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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.

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DIY 75Ω Matching Pads

Recently I found the need to do some measurements on 75Ω CATV equipment, only having 50Ω test equipment to use. For this, matching networks exist to convert 50Ω to 75Ω, but they’re fairly simple, so building them was a viable option.

Matching Pad Schematic
Matching Pad Schematic

Above is the very simple schematic to create the 75Ω match. To help keep any parasitics down, this circuit will be built directly onto the back of BNC connectors, that are soldered back-to-back, before being covered in shielding tape.

Resistors Soldered
Resistors Soldered

Here’s the first 50Ω BNC connector, with the resistor network soldered on. I’ve used 4x 360Ω resistors in parallel to create the 90Ω to ground, and a single 43Ω series resistor on the centre pin.

End View
End View

This end view of the arrangement shows the 4 resistors evenly spaced around the centre pin & soldered to the shell.

BNCs Soldered
BNCs Soldered

The centre pin of the 75Ω BNC connector is trimmed down to match the length needed to touch the end of the series resistor, and it’s soldered in place. It’s a bit tricky, soldering within the gap between 2 of the ground pins!

Completed Matching Pads
Completed Matching Pads

Finally, the internals are shielded with copper tape, soldered at the seams.

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HP 8753C Display LCD Replacement

Completed Install
Completed Install

Since I inherited an old HP 8753C Network Analyser from work, I figured updating a few things to relatively modern standards would be good. The factory CRT, being 28 years old, is definitely getting a little tired, not to mention being slow to warm up. I read over on the EEVBlog forums about a DIY modification to integrate an LCD display into place instead. There was also the option of a ready-made kit for these instruments which would integrate an LCD, but the cost at over £300 was very prohibitive!

CRT Pinout
CRT Pinout

The CRT display unit is a self-contained Sony unit, taking RGBHV signalling from the graphics control card of the analyser. Power is 65v DC which will definitely come in handy for powering the new LCD & control gear, after some conversion.

Test Wiring
Test Wiring

Doing a quick test with some wiring stuck into the video connector from the graphics controller, proved that I could get a decent video signal out of the unit! The only signals used here are RGB, along with the vertical & horizontal sync.

GBS-8200 Converter Board
GBS-8200 Converter Board

The video is converted to VGA by way of a GBS-8200 arcade machine video conversion board, which will take many different video formats & spit out standard VGA signalling. The power supply to the left is a standard 100-240v to 12v PSU, which is happy to run at 6t5v DC input voltage, albeit with a ~5 second delay on output startup when power is applied. This is due to the massive 6.6MΩ resistance of the startup resistor chain, which I did reduce by 50% to 3.3MΩ with no effect. Since it does start OK even with the delay, I think I’ll not tinker with it any further. I doubt I could pull the full rated power from it with such a low input voltage, but all included, this mod draws less than 600mA at 12v.
A custom 20-pin IDC cable was made up to connect to the analyser’s graphics board, and this was then broken out into the required RGB & sync signals. Quite a few of the grounds are unused, I’ve not yet noticed any issues with EMC or instability.

Sync Combiner
Sync Combiner

There is a quad-XOR gate deadbugged to the PCB, which is taking the separate sync signals & combining them into a composite sync. The conversion board does have separate sync inputs, but for some reason doesn’t sync when they’re applied separately. This gate IC is powered from the 3.3v rail of the converter board, with the power lines tacked across one of the decoupling caps for the DRAM IC.

LCD Control PCBs
LCD Control PCBs

The donor 8.4″ LCD came from eBay in the form of a POS auxiliary display. I pulled the panel from the plastic casing, along with the control boards, and attached them all to the back. This LCD also had a sheet of toughened glass attached to the front, no doubt to protect against the Great Unwashed while in use! This was also removed.

Control Boards Mounted
Control Boards Mounted

A cut piece of plexiglas allows the boards to be mounted in the cavernous space the CRT once occupied, with some brass standoffs. 12v power & VGA are routed down to the LCD on the front of the analyser.

LCD Wiring
LCD Wiring

The LCD itself is tacked in place with cyanoacrylate glue to the securing clips for the glass front panel, which is more than enough to hold things in place. The input board which just has the VGA connector & power connector is glued edge-on to the metal back panel of the LCD, and is under little strain so this joint should survive OK.