Weird project: intentional short circuits: how to protect / switch it off?

Other Hardware General Electronics
1

Hello all,

I'm doing a weird project (won't bore you with the details), where I intentionally want to create a short circuit for a brief moment. I need to connect two cables at 24v, and generate a spark. The cables will be brought apart from an actuator; however, I need to protect the whole thing from burning everything down in case the actuator fails.

What I need is: a way of automatically switching off the 24v line, in case the amps get too high (or the short circuit lasts too long). Important is: the switch would switch back on automatically after the short circuit is absent.

... quite similarly to how table power supplies handle this issue (we usually even hear a * clicking * sound when that happens).

The setup should be able to handle such a thing repeatedly, fast, and as indefinitely as possible.

What is the most secure way of doing exactly this? I'm open to both DIY solutions (to incorporate on my PCB), or standalone devices (I am though avoiding having a whole table power supply just for handling this, as this seems.. well, not at all elegant).

Thanks,

lg

2

What is supplying the power? I would expect a power supply to have current limiting, possibly even foldback current limiting, which means that the current drops to a low level when there is a short.

3

You mention 24 volts and make no mention of the current? I also suggest standing by with a good fire extinguisher if anything burning down is even a remote possibility.

Ron

4

You can generate impressive sparks by short-circuiting a large, charged capacitor. This has the advantage of an inherently limited and very short duration. The spark intensity will depend upon the voltage and the size of the capacitor, so it's something you can control experimentally. Sparks from discharging large caps in this manner can definitely start fires, so you'd want to exercise due caution and use the smallest cap consistent with whatever it is you're trying to achieve. The cap is charged between shorts through a resistor or its internal current limiting.

Whether or not this has any relevance to what you're doing depends on what you're trying to accomplish... which is unclear.
S.

5

Sounds like an xy problem.

It appears what you're really trying to do is create a spark (but you ask for short circuit protection). A great way of doing that is indeed by charging a cap and then shorting that out. This cap can be charged through a resistor from the power source, or if you borrow the circuit of a mosquito swatter it's simply the available power that stops the wires from melting.

If you really just need short circuit protection, there are many options for that, depending on a.o. what current level it should activate.

6

Wouldn't an inductor be better for generating a spark than a capacitor? Put your spark gap across a large inductor, charge the inductor with some current, then break the circuit.

7

I guess it depends on what breed of spark you're after. There are high-voltage, low-current sparks typically produced by an inductor's collapsing magnetic field, such as produced by an automotive spark coil. And then there are lower voltage, high current sparks caused by local heating and dislodging of tiny bits of burning metal. Below is an example of the latter, caused by shorting a cap bank of about 0.1 F charged to about 40v to a grounded aluminum heat sink.
It's still not clear to me what the OP wants though.
S.

B252E3.jpg
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B252E3.jpg
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8

Serious sparks

9

A classic favorite of mine: Arcs and Sparks. The 480 volt three phase short is pretty neat. Some of the high voltage switch gears opening are pretty cool too.

Ron

10

How about a fuse ?

11

PerryBebbington:
What is supplying the power? I would expect a power supply to have current limiting, possibly even foldback current limiting, which means that the current drops to a low level when there is a short.

Still not defined, but I would probably use a Meanwell power supply such as: MEANWELL LRS-150-24 150W 24V Switching Power Supply

My issue is really finding something that reliably and indefinably "switches off" when there is a short, and "switches on" automatically when the short is absent.

Ron_Blain:
You mention 24 volts and make no mention of the current? I also suggest standing by with a good fire extinguisher if anything burning down is even a remote possibility.
Ron

Definitely, the fire extinguisher is there ; ).

I would like to experiment a bit with the current, but something around 5 to 10A are more than enough. Sorry for the lack of info initially...

srturner:
You can generate impressive sparks by short-circuiting a large, charged capacitor. This has the advantage of an inherently limited and very short duration. The spark intensity will depend upon the voltage and the size of the capacitor, so it's something you can control experimentally. Sparks from discharging large caps in this manner can definitely start fires, so you'd want to exercise due caution and use the smallest cap consistent with whatever it is you're trying to achieve. The cap is charged between shorts through a resistor or its internal current limiting.

Whether or not this has any relevance to what you're doing depends on what you're trying to accomplish... which is unclear.
S.

This sounds really interesting. Even though I'm going to experiment with it, for my current application, it won't work. The two ends I'm using to produce the brief spark are a copper plate and a copper rod, and they cannot be changed at this point.

wvmarle:
Sounds like an xy problem.

It appears what you're really trying to do is create a spark (but you ask for short circuit protection). A great way of doing that is indeed by charging a cap and then shorting that out. This cap can be charged through a resistor from the power source, or if you borrow the circuit of a mosquito swatter it's simply the available power that stops the wires from melting.

