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This seems like an obvious question but I can't seem to find an answer to it. Can a switch be rated too highly for an application? as in can it be too beefy?

I'm looking for a float switch to use with a RaspberryPI 4. One GPIO pin can source 3.3vdc @ 16mA max (all pins in concert can provide 50mA max) but even the smallest float switches I can find are rated for hundreds of VDC and hundreds of mA. Would such a switch be suitable or would it offer too much resistance? I understand the wiring from the PI to the switch and back again is also a factor.

For reference I'd be looking at getting something like this, and will be connecting two of them to the PI. I'm an EE newbie but am happy to tackle proper answers, so don't hold back!

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  • \$\begingroup\$ the voltage rating is the maximum voltage that can be switched without arcing between contacts, when the contacts separate \$\endgroup\$
    – jsotola
    Oct 26, 2023 at 15:36
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    \$\begingroup\$ Basically there is no too high rating. But large mechanical switches sometimes also demand minimum current/voltage levels to establish an actual contact. Large solid state switches might ask for rather large driving (actuation) requirements. Basically just read datasheets carefully with respect to such requirements. \$\endgroup\$
    – tobalt
    Oct 26, 2023 at 16:17
  • \$\begingroup\$ @tobalt This switch's datasheet doesn't say any minimum current or voltage. Does that mean there isn't one? A few years ago I would have assumed the switch was okay, now I've read more and doubt myself. Is it really a problem, or is it imposter syndrome? \$\endgroup\$ Oct 27, 2023 at 11:31
  • \$\begingroup\$ Since you say that the purpose is to operate a pump to prevent flooding and you are concerned about reliability, is there a good reason that the Raspberry Pi needs to be in the control loop at all? It would sound simpler to have the pump directly controlled by the float switch(es) via a suitable relay. If you want the Pi to monitor whether the float switches operate you could still do that via a separate set of relay contacts. \$\endgroup\$
    – nekomatic
    Oct 27, 2023 at 14:48
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    \$\begingroup\$ related: "Why does a relay have a minimum applicable load?" \$\endgroup\$
    – davidcary
    Oct 27, 2023 at 18:43

5 Answers 5

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Can a switch be rated too highly for an application? as in can it be too beefy?

Yes it can. Many switches that are designed for low currents will operate cleanly (well, switches are never that clean) down to a few micro-amps. It's called "wetting" current and, it's the minimum current that can be reliably switched i.e. break-through any oxide layer or, put another way, the contact doesn't form an oxide that needs breaking through.

the smallest float switches I can find are rated for hundreds of VDC and hundreds of mA. Would such a switch be suitable or would it offer too much resistance?

It will have very low resistance when sufficient current is being switched but, it may form an oxide layer on the contact surfaces that just won't reliably switch a few milliamps.

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  • \$\begingroup\$ Please don't carry on conversations that are not related to the post, it flags the user and can be annoying, please use the chat system. \$\endgroup\$
    – Voltage Spike
    Oct 31, 2023 at 22:36
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Many switches exhibit a phenomenon called the "coherer" effect, which was used in late 19th century radio receivers, but today is more often a nuisance. If two pieces of metal touch at a point which has been exposed to air since the last time current flowed through it, that point may act as an insulator until the potential difference between the pieces of metal exceeds a certain threshold, after which that point will be conductive until the contacts move and expose it to air.

This phenomenon may be demonstrated using a low-voltage (e.g. 1.8V) LED, a current-limited supply, three pieces of aluminum foil, some adhesive tape, and some test leads with clips. Set the supply current for 2 volts and set the current limit to a value appropriate for the LED, fasten two strips of aluminum foil to an insulating surface, connect one strip to the supply, the other to the LED, and connect the other LED to the supply so that when the strips are connected, the LED will light up. Then crumple the third piece of aluminum foil and place it so that it rests lightly on the other metal strips.

Generally, 2 volts will not be sufficient to push any meaningful current through the aluminum foil junction, but if one ramps up the supply voltage the foil will start conducting and it will continue to conduct even if the voltage is ramped down, or even if the supply is disconnected completely and reconnected. The foil may also start conducting if a nearby electric field creates a momentary potential difference. Operating a piezoelectric stove lighter a few centimeters away may be sufficient. If one picks up the crumpled aluminum foil and sets it down again, the junction will often go back to being an insulator until the next time the voltage is raised or an elevated potential is applied.

Most switches aren't made out of aluminum, but out of materials that oxidize more slowly. The same principles may apply, however, for contacts that will sit open for awhile before they are closed.

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Per se, no, there is no "too high rated voltage or current". But as other already mention there can be issues with the mechanical components and their materials with very low current (mA) and voltages (<5V). I recommend using higher voltage, 12V or higher for logic inputs, with a resistor and a zener diode to lower the voltage to CMOS level for the ic. The current can stay in the range of mA.

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I assume you have the pump-drive scheme worked out on the relay side.

In the water industry, there exist several manufacturers of small float switches. Fairly cheap.

------https://www.instrumentation2000.com/flowline-lv20.html---- an example.

They consist of a magnet connected to a contact, inside a sealed, glass tube with wires coming out. This assembly will directly drive the 3.3V Raspi inputs and thus your relay.

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Voltage rating should not be an issue. I have seen 250V toggle switches used in 24V automotive systems. In fact with higher current ratings the contact resistance tends to be lower to avoid power loss.

The switch you linked should work just fine :).

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