112-5779 TO-220 transistor pinout and replacement alternatives

User question

112-5779 pin out and alternative this transistor pls help

Artificial Intelligence Response

Direct answer to the question

I cannot confirm a true pinout or a safe exact substitute for 112-5779 from a reliable public manufacturer datasheet. The only public information I could verify is that distributor listings identify 112-5779 as an ON Semiconductor transistor in a TO-220 package, but they do not publish the actual device type or pin mapping. (veswin.com)

So, the short answer is:

• Do not assume it is NPN, PNP, Darlington, or MOSFET from the part number alone.
• Do not install a substitute yet unless you first identify whether it is a BJT or a MOSFET.
• The safest path is to test the removed part with a multimeter and trace the PCB pads.

If you want the most honest engineering answer: the part number alone is not enough for a guaranteed replacement. (veswin.com)

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Detailed problem analysis

The sample answers conflict badly: one says NPN BJT, another says N-channel MOSFET, another says PNP Darlington. That level of disagreement is a strong sign that public data for 112-5779 is not trustworthy or not publicly documented. The only consistent facts I could verify are ON Semiconductor and TO-220 package. (veswin.com)

That means the correct engineering workflow is:

1. First identify the semiconductor class

You need to determine whether the device is:

• BJT: Base / Collector / Emitter
• MOSFET: Gate / Drain / Source

2. Use diode-test mode on the removed part

With the part fully removed from the board:

If it is a BJT:

• One pin will behave like a base, showing a diode junction to the other two pins.
• For NPN, the base typically conducts to collector and emitter with the red lead on base.
• For PNP, the base typically conducts with the black lead on base.

If it is a MOSFET:

• One pin will be the gate, which should look mostly insulated from the other two.
• The other two pins will show the body diode between drain and source.
• For an N-channel MOSFET, you will usually find the intrinsic diode between source and drain.
• For a P-channel MOSFET, the polarity is opposite.

3. Check the tab / center pin

For many power devices in TO-220-family packages, the center pin and metal tab are the same node.

Official examples:

• onsemi TIP35C BJT: 1 = Base, 2 = Collector, 3 = Emitter. (onsemi.com)
• Infineon IRLZ44N MOSFET: 1 = Gate, 2 = Drain, 3 = Source, tab = Drain. (infineon.com)

So if you ohm out the original device and find the tab tied to the center pin, that still does not tell you whether the part is a BJT or MOSFET; it only helps narrow the pin mapping after the device class is known. A power BJT often has collector on tab, while a MOSFET often has drain on tab. (onsemi.com)

4. Trace the PCB pads

This is often more reliable than internet guesses.

Look where each pad goes:

• If one pad goes through a small resistor from a logic IC and another pad goes to a load/heatsink rail, it may be a MOSFET gate drive arrangement.
• If one pad goes through a resistor network into a driver transistor stage and the load path is through the center/tab, it may be a power BJT.
• If the part drives a solenoid, injector, relay, or valve, also inspect the nearby flyback diode / TVS / snubber.

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Current information and trends

What I could verify today is limited:

• 112-5779 is publicly listed as an ON Semiconductor transistor. (asahi-eng.co.jp)
• Public listings show TO-220 package. (veswin.com)
• onsemi’s own cross-reference guidance warns that even “functionally comparable” parts must still be checked against the full datasheet before substitution. (onsemi.com)

Current repair practice for obsolete power semiconductors is to replace them with a modern part only after matching:

• device class
• polarity
• pinout
• voltage rating
• current rating
• power/thermal limits
• switching behavior
• package/tab connection

That is standard best practice, especially in automotive or heavy-equipment electronics where inductive loads and transients are severe. (onsemi.com)

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Supporting explanations and details

What substitute can you use?

Only after identification.