If you really just need short circuit protection, there are many options for that, depending on a.o. what current level it should activate.

In this case it was really just about the reliable and automatic short circuit protection. Would you have a recommendation for a circuit that could: a) protect reliably a 24v / 5-10A circuit b) turn "on and off" automatically- meaning, when there is a short circuit, it brakes the circuit; when there is not, it closes it.

12

In this case it was really just about the reliable and automatic short circuit protection. Would you have a recommendation for a circuit that could: a) protect reliably a 24v / 5-10A circuit b) turn "on and off" automatically- meaning, when there is a short circuit, it brakes the circuit; when there is not, it closes it.

Sounds like fold back current limiting, which many power supplies, converters and regulators do as standard.

13

srturner:
I guess it depends on what breed of spark you're after. There are high-voltage, low-current sparks typically produced by an inductor's collapsing magnetic field, such as produced by an automotive spark coil. And then there are lower voltage, high current sparks caused by local heating and dislodging of tiny bits of burning metal. Below is an example of the latter, caused by shorting a cap bank of about 0.1 F charged to about 40v to a grounded aluminum heat sink.
It's still not clear to me what the OP wants though.
S.

B252E3.jpg
B252E3.jpg800×608 90.4 KB

Amazing picture! Yes, I believe that is the same case, though at a smaller scale. I'm shorting a 30cm radius copper plate to a 1m long copper rod. When they touch, the spark happens; but at 24v / 5-10a maximum.

Ron_Blain:
A classic favorite of mine: Arcs and Sparks. The 480 volt three phase short is pretty neat. Some of the high voltage switch gears opening are pretty cool too.

Ron

wow! thanks for the link.

hammy:
How about a fuse ?

A fuse (afaik) would need to be changed every time after an overcurrent event; or am I mistaken?

14

A circuit breaker then, that can be reset

15

lgguts:
My issue is really finding something that reliably and indefinably "switches off" when there is a short, and "switches on" automatically when the short is absent.

In this case it was really just about the reliable and automatic short circuit protection. Would you have a recommendation for a circuit that could: a) protect reliably a 24v / 5-10A circuit b) turn "on and off" automatically- meaning, when there is a short circuit, it brakes the circuit; when there is not, it closes it.

You could put a small (0.1 ohm) sense resistor in the high current line and feed the voltage drop into an Arduino. If the current (and voltage drop) goes too high, then the Arduino cuts the power (via MOSFET or something). The trick is that you program in a short delay. Otherwise, the circuit will immediately come back on after power is cut, because the overload condition no longer exists.
I think this would work but should not be your only safety measure. It's hard to beat a slow-blow fuse for simplicity and reliability in the event of failure of the Arduino mechanism.
S.

16

PerryBebbington:
Sounds like fold back current limiting, which many power supplies, converters and regulators do as standard.

Indeed! (and it is great to know its correct technical name, thank you for that!).

But I believe this mechanism may have its limitations too. After "abusing" of this mechanism, it has stopped working at my cheap table power supply. Thus, my idea is to incorporate a safe, durable, reliable form of it directly into my custom PCB. But maybe a very good (= expensive) power supply is best for this case?

17

srturner:
You could put a small (0.1 ohm) sense resistor in the high current line and feed the voltage drop into an Arduino. If the current (and voltage drop) goes too high, then the Arduino cuts the power (via MOSFET or something). The trick is that you program in a short delay. Otherwise, the circuit will immediately come back on after power is cut, because the overload condition no longer exists.
I think this would work but should not be your only safety measure. It's hard to beat a slow-blow fuse for simplicity and reliability in the event of failure of the Arduino mechanism.
S.

Humm nice idea! Though I really don't want to leave this level of security only to the microcontroller's operation, this would give me even more control of the open/closed time...

18

lgguts:
But I believe this mechanism may have its limitations too. After "abusing" of this mechanism, it has stopped working at my cheap table power supply. Thus, my idea is to incorporate a safe, durable, reliable form of it directly into my custom PCB. But maybe a very good (= expensive) power supply is best for this case?

If it's well designed it should not matter how many times you short it out.

19

When I was a boy my dad had a motor - generator* set made from an old electric motor, probably about 1/3HP, and a generator from a car, along with the associated electromechanical control system. I used to abuse this relentlessly as I thought the sparks were good fun. It never complained.

*Generator as in DC generator, as in armature with a commutator and brushes, and variable field current to control the output.

20

Other than the Arcs and Sparks I linked to previously I like using this video as an example of what relay and switch contacts see when switching AC verse DC current.

They are using a common heater element as the load and a common knife switch. Note the use of gloves. :slight_smile:

Ron