Tech Directions- October 2008

R LED Cathode R2 100 K #24 AWG bare S #24 AWG bare D L LED Cathode R3 47 Ω word processor, scissors, and a razor knife to create these decals. Clear tape over white paper with hand-printed letters also works quite well. G S D Using the JFET Checker #22 AWG insulated Whether you are using the checker constructed on your proto-board or the one in the enclosure, the testing R1 C1 procedure will be the same. S2 S1 Gather some JFETs. Select an N-channel JFET. You Back PTT #22 AWG #24 AWG side will need to know the JFET’s pin-out. Make sure that the bare view insulated JFET’s leads are straight. Insert the JFET into the 5-pin test socket. The JFET’s drain must be plugged into the Red Twisted [D], the gate into the [G], and the source into the [S]. -+ Place the toggle switch in the N-channel position and Blk Battery clips the N-channel LED should be on. Press the push-button -+ Red marked press-to-test. The LED should turn off then slowly Fig. 3—Assembly drawing return to its original brilliance, which indicates that the JFET is good. lated wire (about #22 AWG) from ML of the toggle switch Repeat the procedure using a P-channel JFET. After to the bottom pin of the push-button switch. inserting the P-channel JFET properly and changing the 12. Bend the leads of resistor R2 so you can solder it position of the toggle switch, you will see similar testto the [G] pin and [S] pin of the 5-pin socket. Solder the ing results. Removing an N-channel JFET and replacing it resistor to the socket. Place capacitor C1 between the with a P-channel JFET will still allow the N-channel LED 5-pin socket and the push-button switch. Carefully bend to light. However, the P-channel JFET cannot be checked. one lead so that you can solder it to the top pin of the The P-channel JFET can only be checked if the toggle push-button switch. Bend the other lead so you can solswitch is switched to the P-channel position. der it to the [S] pin of the 5-pin Get several JFETs of the socket. Solder the capacitor in same type and test them. place. With different JFETs, you 13. Referring to Figs. 1 and will note a slight difference 3, solder the two battery clips in the LED’s brilliance. to the circuit. The JFET checker This is because each JFET should resemble the unit shown has a slightly different in Photo 3. transconductance. A dim 14. While the JFET checker is glow indicates a transconstill apart, attach two new ductance of about 1,500 9 V batteries to the battery clips. micro-mhos while a very Insert either an N-channel or a Pbright glow indicates a channel JFET into the test socket. transconductance of about If it works—great! If not, ﬁx it. 6,000 micro-mhos. A mePhoto 3—Checker with batteries Make sure that no soldered wires dium glow would indicate touch other points. If necessary, refer to your still-intact a transconductance of about 3,000 micro-mhos. proto-board circuit. You will note that some JFETs will not allow the LED to 15. Once you have satisﬁed yourself that everything completely turn off. This is due to that particular JFET’s works properly, carefully place the attached batteries in pinch-off voltage exceeding the gate-source voltage prothe plastic box. The battery closest to the top should be duced in the JFET checker’s circuit. on its narrow side and the other battery should be on its wide side. Put a piece of sponge over the batteries to Final Thoughts keep the batteries from moving around inside the box. Whether you are a student or an instructor, this handy 16. Once the lid is in place, tighten the two hex nuts JFET checker can be of great use to you. It will allow you that secure the two switches to the perfboard. Place the to check and/or test JFETs at the proto-board stage. With perfboard on top of the box. Insert the four mounting a little experience, you will learn that you can also disscrews through the mounting holes of the perfboard and cern different JFETs without knowing their pin-outs. With tighten the screws. the JFET checker, you can estimate a JFET’s transconducYou can spiff up the appearance of the JFET checker tance as well as its pinch-off voltage. LED slightly on, Vpo by applying labels to identify the switches, as well as to is greater than 5 VDC. LED completely off, Vpo is less than mark the 5-pin socket polarity locations. This will help 5 VDC. make the JFET checker more functional and professional If you discover anything interesting, keep me posted looking. You can use labels with adhesive backs, your via e-mail. Good luck and happy checking! 100 K 10 μF 26 techdirections ◆ OCTOBER 2008

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