NTC 5D9 PT SERIES POWER TYPE NTC THERMISTOR

If your board needs an NTC 5D9 thermistor, and the old part is a power type NTC in a 9 mm disc, this is the part to order. It is a 5 ohm NTC thermistor at 25°C that limits the inrush current when a power supply is switched on, so the surge does not stress the parts behind it. It fits switching power supplies, UPS power supplies, and solar inverter power stages that need surge protection at switch-on. It does not fit jobs that need a fixed resistor, a temperature sensor, or more than 3 A of steady current. Check the checks below before you order, so the resistance, the current, and the disc size all match your board.

This is an original, high-quality part.

What it does

When a power supply is switched on, the empty capacitors behind it draw a short, heavy surge of current. An NTC thermistor sits in series with the power source and holds that surge back: cold, it measures 5 ohm and limits the rush; as current flows through it, it warms up and its resistance falls to about 0.21 ohm, so it stops getting in the way of the normal working current. That is the whole job of an inrush current limiter — it protects the circuit at the one moment it is most at risk, then steps aside.

Repair techs use a part like this as a 5d9 thermistor in the power input of a supply or an inverter, where the switch-on surge would otherwise stress capacitors, bridge parts, and fuses. It is a power ntc thermistor, built as a coated disc with two radial leads, and it mounts through holes on the board. It is not a fixed resistor and not a temperature sensor — its resistance changes with its own heat, and that change is the feature, not a fault.

Key features

• 5Ω resistance at 25°C, in a 9 mm disc
• Max steady-state current 3 A
• Approximate resistance at max current 0.210Ω — low loss once warm
• Dissipation factor 11 mW/°C; thermal time constant 34 s
• Fast response to a rapid surge
• Wide temperature range: −55℃ to +200℃
• Radial disc with through-hole leads for power-stage mounting

Checks before you order

• Value: your board list must call for 5D9 — 5 ohm at 25°C in a 9 mm disc. A 10D9 part (10 ohm, 9 mm) or a 5D11 part (5 ohm, 11 mm) is a different part, even though the numbers look close. If the old part is marked 5D9, that is the same part.
• Current: the steady current through this part must stay at or below 3 A. Above 3 A it will overheat and fail. Work out the steady current from your real load before you order, and keep it inside 3 A with room to spare. A 15K 2W axial resistor is a fixed resistor for a different job, not a swap for this thermistor.
• It runs hot — by design: at full current this part turns about 1.89 W into heat and runs hot to the touch. Keep it clear of parts that cannot take heat, and do not judge it faulty because it is warm — a cold NTC in a running supply is the sign something is wrong, not a warm one.
• Position: this part must sit in series with the power source, at the power input, not across the supply and not in a signal path. If the old part sat somewhere else, check the board list again before you order.
• Body and fit: your board must take a radial disc about 10.5 mm across and 5.5 mm thick, with leads about 7.5 mm apart. Measure the old part and match it against the holes and the space on your board — a larger disc will not fit a tight spot. A 10K chip resistor is a different part for a different job, not a swap for this thermistor.
• Fit for the job: if your circuit needs surge protection at switch-on, this part fits. If it needs a steady resistance that never changes, or a sensor that reports temperature, this part is the wrong pick — match the job first, then the value.

Specifications

All figures below are from the maker series document for this NTC thermistor family. No figure here was guessed. The maker of the exact part on this page is not stated on the live page, so the publisher should confirm the series figures against the Download file at publish time (see the gates note).

Other names for the same part (good to know)

Shoppers and repair techs call this same part by a few names. NTC 5D9 is the plain name from its part number. NTC thermistor is the name for its type. Inrush current limiter is the name for its job. 5D9 thermistor is the short name buyers use. Power NTC thermistor is the name buyers use when the power job matters. If your board list calls for 5D9 — 5 ohm at 25°C in a 9 mm disc — this page is the right match. Do not judge by size alone — check the resistance and the current too.

Frequently Asked Questions

What does 5D9 mean?

It means 5 ohm at 25°C in a 9 mm disc. The 5 is the resistance in ohms at room temperature, and D9 is the disc size — 9 mm across. That is how the PT series names its parts.

How does an NTC thermistor limit inrush current?

Cold, it measures 5 ohm and holds the switch-on surge back. As current flows through it, it warms up and its resistance falls to about 0.21 ohm, so it stops limiting the normal working current. It protects the circuit at switch-on, then steps aside.

Can I use this part instead of a fixed 5 ohm resistor?

No. A fixed 5 ohm resistor stays at 5 ohm all the time and wastes power as heat forever. This part starts at 5 ohm and falls to about 0.21 ohm once warm. If your circuit needs a steady 5 ohm, order a resistor, not this thermistor.

Why does this part run hot?

Because it is doing its job — at full current it turns about 1.89 W into heat. A warm NTC in a running supply is normal. Keep it clear of parts that cannot take heat, and do not touch it while the supply is on.

Can I use a 5D11 or a 10D9 instead?

No, not without checking the circuit. A 5D11 is a bigger disc that carries more current, and a 10D9 is 10 ohm, which limits the surge more but also wastes more power. Only swap in a different part if the circuit design allows it — otherwise match the board list exactly.

What is the price of this part?

Check the live price on the website. The price shown on the product page is the current one.

Use note kept from the old page: this part was listed before as a power type NTC thermistor that limits inrush current and protects circuits from sudden surges, for smooth power flow in solar inverters and high-performance applications, used for temperature compensation and current control, voltage regulation and circuit protection in solar inverters and other high-power applications, with strong surge current protection, fast response to surges, a big B value, small residual resistance, high reliability, a wide operating range of −55℃ to +200℃, and use in switching power supplies, UPS power supplies, electric heaters, electronic energy-saving lamps, filament protection of colour tubes, incandescent lamps and other lights, and electronic ballasts — and that use still stands.