175: Saturday 12th February 2011

I have been putting off working on the Probatron until I had tidied up my garage a bit. Today I set out to get my bench organised. I took my arc welder off and got rid of a load of rubbish, put some tools away and generally moved stuff around so I could work in comfort. That took me a good few hours. Then I got stuck in to building the display driver for the Amps x 100 bar graph display. You may remember from post 148: Friday 23rd July 2010, that I have mounted ultra bright blue LEDs behind each of the numbers around the rev counter (tacho). These are wired up to a ribbon cable ready for connection to the driver circuit that I have now made. The photo shows the circuit connected up on the bench with a temporary display for bench testing.
You may wonder how the current is going to be sensed? The device in the photo below is a hall effect current sensor and it measures the magnetic field surrounding a current carrying conductor (the power cable). This device takes nothing away from power source as it is an active measuring device to measure a magnetic field, not current directly like a shunt does. However even a shunt does not measure current directly as the shunt gives its meter a voltage across it depending on the current through it. This is achieved by applying a small load through a small resistance. This does not take much from the batteries, but it does cost about £50 for a shunt resistor and £10 for a hall effect sensor. The magnetic field is directly proportional to the current therefore once the display is calibrated, it should stay accurate. This is the back view. If you enlarge this photo you will see the connections at the top.
The next photo is just the front view of the same sensor. As you can see all you need to do is disconnect the battery negative and pass it through the centre of this sensor. The sensor itself needs a steady 5v supply and is supposed to give out a signal that is a dc voltage up to 5v when 800A is being pulled, and proportionally less down to 0v when no current is being drawn.
The photo below is a close up of the driver circuit and the temporary display. I have my voltage meter hooked up to the signal into the circuit and it was reading 2.5 volts. This you can see this lights up 5 LEDs (the numbers 0 to 4 on the tacho) and this will then indicate half the maximum i.e. 400A. The gauge is therefore displaying Amps x 100. Just like the tacho displays revs x 1000. It was convenient that my tacho happened to go up to 8. Also you can see the blue trimmer pot on the circuit board. This is a multi-turn pot that can be adjusted in very small increments to set the bar graph to correspond to the current being drawn. This is simply adjusted with a jewellers screwdriver using the screw on the top. You may need to enlarge the photo to see this properly. I have circuit diagrams and board layouts for vero-board if anybody is interested.

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