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techdirections August 2013 : Page 22

Build a Handy MOSFET Checker By Charles Harman E LECTRONICS students and instructors find being able to check the operation of a MOSFET (metal oxide semiconductor field effect transistor) at the proto-board stage a real blessing. Most students do not have the experience or knowledge it takes to recognize whether or not a MOSFET is operating normally. With the handy, easy-to-make MOSFET checker described in this article, a student or instructor can simply remove the component in question from the proto-board, carefully at-tach the component to the checker’s test leads, push the appropriate switch, and observe the MOSFET checker’s LEDs (light emitting di-odes). It’s that simple. devices. Another type of MOSFET is the depletion mode device. They are mostly N-channel types. The D-MOSFET is normally on until turned off. Although not used as much, the depletion type MOSFET is still taught in many tech schools. An E-MOSFET or a D-MOSFET de-vice under test in the MOSFET check-er will either conduct or not conduct after the user correctly connects the device to the MOSFET checker’s test leads. Test leads are used in lieu of a test socket because of the many different case types associated with MOSFET devices. The user can select an N-channel [N-ch] or a P-channel [P-ch] opera-tion by setting a switch. This sets the MOSFET’s drain voltage polarity Vd to be either positive or negative with respect to the MOSFET’s source. Photo 1 How It Works The MOSFET checker allows a user to dynamically check an N-channel or a P-channel MOSFET, both of which are voltage-controlled de-vices. To facilitate accurate testing, the MOSFET’s pin-out configuration should be known by the user, and it would be helpful to the user to know if the MOSFET is an enhancement type (normally off) or a depletion type (normally on) device. Most MOSFETs are enhancement mode devices. This means that an E-MOSFET is normally off until turned on. E-MOSFET devices are available as N-channel or P-channel Charles Harman taught electronics at ITT Technical Institute in Houston and Garland, TX, and served as a proctor for the Electronics Technician Association. Qty. 1  1  1 pkg.  1  1 pkg.  1 pkg.  1 pkg.  1 pkg.  1  2  1 pkg.  2  1  Parts List Description Enclosure, 270-283, Radio Shack Perfboard, 176-1395, Radio Shack Push-button switch, NO, 275-1547, Radio Shack Toggle switch, SPDT, 275-613, Radio Shack Clip leads, 14", 278-1156, Radio Shack R1, resistor, 1 M 1 , 271-1356, Radio Shack R2, R3, resistor, 100 K 1 , 271-1347, Radio Shack R4, resistor, 1 K 1 , 271-1321, Radio Shack C1, capacitor, 10 + F, NP, 272-999, Radio Shack LED, green, 276-022, Radio Shack Battery clips, 270-325, Radio Shack 9 V batteries Small sponge 22 tech directions X AUGUST 2013

