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I am trying to understand a circuit diagram for a device that I have that has an h-bridge connected to a motor. Each of the wires going to the motor is connected between two diodes in series which are connected to ground and a fixed voltage. The searches that I have done have also shown this same setup connected to a pair of charlie-plexed LEDs.

I am having a hard time understanding the purpose of these diodes. I have two hypotheses. 1) Noise suppression. 2) Circuit protection for when the motor is connected and disconnected from the circuit.

enter image description here

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I redrew the circuit in a way that is more commonly presented. The reason why we like this form is that it makes it very easy to see what is happening. It's the same reason why we have standard form for equations because you can visually extract the necessary information. So using "standard topologies" also make it easy to see what's happening.

If the voltage on the HBRIDGE A line were to exceed VCC+VD, then D3 would conduct and thereby clamping the voltage to VCC+VD.

If the voltage on the HBRIDGE A line were to drop below -VD, then D2 would conduct and thereby clampig voltage to -Vd.

The same logic applies to the HBRIDGE B line.

Vcc is whatever the supply voltage is. Vd is whatever the diodes forward drop is.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for redrawing the circuit. This makes perfect sense. One more quick question. Are there any special considerations that need to be taken into account with respect to the power supply/voltage regulator having the excess current being dumped into them? \$\endgroup\$ – ElectricalFail Oct 28 '16 at 22:14
  • \$\begingroup\$ Not all supplies can sink current. So you need make sure that whatever source you are using can sink current otherwise you may risk damaging your supply. (see: electronics.stackexchange.com/questions/188510/…) \$\endgroup\$ – efox29 Oct 28 '16 at 22:16
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You were correct with circuit protection. This way if the voltage momentarily jumps above 5V the leftmost diode will clip it to ~5.7 V, or alternatively if it drops below 0V, the rightmost diode wil lclip it to -0.7 V.

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