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Testing Diodes, Transistors and Regulators

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L7805CV regulator on a circuit board (Raimond Spekking)
An STMicroelectronics L7805CV 5 V regulator in a TO-220 package on a circuit board, with the board marking pins 1 and 3 (photo: Raimond Spekking, CC BY-SA 4.0).

Testing diodes, transistors and regulators covers the quick checks that find failed discrete semiconductors and three-terminal voltage regulators: the multimeter's diode-test range for diodes and transistor junctions, and voltage measurements at a regulator's input and output with the power on. The diode test finds parts that have failed open or short. It does not prove that a part works at full current or full speed, and it does not fully test zener diodes or LEDs.[1] The last part of the page covers replacing a linear 78xx regulator with a switching module, where the module's maker documents it as a drop-in replacement.

When to use it

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  • A supply rail is missing, low or high. Check the regulator that makes it, and the rectifier and protection diodes in front of it. The Super Nintendo Entertainment System Power Circuit Repair follows a supply through its fuse, diode and regulator in this way.
  • A fuse blows. Goldwasser lists shorted rectifier diodes and a shorted switching transistor among the common causes in a switch-mode supply.[2] See Fuse Identification and Replacement and Finding Short Circuits.
  • A stage built round a transistor (a video amplifier, a sound output or a DC-DC converter) is dead. Sinclair's first check on a Spectrum that smokes at switch-on is its converter transistor TR4, shorted base to collector.[3]
  • A linear regulator runs hot enough to be a problem, and the machine's documentation allows a replacement.

Logic chips are covered in Logic Chip Testing and electrolytic capacitors in Capacitor Failure Symptoms.

Safety

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  • Make diode and transistor tests with the power off and every capacitor discharged; a charged capacitor gives false readings.[4]
  • Regulator checks are made with the power on. On a machine with an internal mains supply, keep to the low-voltage side and see Recommended Tools on isolation transformers. On a machine with a CRT, follow the CRT Discharge Procedure.
  • The metal tab of a TO-220 regulator is connected to one of its pins: to ground (pin 2) on the MC7800 series, but to the input (pin 2) on the negative MC7900 series.[5][6] A 79xx tab bolted to a grounded heat sink without its insulating washer shorts the input.
  • A regulator that loses its ground connection, or that has failed internally, can pass its unregulated input straight to the output, and that can damage every chip on the rail. TI warns that with the ground pin disconnected the output approaches the unregulated input.[7]

Tools and materials

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  • A digital multimeter with a diode-test range, marked with the diode symbol. It may share a position on the dial with another function, often continuity.[4]
  • A known-good silicon diode (Goldwasser suggests a 1N4007 or 1N4148, and a 1N34 if germanium parts are expected) to check the meter's lead polarity and to see what a good junction reads on that meter.[1]
  • The datasheet or pinout of each part. Regulator pinouts differ between families; see the table below.[5][6][8]
  • A bench power supply with current limit, for testing a regulator off the board.[9]
  • For a switching replacement: a module made as a 78xx replacement, such as a Traco Power TSR 1-2450 or a RECOM R-78E5.0-1.0 for a 7805.[10][11]
Linear regulator and switching module pinouts (diagram)
Pin connections of a 78xx and a 79xx regulator in TO-220, and of a TSR 1 or R-78E switching module, from the onsemi, Traco Power and RECOM datasheets. Drawn for this wiki.
Regulator pin connections
Part Package Pin 1 Pin 2 Pin 3 Tab
78xx positive regulator (MC7800, LM7805) TO-220 Input Ground Output Ground (pin 2)[5][7]
79xx negative regulator (MC7900) TO-220 Ground Input Output Input (pin 2)[6]
78Lxx low-power positive regulator (UA78L) TO-92 Output Common (ground) Input None[8]
Traco Power TSR 1 switching module SIP3 +Vin GND +Vout None[10]
RECOM R-78E switching module SIP3 +Vin GND +Vout None[11]

Procedure

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Test a diode

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  1. Switch off and discharge the circuit.[4]
  2. Set the meter to diode test.[4]
  3. Put the red lead on the anode and the black lead on the cathode (the end with the band). This is forward bias.[4]
  4. Read the display. A good silicon diode reads 0.5 to 0.8 V; a germanium diode lower, about 0.2 to 0.3 V according to Fluke, or 0.2 to 0.4 V according to Goldwasser.[4][1]
  5. Swap the leads. A good diode reads OL (open).[4]
  6. Judge the result. OL both ways means the diode has failed open. The same low reading both ways means it has failed short; Fluke gives about 0.4 V for a shorted diode, and a hard short reads close to zero.[4][1]
  7. If the readings in circuit do not make sense, lift one end of the diode and test again. Other parts in parallel can give false readings, and Fluke says the resistance ranges in particular should not be trusted in circuit.[4]

A bridge rectifier is four diodes, and each can be tested the same way between its pins.[4] A zener diode tests like an ordinary diode in the forward direction, but its zener voltage is too high for a meter's diode range to show, and most LEDs need more forward voltage than the meter supplies.[1]

Test a bipolar transistor

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Bipolar transistor as two diodes for meter testing (diagram)
An NPN and a PNP transistor drawn as two diodes, with the diode-test readings Goldwasser gives for a good silicon part out of circuit. Drawn for this wiki.

