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Effect of adding another another motor in parallel

A PMDC motor M2 is connected to a constant regulated voltage source. To protect the regulator chip from the flyback voltage, a diode has been added. From the same unregulated voltage source another motor is also driven. The voltage regulator has an inbuilt polarity protection and as such I don't want to add another diode in series with the circuit so as to have an unnecessary drop. Also if a flyback diode is used across M1 motor, it will create a short under reverse polarity.

What will be the effect of flyback voltage generated across M1 motor on the regulator chip? If possible, please show the conventional current flow in the circuit.

[![Block Diagram][2]][2]

Reverse Polarity

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2 Answers 2

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Interpretation of OP's circuit with additional diode D2.

A PMDC motor M2 is connected to a constant regulated voltage source. To protect the regulator chip from the flyback voltage, a diode has been added.

That much is clear.

From the same unregulated voltage source another motor is also driven.

M1 is powered directly off the solar panel.

The voltage regulator has an inbuilt polarity protection and as such I don't want to add another diode in series with the circuit so as to have an unnecessary drop.

OK. No series diode added.

Also if a flyback diode is used across M1 motor, it will create a short under reverse polarity.

This isn't clear. How will reverse polarity occur? There are no switches anywhere in your circuit.

What will be the effect of flyback voltage generated across M1 motor on the regulator chip? If possible, please show the conventional current flow in the circuit.

Just add D2. It's normally reversed biased. If the solar panel is suddenly disconnected current will continue to flow in the motor and D2 until the motor stops. No problem.

Without D2 a negative voltage spike will be generated on disconnection of the power source. This may destroy the regulator.

If you need further clarification then please add a proper schematic to your question. There's a schematic button on the editor toolbar. Keep positive at the top, negative at the bottom and, in general, power flow from left to right.

schematic

simulate this circuit

Figure 2. Zero volt-drop reverse polarity protection.

The scheme of Figure 2 protects the circuit against reverse polarity connection of the solar panel or battery but has zero voltage drop.

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  • \$\begingroup\$ Battery or solar panel is an external source to the circuit. It is not connected to the circuit by a permanent connector or so. In your above schematic, flip the plus terminal, and its a short. Motor M1 will start rotating in the opposite direction which is not a problem in my case. \$\endgroup\$
    – Akash
    Commented Mar 30, 2016 at 10:56
  • \$\begingroup\$ Well then just use a polarised connector for the solar panel so that it can't be reverse connected. \$\endgroup\$
    – Transistor
    Commented Mar 30, 2016 at 11:07
  • \$\begingroup\$ That is impossible in my case. A battery has terminals which are connected through wire, and that too by a novice. I cant expect them to connect the system appropriately and that is what reverse polarity protection stand for. Also there are several ways to eliminate this problem permanently. For e.g using a high current flyback diode and a fuse in series so that when its connected the wrong way, fuse blows. But I dont want to know the method, I want to clarify the effect of the other load M1 in my case. \$\endgroup\$
    – Akash
    Commented Mar 30, 2016 at 11:14
  • \$\begingroup\$ All of this additional information should have been in the original question. See update. \$\endgroup\$
    – Transistor
    Commented Mar 30, 2016 at 11:26
  • \$\begingroup\$ Even a NMOS or PMOS can be used for reverse polarity protection and that will lead to negligible drops. But @transistor its all about saving parts. Why to add one when the need is still not clear. I'm attaching the Block Diagram of the chip. Check my original post... \$\endgroup\$
    – Akash
    Commented Mar 30, 2016 at 14:41
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A flyback diode is required to shunt back-emf caused by the inductance of the motor (and wiring) when power is removed. Since you are applying DC this will be a one time spike that only occurs at the moment you disconnect the battery or solar panel.

If the regulator has a large filter capacitor directly across its input then it may be sufficient to soak up the spike. If it doesn't, add a bidirectional transient suppressor diode (TVS) rated slightly above the maximum input voltage.

You say that the regulator has reverse polarity protection, but can it handle a continuous reverse voltage? If not then you should wire a Schottky diode in series with its input (after M1). Then this diode has to take whatever back-emf voltage M1 produces, so to protect it put a suitably rated TVS across the input.

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