If your test shows…	Possible candidate	Why	Important warning
NPN power BJT	TIP35C	Officially a high-power NPN transistor; onsemi datasheet gives 1=B, 2=C, 3=E and up to 100 V class for TIP35C. (onsemi.com)	Modern TIP35C is commonly TO-247 / TO-218 family, not necessarily a physical drop-in for every TO-220 layout.
Logic-level N-channel MOSFET	IRLZ44N	Officially 55 V, 47 A, TO-220, with 1=G, 2=D, 3=S. (infineon.com)	Only valid if the original is really an N-channel MOSFET and the voltage margin is sufficient.
Stronger logic-level N-channel MOSFET	IRL3705N	Officially 55 V, 89 A, TO-220. (infineon.com)	Again, only if the original is confirmed MOSFET and 55 V is enough for the circuit transients.

Very important correction

Do not replace:

• a BJT with a MOSFET
• a MOSFET with a BJT
• an NPN with PNP
• an N-channel with P-channel

unless you have confirmed the circuit is redesigned for it.

That is why some of the sample answers are unsafe: they give exact substitutes while disagreeing on the basic device type.

Practical quick-test summary

If you have the part on the bench:

• Two diode junctions from one pin to the other two → probably BJT
• One insulated pin + one body diode between the other two → probably MOSFET
• Tab tied to center pin → normal for many power TO-220 devices, but not enough by itself to identify type. (onsemi.com)

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Ethical and legal aspects

For heavy-equipment or automotive control boards:

• A wrong substitute can damage the board, wiring harness, or actuators.
• If this transistor is part of a fuel, injector, solenoid, motor, or safety-related output stage, an incorrect replacement can create a hazardous machine state.
• Use an isolated current-limited bench supply for first power-up.
• Never test such a repair directly on the machine at full battery current without verifying surrounding components.

If this is from a commercial machine under warranty or regulated repair conditions, also consider OEM service documentation and liability.

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Practical guidelines

Best repair method

1. Remove the old part.
2. Identify device type with diode test.
3. Check if tab = center lead.
4. Trace the PCB pads:
   • ground?
   • supply rail?
   • load output?
   • resistor from logic stage?
5. Check nearby components:
   • gate/base resistor
   • flyback diode
   • TVS
   • driver transistor/IC
6. Choose replacement with:
   • same device class
   • same polarity
   • same pinout
   • same or higher voltage/current rating
   • suitable thermal rating
   • same mounting/isolation arrangement

First power-up

• Use bench supply with current limit
• Start at lower current
• If possible, use a dummy load instead of the real load
• Monitor:
  • supply current
  • transistor temperature
  • gate/base drive waveform
  • voltage spikes at turn-off

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Possible disclaimers or additional notes

My confidence is low on any exact part number substitute for 112-5779 because I could not verify a real manufacturer datasheet for that exact code. Public sources only gave package/manufacturer listing, not the electrical identity. (veswin.com)

So the safe statement is:

• Pinout is not confirmed
• Alternative is not confirmed
• Testing the original part is required

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Suggestions for further research

To narrow it down, send:

• a clear photo of the front marking
• a photo of the PCB around the part
• the board name / machine model
• a photo showing where each pad goes
• whether it is attached to a heatsink
• what the transistor is driving: relay, solenoid, injector, motor, lamp, etc.

With that, I can usually tell you whether it is BJT or MOSFET and suggest a much safer replacement.

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Brief summary

• 112-5779 appears publicly only as an ON Semiconductor TO-220 transistor, but I could not verify a trustworthy exact datasheet or pinout. (veswin.com)
• Because online claims conflict, do not trust an exact replacement yet.
• First determine whether it is BJT or MOSFET.
• If it proves to be:
  • NPN BJT: a part like TIP35C may be a candidate. (onsemi.com)
  • logic-level N-channel MOSFET: IRLZ44N or IRL3705N may be candidates. (infineon.com)
• Do not substitute across device families.

I searched for official manufacturer information and found only distributor listings for 112-5779, plus official datasheets for possible replacement families. That was enough to confirm ON + TO-220, but not enough to confirm the exact pinout or exact replacement. If you upload a photo, I can help identify it more precisely.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.