Goldwasser's method treats the transistor as two diodes joined at the base, and is for transistors out of circuit only.[1]

  1. Find which lead is base, collector and emitter from the datasheet.
  2. Set the meter to diode test.
  3. For an NPN transistor, put the red lead on the base and the black lead on the emitter. A good part reads a junction drop of 0.45 to 0.9 V. Move the black lead to the collector; the reading should be about the same.[1]
  4. Put the black lead on the base and the red lead on the emitter, then on the collector. A good NPN part reads open both times.[1]
  5. For a PNP transistor the polarities are the other way round: black on the base gives the junction drops, red on the base gives open.[1]
  6. Measure from collector to emitter in both directions. A good transistor reads open both ways.[1]
  7. A reading of zero, or any reading that differs from the pattern above, means a failed transistor.[1]

Some transistors do not follow the pattern. Power transistors with a built-in damper diode read low in one direction from collector to emitter, and those with a resistor of about 50 Ω between base and emitter read almost zero across that junction without being faulty. A Darlington reads 1.2 to 1.4 V from base to emitter, because it has two junctions in series. Compare with a known-good part or the datasheet before condemning one of these.[1]

Check a linear regulator in circuit

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  1. Identify the regulator and its pinout from the datasheet and the table above.
  2. Power up and measure from the regulator's ground pin to its input. The input must be higher than the output by at least the dropout voltage: TI and onsemi give a typical dropout of 2 V at 1 A for a 7805, and TI specifies the LM7805's output for inputs between 7.5 and 20 V.[7][5]
  3. Measure from the ground pin to the output. TI specifies an LM7805 at 4.8 to 5.2 V at 25 °C, and 4.75 to 5.25 V over load, line and temperature.[7] Compare with the machine's own figures where its manual gives them.
  4. Read the result:
    • Input correct, output correct: the regulator is working. Look downstream for the fault.
    • Input missing or low: the fault is in front of the regulator, in the supply, fuse, switch or rectifier. See Continuity Testing and Signal Tracing.
    • Input correct, output low or zero: either the regulator has failed, or the load is shorted and the regulator is limiting. The LM7805 has thermal shutdown and short-circuit protection, so a shorted rail pulls its output down, and a regulator running very hot is a sign that the circuit is drawing too much current.[7][12] Switch off, measure the output rail to ground, and if it is low go to Finding Short Circuits; if not, replace the regulator.
    • Output close to the input voltage: the regulator has failed, or its ground pin has lost its connection.[7] Switch off at once, before the over-voltage damages the chips on the rail, and check the ground connection before replacing the regulator.
  5. Feel the regulator after a few minutes. A linear regulator turns the difference between input and output voltage into heat, so a hot regulator with correct voltages may be normal, while one that is very hot with a low output points to an overload.[13][7]

Test a regulator off the board

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  1. Connect the regulator to a current-limited bench supply, with an input capacitor of 0.22 µF and an output capacitor of 0.1 µF close to its pins, the values TI uses for its datasheet measurements.[7]
  2. Set the input to at least 2 V above the rated output (7.5 V or more for a 7805) and well below the 35 V maximum.[7]
  3. Measure the output with no load, then with a resistive load drawing a known current, and compare with the datasheet figures.[7]

Replace a linear regulator with a switching module

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A switching regulator wastes much less power as heat. Traco Power describes its TSR 1 series as a drop-in replacement for 78xx linear regulators, pin compatible and in a SIP package that fits the TO-220 footprint, with an efficiency high enough to run at full load up to 60 °C ambient without a heat sink.[10] RECOM describes its R-78E series in the same terms.[11] Retroleum sells Traco and Murata/OKI modules as 7805 replacements for the Sinclair ZX Spectrum, and one Super Nintendo Entertainment System restoration replaced the console's 7805 with a TSR-1-2450.[14][15]

Check these limits before choosing one:

