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Quickie Teardown – ShopGuard Anti-Theft Tag

ShopGuard Anti-Theft Tag
ShopGuard Anti-Theft Tag

Everyone at some stage must have seen these EAS security tags in shops, usually attached to clothing with a steel pin. As some of this year’s presents had been left with the tags attached, I had to forcibly remove them before wrapping could commence.

Reverse Side
Reverse Side

These are just a plastic disc about 50mm in diameter, with an internal locking mechanism & RF tag inside.

RF Coil
RF Coil

After some careful attack with a saw around the glue seam, the tag comes apart into it’s halves. The RF coil & it’s ceramic capacitor can be seen wrapped around the outside of the tag. The capacitor in this case isn’t even epoxy dipped to save that extra 0.0001p on the manufacturing price. In the top centre is the pin locking mechanism, enclosed in a small plastic pill.

Lock Pill
Lock Pill

Popping off the back cap of the lock shows it’s internals.

Ball Bearing Lock Assembly
Ball Bearing Lock Assembly

The lock itself is very simple. The centre section, held in place by a spring, carries 3 small ball bearings. The outer metal frame of the lock is conical in shape.

When the pin is pushed into the tag, the conical shape of the lock chamber causes the ball bearings to grab onto it, helped by the action of the spring that pushes the ball bearing carrier further into the cone.
This also means that any attempt to force the mechanism causes it to lock tighter onto the pin.
In normal operation, removal is achieved by a strong magnet that pulls the ball bearing carrier back slightly against it’s spring, allowing the pin to disengage & be pulled out.

This design is incredibly simple & cheap to make, and gains it’s locking strength from friction alone.

I would consider the RF coil being around the outer edge of the device a bit of a security risk – a quick chop with a sharp pair of wire cutters would disable the tag’s alarm functionality instantly. Making the coil slightly smaller & keeping it out of reach of the edge of the tag would help in this regard.

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12v CFL Lamp Failure Analysis

On the boat I have installed custom LED lighting almost everywhere, but we still use CFL bulbs in a standing lamp since they have a wide light angle, and brightness for the size.

I bought a couple of 12v CFLs from China, and the first of these has been running for over a year pretty much constantly without issue. However, recently it stopped working altogether.

12v CFL
12v CFL

Here’s the lamp, exactly the same as the 240v mains versions, except for the design of the electronic ballast in the base. As can be seen here, the heat from the ballast has degraded the plastic of the base & it’s cracked. The tube itself is still perfectly fine, there are no dark spots around the ends caused by the electrodes sputtering over time.

Ballast
Ballast

Here’s the ballast inside the bottom of the lamp, a simple 2-transistor oscillator & transformer. The board has obviously got a bit warm, it’s very discoloured!

Failed Wiring
Failed Wiring

The failure mode in this case was cooked wiring to the screw base. The insulation is completely crispy!

Direct Supply
Direct Supply

On connection direct to a 12v supply, the lamp pops into life again! Current draw at 13.8v is 1.5A, giving a power consumption of 20.7W. Most of this energy is obviously being dissipated as heat in the ballast & the tube itself.

Ballast PCB
Ballast PCB

Here’s the ballast PCB removed from the case. It’s been getting very warm indeed, and the series capacitor on the left has actually cracked! It’s supposed to be 2.2nF, but it reads a bit high at 3nF. It’s a good thing there are no electrolytics in this unit, as they would have exploded long ago. There’s a choke on the DC input, probably to stop RFI, but it doesn’t have much effect.

Supply Waveform
Supply Waveform

Here’s the waveform coming from the supply, a pretty crusty sinewave at 71.4kHz. The voltage at the tube is much higher than I expected while running, at 428v.

RFI
RFI

Holding the scope probe a good 12″ away from the running bulb produces this trace, which is being emitted as RFI. There’s virtually no filtering or shielding in this bulb so this is inevitable.

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Chinese 12v 10A Power Brick Analysis

I recently ordered a PSU to run one of the TVs I converted to 12v operation, and being an older TV, it’s a fairly heavy load at 6.5A. eBay to the rescue again, with a cheap 10A rated supply.

Power Brick
Power Brick

Like all similar supplies these days, it’s a SMPS unit, and feels suspiciously light for it’s power rating.

Cover Removed
Cover Removed

Luckily this one is easy to get into, no ultrasonic welding on the case, just clips. Here’s the top cover removed, big alloy plate between the heatsinks.

PCB
PCB

The top heatsink plate was glued to the top of the transformer with silicone, some gentle prying released it. From the top, things don’t look too bad. There’s some filtering on the mains input & it’s even fused!

Primary Side
Primary Side

Here’s a closeup of the primary side of the PSU, the main DC bus capacitor is a Nichicon one, but it’s clearly been recovered from another device, look at the different glue on the end!
it’s also flapping about in the breeze, the squirt of silicone they’ve put on does nothing to stop movement.
Also here is the mains input fuse, filter capacitor & common mode choke. At least there is some filtering!

The main control IC is a UC3843B High Performance Current Mode PWM Controller, operating at a switching frequency of 250kHz.
The main switching transistor is visible at the bottom left corner, attached to the heatsink.

Secondary Side
Secondary Side

Here’s the secondary side of the supply. The transformer itself is OK, nice heavy windings on the output to suit the high current.
It’s using proper opto-isolated feedback for voltage regulation, with a TL431 reference IC.
The output diodes are attached to the heatsink at the top of the photo, I couldn’t read any numbers on those parts.

The output filter capacitors are low quality, only time will tell if they survive. I’ll put the supply under full load & see what the temperature rise is inside the casing.

PCB Bottom
PCB Bottom

On the bottom of the PCB things get a little more dire. There isn’t really much of an isolation gap between the primary & secondary sides, and there’s a track joining the output negative with mains earth, which gets to within 2mm of the live mains input!

As with all these cheapo supplies, there’s good points & bad points, I will update when I’ve had a chance to put the supply under full load for a while & see if it explodes!