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Author Topic: Single Coil Latching Relay Protection  (Read 464 times)
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Hello,

I need a little bit of advise as I just hit a wall while working on a project. So I've misread that zener diodes allow current in the reverse direct as long as the current is under breakdown voltage. This is incorrect (my fault of course) as it allows current to flow over the breakdown voltage. Now those that do know coils do have the capability to discharge voltage at a higher rate when the coil discharges. I do not want to damage that arduino so I need 2 zenor diodes between the coil ( 1 for each end) to current higher than 5V.

So I need a device that allows current in both directions as the pins will be changing throughout the program from high to low to reverse the current.

Any ideas will be helpful. The relay works fine on 5 volts I just need to protect the atmega as These will be used in long term products.
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Shouldn't a capacitor be fine for this job?  smiley-eek IDK why I did not think of this before? I'm going to try two capacitors.
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Well I guess a capacitor won't make the cut since the voltage is barely making it anyways. Any real dangers with such a tiny coil at 4.5v?
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I don't completely understand your question, but you don't need Zener diodes to protect the Arduino or transistor controlling the relay. You need an ordinary diode, placed in parallel with the relay coil, to prevent inductive kick.

Keep in mind that you should never try to draw more than 20 mA from the Arduino output. Most relay coils draw more, so almost all relay circuits require a transistor to amplify the current.

http://www.electroschematics.com/8975/arduino-control-relay/
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As you have a latching relay, you need to be able to power the coil in both directions. For this you need a H-bridge.

Find a suitable H-bridge like the L293D (the D means built-in protection diodes) or a similar newer part.

// Per.
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I don't completely understand your question, but you don't need Zener diodes to protect the Arduino or transistor controlling the relay. You need an ordinary diode, placed in parallel with the relay coil, to prevent inductive kick.

Keep in mind that you should never try to draw more than 20 mA from the Arduino output. Most relay coils draw more, so almost all relay circuits require a transistor to amplify the current.

http://www.electroschematics.com/8975/arduino-control-relay/

This is a latching single relay so I will need to reverse the current on demand which a diode would prevent. On the specs the coil is rated at 33.3mA when the Arduino pins can support up to 40mA so what router should I take from there? Just so you know its up to 40mA. 20mA to be safe I guess 33.3 should be safe as well.
« Last Edit: March 23, 2014, 05:07:05 pm by geant90 » Logged

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There are single coil latching relays which change states every time they receive an impulse and hence, coil current flow is unidirectional (except  for the inductive spike). Google will provide a number of schematics for single-coil relays which require reversal of coil current, see attached for one.

40 mA is the absolute maximum for an ATmega output, above which you are more or less guaranteed to destroy the pin circuitry. So in the long term it is NOT safe to power a 33 mA relay coil directly from the Arduino output pin. That is especially true if you use Zener diodes for kickback protection, as they cannot completely eliminate an inductive spike.


* relay.jpg (4.61 KB, 240x144 - viewed 34 times.)
« Last Edit: March 23, 2014, 06:49:54 pm by jremington » Logged

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That circuit (relay.jpg) requires a higher voltage supply that the relay normally
takes to overcome the additional losses in the resistors, note.  It doesn't need
protection diodes because the resistors serve to carry and limit the inductive spike.
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So with the L293D to control a single relay I could set outputs on the arduino, say pin 8 and 9. These are connected to the L293d pin 2 and 7. One pin on high sets current in one way and alternate to reverse the current. Seems simple enough Just waiting on one to get here. I was just looking for a cheaper route as when it comes time for a mass production those damn l293d are pretty pricey. Also couldn't  the same thing be accomplished with a voltage regulator? I'm already convinced the L293D is the safer route especially with kick. I just have to rewrite some simple code.
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As I assume you won't be activating the relays that often I would say driving 33mA is OK, I wouldn't do it for a commercial product though.

As for the protection, wouldn't a clamping diode to 5V (and maybe also to GND) on each pin do the job?

EDIT:
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when it comes time for a mass production
Oh. looks like it might be a commercial product smiley

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« Last Edit: March 26, 2014, 07:42:35 pm by Graynomad » Logged

Rob Gray aka the GRAYnomad www.robgray.com

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