Build a Handy MOSFET Checker

Charles Harman

<br /> ELECTRONICS students and instructors find being able to check the operation of a MOSFET (metal oxide semiconductor field effect transistor) at the proto-board stage a real blessing. Most students do not have the experience or knowledge it takes to recognize whether or not a MOSFET is operating normally. With the handy, easy-to-make MOSFET checker described in this article, a student or instructor can simply remove the component in question from the proto-board, carefully attach the component to the checker’s test leads, push the appropriate switch, and observe the MOSFET checker’s LEDs (light emitting diodes). It’s that simple.<br /> <br /> How It Works<br /> The MOSFET checker allows a user to dynamically check an Nchannel or a P-channel MOSFET, both of which are voltage-controlled devices. To facilitate accurate testing, the MOSFET’s pin-out configuration should be known by the user, and it would be helpful to the user to know if the MOSFET is an enhancement type (normally off) or a depletion type (normally on) device.<br /> <br /> Most MOSFETs are enhancement mode devices. This means that an E-MOSFET is normally off until turned on. E-MOSFET devices are available as N-channel or P-channel devices. Another type of MOSFET is the depletion mode device. They are mostly N-channel types. The DMOSFET is normally on until turned off. Although not used as much, the depletion type MOSFET is still taught in many tech schools.<br /> <br /> An E-MOSFET or a D-MOSFET device under test in the MOSFET checker will either conduct or not conduct after the user correctly connects the device to the MOSFET checker’s test leads. Test leads are used in lieu of a test socket because of the many different case types associated with MOSFET devices.<br /> <br /> The user can select an N-channel [N-ch] or a P-channel [P-ch] operation by setting a switch. This sets the MOSFET’s drain voltage polarity Vd to be either positive or negative with respect to the MOSFET’s source.Pushing a push-button switch, indicated as [+Vg] or [-Vg], allows a negative or a positive gate voltage, +/-Vg, to turn on and/or off the MOSFET device. When an enhancement-type MOS device turns on, an LED will glow when the appropriate Vg switch is pressed. When a depletion-type MOS device turns off, an LED will stop glowing when the appropriate Vg switch is pressed. Pushing either Vg switch steers current into capacitor C1 connected in parallel with the gate-source resistor R1, thus, providing a changing voltage at the MOSFET’s gate.<br /> <br /> Parts List<br /> <br /> Qty.Description<br /> <br /> 1 Enclosure, 270-283, Radio Shack<br /> 1 Perfboard, 176-1395, Radio Shack<br /> 1 Toggle switch, SPDT, 275-613, Radio Shack<br /> 1 pkg. Push-button switch, NO, 275-1547, Radio Shack<br /> 1 pkg. Clip leads, 14", 278-1156, Radio Shack<br /> 1 pkg. R1, resistor, 1 MΩ, 271-1356, Radio Shack<br /> 1 pkg. R2, R3, resistor, 100 KΩ, 271-1347, Radio Shack<br /> 1 pkg. R4, resistor, 1 KΩ, 271-1321, Radio Shack<br /> 1 C1, capacitor, 10 µF, NP, 272-999, Radio Shack<br /> 2 LED, green, 276-022, Radio Shack<br /> 1 pkg. Battery clips, 270-325, Radio Shack<br /> 2 9 V batteries<br /> 1 Small sponge<br /> <br /> Building the Checker<br /> 1. I suggest that you first build the MOSFET checker on a proto-board so it resembles the MOSFET checker shown in Photo 1. This will allow you to physically hook up all necessary components of the circuit as well as better understand the theory of the MOSFET checker’s operation.<br /> 2. Refer to the schematic diagram (Fig. 1) as you build your circuit. The parts needed to build the MOSFET checker are, for the most part, available at Radio Shack and are listed on the parts list. At this stage of the project, you will not need the perfboard or the enclosure.<br /> 3. After you have built the MOSFET checker on your proto-board, tested it, and proven it to be lab worthy, it is time to construct the MOSFET checker that looks like the unit shown in Photo 2. DO NOT disassemble your proto-board circuit. If there is a wiring error in the new MOSFET checker, having a unit that works will be helpful to you.<br /> 4. Refer to the panel layout drawing (Fig. 2). Carefully measure and mark the perfboard per the indicated dimensions. Double-check your measurements, then carefully cut the perfboard. Place the cut perfboard on top of the plastic box. It should fit nicely. If not, fix it. Use a piece of medium grit sandpaper to smooth out any rough edges on the perfboard.<br /> 5. Carefully measure and mark the locations to be drilled on the perfboard for the two push-button switches and the toggle-switch as shown on the front panel drawing (FIg. 3). Double-check your measurements then carefully drill the two holes for the push-button switches using a #F drill bit. If you do not have a #F drill bit, use a drill bit that will allow a 1/4" thread to pass through the holes. Drill the hole for the toggle switch using a #1 drill bit. If you do not have a #1 drill bit, use a bit that will allow a 15/64" drill bit to pass. Do not attach the three switches at this time.