DC load in an aluminium enclosure

The DC load needs an enclosure and finding one always turns out more difficult than you think. This is a common fate of many electronics project, often you start with a prototype, the PCB and wires etc are on the workbench, and in a second iteration you rebuild everything so that it fits in a standard Hammond enclosure. These boxes are expensive, some of my ham friends make them from double sided PCB. An alternative is to look for a machining tool which allows you to make almost any aluminium enclosure that you like.

For this project I made an aluminium box, something I’ve never done before. To machine 1 mm aluminium sheets you need a tool that cuts and bends the material, the used HBM tool can even roll sheets and bars. There is only one drawback, it weighs 45kg and I need it on the second floor, so I threw my back out but a week later I’m doing a lot better. Aluminium sheets are affordable, 1000 by 1000 mm aluminium at 1mm is about 36 euro, please ask them to cut it as 300 by 1000 mm because of a limitation with the HBM tool unless you got more space in your workshop to get something larger. But 300 mm is a nice size, if you need something larger then consider to reuse 19 inch rack enclosures. Also, if you go beyond 300 mm then 1 mm sheets are not enough to provide stability so that you need a frame within the box. Up to 300 mm you don’t need thicker aluminium or a frame unless something heavy like a transformer is part of the project.

plaatwerk
HBM combi machine for cutting, bending and rolling, weight: 45kg

The first thing to machine for this project is a conduit for the heater coil in the DC load. The maker-beam frame in the second image is only used for orientation and placement of all components before you start on the box. The conduit consists of two L shaped parts and in the end I used rivets to attach them together. Forget about 3mm nuts and bolts, instead use self drilling screw and rivets.

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Left: conduit with heater coil and computer fan,  right: PCB with relays.

And after about 2 days of work you get the following:

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The aluminium enclosure consists of two U shapes that fit into another. Due to the limitation of the HMB combi machine you have to make each U shape from three parts. Finally use rivets, self drilling screws and washers to keep everything together.
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Display and rotary encoder / push button switch installed, separate system fuse and power and measurement connectors on front and backside,
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DC load in action discharging a battery

Schematics:

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The software and hardware is rather straightforward Arduino stuff, you select an resistance (2.5, 5, 10 or no Ohm) and the discharging starts, every minute it measures the unloaded and the loaded voltages and both values are shown on the display.
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This is the correct relay logic. The monitor port is not used by the Arduino, it is intended for a separate volt meter to obtain the current. The conversion of voltage over the 0.15 Ohm resistor to amperes requires a calibration constant.

Mind that the relays have a 8 Amp limitation up to 250 Volt. All loads that you connect should have a separate fuse. If you go over 8 amps then chances are that the relays start to fail, contacts weld together etc. A worst case failure is that relay R1 and R2 and R3 stay in a M (make) position (the used relays have a make and a break contact). In case of a failure the 0.15 Ohm resistor is your only savior, and a fuse between the battery or power supply and the DC load should take over. It actually happened once which is the reason to print the warning on the front plate of the enclosure.

Last update: 13-Jan-2020 7:20 (spelling correction, and additions)

Voltage limiter

It is a reminder for myself, this is how you make it with a LM358 (any opamp will do in fact):

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And this is what it does:

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Last update: 28-Dec-2019

Heavy duty DC load

Here is a heavy duty DC load that can be used up to 800W, something you can not do with a single MOSFET since they usually give up the ghost at about 100W or at a junction temperature of 125C (which you cannot directly measure). If you want to go beyond this limit then either use more MOSFETs (please say no here), or do something simpler. I believe in the KISS approach (google for it) and came up with this design:

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The schematic of the high power DC load is embarrassingly simple and thus part of the KISS philosophy:

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To build the heavy duty DC load is a bit of a challenge because the resistors will become hot (temperatures like 200 C), so that you need a blower. The essence of the design is resistance wire (I used 2.2 Ohm per meter) and a ceramic coil element which can withstand the heat of the wire.

