rather than the rest of the circuit.
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...If this diode is placed across the switching transistor this will still short out reverse voltages from the coil but it relies on the fact that the two terminals of a power supply present a very low impedance....
I don't think so. The diode connected across the transistor is the wrong way round to conduct reverse voltage from the coil when the current through it is interrupted, unless it is a Zener diode.However, in an H-bridge with diodes across all the transistors (or the body diodes of mosfets), you're right, those diodes contain the back EMF by feeding it into the power supply.
The diodes are doing the same job in either position. the diode across the transistor is there specifically to protect the transistor from reverse transients from switching off an inductive load.