Inside the surge protector strip and USB power supply test - SFS133 Defender Surge Protector

p.kaczmarek2 09 Apr 2026 09:53 1 1095 Cool? (+4)

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The SFS 133 surge protection power strip (product 99457) is available to buy for around PLN 80. It offers 4 230 V sockets, three USB sockets with a claimed current capacity of up to 2.1 A and a main switch-off button. Here I'll show its interior and check that it matches its description from the seller.

Initial discernment has not revealed any major nonsense to me. The only thing I have doubts about is that current from the USB ports - in the image below I show an excerpt from the product description. It says "Maximum USB port voltage: 2.1 A". These 'voltage', or 'voltage', could happen to anyone, so I'll turn a blind eye, what puzzles me more is why there is a singular number there? That suggests 2.1 A per port, a good 6 A in total. I'll check it out.

Here's the strip in all its glory. The plastic is sturdier than I thought, and there's an LED slot next to the button too - could it be a state of protection?

Main button:

Markings - the fuse is referred to here:

The centre is guarded by screws requiring a triangular head screwdriver:

Fortunately I have one in stock. A first glance reveals that the thickness of the wires is not as bad as in the cheapest products.

We also have a board with a varistor and with a protection indicator - the varistor is in a thermal fuse tee, its overheating breaks the circuit and extinguishes the LED.

The circuit is simple enough to draw a schematic from the pictures of the paths.

The power supply section remains.

PCB designation: HL-12W-AGK3A REV:01 2021-8-11 These 12 watts already suggest that it will be 2.1 A for all three ports though.

The power supply is as usual in flyback topology and is based on the ATC9307, a controller with an integrated keying transistor. The feedback is taken from the primary side, there is no optocoupler.

A CM1601 is used on the secondary side, this circuit looks to me like a synchronous rectifier (based on a MOSFET transistor). There is no Schottky diode there.

The USB port section uses dedicated circuitry to 'report' the power supply to the phone, rather than using the cheapest solution with resistors or shorted pins. This will make the phones "more willing" to draw current. This has already been discussed in my other topic:
Two-port USB charger that accelerated the charging of my iPhone

It remains to perform the test with an artificial load:

It is possible to draw up to around 2.5 A.

In summary , protection is present inside, but it is more poor than in slightly better strips. The varistor is one and the protection circuit is not connected to earth at all, in Brennenstuhl 13.500A Super Solid it was better. Wire thickness is better than in the cheapest strips, and the ground is connected, so here I won't criticise so much, but the USB power supply is excessively weak, even useless in many cases. I'm not complaining about the lack of support for higher voltages (QC standard), as the description didn't promise that, but I'm puzzled by the fuse that's supposed to be inside. I assume it is integrated into the switch. I would potentially check it, but the strip is already mounted in its intended location. The switch itself cuts off one wire, not two, as in the better strips.
Anyway - depends on who needs what, as a modern phone these USB ports won't charge quickly.

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CosteC 10 Apr 2026 06:52

I have my doubts that it works like this. The long piece in a separate heat shrink cage looks like a fuse. But a fuse only for the varistor. I'm under the impression that the LED will go off when the... [Read more]

FAQ

TL;DR: At about 2.5 A total, the three USB ports share one modest 12 W supply, and the teardown concludes "protection is present inside" but basic. This FAQ is for buyers who want to know whether the SFS133 is good enough for everyday mains protection and slow USB charging, or too weak for modern phones. [#21880314]

Why it matters: A strip can look premium at roughly £80, yet still use a shared low-power USB section and a simpler surge design than stronger alternatives.

Feature	SFS133 Defender	Brennenstuhl 13.500A Super Solid
AC sockets	4 × 230 V	Not specified in this thread
USB ports	3 ports, shared output	Not discussed in this thread
Surge design	One varistor, not connected to earth	Described as better internally
Overall thread verdict	Acceptable wiring, weak USB	Better surge protection design

Key insight: The SFS133 works as a basic surge strip with USB, but its USB section is the main compromise. The internal 12 W supply makes the three ports suitable for light charging, not fast charging for modern phones.

Quick Facts

Approximate street price is £80, and the strip offers 4 × 230 V sockets, 3 USB ports, and a main power switch. [#21880314]
The USB board is marked HL-12W-AGK3A REV:01 2021-8-11, which strongly indicates a shared 12 W budget across all three USB outputs. [#21880314]
The teardown measured up to about 2.5 A from the USB section under artificial load, higher than the listed 2.1 A but still far below 2.1 A per port. [#21880314]
Surge protection uses one varistor with a thermal fuse and a status LED; if the varistor overheats and the thermal fuse opens, the LED goes out. [#21880314]
Internal wiring looked thicker than in the cheapest strips, and the protective earth for the sockets is connected, but the surge circuit itself is not connected to earth. [#21880314]

How much USB current can the SFS133 Defender Surge Protector actually deliver across its three ports in real use?