Settings:

  • For a 10 Ohm load use terminal 0 and C. This is tested up to 60 Volt where you can initially get a current of 6 Amp, which quickly reduces because resistance wire that heats up will lose its conductivity, so the resistance will increase. Atoms in the wire become more chaotic, and the efficiency by which a current can flow will reduce.  You can model this effect with the Stefan-Boltzmann law which says that anything that is hot will radiate and the hotter it becomes the more it will radiate effectively by temperature to the power 4 times a constant.
  • For a 5 Ohm load use terminal 0 and B
  • For a 2.5 Ohm load use terminal 0 and B and short terminals A and C
  • In any configuration you measure the voltage between 0 and A, and multiply times 0.159 to get the current. You can also use this feature separately for current measurements up to 15 Amp or so, the 0.15 Ohm resistor is rated up to 25W, so 10*10*0.159 = 15.9W. You can not go beyond approximately 15 Amp with this device, but this is more than enough for its intended use which is to test power supplies and to discharge batteries.

Some experiments:

  • Take any power supply or battery and measure the internal resistance, that is step-wise increase the load and measure the voltage over the source, plot the current and voltage in excel and compute the slope of the regression line, this gives the internal resistance of the source which is a quality of the power source.
  • Measure the ripple of a power supply for various loads, for this use the AC voltage measurement of your multimeter or a scope.
  • Measure the efficiency of a power supply under various loads, so, what is the ratio between the power put into the supply, and the power that you measure over the load. Also this is a quality factor of a system.
  • Connect several identical batteries in series and discharge them, then measure the voltage across each battery to find out whether they are all ok. In a pack of four dryfit 12Volt batteries I was able to locate one bad cell which was otherwise hard to find.
  • Discharge any battery and charge it again, try it several times, this should be a habit in your collection of lead acid batteries, otherwise you can toss them after several months. All NiCd and Pb batteries need this type of maintenance.
  • If you want to do more then demonstrate the Stefan-Boltzmann law, for a description see http://fisica.uc.cl/images/stefan-boltzman_lamp.pdf

Last update: 25-Dec-2019 12:30

Adjustable DC load

The Adjustable DC load is on the left, I use it to test power supply circuits such as the one on the right. The question with any power supply is always: 1) what is the internal resistance, 2) what is the ripple voltage, 3) what happens when we gradually increase the current, i.e. what part gets warm in the power supply 4) does the power supply limit the current at some point, or, will it trip a fuse, and finally, 5) how does a power supply react to a short circuit?

The adjustable load design is inspired by https://forum.arduino.cc/index.php?topic=90343.0 The idea is that you set the pot-meter, the first part of the opamp is a voltage follower, next you take half of that voltage and it goes in the second opamp which has a negative feedback of the voltage over a 1 ohm resistor at the source of a N channel power MOSFET. Effectively the both opamps will control the N channel MOSFET such that the current running from Drain to Source is proportional to the pot. meter setting.

Any IRF N channel power MOSFET will do, the used IRF840 can take up to 8 Amps  as long as you keep the junction temperature under 150C. Temperature is more you problem than Amps. With the IRF840 and the used heatsink you can continuously dissipate 50W, in that case the heatsink becomes something like 90C. If you want to you can dissipate 100W shortly, but at some point the MOSFET will give up. You should not dissipate more than 125W into a IRF840 at 25C. The alternative is to also put a separate resistor above the drain so that you can use the adjustable DC load for higher voltages. A drain resistor of 15 Ohm could be handy for voltages up to 60V, 5 Ohm would be helpful up to 30V, etc.

A separate PC fan  will increase the efficiency of the heatsink and the range for the DC load, you can put several MOSFETs and resistors and PC fans on different dummy load to even further increase the range of the adjustable load. Imagine what it will look like when you try to dissipate 1KW at 60 Volt, it should be a lot of plumbing.

Power supply

Lets build a power supply, target 50V 10A, challenge, most parts come from the scrapyard, some parts had to be purchased, other parts are from hammarkets.

