Tag Archives: Thyristor

Component Tester FISH 8840 Review


Alan Parekh @ hackedgadgets.com has a review of a cheap component tested he found on ebay. This device can test bipolar transistors, MOSFET, diodes, thyristors, resistors and capacitors. He writes:

This is an inexpensive component tester called the FISH 8840 which you can find from many online eBay retailers for around $30. The interface is very simple, attach a device to be tested and press the test button. It turns off after about 20 seconds, pressing the off button puts it into sleep mode immediately. There is a ZIF socket that allows you to insert leaded devices and pads that allow you to press SMD devices directly onto the tester.

Component Tester FISH 8840 Review – [Link]

Basic Types of Thyristors and Applications

In this article we will discuss about the various types of thyristors. Thyristors are 2 pin to 4 pin semiconductor devices that act like switches. For example a 2 pin thyristor only conducts when the voltage across its pins exceeds the breakdown voltage of the device. For a 3 pin thyristor the current path is controlled by the third pin and when a voltage or current is applied to this pin the thyristor conducts. In contrast to trasistors, thyristors only work on ON and OFF states and there is no partial conduction state between these two states. Basic types of thyristors are: SCR, SCS, Triac, Four-layer diode and Diac.

Silicon Controlled Rectifier (SCR)


Silicon controlled rectifier is normally in OFF state but when a small current enters its gate G it goes to ON state. If the gate current is removed the SCR remains in ON state and to turn it of the anode to cathode current must be removed or the anode must be set to a negative voltage in relation to cathode. The current only flows in one direction from anode to cathode. SCRs are used in switching circuits, phase control circuits, inverting circuits etc.

Silicon Controlled Switch (SCS)


Working of SCS is similar to SCR but also it can be turned off by applying a positive pulse on the anode gate. The SCS can also turned ON by applying a negative pulse on anode gate. The current flows only from anode to cathode. SCS are used in counters, lamp drivers, logic circuits etc.



Triac is similar to SCR but it conducts in both directions, means that it can switch AC and DC currents. The triac remain in ON state only when there is current in gate G and switched OFF when this current is removed. Current is flowing in both directions between MT1 and MT2.

Four layer diode


Four layer diode has 2 pins and works like a voltage-sensitive switch. When the voltage between the two pins exceeds the breakdown voltage it turns ON, otherwise it’s OFF. Current flows from anode to cathode.



Diac is similar to four-layer diode but it can conduct in both directions meaning it can contact both AC and DC currents.

Basic SCR Applications

Basic Latching Circuit


In this circuit a SCR is used to form a basic latching circuit. S1 is a normally open switch and S2 is a normally close switch. When S1 is pushed momentary a small current goes into the gate of SCR and turning it ON, thus powering the load. To turn it off we have to push the S2 push-button so the current through SCR stops. Resistor RG is used to set the gate voltage of SCR.

Power Control Circuit


In this circuit a SCR is used to modify a sinusoidal signal so that the load receives less power than of what would receive if source voltage was applied directly. The sinusoidal signal is applied to the gate of SCR via R1. When the voltage on the gate exceeds the trigger voltage of SCR, it goes to ON state and Vs is applied to the load. During the negative portion of the sine wave the SCR is in OFF state. Increasing R1 has the effect of decreasing the voltage applied to the gate of SCR and thus creating a lag in the conduction time. In this was the load is receiving power for less time and thus the average power to load is lower.

DC motor Speed Controller


This is a variable speed DC motor controller using a UJT, a SCR and few passive components. UJT along with resistors and capacitor form an oscillator that supplies AC voltage to the gate of SCR. When the gate voltage exceeds the triggering voltage of SCR, the SCR turns ON and motor is running. By adjusting the potentiometer the output frequency of oscillator is changing and thus the times the SCR triggered is changing, which in turn changes the speed of the motor. In this way the motor is receiving a series of pulses that average over time and the speed is adjusted.

Basic TRIAC Applications

AC Light Dimmer


This is an AC light dimmer formed by a diac, a triac and some passive components. The capacitor is charging through the two resistors and when the voltage on one end of the diac exceeds the breakdown voltage it goes ON and sends a current to the gate of triac putting the triac to ON state and thus powering the lamp. After the capacitor is discharged to a voltage below the breakdown voltage of diac, the diac, triac and lamp turn off. Then the capacitor is charged again and so on. So the lamp is only powered for a fraction of time during the full sinewave. This happens very quickly and the lamp seems dimmed. Brightness is adjusted using the potentiometer.

32 channel mains lamp controller


Jaanus Kalde made this 32 channel light dimmer project, that is available at Github:

I needed a computer controllable 32 channel light dimmer for an art installation. After looking around a bit I found out that there isn’t even a Arduino shield for the work. So I made a quick 4 channel stackable board to control lights. The board uses SHARP thyristor based solid state relays to switch mains voltage. As normal with thyristors – all the outputs can be used as dimmers through zero-crossing detection. All outputs are able to handle 0.9 A / 200 W. We connected 40 W incandescent light bulbs to it but you can control whatever with it – lights, electronics, computers, motors etc.


32 channel mains lamp controller – [Link]

555 timer triggers phase-control circuit


JN Lygouras, University of Thrace writes:

The control circuit in Figure 1a allows you to manually adjust the power delivered to a load. By changing the setting of potentiometer R3, you change the phase angle at which the thyristor (Q3) fires (Figure 1b), thereby altering the load current’s duty cycle. The adjustment range is about 0 to 180°. Q3’s off time is linear with R3, but of course the resulting load power is not linear with R3.

555 timer triggers phase-control circuit – [Link]

Component tester

coremelt.net writes:

With this tool you can test various electronic components like diodes, LEDs, all kinds of transistors (PNP, NPN, several types of MOSFETs), capacitors, resistors as well as triacs and thyristors. It will show you several physical characteristics after the test was completed, like forward voltages, (gate) capacity and amplification factor. More over, it will show the polarity of the component and identifies the several pins of a package. A very nice and sophisticated project I host for Markus Frejek. I’ve done an additional layout for the device you can see on the left side. This project has found a lot of fans, including myself. The device is powered by an AVR ATmega 8 MCU.

Component tester – [Link]

AVR-based transitor tester

dangerousprototypes.com writes:

After our recent post about the commercial semi-conductor tester we started a discussion about building a similar open source project. What came up is this AVR based transistor tester (machine translation) by Markus.

It’s built around an ATmega8 IC that interfaces with a standard HD44780 16×2 character LCD. The circuit that does the testing is simplicity itself. Three pairs of resistors are connected to 6 pins of the microcontroller, and each pair is connected on the other end to one of the transistor pins.

The theory of operation is also relatively simple. The microcontroller cycles through different patterns on its output pins until a recognizable pattern is read on its input pins. It supports a very large range of devices:

Arup and Fcobcn have already built one for themselves. Join the discussion and add your input to our development of an open source part tester.

AVR-based transitor tester – [Link]