<br /> 6. Locate where each wire of the three test leads will be inserted and mark the locations with a pencil. Using a #45 drill bit, drill the three holes.<br /> 7. Find the locations where the four mounting screws will be inserted through the perfboard and attach to the plastic enclosure. Mark each spot with a pencil. Double-check the measurements, then drill the holes using a #30 drill bit.<br /> 8. Using Fig. 3, attach the two push-button switches and the toggle switch to the perfboard. Position the three switches so they are orientated like the ones shown on the assembly drawing (Fig. 4). Tighten the hex nuts.<br /> 9. From the five-piece test-lead set (RS 278-1156), select the red, black, and green test leads. Using wire strippers, cut the mini-alligator clips off one end of the three test-leads. Strip away about 1/8" of insulation from the test leads. Using your soldering iron and solder, tin the exposed wires of the three test leads.<br /> 10. Referring to Fig. 4 and Photo 3, solder R1, R2, R3, and C1 to the three switches. Following the layout of the assembly drawing will insure that the wired circuit will fit between the two 9 V batteries (not yet installed).<br /> 11. Per Fig. 2, carefully insert each test-lead wire through the perfboard: black on the left, green in the middle, and red on the right.<br /> 12. Insert the two LEDs into the panel, left LED’s cathode at the top and right LED’s anode at the top. Bend the LED’s leads as shown on Fig.4. Solder the leads together. Solder R4 to the LEDs.<br /> 13. Solder the tinned ends of the test leads to the points indicated on Fig. 4.<br /> 14. Per Figs. 1 and 4, solder the two battery clips to the circuit. The MOSFET checker should resemble the unit shown in Photo 3.<br /> <br /> Checking the MOSFET Checker<br /> While the MOSFET checker is still apart, attach two 9 V batteries to the battery clips. Place the toggle switch in the N-channel position and touch the red test lead’s alligator clip to the black test lead’s alligator clip. The right LED [P] should glow. Changing the toggle switch’s position should make the left LED [N] glow. With your DMM or VOM set to indicate voltage, measure the voltages across R1. The voltage should be about +/– 7.5 V (polarity will depend on the toggle switch’s position). If everything is OK, great! If not, fix it. Insure that no soldered wires are touching other points. If necessary, refer to the proto-board circuit that is still intact.<br /> <br /> Once everything is working properly, carefully place the attached batteries in the plastic box. Both batteries should be on their narrow sides. Put a small piece of sponge between the batteries to keep the batteries from moving around inside the box once the lid is in place. Place the perfboard on top of the box. Insert the four mounting screws through the mounting holes of the perfboard and tighten them.<br /> <br /> You can spiff up the appearance of the MOSFET checker by applying labels to the switches and marking the functions of the three wire test leads. This will make the MOSFET checker more functional and professional looking.<br /> <br /> Using the MOSFET Checker<br /> Whether you are using the MOSFET checker constructed on your proto-board or the one in the enclosure, the testing procedure is the same. Regardless of the MOSFET checker you use, I recommend inserting the MOSFET you want to test into a portion of the proto-board.<br /> <br /> Gather some MOSFETs, and select an N-channel E-MOSFET. You will need to know the MOSFET’s pin-out. Make sure that the MOSFET’s terminals are straight. Because of possible static electricity discharge, DO NOT touch the MOSFET’s terminals. Carefully attach the three test leads to the MOSFET’s terminals. The MOSFET’s drain must be attached to the red testlead [D], the MOSFET’s gate must be attached to the green test lead [G], and the MOSFET’s source must be attached to the black test lead [S]. Move the toggle switch’s handle to the N-channel position [N-ch]. The [+] LED and the [-] LED should be off. If an LED is glowing, the MOSFET is not hooked up correctly. Press and release the [+Vg] push button. The [+] LED should turn on, then slowly turn off, which indicates that the MOSFET is good.<br /> <br /> Repeat the procedure using a P-channel E-MOSFET. After attaching the three test leads to the P-channel MOSFET’s terminals and changing the position of the toggle switch to [P-ch], press and release the [-Vg] push button. The [-] LED will turn on then slowly turn off.<br /> <br /> Final Thoughts<br /> This handy MOSFET checker can be of great use to students and instructors. It enables you to test Nchannel or P-channel MOSFETs at the proto-board stage. Either enhancement types or depletion types can be checked. With a little experience, you will learn to discern different types of MOSFETs, as well as their terminals, without knowing their pin-outs.<br /> <br /> Charles Harman taught electronics at ITT Technical Institute in Houston and Garland, TX, and served as a proctor for the Electronics Technician Association.

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