How to Properly Test FTD01N & FTD02P Transistor Pairs in Epson Printers (Step-by-Step Guide)

Question:

How do I test the FTD01N and FTD02P transistor pair used in Epson printers?

A customer asked how to correctly test the FTD01N and FTD02P transistor pair commonly found in Epson printer boards. These components often fail when the carriage motor or paper feed motor overloads, or when a short occurs on the main board. The question was essentially how to verify whether these transistors are still functioning and how to test them using standard tools.

Answer: 

Testing the FTD01N (N-channel IGBT) and FTD02P (P-channel IGBT) transistor pair is a crucial step when diagnosing Epson printer failures-particularly when symptoms include no carriage movement, no paper feed, sudden shutdowns, or blown fuses. These transistors handle the high-current motor drive stages, so they are among the first components to fail during an electrical event.

Below is a detailed breakdown combining your proposed answer with additional technical insights.

What You Need

• A digital multimeter capable of diode-mode testing

• A static-safe work area

• Transistors removed from the circuit board (strongly recommended-on-board testing gives unreliable results)

1. Identify the Pins

Each transistor (FTD01N and FTD02P) has three pins:

• Collector (C)

• Emitter (E)

• Gate (G)

Pin orientation differs slightly depending on the brand or manufacturing batch. If in doubt, reference the markings or request a pinout diagram-I can generate one if helpful.

2. Perform a Basic Diode Test (Collector → Emitter)

This step checks the most common failure: a direct short.

1. Set the meter to diode mode.

2. Red probe → Collector

3. Black probe → Emitter

Normal Readings:

• Some IGBTs show open circuit (OL).

• Some show a diode drop (0.3-0.7 V).
  Both behaviors can be normal depending on internal structure.

Fail Readings:

• 0.000 V → shorted

• Very low resistance → shorted

• OL in both directions AND no response from gate-charging test → dead / non-functional

3. Reverse the Probes (Emitter → Collector)

1. Black probe → Collector

2. Red probe → Emitter

Normal:

• Opposite behavior from the earlier test

• Typically open in this direction

Fail:

• Same reading in both directions

• Conduction in both directions → definite short

Any IGBT showing symmetrical conduction is defective.

4. Gate Charging Test (Critical Test of Transistor Function)

This determines whether the device actually switches-its primary job.

1. Keep the meter in diode mode.

2. Touch red probe to Gate, black probe to Emitter for 1 second.

   • This "charges" the gate.

3. Move red probe to Collector, black to Emitter.

Normal:

• You should now see a diode drop or partial conduction.

• This shows the device is responding to the gate charge.

Fail:

• No change compared to before charging

• Still shows OL

• OR suddenly shows a short after gate charge → internal damage

This is the most reliable way to determine if the device can still turn on.

5. Discharge the Gate

To reset the transistor:

• Touch all three pins together with your finger or a metal tool

• Retest to confirm repeatable switching behavior

A healthy transistor should turn on and off consistently each time the gate is charged and discharged.

Signs the Transistor Is Bad

Replace the FTD01N and FTD02P pair if you find any of the following:

• Direct short between Collector and Emitter

• The same readings in both directions

• Gate-charging test does not activate conduction

• Conduction occurs even without gate charging (internal short)

• Physical damage (burn marks, cracks, warping, discoloration)

• Printer symptoms:

  • No carriage movement

  • No paper feed

  • Printer shuts off immediately

  • Fuses blow repeatedly

Important: Because these two transistors work together as a complementary pair, always replace both-even if only one appears bad.

A quick note about hands-on repair

Addressing printer issues can be complicated due to the hands-on nature of the repairs. Therefore, we're not able to provide remote diagnostics or detailed one-on-one troubleshooting. If you need professional assistance, we offer an in-person diagnostic and repair service at our local facility, which you can learn more about through our printer repair service page (https://bchtechnologies.com/printer-repair-service).
We operate on a first-come, first-served basis, and depending on demand, it may take a few weeks before we can begin work after drop-off. We can repair entire printers or specific components based on your preference, though we recognize our service might not be the most budget-friendly.

For DIY support, we strongly encourage searching online and exploring our YouTube channel's homepage (https://youtube.com/@bchtechnologies). Use the search icon next to the "About" tab to quickly locate specific repair topics. With nine years of content and hundreds of videos, searching is often faster than waiting for a reply, and YouTube may also recommend helpful videos from other creators.

Thank you again for reaching out and for supporting our work. Your engagement helps us continue developing high-quality technical resources for the printer repair community. If you need anything else, feel free to contact us anytime.