  • Current. The TSR 1-2450 and R-78E5.0-1.0 are 1 A parts. A machine that draws more from its 7805 needs a larger module; Celso Martinho lists the Traco TSR 2-2450 (2 A) among the options for a Spectrum 128.[10][11][13]
  • Input voltage. The TSR 1-2450 needs 6.5 to 36 V, with an external 22 µF 50 V input capacitor above 32 V; the R-78E5.0-1.0 needs 8 to 28 V.[10][11] Measure the regulator's input in the machine first, at its lowest and highest.
  • Polarity. These modules replace positive regulators only. RECOM states that the R-78E cannot be used as a positive-to-negative converter, so a 79xx needs a different part.[11]
  • Capacitance on the output. Traco allows up to 470 µF on the TSR 1's output and RECOM up to 220 µF on the R-78E5.0-1.0. Add up the capacitors on the rail before fitting one.[10][11]
  • Ripple. Traco gives typical output ripple and noise of 50 mV peak to peak for its 12 V-input models, and RECOM gives 120 mV peak to peak for the R-78E. Celso Martinho found that a generic buck converter fitted in place of a Spectrum 128's 7805 let supply noise show on the video output, and suggests a 100 µF electrolytic with a 100 nF ceramic across its output.[10][11][13]
  • Space. Retroleum notes that its vertical-mount module does not fit an Issue 2 Spectrum board in the original rubber-key case.[14]

To fit the module:

  1. Switch off, unplug the supply and discharge the board.
  2. Note the regulator's orientation and the board's pin 1, and photograph them.[16]
  3. Remove the regulator. See Through-Hole Desoldering.
  4. Fit the module with its pin 1 (+Vin) in the hole that the regulator's input (pin 1) used, its pin 2 to ground and pin 3 to the output. Traco marks pin 1 with a dot.[10]
  5. Add capacitors only where the maker calls for them. The TSR 1 has built-in filter capacitors and needs no external ones below 32 V input; RECOM shows 10 µF ceramic capacitors on the input and output of the R-78E as optional. Keep the total capacitance on the output rail within the module's limit.[10][11]
  6. Power the board from a current-limited supply first, and measure the output at no load and in use.[16] RECOM advises a soft-start supply to protect its module at power-up.[11]

Checking the repair

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  1. After replacing a diode or transistor, repeat its diode test in circuit and compare with a good part of the same type, if one is fitted elsewhere on the board.[1]
  2. After replacing a regulator, measure its input and output with the machine running, and compare the output with the datasheet and the machine's own specification.[7]
  3. Measure every rail that the regulator feeds, at the chips' own supply pins.[16]
  4. For a switching module, check the output with an oscilloscope for ripple, and check the machine's picture and sound for new noise.[13]

Common mistakes

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  • Testing a diode in circuit on a resistance range, or trusting an in-circuit diode reading that a parallel part could explain.[4]
  • Condemning a power transistor with a built-in damper diode or base resistor, or a Darlington, because it does not read like a small-signal transistor.[1]
  • Expecting the diode test to show a zener's voltage or light every LED.[1]
  • Assuming every three-pin regulator has the 78xx pinout. A 79xx has its input on pin 2 and its tab at input potential, and a 78Lxx in TO-92 has its output on pin 1.[6][8]
  • Replacing a regulator whose output is low because the rail it feeds is shorted.[7]
  • Leaving the machine running after finding a regulator's output at its input voltage.[7]
  • Fitting a switching module whose current, input range or output capacitance limit does not suit the machine, or fitting one in place of a negative regulator.[10][11]

