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Inductive Load Switching and Step-Down Power Supply


JavaLaurence
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The documentation for the Dual Relay warns that switching inductive loads may require external protection of the stack.

 

Having tried, and failed, to find a shop that can sell me a suitable varistor, I think that it would help future customers if the Step Down Power Supply contained one or more of these things. There appears to be physical room left on the board for at least one.

 

If this idea is a non-starter, then maybe it would be nice if the online documentation could be enhanced to contain instructions for non-electronic buffs on how to protect stacks that switch inductive motors..

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Having tried, and failed, to find a shop that can sell me a suitable varistor, I think that it would help future customers if the Step Down Power Supply contained one or more of these things. There appears to be physical room left on the board for at least one.

Are you sure that you mean Step Down Power Supply? Typically you should use Varistors at the point where the problem occurs. So the Dual Relay should be equipped with some protection circuitry when switching inducitve loads.

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Forgive me for my ignorance of electronics, but I thought I understood that the inductive load problem boils down to a brief (ns?) Voltage spike generated by the (AC-powered) pump, which travels along the wires to affect the power supply input to the stack, i.e. the Step-Down brick. Am I wrong in this understanding?

 

If I am wrong, please could you explain how my stack crashes (sometimes) when the pump is switched off?

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The idea of preventing inductive load problems is that you prevent to get these spikes. The best way is that these spikes can't emerge. This way they can't travel through the circuit and can't interfere with it.

Therefore you try to prevent them at the source.

 

If I'm correct you try to switch this AC Pump with an Dual Relay Bricklet. Since it is an inductive load there might be some sparks inside the relay during on/off switching or other feedback. This must be prevented. Therefore the varistor stuff should be placed at the Dual Relay

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OK, let me then turn the tables for a brief moment.

 

I'm suspecting that TinkerForge gear attracts a lot of people to your products because they are so damn simple to click together and program. You must be attracting a fair proportion of people with little or no electronics experience, like myself.

 

So why doesn't TinkerForge sell its relay bricklets with built-in protection against this, apparently inevitable, reality of electronics? Your docs hint/warn at the problem, but what most customers would probably prefer is simply that the problem goes away.. while keeping the click-and-play simplicity of TinkerForge kit.

 

Any chance of a Dual Relay revision with built-in varistor?

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A future version of the Dual Relay Bricklet will have more protection against EMV. But it is at the moment planned without any varistors or snubbers.

 

 

Varistors will not be on the PCB since you will need other Varistors then our 230V Varistors in the U.S. when switching 115V. One option will be to put different Varistors in the Shop.

 

 

Another possibility would be to add a snubber (RC network) on the PCB. Unfortunately the dimension of the snubber depends on the load and voltage. See this graphic: http://www.mikrocontroller.net/attachment/45846/RC_Glied.pdf

 

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