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I'm having a little trouble finding a way to activate my circuit. I don't have a proper circuit drawing tool available however hopefully my drawing from MS Paint will do.

enter image description here

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EDIT: Switch should be between capacitor and solenoid, not between boost converter and capacitor.

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I have a 12 VDC power supply, that goes through a boost converter, and charges capacitors of 15000 uF combined to 50 V. When I activated my physical switch in the past (with lower voltages), the current would flow from the capacitors and power my solenoid (linear kicker). However, ever since I raised the voltage up to 50 V, my physical switch breaks down (blew up...) and is unable to handle the higher voltages.

I don't have a very good understanding of transistors, but I want to be able to activate this circuit through a 5v input current (from an Arduino).

I have seen a couple transistors, however I am unsure whether it is capable of handling these high voltages, and high currents (i = C*dv/dt) (I don't know di/dt), nor do I know how to hook the circuit up.... I am very scared of linking a 50V circuit to the Arduino... Or do I need a relay instead of a transistor?

V. Kuo
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    *I don't have a proper circuit drawing tool available* Actually **you do** ! Edit your question and use the circuit editor, it is activated with the small resistor/capacitor icon (or press Ctrl M). – Bimpelrekkie Jul 11 '16 at 10:18
  • Oh indeed! However I do not exactly know how to draw up the circuit for the boost converter or the solenoid... – V. Kuo Jul 11 '16 at 10:20
  • The switch operates at 50 V, it is difficult and cumbersome to use a transistor to switch like this from an Arduino. I suggest you use a **relay** or a **solid state relay**, see: https://en.wikipedia.org/wiki/Solid-state_relay Do get a solid state relay which can handle DC at switching side (many only work with AC). – Bimpelrekkie Jul 11 '16 at 10:22
  • The boost converter circuit is irrelevant. 50 V comes out, that is enough. – Bimpelrekkie Jul 11 '16 at 10:23
  • Thanks. I have taken a look at SSR's, however they seem quite expensive. Would there an alternative method? – V. Kuo Jul 11 '16 at 10:30
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    A relay but that also might blow up. Charging 15000 uF **directly from a boost converter** is also **not a good idea**, I would add a (power) resistor in series to limit the charging current to a safe value. Boost converters have an inductor between in and output allowing for very high currents. You should try to avoid those high currents. – Bimpelrekkie Jul 11 '16 at 10:36
  • I see. Thank you for the precaution! Will add a resistor. I will check to see if there are any relays that can handle 50V. Thanks. – V. Kuo Jul 11 '16 at 10:42
  • How much current do you want to control? – JimmyB Jul 11 '16 at 12:10
  • *I will check to see if there are any relays that can handle 50V* The problem is **NOT** the 50 V. The problem is the **very large current** which will flow when the **empty** capacitors are connected directly **without a resistor** to the boost converter. – Bimpelrekkie Jul 11 '16 at 12:33
  • What is the part number of the boost converter? Many DCDC converters have an enable pin which can be used to shut down the output. – Bence Kaulics Jul 11 '16 at 12:43
  • If you replace the MS pain circuit with a proper schematic and ping me, I'll remove my downvote. – skrrgwasme Jul 11 '16 at 17:03

1 Answers1

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Your drawing does not match your description. You say the boost converter charge up the capacitors, then you activate your switch. If the switch is open and located as you've drawn, then there is no way the boost converter could charge anything.

Also, as you've drawn your circuit, the solenoid would always be connected to the capacitors, so I don't see how the capacitors would ever become charged, you'd just be shorting the boost converter through your solenoid with the caps in parallel.

Please draw an accurate picture if you're going to draw one at all. As it stands now, it would actually have been better without a picture, as the one you've drawn is wrong and just confuses things.

I'm assuming you meant to draw the switch between the capacitors and the solenoid, not between the capacitors+solenoid (permanently tied to each other) and the boost converter. In other words, the switch causes the capacitors to dump their stored energy into the solenoid when closed.

It's no surprise your switch destroyed itself, higher voltages translate into much higher currents, especially with that much capacitance. You can ablate pits in screwdrivers with a capacitor charged with 50V. The problem is that mechanical switches begin conducting at the earliest possible time, which is going to be before it has established good contact. The initial connection will not be nearly as low resistance as you'd expect, and with lots of current, it will get quite hot, and you will either vaporize some of the metal contact area, or if the switch is very underrated for the application (like in your case) it will fail in a variety of spectacular ways.

You're doing a one-shot, right? As in, you flip the switch, you dump the energy in the capacitors into your solenoid, and that's it? That was the 'one-shot'. Now you have to start over and recharge before you go again. If that is the case, you should use the component that is designed specifically for this. An SCR. They are built like tanks. The inductance of the solenoid likely limits the current to as little as 50A, but lets assume you have a really badass setup, and the total resistance of the capacitors AND the solenoid is about 3mΩ because you're using 1" square solid copper bars as wiring, and lets say this is a low turn solenoid, and only has 2mH of inductance (unusually low for a solenoid). In those unlikely circumstances, you won't even hit 150A peak. 100A is far more likely.

So, just use this.

It's $1.39, it can handle 160A 60Hz half cycle, which is more than even the most pessimistically (optimistically? Depends if this is a solenoid or a coil gun) high currents you might see and on the time scales they will happen. Those time scales being less than 60Hz half cycle equivalent, thanks to all that inductance. It is solid state so will not slowly be destroyed every time it makes contact like a mechanical switch or relay, and it is specifically built to withstand the stresses you will be subjecting it to. Over and over.

They are incredibly easy to use too - you just squirt a little current into the gate pin, and it will trigger it. The gate is cathode referenced, so if you used it high side (on the positive rail of the capacitors) and had a common ground with an arduino, then the cathode would be effectively ground referenced through the solenoid. You need a little kick to trigger it, the datasheet says 9-30mA. An arduino can do 40mA on one I/O pin, so just put a 150Ω resistor between it and the gate, and you're good to go. Take the IO pin high and the SCR will trigger. Once the current through the solenoid has fallen below the holding current, the SCR will turn off and resume blocking any current from flowing through it. At least, until you trigger the gate again.

In your comments, you mention "SSRs", which I assume is referring to SCRs, are expensive. Expense is relative, but you will not find a cheaper solution than an SCR, so hopefully $1.39 is not too expensive. If it is, there is always taping two wires to a piece of wood and grabbing a shovel. Just close your eyes first. (I speak from experience hehe).

Almost forgot: Put a few beefy diodes the 'wrong way' across your solenoid coil, so they have their cathode going to the positive terminal of the capacitors, and anode to ground. Or your Arduino will explode.

metacollin
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  • Thank you for the long answer! Sorry, I did mean to have the switch between the capacitor and solenoid, will add a note in the original post. Thank you for suggesting an SCR. I did not know these existed (high school student) and will definitely look into it. I am still a little unsure how to wire it all up, and am in particular confused with your last sentence in regards to the diodes. http://electronics.stackexchange.com/questions/33400/my-mosfet-operated-solenoid-circuit-destroys-my-arduino-inputs Would the circuit be similar to the one in this link? – V. Kuo Jul 11 '16 at 14:20
  • I think by SSR he meant "solid state relay". – scanny Jul 14 '16 at 22:40