  • Frame without a top plate and a backplate : scrapyard
  • Toroidal transformer 1KW which I estimated from its size because there is no documentation,  input 2 times 110V, output 2 times 43V and 18V (unused), powerswitch : scrapyard
  • A 100 Volt, 22000 micro farad capacitor : hammarket
  • Netzteil + other capacitors, rectifier, 2N3055’s, power diodes : Reichelt
  • Two 220V fans : hammarket
  • A high voltage LM317HVK in TO-3 case: Ebay (UK)
  • 10 mm Pertinax plate, scrapyard
  • Nuts and bolts, 1.5mm2 copper wire, hinges,  aluminum/plastic piping : all comes from the hardware store
  • Top / Back plate 2mm aluminum including the work to drill two 76mm diameter openings for the fans: metal workshop in the city.
  • All heat sinks come from the scrapyard or hammarkets. Just find something with a rating of about 1 to 2C / Watt.

The circuit is not that difficult, nothing fancy here, not drawn is a power switch and a 10S 385K varistor over the power plug:

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The blue netzteil from Reichelt is a delay switch that you install before the transformer, it reduces the transient current caused by induction of the toroidal transformer when you turn it on. Essentially the blue block is a series resistor that is short circuited after 60 msec. The image below is my test setup for any unknown transformer that I get, it is a variac where the primary has a manual protection against run-up induction currents. In that case the resistor is a lamp in series with the primary of the variac which you short circuit with a push button automatic fuse (called a Stotz, this is the 1960 version). If you don’t use the Stotz then the transient induction current goes in the net, usually it blows the fuse on the meter case in the hallway which you want to avoid. The transient of a 1Kw toroidal transformer does the same, so this is why you need the blue netzteil from Reichelt which is a modern solution for the problem.

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The 100 Ohm bleeder resistor over the 22000 micro Farad initial capacitor is on a separate heat sink (maybe it is a bit overrated), a 0.05 Ohm series resistor is mounted to the bottom plate of the case behind the first capacitor, it is there to allow reduce a transient caused by charging and discharging a large capacitor, that is, within 5 seconds it is stabilized at the required 60V (approx). Rapidly discharging any capacitor over 10 micro Farad at 60V is an issue, if you want to see sparks then start here. So the bleeder resistor is a must for safety.

The LM317HVK regulator is a TO-3 version that can the used up to 60V, don’t get them in China, buy them here (thus second hand on eBay). The LM317HVK with the rectifier go together on a separate heat sink, and the 2N3055 transistors are as well on a separate heat-sink. Both diodes are 10Amp 1kV, and all capacitors are rated at 100 Volt. The measured ripple reduction is better than 10mV with a load of 2Amp or 100W. I will still try to find a solution to test this supply under higher loads.

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The weight of this supply is mostly caused by the transformer, the top plate has two hinges and is closed with six Parker screws. Also, you need air circulation because of the heat dissipated in the power supply. All wiring is 1.5 mm2 copper, clamp screws on 10mm pertinax slightly elevating above the base plate. Also the transformer has a 8 mm isolation from the case to reduce stray volatage on the case, it is mounted with two screw threads to the base plate. Pertinax is not used in modern designs because it is hygroscopic, epoxy resin is the preferred solution. But this is the reason why you find pertinax on scrapyards.
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The blue block is the Netzteil from Reichelt, Toroidal transformer with two 43V secondaries in parallel, the high amp regulator,  a steel LM317HVK on the same heatsink, top center is the 22000 micro Farad capacitor, to the right the bleeder resistor on a separate heat sink, on the third heat sink, the largest, you find both 2N3055 transistors, it is a current amplifier.
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The way it looks when the case is closed.

