Building a Test Bench

So we've got a parts list, a schematic, and you could just jump in and build this bridge to see if it will work. However, as this is both a theory and a practice tutorial I'll dive in a bit deeper than that.

To understand how this bridge performs in practice, I built one half of it on a piece of proto-board. I only had to build "half" the circuit because the circuit is symmetrical, anything I can learn about this half is equally applicable to the other half. The only thing this won't tell me is what the behavior would be if the bridge switches from full forward to full reverse in one go. The picture on the right shows the half bridge and a Basic Stamp II board next to it on the left. The Basic Stamp is used as a test pattern generator for my experiments. The schematic of the test circuit is shown on the right. Inputs to this circuit are P0 and P1 which reflect the names of the Basic Stamp 2 pins to which those test points are connected. On the right side of the schematic are four test points labeled TP1 through TP4 which correspond to the battery and motor connections in the full schematic.
By building one half of the full h-bridge, I can turn on one low side (sink) transistor and one high side (source) transistor and then look at the waveforms and voltages under controlled conditions. The difference between this schematic and the one in the previous section is that half of the bridge is missing, and I've added independent control of the transistors. thus the FWD, REV, and ENA* lines aren't present.
For the static tests I have the Basic Stamp II simply turn on the transistors and leave them on. One additional piece of information gathered was that 470 ohm resistors work as do 560 ohm resistors. (560 turned out to be my final value that I used). If you don't have a Stamp or some other piece of equipment that generates signals you can simply connect the 1K resistors to TP1 and TP4 respectively. That will force the transistors on while you are doing the tests.

Static Tests.

The first set of tests are static analysis of the transistor behavior. The three parameters I was most interested in were the voltage drop across the "source" transistor, the voltage drop across the "sink" transistor, and heat dissipation in the transistors. These tests are run with a voltmeter and an adjustable power supply. The datasheets include graphs of these parameters because they change for different currents. I thought I would pick three currents from which to take measurements of 10mA, 100mA, and 1000mA then plot the results. Note that the next step for current in a log scale would be 10 amps but the circuit is not rated for 10 amps. You could go to 2 amps (pretty much the limit without using heatsinks) or to 6 amps if the transistors had heatsinks installed.

Test #1 : Source side Voltage Drop

Purpose:The purpose of this test was to ascertain the voltage drop across the high side (source) transistor. The motor will see the battery voltage minus this voltage and the voltage drop across the sink transistor so knowing these values help define the lowest voltage that is practical with this bridge.

Description: A variable output bench top power supply is connected to TP1 and TP4. To create a load, a 4.7 ohm, 10 watt, power resistor is connected between TP2 and TP4. A digital voltmeter was connected between TP1 and TP2 and set to "DC Volts".
The power supply was turned on and its current output adjusted to 10 mA, a reading was taken on the voltmeter, then the current is increased to 100 mA and finally 1000 mA.
Test Results:

The results of this test are collated in the table below. As the current through the transistor is changed from 10mA to 1000mA the voltage drop changes from a low of about .6 V to a high of about .9V. This change is plotted in the chart on the right side of the table.

What this tells us is that as the current gets larger the transistor is less like a conductor and more like a resistor. The physics behind this change are due to the fact that silicon, unlike other metals, is transferring charge carriers (electrons and holes) through a crystal lattice. The larger the current, the more charge carriers are required, and the space in the lattice is finite. The result is that more charge carriers collide with the lattice generating heat, and the difference in charge going in vs. that going out is measured as a voltage drop. (recall that the voltage potential between two points is simply the difference in charge between those points)

Test #2 : Sink side Voltage Drop

Purpose: The purpose of this test was to ascertain the voltage drop across the low side (sink) transistor. The motor will see the battery voltage minus this voltage and the voltage drop across the source transistor so knowing these values help define the lowest voltage that is practical with this bridge.

Description:A variable output bench top power supply is connected to TP1 and TP4. To create a load, a 4.7 ohm, 10 watt, power resistor is connected between TP1 and TP3. A digital voltmeter was connected between TP3 and TP4 and set to "DC Volts".
The power supply was turned on and its current output adjusted to 10 mA, a reading was taken on the voltmeter, then the current is increased to 100 mA and finally 1000 mA.
Test Results: Again I've collected the results into a table below. To bring this back to a global point about this web site, there is a page in my "physical" robotics notebook where I hand drew a box with 10, 100, and 1000 in the left hand side, then as I got measurements I wrote them to the right. Around the box I identified these results as being "Sink Experiment #2" and the date. I can go back later and refer to the notebook pages I created during this project and easily re-create my experiment or refresh my self on the results of that experiment.