Categorized | bbboost, projects

bbboost chapter four – the digital ammeter

[3 July 2010 – this project has been retired, but the posts left for reference]

Greeting again to followers of the bbboost journey. Finally the required parts arrived today so now the project can move forward. (Living in a country of 22 million people, you would think 5W resistors would be easy to come by. Think again). If this is your first brush with the bbboost project, please visit here to see what it is all about!

So today we are going to modify the voltmeter module to convert it to an ammeter (current meter) and therefore a very useful thing to have on a desktop power supply. By having an ammeter, once  your project or prototype is running you can use the current readout to determine the power supply or battery requirements for your project, or dance dangerously close to the limits of the circuitry. Careful!

However converting the voltmeter is a simple process. Using Ohm’s law, we know that current = voltage over resistance. So our problem requires us to determine the current. If we measure the voltage potential across something of a known resistance (say, a resistor), and divide the voltage by the resistance, we have the current flowing through the resistor.

I = current in Amps; V = Volts; R = resistance in Ohms.

So for example, if we have the current of our supply circuit running through a 1 ohm (5 watt – as the wattage will increase at full load) resistor, and the voltage potential across the resistor is 0.0084 volts (8.4 millivolts), the current will be 8.4 milliamps (or 0.0084 A).

Now that we already have a voltmeter, a simple removal of the 1M ohm resistor from the terminals and replacing it with a 10k ohm resistor allows the meter to measure much smaller voltages. Therefore the maximum is will measure is 999.9 mV (which we will note as mA when being used in ammeter mode). Here is the circuit diagram for the ammeter. Note the only change from the voltmeter is below the 10 nF capacitor at the bottom-left of IC1. The supply current will be running through the 1 ohm 5 watt resistor. In the next chapters we will discuss a switching solution to flip between voltmeter and ammeter without any rewiring by the end user.

ammeter

Also note the change of wiring on our breadboard, it is much neater and easier to follow. The solid wires are much more reliable than the looser ones used previously. Although they can be more difficult to route around a breadboard, they will be more reliable – especially if you move the board around a lot.

ammeter_small

There we have it! A simple conversion has made our voltmeter an ammeter with a range of 0~999.9 milliamps (basically 1 amp). Which matches nicely with the original specification of the bbboost power supply of 1 Amp. But now for the action test: measure some current! Our test subject is an LED in series with a 10k variable resistor and a 6v battery. In the video clip (no audio) we will measure the current through the circuit at three different rates, changing the resistance to alter the current three times, then comparing the readout on the bbboost and a multimeter set to mA scale. The test current values are: 0.8 mA; 12.4~5 mA (third time lucky) and 118.5 mA (bbboost) vs. 114.8 mA (multimeter). That’s a difference of 3.7 mA – for the purpose of this project, quite negligible. There is always Fluke!

The extra parts required for this section are a 10k ohm 0.25 W resistor and a 1 ohm 5W resistor.

Well that was nicely successful – except for that pesky decimal point – we didn’t do anything about it in the change from a voltmeter to an ammeter. That, my friends, is for the next instalments.

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John Boxall

Person. Author of http://arduinoworkshop.com Director of http://tronixlabs.com.au Rare updater of http://tronixstuff.com VK3FJBX

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