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Undercounter Fridge – Conversion for 12 Volt Use

Since I do a lot of camping, and several festivals per year (when there isn’t a pandemic on!), I identified the need for a proper fridge that can be powered from my solar setup. Such fridges & coolers already exist, that run from either mains AC, 12/24v DC, and some of them (absorption cycle) will run from bottled gas.

The last option is out, as they’re hideously inefficient, and this would require the carrying around of a flammable gas source. Ready-made units using the vapour-compression method used in all domestic & industrial refrigeration, but they are very expensive. For an upright fridge type unit that could store enough to feed a family of 4, I was looking at over £550+VAT. A cheaper option was definitely required.

Since I already have a couple of spare Danfoss BD35 DC refrigeration compressors, I decided to grab a cheap domestic mini-fridge, and perform a compressor-ectomy to make the unit operable on a low voltage supply.

Russel Hobbs Mini-Fridge

Russel Hobbs Mini-Fridge

Here’s the fridge I obtained from one of the many suppliers of domestic kit, this is a Russel Hobbs branded mini-fridge.

Factory Compressor

Factory Compressor

I was careful to select a unit with no Aluminium pipework – the stuff is damn near impossible to join onto with soldering. Brazing is impossible due to the temperatures involved. These units have copper & steel in their circuit, so this will be easy. Factory charge is 16g of R600a (Isobutane). This one isn’t even going to make it to the point of being plugged in before modification!

BD35 Fitted

BD35 Fitted

I evacuated the factory charge, and removed the original compressor. To avoid having to disturb the capillary tube, I ensured the system was in continual nitrogen purge to keep moisture out – this meant I could retain the factory filter-drier. The condenser in this fridge is skin-type, on both sides of the outer shell, and formed from steel tube. This connection required the use of silver braze to connect to the compressor.
The suction line from the evaporator is copper, so that’s an easy braze onto an extension to the compressor.

New System Charging

New System Charging

Once the new compressor was brazed into place, a full leak & pressure strength test is performed. I’m using isolation valves on the charging hoses here – they’re quite nifty. Backseat them all the way & the charging hose is isolated from the system. Front seat all the way & the hose valve is opened, and the Schrader valve core is depressed in the service port. They really cut losses when charging systems with Schraders!

Vacuum Stage

Vacuum Stage

Next step is applying a vacuum to the system. I aimed for a final vacuum of 250 microns. This by far takes the longest amount of time in a refrigeration job. For reliability & longevity of the system, it’s imperative that all contaminants such as water vapour & air are removed from the circuit.

Refrigerant Bottle

Refrigerant Bottle

The final step is a refrigerant charge. Since I’m not at all fond of flammable refrigerants in this use case (camping), I broke out the bottle of R-134a. This isn’t ideal, as the capillary tube will be sized for the original charge of R600a, but the effect on efficiency shouldn’t be too terrible. (There will be a drop in COP, but I haven’t yet measured the actual COP of the re-engineered system). Unfortunately, as this uses a plate evaporator with a built in capillary tube, there’s no way to resize this for another gas. The capillary tube is fed down the centre of the suction line in these systems, to increase efficiency of the cycle.

Evaporator

Evaporator

A few minutes after an initial charge of 45g R-134a, there’s frost on the plate evaporator! Since this is a gas change as well as the compressor, there’s no other way than to charge slowly, and wait for the system to stabilize at temperature. Then gas is added until there’s an even frost all over the evaporator surface. I would have measured the charge by suction line superheat, but I have no idea of the original system specifications.

Suction Line Icing

Suction Line Icing

In this case, when running the cabinet down to the minimum temperature possible, a slight overcharge became evident. Releasing a small amount of the refrigerant back into the charging bottle sorted this out.

I may yet make another modification to this unit, to remove the skin-condenser from the circuit. While cheap, and difficult to damage as they’re buried behind the outer case metal, they’re not very efficient. I have some small fan-cooled condenser coils that will probably end up in the back next to the compressor to improve efficiency. This will also take some of the heat load off the cabinet insulation, as there won’t be a coil of hot refrigerant next to it.

