crystal freq) as mentioned in the figure. Using a normal voltage divider, you will see the drive will be reduced by half. Keep in mind that adding the resistor can add any additional phase shift to the oscillator circuit so you would want to double-check the crystal frequency once again in the circuit. For digital uCs these days, drive values from the Xout pin are set very low and hence you don’t see them in most datasheets.

If you liked the post, Share it with your friends!
1 … 53 54 55 56 57 … 66
Copyright © 2025 Amaldev. All Rights Reserved.