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I'm designing a circuit to drive two 12V dc motors. I've got two low current switches connected to relays as can be seen in the schematic. These are for power isolation and local motor isolation.

Can you tell me if relays are an appropriate switching mechanism for these loads?

Considering the 24V relaym aware a typical 24V relay requires 200mA to drive. Is there something else I should be using with less current drain? (more battery life - I'm using two 12V lead acid batteries)

Segway schematic

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  • \$\begingroup\$ The question is how much amps you switch and what relays you use (i.e. if they are suited for those amps and inductive loads). \$\endgroup\$ – PlasmaHH Jan 14 '16 at 21:30
  • \$\begingroup\$ The motors will be approx. 28A at full load. Am I correct in saying that a typical relay needs approx 200mA to drive? Considering that RY1 will be constantly active and RY2 close behind, are relays the best choice here when aiming to conserve battery power? \$\endgroup\$ – adamp524 Jan 14 '16 at 21:42
  • \$\begingroup\$ and then you worry about an additional 200mA ? Also you probably want a contactor instead of a simple relay then \$\endgroup\$ – PlasmaHH Jan 14 '16 at 21:46
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    \$\begingroup\$ What is the LRA (stall current) rating on the motors? What is the relay rating? \$\endgroup\$ – Spehro Pefhany Jan 14 '16 at 21:46
  • \$\begingroup\$ I didn't go for a simple switch as opposed to relay as there was potential to be driving high currents (28A). Would a switch better then? \$\endgroup\$ – adamp524 Jan 14 '16 at 21:50
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RY1 seems silly since S3 can perform its function directly, unless the issue is that you can't find a suitable switch to handle the current. A relay will be more expensive than just a switch of the same rating.

However, RY2 looks like a reasonable use of a relay. This is assuming the motors are not "frequently" switched on and off, like they would be with PWM control. Of course the relays need to be rated for the maximum motor current, which will occur when first turned on. This is often called the "stall current". See the motor datasheet.

The resistor in series with the RY2 coil doesn't make sense. From the labels, it appears you are driving the relay from 5 V. Get a relay with a 5 V coil. Most relays come in a family with the only difference being the coil voltage. 5 V is a common voltage, so you should not have trouble finding the right relay. 200 mA also sounds high unless this relay needs to handle 10s of A.

You might also consider a snubber across the relay contacts. That will extend life. If the motors are driven with DC and only one polarity, then a reverse diode across each motor will also help.

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  • \$\begingroup\$ Hi, thanks for your comments Yes RY1 is purely for switching the whole circuit on and off. A relay and small switch was looking cheaper than a single switch which would handle 30A. Looking at RY2, the motors will not be regularly switched on and off i.e. PWM. Can you explain what a snubber would be across the relay contacts? \$\endgroup\$ – adamp524 Jan 15 '16 at 19:35

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