ATC-1000 Serial to IP Converter Teardown

Front Cover

Front Cover

Just a quick teardown this month! This is an industrial RS-232/RS-485 to Ethernet serial converter.Not much to say about the outside of the unit, there’s the DB-9 connector for the 232 interface, and Phoenix connector blocks for the 422/485 interface. The main power input, and the Ethernet jack are on the other side.

PCB

PCB

Under the hood, there’s a pretty densely populated board. The brains of the operation is a IP210T Serial to Ethernet SoC with A/D Converter. This is an 8051 based core, wuth 10/100 MAC. No flash memory here either, only 64KB of OTP EPROM. On the left there’s a few bus transceivers to interface the serial ports, along with some glue logic. At the top we have the Ethernet magnetics, configuration EEPROM for the SoC & a trio of indicator LEDs.
Power supplies are dealt with via an LM2576-3.3 DC-DC buck converter, providing the main 3.3v rail for the logic.

Siemens Orthphos 3 X-Ray Head Teardown

X-Ray Filter Carousel

X-Ray Filter Carousel

On the front of the head is the filter carousel. In this case it’s only fitted with a 2.5mm Aluminium filter. These are used to filter out the low-energy X-Rays from the beam, reducing the overall dose to the patient of radiation that does not contribute to the resulting image, as these lower energy rays are totally absorbed.

Top Cover Removed

Top Cover Removed

Removing a lot of cap screws later & the O-Ring sealed cover comes off the head. Unsurprisingly, this is full of dielectric mineral oil for insulation & cooling. There’s not much to see yet, as most of the components are hidden inside the plastic housings. The expansion bellows is at the top left, and the main HV transformer bottom left.

Wiring Feedthrough

Wiring Feedthrough

All the wiring to the head is fed through this plastic plug in the side, which is O-Ring sealed into the casting to prevent oil leaking out.

Tube Assembly

Tube Assembly

A clean bottle, syringe & some oily mess later, I manage to get the core out of the housing. This unit had about 3L of oil! The main HV transformer is hanging off the lower left corner of the plastic frame here, with the rest of the PSU behind it. The X-Ray tube is hiding within a metal shield in the tube to the right. There are no primary drive components present in the head – all of that is in the rest of the X-Ray machine.

X-Ray Tube Sleeve

X-Ray Tube Sleeve

A look down filament end of the X-Ray tube. There’s a shield around this, likely to help stop stray X-Rays, and to provide some electric field strength distribution, as this is connected to centre of the HV PSU’s voltage multiplier, biasing it at around +40kV.

Voltage Multiplier

Voltage Multiplier

Behind the transformer lies the rest of the PSU – a 4-stage voltage multiplier. There are 4 separate secondary windings on the transformer, each feeding a voltage doubler. This is to keep the high-voltage stress on the transformer windings to a minimum, as the windings themselves only see 10kV per section, instead of the full output voltage of 80kV. This is increased to 20kV per section by the full-wave doubler, and as these are all in series the output end has the full 80kV.

Ballast Resistor PCB

Ballast Resistor PCB

Over at the anode end of the X-Ray tube, is this board with 6x 6.8kΩ resistors in a series string for 40.8kΩ total. As this is in series with the tube anode, I presume it’s there for current limiting. At full power of 80kV 10mA, this stack will be dropping almost 500v & 4W of power.

X-Ray Aperture

X-Ray Aperture

At the bottom of the unit is the aperture where the X-Rays emerge. The connection to the centre of the voltage multiplier is also visible here.

Filament Powered

Filament Powered

Here is the view through the side of the tube with the filament powered at 4v. Filament current is about 2.6A. The copper heatsink block surrounding the Tungsten target is visible in the centre of the picture. The target is probably alloyed with Rhenium to give better longevity.

X-Ray Tube

X-Ray Tube

The tube is held into the frame by a single bolt in the middle of the anode spider. Removing this bolt allows the tube to come out, and it’s heavy. This is not that surprising, since the anode of the tube is a solid chunk of Tungsten & copper!

Orthophos 3 Head Schematic

Orthophos 3 Head Schematic

Finally, here is the internal schematic of the head itself, with the pinout marked. The operating frequency of the transformer is 35kHz, and it is my understanding that these are operated in resonant mode. There’s no filament transformer, the drive for this is supplied externally. Output voltage feedback is via an auxiliary winding on the transformer.