MMSZ5248 (J3)18V Zener Diode Sod-123

If your board calls for an 18 V Zener diode in a small SOD-123 pack, and the part on the board is marked J3, this is the part to order. It holds the line near 18 V and guards the parts after it from voltage spikes. Check the checks below before you order, so the pack, the pins, and the power all match your board. This is an original, high-quality part.

What it does

A Zener diode works in a simple way. In normal use it blocks current one way. When the voltage across it climbs past its set point, it starts to pass current and holds the voltage near that point. For this part, that point is 18 V. That is why repair techs use it to steady a supply line and to clamp spikes before they reach chips that cost far more than this diode.

This is a small surface-mount part. You solder it flat on the board. It has two ends. One end has a band on it. That band marks the cathode end. Put it the wrong way round and the circuit will not work as planned, so match the band to your board print.

Who it fits

This part fits you if you repair or build power boards, solar inverter boards, UPS boards, or small power supply boards, and your board print shows a SOD-123 Zener diode at 18 V. It also fits if you are replacing a burnt or shorted J3-marked diode and your old part sat in the same SOD-123 spot.

It does not fit every job. If your board needs a different voltage, a bigger pack, or a part that can handle much more heat, this is not the right pick. Use the spec table below and match it line by line.

Checks before you order

• Voltage: your board must need 18 V. A 12 V or 24 V part will not do the same job.
• Package: your board footprint must be SOD-123. A SOT-23 part has a different shape and will not sit right on this footprint.
• Pinout and polarity: match the cathode band to the band mark on your board. The band end is the cathode.
• Current and power: work out the current your circuit will push through the diode, then check that 18 V times that current stays well inside the 500 mW limit in the table below. If your circuit runs hot, pick a part rated for more power, not this one.
• Marking: the B-suffix part is marked J3 on the body. If your old part shows a different mark, stop and check your board list again.
• Tolerance: the B suffix means ±5%. That gives 17.10 V to 18.90 V. If your circuit needs a tighter range, ask about the C-suffix variant before you order.

Specifications

All figures below are from the MMSZ52xx series datasheet, B-suffix row for this part. No figure here was guessed.

You can view and download the datasheet for this part from the Datasheet button on this page.

If this part is not available, use one of these instead

These are not exact matches. Read the warning under each one before you swap. For current availability, open the linked SunTech page for that part.

Substitutes you can buy at SunTech Mall

• BZT52C18 WL 18V SMD Zener Diode SOD-123 — practical substitute, not exact. It is also 18 V, also SOD-123, and also ±5% tolerance, so it fits the same footprint and the same board pads. But its Zener impedance is 45 Ω, not 21 Ω, and its test points differ. In a plain clamp job it can work. In a tight regulation job, where the voltage must hold steady while the current changes, the higher impedance means the voltage moves more — test it first.
• BZX84C18-V Y6 18V Zener Diode SOT-23 — practical substitute only if your board fits SOT-23. It is also 18 V, but it comes in SOT-23, not SOD-123, its power limit is lower at 250 mW (half of this part’s 500 mW), and its impedance is 45 Ω. Its own SunTech page calls it a low-power part. Do not buy it for a SOD-123 footprint unless you have a plan to fit it right.

Warning: never swap on voltage alone. Check pinout, voltage, current, package, and polarity first. If any one of those does not match, do not swap.

Other names for the same part (good to know)

Shoppers and repair techs call this same part by a few names. If you see J3 printed on a tiny SOD-123 diode, that mark points to this part. MMSZ5248B is the same part with the B suffix, which means ±5% tolerance. Do not mix this up with a part in a different pack even if the number looks close — a different pack will not fit your board the same way.

Frequently Asked Questions

What does J3 mean on this diode?

J3 is the small mark printed on the body of the MMSZ5248B diode. When you see J3 on a SOD-123 Zener diode, it tells you the part is the 18 V MMSZ5248B.

Is MMSZ5248B the same as MMSZ5248?

Yes, for this listing. The B at the end means the voltage tolerance is ±5%. So MMSZ5248B is the MMSZ5248 with a ±5% range of 17.10 V to 18.90 V.

Can I use BZT52C18 instead of this part?

It can work as a practical swap, but it is not exact. It shares the 18 V value and the SOD-123 pack, but its impedance is 45 Ω against 21 Ω here. Check your circuit first, and test it in a tight regulation job.

Can I use BZX84C18 instead of this part?

Only if your board fits SOT-23. BZX84C18 is also 18 V, but it comes in SOT-23, its power limit is lower, and its impedance is 45 Ω. It will not drop into a SOD-123 footprint.

What should I check before I order?

Check five things: voltage (18 V), package (SOD-123), pinout and polarity (match the cathode band), current and power for your circuit, and the J3 mark on the part.

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.