I find the best way for me to learn anything is through practical application. To that end I came across an interesting video blog by Dave Jones (www.eevblog.com), detailing the design of a electronic dummy load. For my next project I thought it would be useful to have a constant current dummy load in the lab (aka garage), so I decided to build up the circuit Dave Jones demonstrates in his video. I followed the original circuit faithfully, only changing out the LM324 Op Amp with a MCP6004 Rail to Rail Op Amp and I added a couple of trim pots for calibration. After procuring the components, I built up the circuit on a breadboard. I found that I could easily handle a couple amps with this design without many issues regarding heat. For this project I wanted to try my hand at designing a circuit board and getting it professionally manufactured. To have the board manufactured I designed it using CadSoft’s Eagle PCB application and sent the board file to Osh Park for manufacturing. Osh Park made it really easy to just upload the board file online with no need for producing Gerber files or anything else. Turn around time on the board was pretty good to, I think it took about two weeks. Once I had the boards in hand it didn’t take long to drill out a standard Radio Shack project box to house the final device. Below you will find the eagle files I used in this build along with a few pictures of the entire process.
A few additional consideration in the development of this device:
- The MOSFET is mounted directly on an external heat sink and not on the PCB.
- The onboard 7805 regulator feeds both the Op Amp and adjustment pot.
- The device is powered via an external wall adapter. I am using a 7.5 volt 800 mA adapter from my parts bin and I have no idea what it went to originally.
- The 10 resistors get a lot warmer then I thought they would. I have a heat sink I can put on top of them, but need to get some thermal compound that will hold it in place.
- The only way to know what the current draw is putting a load on it. In future designs I would like to have this as an independent control.