It can deliver about 2.5 A total across all three USB ports in the author's load test. That result fits the 12 W USB board marking far better than a per-port 2.1 A claim. In practice, the ports share one modest power budget, so charging speed drops when multiple devices draw current together. [#21880314]

Why does the SFS133 product description say "Maximum USB port voltage: 2.1 A," and does that rating apply per port or to all USB ports combined?

The wording is a labeling error, because 2.1 A is current, not voltage. The teardown shows that 2.1 A matches the whole USB section much better than each port individually, since the board is marked 12 W and tested near 2.5 A total. So the rating should be read as a combined figure, not 2.1 A per port. [#21880314]

What surge protection components are inside the SFS 133 power strip, and how effective is its protection design?

It uses one varistor, a thermal fuse, and a protection-status LED on a small board. The design provides real protection, but it is basic because the surge circuit is not connected to earth at all. The reviewer explicitly judged it weaker than the internal design in the Brennenstuhl 13.500A Super Solid strip. [#21880314]

How does the protection LED in the SFS133 indicate the condition of the varistor and thermal fuse?

The LED shows that the varistor protection path is still intact. If the varistor overheats, the thermal fuse opens the circuit and the LED goes out. That means a dark LED is not just cosmetic; it indicates the strip has lost its active surge-protection path. [#21880314]

What is a varistor, and what role does it play inside a surge protector strip like the SFS133 Defender?

"Varistor" is a surge-protection component that clamps overvoltage spikes, changing its resistance when voltage rises sharply. Inside the SFS133, a single varistor is the core protective part, and it works with a thermal fuse and LED to absorb surges until overheating disconnects it. [#21880314]

What is flyback topology, and why is it commonly used in compact USB power supplies?

"Flyback topology" is a compact switched-mode converter that stores energy in a transformer and releases it to the output, allowing isolation with few parts. The SFS133 USB supply uses a flyback design because it fits a small board and supports low-cost mains-to-USB conversion in a 12 W product. [#21880314]

How do you open the SFS133 Defender Surge Protector safely, and what kind of triangular screwdriver is needed?

You open it by removing the center screws with a triangular-head screwdriver. 1. Unplug the strip completely. 2. Remove the triangular screws from the center section. 3. Lift the housing carefully and inspect the mains wiring without stressing it. The thread shows that standard opening tools will not fit these security-style screws. [#21880314]

Which controller ICs are used in the SFS133 USB power supply section, and what do the ATC9307 and CM1601 do?

The USB power section uses an ATC9307 on the primary side and a CM1601 on the secondary side. The ATC9307 is the main flyback controller with an integrated switching transistor, while the CM1601 appears to handle synchronous rectification on the secondary instead of using a Schottky diode. [#21880314]

Why does the SFS133 USB charger use dedicated USB identification circuitry instead of simple resistor coding or shorted data pins?

It uses dedicated identification circuitry so phones recognize the charger more willingly and draw current more consistently. The author notes this is better than the cheapest schemes that use only resistors or shorted data pins. That choice improves compatibility, even though the actual power budget remains limited to roughly 12 W total. [#21880314]

How can you test the real output current of a USB power supply in a surge protector strip using an artificial load?

You test it by increasing load current until the USB output reaches its practical limit. 1. Connect an artificial USB load to one port. 2. Raise current gradually while monitoring stability. 3. Note the highest sustained current before voltage regulation worsens or the supply limits. In this case, the tested ceiling was about 2.5 A total. [#21880314]

In SFS133 Defender Surge Protector vs Brennenstuhl 13.500A Super Solid, which one has the better internal surge protection design?

The Brennenstuhl 13.500A Super Solid has the better internal surge protection design in this comparison. The reviewer states the SFS133 has only one varistor and its protection circuit is not connected to earth, while the Brennenstuhl unit was explicitly described as better in those respects. [#21880314]

Why is a surge protector design that is not connected to earth considered weaker protection?

It is weaker because the strip loses an earth-referenced path in its protection network. In the SFS133, the sockets' ground is connected, but the surge circuit itself is not connected to earth at all. That makes the protection simpler and is the main reason the reviewer ranked it below the Brennenstuhl design. [#21880314]

What does primary-side feedback mean in a USB charger, and how is it different from an optocoupler-based design?

"Primary-side feedback" is a regulation method that senses output conditions from the primary side of an isolated converter, avoiding a separate optical feedback link. In the SFS133 charger, feedback comes from the primary side and there is no optocoupler, which reduces parts count compared with an optocoupler-based isolated feedback design. [#21880314]

Why might the built-in USB ports on the SFS133 feel too weak for charging a modern phone quickly even if they still work?

They feel weak because all three USB ports share a supply marked 12 W, not a high-power fast-charge design. The reviewer says the ports are effectively too weak for many modern uses and notes that fast higher-voltage charging standards such as QC are not supported. Phones may still charge, but not quickly. [#21880314]

Where is the fuse likely located in the SFS133 power strip, and how can you identify whether it is integrated into the main switch?

The fuse is most likely integrated into the main switch assembly. The housing markings refer to a fuse, but the reviewer did not find a separate accessible fuse and inferred it is built into the switch. You can identify that by tracing the live path from the input through the switch module and checking whether no standalone fuse holder exists. [#21880314]

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