The appropriate voltage from the motor power supply should be present at each of the respective two-way terminal blocks. If not, check:
–Powerpack or external power supply wiring/operation, and wire links are fitted across two-way terminal blocks.
The four ‘transistor switches’ used in each H-bridge operate independently - each ‘closes’ when its logic input is taken to a high level. Each H-bridge circuit uses two types of transistor:
1.Two identical lower transistor switch circuits, each using a npn darlington transistor (e.g. Q5).
2.Two identical upper transistor switch circuits, each using a npn transistor (e.g. Q2) driving a pnp darlington transistor (e.g. Q8). Without the npn transistor, the logic input signal could not drive the pnp darlington transistor, since this darlington configuration needs to be controlled by an input voltage capable of rising close to the motor supply voltage (Vm1) to turn off the transistor. The npn transistor ‘inverts’ the logic input signal and can switch off at voltages up to Vm1. The ‘inverted’ signal then drives the pnp darlington transistor.
Test each of the transistor switches in the two H-bridge circuits in-turn. Fit a resistor, say 1K: (1000 :) as a light load in place of an actual motor across the four way terminal block contacts marked M1 and M2.
One lower npn darlington transistor and its diagonally opposing upper pnp darlington transistor circuitry will be tested at a time, all other logic inputs must not be connected. For example, logic input C and logic input B will be switched as follows:
When logic input C is wired to +5V (Q5 ON) and logic input B is wired to +5V (Q10 ON), the voltage at the four way terminal block contact, M1 should be equal to approximately 1V and M2 should be equal to approximately Vm1 minus 1V.
Repeat this test for the opposite transistor circuits in the H-bridge controlled by logic inputs A and D. This circuit test will give opposite voltages at terminal block contacts M1 (Vm1 minus approximately 1V) and M2 (approximately 1V). Should any of these tests fail, check:
–Power supply operation, poor lead connections, missing wire links, shortcircuits, open-circuits, faulty soldering of components, components having incorrect value or incorrect orientation, and faulty components.
Note that the BD649 and BD650 darlington transistors used in the H-bridge (shown on the schematic with two arrows) are capable of switching currents up to 8A. Other darlington transistors with similar current capacity and matching pin configuration can be used. A 24V zener diode in series with a regular diode is connected across the BD650 darlington transistors to absorb spikes of back-emf generated during switching. The diodes across the BD649 darlington transistors limit negative voltages across the transistor to one forward-biased diode drop approximately (0.7V).
