I have a capacitive system which I need to power via resonnance using a coil. Resonance occurs at several frequencies, therefore I need a different coil for each frequency. So far I've manually switched the coil whenever I needed to use the other resonance frequency. However I feel like this process can be automated, so I've designed a shield for my arduino that allows me to do just that, to switch coils using a button or later an automated command coming from the computer to the arduino's serial. (The arduino here is an UNO).

Since it's using alternative current, I needed to use a TRIAC. The design I've come up with so far looks like this

Coil switcher schematic

It works this way : The Signal + connector at the top goes into a function generator with an amplifier which outpus an AC voltage which can range from 5V to 20V in higher voltage uses. The Signal - connector at the bottom goes to my capacitive circuit, which itself is connected to a shunt resistor and eventually back to the function generator.

On the shield, as can be seen is a small 2 poles connector onto which the coil is plugged. There's another coil-switcher right next to this one as I'll be using 2 coils on this system but that I haven't shown as it's exactly the same as the first one. There's a small opto-TRIAC (the MOC3031M) to separate the arduino from the rest and to trigger the actual TRIAC. I drive the opto-TRIAC using a simple MOS from the arduino.

My question now is the following : Is that a good way to control such a system?

What I really only want to do is to be able to select between one coil or the other without having to physically change the component.

  • \$\begingroup\$ I changed the part about the voltage being used, as it was unclear before. It does not oscillate between 5 and 20V. These two values actually indicate some of the possible voltages used in this device. \$\endgroup\$
    – saeleko
    Nov 29 '16 at 17:14

One problem with this circuit is to turn off the TRIAC after it has been turned on. To turn off the TRIAC, the voltage between MT1 and MT2 needs to be zero. This circuit has the voltage from the amplifier, which only goes down to 5 V, and whatever voltage is left on the capacitor.

I would use an analog switch instead.

  • \$\begingroup\$ Ah I meant the maximum voltage was of 20V in higher-voltage uses, and 5V for lower voltage uses, it's actually "true" ±AC current, it does go down to 0V so the TRIAC is able to shut down. I was more worried about the TRIAC not turning on. I could use an analogue switch, but I was worried about the high voltage peaks caused by the resonnance, that's why I went with a TRIAC. I'll edit my post. \$\endgroup\$
    – saeleko
    Nov 29 '16 at 17:11
  • \$\begingroup\$ You can use the same circuit from an analog switch IC, but made from higher voltage discrete components or FET arrays. See MT-088 Analog Switches and Multiplexers Basics analog.com/media/en/training-seminars/tutorials/… for schematics of what is inside an analog switch. What values of capacitance across the switch and switch on-resistance can you tolerate? Would need to know the L, C and Q values of the resonance for this. Sometimes a relay is the best solution, or maybe a TRIAC is okay, but the TRIAC will be switching off and back on every half-cycle. \$\endgroup\$ Nov 30 '16 at 8:10
  • \$\begingroup\$ Thanks for the link. The issue with the analog switches is that the ones I've found don't really tolerate very high voltages, also the resistance should be low as well. During resonance my circuit can easily reach 100V and most of the analog switches I've found barely support ±22V (±25 in your link). Seems like it's their limit. I'm not worried about the TRIAC turning off and on as the opto-TRIAC that drives it will always be on. I guess I'll go for a relay instead, it seems more adapted to that situation, once ON it's like a wire. \$\endgroup\$
    – saeleko
    Nov 30 '16 at 9:13
  • \$\begingroup\$ Turn off the AC and let the signal die out before you switch the relay. Otherwise, the resonator will dump its energy into an arc across the relay contacts, which will lead to premature failure of the relay. This technique is called 'dry switching' and might be mentioned in the relay datasheet. \$\endgroup\$ Nov 30 '16 at 17:06
  • \$\begingroup\$ Oh right, I'll keep that in mind, I remember that from my electronics lessons but it has been a while since then, I totally forgot about that. \$\endgroup\$
    – saeleko
    Nov 30 '16 at 18:53

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