Introduction: DIY Adjustable Electronic DC Load
In this project I am building an analog adjustable dc load with parts easily obtainable from ebay and banggood. The advantage of such a dc load is that you can understand how it works, modify or repair it if necessary far easier than you would with a digital one. I was able to push mine up to 60W dissipation, but it is recommended to stay under 50W to protect the mosfet.
Step 1: Watch the Video!
The video describes the entire build so I recommend watching the video first to get an overview of the project. Then you can come back and read the following steps for more detailed explanation.
Step 2: Get All the Required Parts
Here you can find a list with links to all the parts I used in the project. Feel free to adjust and make changes according to your own needs.
- Constant Current Analog DC Load PCB Module
- PZEM-031 100V 20A LCD Panel Meter
- Metal Enclosure 170mm x 130mm x 75mm Blue
- 10 Turn 10K Potentiometer
- Brass Standoffs Kit
- M3 Screw Kit
- KSD9700 NO Thermal Switch
- Cooling Fan 12V
- JST XH 2.54mm 3 Pin Connector
- 4mm Banana Plug High Current M5
- 2.1mm DC Jack Panel Mount
- Rocker Switch
- Screw Terminal 3 Pin
- 16AWG Silicone Flexible Wire
Step 3: Prepare the Enclosure
Some work was needed to prepare the enclosure and the heatsink for the final assembly. I started by punch marking all the required holes and slots onto the panels next I started drilling the 3mm holes into the bottom of the enclosure. I continued with drilling 2.5mm holes into the heatsink, these will be tapped later with an M3 thread and some standoffs will be mounted.
Next I drilled the required pilot holes into the front and back panel. I started with a 2.5mm drill and them enlarged them to their final size with an 8mm drill. I then continued with cutting the required slots using the dremel tool and a cutting disk. Although I'm not very good at this kind of metal work the end result was pretty nice.
Step 4: Wiring & Assembly
I continued by assembling the front and back panel. On the back panel I had to install the rocker switch and the dc input jack as well as wire the switch in series with the positive line. On the front panel I had to install the panel meter, the two 4mm banana jacks and for these you need to make sure they don’t touch the metal panel, in this case they have a nice insulating jacket that goes through the hole and also prevents the jack from spinning on the panel.
We also have the 10 turn pot on the front panel, it was quite easy to install but a standard 6mm knob doesn’t fit over the shaft because the shaft is 6.3mm. So I had to grind the brass insert on the inside of the knob with the dremel and enlarge it to 6.3mm.
The panel meter was modified because we are powering the panel circuitry from a separate 12V rail while we are sensing voltage and current from different points. The mod is detailed in the video so make sure you watch the full video for instruction on how to do that.
The mosfet was attached to the heatsink using thermal paste and although I didn't use any, you could also use a silicone thermal pad and insulating washer just to isolate the transistor electrically from the heatsink and the metal enclosure. Next to the mosfet the thermal switch was glued in place using thermal adhesive. This is like silicone adhesive but with good thermal conductive properties.
Everything was assembled into the final position and wired accordingly. To help with your wiring I have attached an image with all the wiring info you need.
Step 5: Testing & Final Thoughts
I did some tests on the dummy load and it looks like it can handle 60W while keeping the mosfet under 60 degrees celcius. Depending on your setup, what size of heatsink and cooling fan you decide to use you might be able to dissipate over 100W but keep in mind, that will probably shorten the life of the mosfet considerably because it will operate outside its Forward Bias Safe Operating Area. A solution to that would be to use multiple paralleled mosfets, I would go with one mosfet each 50W.
There is no failsafe on this dummy load, but that could be implemented with a second thermal switch that will cut-off one of the inputs if a certain temperature, let’s say 90 degrees celcius is exceeded on the heatsink.
The total cost of parts used in this dummy load was about $40 but you probably have some of the parts already and you don’t need to purchase them. There is no real advantage to an Analog dc load, except for they are very simple and you can easily understand how they work and possibly repair them if needed.
I'm happy with how the dc load looks in the end, I have added labels as a final touch, they make the front and back panel a bit nicer and provide useful info at the same time. The load should be able to handle up to 100V, but I decided to keep it on the safe side and selected 60V as the maximum input voltage.
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