Tech Directions- October 2008

Check That JFET! Easy-to-Build Tester Makes It Simple By Charles Harman Charman@sbcglobal.net N articles published earlier, we learned how to make an op-amp checker (December 2007) and a bipolar junction transistor (BJT) checker (February 2008). In this article, we learn how to make a junction ﬁeld effect transistor (JFET) checker. Most electronics students do not have the experience or knowledge that it takes to recognize whether a JFET is operating normally. Both instructors and students will ﬁnd having the means to check the operation of a JFET at the proto-board stage a blessing. With this handy JFET checker, you can simply remove the component in question from the proto-board, insert the component into the JFET checker, push the press-to-test button, and observe the JFET checker’s light-emitting diode (LED). It’s that simple. I that the JFET is normally on until turned off. The JFET checker’s circuit allows a JFET under test to conduct when the JFET is correctly inserted into the checker’s test socket and not to conduct when the press-to-test switch is pressed. A 5-pin test socket accommodates most JFET case types (e.g., TO18, TO39, TO92). When the user cor- How It Works The JFET checker allows the user to dynamically check an N-channel- or a P-channel-type of JFET, which is a voltage-controlled device. To facilitate accurate testing, the user should know the JFET’s pin-out conﬁguration. The user also needs to select whether the JFET is an N-channel or a P-channel device. The user accomplishes this action by actuating a two-position toggle switch. The panel has two LEDs. One LED will light if the correctly inserted JFET is an N-channel type and the other will light if the correctly inserted JFET is a P-channel type. A JFET is a depletion mode device, which means Photo 1—JFET checker on proto-board rectly inserts a JFET to be tested into the checker’s test socket, the LED should be lit. When the user presses the JFET checker’s press-to-test switch, the gate-source junction of the JFET will become reverse biased and the LED of the JFET checker will turn off. This action also charges a capacitor connected across the JFET’s gate-source junction. Releasing the push-button switch will allow the charged capacitor to slowly discharge through a resistance connected in parallel to the capacitor. When the gate-source voltage is zero, the JFET will conduct its maximum drain current and allow the glowing LED to be at its maximum brilliance. When the JFET is conducting, the brightness of the LED is a function of the JFET’s drain-source resistance or transconductance. A Charles Harman taught electronics at ITT Technical Institute in both Houston and Garland, TX, before retiring. He is currently a proctor for the Electronics Technician Association. Parts List Qty. 1 1 1 2 1 2 1 2 1 1 roll 1 roll 1 2 Description Enclosure Perfboard Push-button switch, NO LED, red Capacitor, 10 μF Resistor, 100 kΩ Resistor, 47 Ω 9 V battery connector 14-pin wire-wrap socket (cut down to 5 pins) Solid wire, bare, #24 AWG Solid wire, insulated, #22 AWG Sponge, about 1" × 2" × 3/4" Batteries, 9 V Part Number Radio Shack 270-283 Radio Shack 276-1395 Radio Shack 275-1547 Radio Shack 276-330 Radio Shack 272-999 Radio Shack 271-1347 Radio Shack 271-1105 Radio Shack 270-325 Radio Shack 900-7243 Radio Shack 278-1341 Radio Shack 278-1215 Supermarket Radio Shack 23-824 24 techdirections ◆ OCTOBER 2008

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