Worked examples on this wiki

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 Samuel M. Goldwasser, "Testing Semiconductor Devices with a VOM or DMM" (Notes on testing semiconductors), version 2.45, 19 November 2006: silicon diodes 0.5 to 0.8 V and germanium 0.2 to 0.4 V on a DMM's diode test; confirm the meter's lead polarity with a known diode (1N4007, 1N4148, 1N34); the two-diode model; NPN and PNP readings of 0.45 to 0.9 V; collector to emitter open both ways; out-of-circuit only; built-in damper diodes and 50 Ω base resistors; Darlingtons read 1.2 to 1.4 V; LEDs and zeners cannot be fully tested on a meter. Retrieved 25 October 2026.
  2. ↑ Samuel M. Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", version 2.86, 28 January 2014: faulty primary-side rectifier diodes, shorted switchmode transistors and shorted secondary rectifiers among the common faults that blow the fuse. Retrieved 25 October 2026.
  3. ↑ Sinclair ZX Spectrum DC-DC Converter Repair, on this wiki, citing Sinclair's service manual: for a dead machine with smoke at switch-on, the first check is TR4 shorted base to collector.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Fluke Corporation, "How to Test Diodes with a Digital Multimeter": power off and capacitors discharged; red lead on the anode for forward bias; a good silicon diode reads 0.5 to 0.8 V forward, germanium 0.2 to 0.3 V, and OL reversed; an open diode reads OL both ways; a shorted diode reads about 0.4 V both ways; it may be necessary to remove one end of the diode; the resistance mode should not be used in circuit. Retrieved 25 October 2026.
  5. ↑ 5.0 5.1 5.2 5.3 onsemi, MC7800, MC7800A, MC7800AE, NCV7800: 1.0 A Positive Voltage Regulators datasheet, MC7800/D, rev. 29, November 2021: TO-220 pin 1 input, pin 2 ground, pin 3 output, heatsink surface connected to pin 2; dropout 2.0 V typical at 1.0 A. PDF. Retrieved 25 October 2026.
  6. ↑ 6.0 6.1 6.2 6.3 onsemi, MC7900 Series 1.0 A Negative Voltage Regulators datasheet, MC7900/D, rev. 21, October 2021: TO-220 pin 1 ground, pin 2 input, pin 3 output, heatsink surface connected to pin 2. PDF. Retrieved 25 October 2026.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 Texas Instruments, LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators datasheet, SNOSBT0L, February 2000, revised September 2016: TO-220 pins 1 input, 2 ground, 3 output; DC input voltage 35 V maximum; LM340/LM7805 output 4.8 to 5.2 V at TJ = 25 °C, 4.75 to 5.25 V for PD ≤ 15 W, 5 mA ≤ IO ≤ 1 A and 7.5 V ≤ VIN ≤ 20 V; dropout 2 V typical at 1 A; measured with 0.22 µF input and 0.1 µF output capacitors; thermal and short-circuit protection; with the ground pin disconnected the output approaches the unregulated input. PDF. Retrieved 25 October 2026.
  8. ↑ 8.0 8.1 8.2 Texas Instruments, UA78L Series 100-mA Positive Voltage Regulators datasheet, SLVS010X, January 1976, revised June 2023, section 5, pin configuration: TO-92 pin 1 output, pin 2 common, pin 3 input. PDF. Retrieved 25 October 2026.
  9. ↑ Recommended Tools, on this wiki: a bench power supply with current limit.
  10. ↑ 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 Traco Power, TSR 1 Series, 1 A datasheet, 8 October 2026: drop-in replacement for 78xx linear regulators, pin compatible, SIP package fits the TO-220 footprint; full load to +60 °C ambient without a heat sink; built-in filter capacitors and no external capacitors required; TSR 1-2450 input 6.5 to 36 V, output 5 V, 1,000 mA, 94 % efficiency at minimum input; external 22 µF 50 V input capacitor above 32 V; ripple and noise 50 mVp-p typical for 12 V-input models; capacitive load 470 µF maximum; continuous short-circuit protection; pinout 1 +Vin, 2 GND, 3 +Vout, pin 1 marked with a dot. PDF. Retrieved 25 October 2026.
  11. ↑ 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 11.10 RECOM, R-78E-1.0 Switching Regulator datasheet, rev. 9/2024: pin-out compatible with LM78xx linears; R-78E5.0-1.0 input 8 to 28 V, output 5.0 V, 1.0 A, 220 µF maximum capacitive load; output ripple and noise 120 mVp-p; optional 10 µF MLCC on input and output; cannot be used as a positive-to-negative converter; use a soft-start power supply to protect the converter during power-up; pins 1 +Vin, 2 GND, 3 +Vout. PDF. Retrieved 25 October 2026.
  12. ↑ SparkFun Electronics, "How to Use a Multimeter", section "Continuity" (CC BY-SA 4.0): a regulator running very hot indicates the system is pulling too much current. Retrieved 25 October 2026.
  13. ↑ 13.0 13.1 13.2 13.3 Celso Martinho, "Restoring a ZX Spectrum+ Toastrack", 28 June 2025: the 7805's heat comes from the difference between input and output voltage; options of a 78S05, a Traco TSR 2-2450 (5 V 2 A) or a DC-DC buck converter; a buck converter fitted in place of the 7805 let noise from the supply show on the video output; a 100 µF electrolytic and 100 nF ceramic across its output suggested. Retrieved 25 October 2026.
  14. ↑ 14.0 14.1 Retroleum, "Spectrum repairs and mods" (retailer's page): Traco and Murata/OKI switch-mode replacements for the Spectrum's 7805; the vertical-mount version cannot be used in a rubber-key Spectrum with an Issue 2 board. Retrieved 25 October 2026.
  15. ↑ Super Nintendo Entertainment System Power Circuit Repair, on this wiki, citing Adam's Vintage Computer Restorations (9 December 2024): a PAL SNES whose 7805 was replaced with a TSR-1-2450.
  16. ↑ 16.0 16.1 16.2 Battery Explosion, Capacitor or Corrosion Damage, on this wiki, sections "Assessing the damage" (photograph both sides of the board so that component orientation is on record) and "Checking the repair" (power from a current-limited bench supply first and measure the rails against the machine's own figures).
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