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Question

Will this switch spark when it is switched?

Intro

I am currently working on a electric longboard and I came across this problem. I have a LiPo battery providing power to the motor through the ESC (electronic speed controller). Now, when the batteries would run out of juice I'll have to charge them and in doing so I'll have to connect those leads to the charger. Most E-board builders use a loop-key to cut the circuit between the battery and the ESC. This is a good and working solution but the only problem is that if you were to forget to remove the loop-key you would potentially harm the ESC. An anti-spark on/off switch is more elegant solution, but still has the same problem.

My question: would an SPDT switch with the following specifications suffice?

Rated Voltage @ 15A 250VAC, 20A 125VAC

Schematic

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ You're using the switch in a DC application, so its DC specs would be far more informative than it AC specs. \$\endgroup\$ – brhans Feb 8 '16 at 0:16
  • \$\begingroup\$ lmao, yes indeed \$\endgroup\$ – Koen Demonie Feb 8 '16 at 0:17
  • \$\begingroup\$ I can't seem to find any of those on ebay :/ ,Do you perhapse have a better site to find them ? \$\endgroup\$ – Koen Demonie Feb 8 '16 at 0:24
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    \$\begingroup\$ should be fine so long as you stop charging or running the motor before switching. \$\endgroup\$ – Jasen Feb 8 '16 at 1:14
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    \$\begingroup\$ That's ok! But fear not - the power of the internet has great leverage here. I personally recommend Mouser, Digi-key and Newark for all your needs - then again, I'm in the US, so I don't know if that will work quite as well for you, not knowing where you are.. RC hobby websites can point you in even more directions. Remember that having a high enough voltage rating isn't necessarily good enough for a switch - it should actually list a DC rating, especially if you intend to open it under load. \$\endgroup\$ – Sean Boddy Feb 8 '16 at 17:56
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If I read between the lines of your question correctly, you're not directly concerned about the spark (e.g. because it might ignite flammable gases), it's the voltage spike that would accompany it that might damage the speed control circuit. Have I interpreted your situation correctly?

If that's the case, you might consider adding a simple snubbing circuit, such as a capacitor across the switch terminals. Such a circuit absorbs the energy that would otherwise create the spark (and the voltage spike that gives rise to it), and dissipates it more gradually.

A capacitor across the terminals is probably the simplest type of snubber, but there are many more sophisticated ones; the next step up might be a simple RC network, perhaps including a diode. It depends on what's drawing the current, how much is being drawn, and how much inductance you've got in the circuit.

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  • \$\begingroup\$ Yes, that is spot on ! Would this switch still be suspicable to being spot-welded closed when my motors will ask for more power? like @jms answered here below ? \$\endgroup\$ – Koen Demonie Feb 8 '16 at 11:09
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    \$\begingroup\$ A switch has four modes: on, off, switching from on to off, and switching from off to on. I think the only mode in which you'd be at risk of contact welding is off-to-on, because there you might have arcing to perhaps do some melting and the contacts are then pressed and held together. If the switch is rated for the maximum DC current you have (including possible surge current), I don't expect they'd spontaneously weld in the on state. I found the resources here useful when I was studying toggle switches: carlingtech.com/switch-learn-more particularly the one on amp ratings. \$\endgroup\$ – scanny Feb 8 '16 at 20:51
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    \$\begingroup\$ continued ... But arcing is typically associated with moving from on-to-off, when you're interrupting a current flow and the circuit inductance wants to keep that current flowing (hence producing the voltage spike). As long as your switch is DC rated for the surge current @jms mentions you should be safe from contact welding. Regarding a possible snubbing circuit, that would make a nice follow-on question, something like: "What snubbing network is appropriate for battery charger switching?". You'd want to describe the voltage and currents involved and the on/off state of the motor and charger. \$\endgroup\$ – scanny Feb 8 '16 at 20:53
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No matter whether you use an AC or a DC rated switch, it will always bounce and it will always spark.

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  • \$\begingroup\$ Is this considered a bad thing? \$\endgroup\$ – Koen Demonie Feb 8 '16 at 0:29
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    \$\begingroup\$ Generally, yes, and more so under DC since since there's no zero crossing and the arc will persist until the the contacts separate enough to quench the arc. Also, under DC, contact metal will always migrate in the same direction, while under AC metal migration direction will be largely random. \$\endgroup\$ – EM Fields Feb 8 '16 at 0:43
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    \$\begingroup\$ But once the switch is fully set to the opposite direction, the spark is gone. So is this still seen as a bad circuit ? \$\endgroup\$ – Koen Demonie Feb 8 '16 at 1:14
  • \$\begingroup\$ It would be better if it didn't spark, but you live with what you have to, and having a sparking switch is better than no switch at all... Most of the time. Plus, there are ways to eliminate the sparking, using external semiconductor switches across the contacts, but that's a whole 'nother subject. \$\endgroup\$ – EM Fields Feb 8 '16 at 2:02
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I don't know what components you have chosen for your longboard, but if the motor consumes say 1 kW of power in bursts, it will draw ~45 A and likely spot weld that switch closed.

If you are concerned about sparking, you can buy connectors that have integrated spark suppression, for example the XT90-S.

XT90-S

Inserting the male connector into the female first connects the negative side and an auxiliary positive contact on the positive side. This auxiliary contact is connected to the positive main contact with a resistor, which allows the ESC capacitors to charge slowly. Inserting the connector further engages the main contacts.

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  • \$\begingroup\$ Yes, i've read about these. but then again ill still be stuck with the problem that I might farm my esc if I start charging my battery without first removing the XT90-S loop key \$\endgroup\$ – Koen Demonie Feb 8 '16 at 11:07
  • \$\begingroup\$ I don't understand the need for a "loop key". Is it some safety feature so that if you fall from the board you yank out a series connector in the process? If you would install the XT90-S female on the battery, then when you would need to charge you would disconnect the battery connector from the longboard and connect it to the charger. \$\endgroup\$ – jms Feb 8 '16 at 12:00
  • \$\begingroup\$ It is used to cut the circuit connecting the battery to the esc. Yes your way is easier to construct but I don't want to open up my board to remove the batteries for charging every time it's needed. \$\endgroup\$ – Koen Demonie Feb 8 '16 at 12:26
  • \$\begingroup\$ Okay, thanks for the explanation. Can't you have a little pigtail sticking out of the side of the longboard with the XT90-S female attached to it? You would plug it to a closely located male XT90 when riding and to the charger when charging. How do you balance the pack, with balance lead extension? \$\endgroup\$ – jms Feb 8 '16 at 12:36
  • \$\begingroup\$ Yes, the balance leads are no problem as I'm only using them for charging. I was thinking of this solution too as I was reading your post. I'll have to draw it out when I'm home to see if it would work \$\endgroup\$ – Koen Demonie Feb 8 '16 at 13:15

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