The ripple voltage to current ratio for this circuit follows from the equation:

du = I / C * dt;  Vrms = du/2*0.7

where dt is 10 msec for a 50 Hz powergrid, Vrms to be measured at the input pin of the LM317 and the common collector of the 2N3055, it will become 0.8 Volt at a load of 5 Amp and it scales linearly. The power dissipated by the 2N3055’s is the difference between 60V and 50V times the current used, so at 10Amp a total of 100Watt is dissipated while 500W is provided to the load. The measured ripple at the output of the transformer is a lot less than Vrms because of the LM317 regulator.

Open points to be improved or tested in this experimental design are:

  • What is the temperature doing at high loads, for this I need to build (or find) a regulated load to test it up to higher currents. Without the fans in a open case the heat sink of the bleeder resistor goes up to 90C which is still acceptable although it is too hot to touch, the bleeder resistor can handle up to 50 Watt and it is dissipating 36W. With the fans in a closed case the measured temperature at the backside of the heatsink stays under 40C at a room temperature of 18C. The other heat sinks don’t really get warm at a 2Amp load.
  • Don’t try to short circuit this power supply, even with a fuse it will kill both 2N3055’s. PH0BAS (Bas) has a solution for me and this will be the next improvement.

17-Nov-2019

Performance specs of this power supply:

  • V1 measured after the 0.05 Ohm resistor is depending on the power provided by the supply, the relation is V1 = -0.925 A + 57.5 Volt, at 10 Amperes we end up at approximately 48 Volt.
  • The ripple measured at 55V after the 0.05 Ohm resistor is 0.5 V rms at 3 Ampere
  • The ripple measured after the 2N3055’s is 10 mVolt at 50 Volt and at 3 Amperes, so the regulator is doing what it is supposed to do.
  • The thermal resistance of the small heat sink on the north east side is 1.88 C/w, probably 1 C/W is the thermal resistance of the large heat sink. More than 100 W dissipation in the 2n3055’s looks like a no go.

This looks like a 500W lab power supply, don’t think this transformer can handle a lot more. Added a fuse holder before the 0.05 Ohm resistor, so this is what we currently have:

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Still to do

  • add a current limiter
  • add a pot meter / and or selector to the front panel

Last update: 17-Nov-2019 6:45

Trap dipole for 40 and 80 meter

A dipole is resonant on one frequency (and all odd harmonics), with a trap you can design it for more frequencies. Here is the trap dipole for 40 and 80m:

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Trap circuit in the dipole

This is how you make it:

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The capacitor fits in the tube

Next you go in the field to adjust the length of the arms to make it resonant at the right frequencies:

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Field test, made a handful of QSOs on both bands.

Construction details: You start with the 40m wire, that is simple, next you do the 80m extension, this is what I found:

  • Arm 1: 9.67 meter from the balun to the trap
  • Trap takes 14 cm of length, so 9.81 meter from the balun to the start of arm 2
  • Arm 2: 16.45 – 9.81 = 6.44 meter from the end of the trap to the isolator

These dimensions make the antenna resonant over the entire 40m band, you don’t need a tuner there, for the 80m band the antenne is resonant in the lower part of the band, but you need a tuner to get it to work over the entire 80m band.

The trap is a parallel inductor-coil circuit wound around a PVC tube, use a dip meter or a VNA to construct it. I used 0.83 mm enamelled copper wire, roughly 20 turns on a 32mm PVC tube, inside the tube you place a 47 pf high voltage (10 kV Russian) capacitor. Use the dip meter and/or the transmission program on your VNA to check the resonance frequency of the trap. I found approximately 7050 kHz with the VNA transmission program. The dip meter ends up somewhat lower, more like 7000 kHz, the Q is around 100.

The trap will work as a stopper for 40m, for this band arm 1 is only used because the LC circuit will start to resonate absorbing any energy going into arm 2. On 80m the trap is mostly a resistor, or better a lengthening (or loading) coil so that you can shorten arm 2 until the SWR of the antenna becomes acceptable for 80m. Start with 11m for each arm and adjust with a pair of pliers until the antenna analyser says that it is ok.

Last update: 16-Sep-2019 11:36