3
\$\begingroup\$

I have looked at many switches, most recently I considered snap action switches (micro switches). What I prefer to use is a PCB mounted switch, but I cannot find any that can tolerate the V DC ratings requirements.

What are some other options I should consider?

Or is it not a big problem to use a switch rated for 250 V AC in a DC circuit, especially if the number of times the switch will be actuated with an energized circuit is low (few dozen)?

For example, this D2S-10L13D...datasheet. It is rated at 250 VAC 10 A.

Also, will two switches in series allow for double the switching voltage capacity?

\$\endgroup\$
  • 2
    \$\begingroup\$ The switch you mentioned has DC voltage rating too. \$\endgroup\$ – Long Pham Feb 8 at 3:30
  • \$\begingroup\$ Is there a specific reason for a mechanical switch? \$\endgroup\$ – Peter Smith Feb 26 at 16:03
  • 4
    \$\begingroup\$ How about a normal mechanical switch used to close a relay? \$\endgroup\$ – Scott Seidman Feb 26 at 16:05
  • \$\begingroup\$ The switches are NC. Placing the enclosure cover actuates them to open. Their purpose is to discharge high voltage capacitors through power resistors when the cover is removed. \$\endgroup\$ – A.S. Feb 26 at 18:36
  • \$\begingroup\$ If you only have one polarity, why not a MOSFET? \$\endgroup\$ – winny Feb 27 at 8:27
5
+50
\$\begingroup\$

With switches reliability is an issue, if a switch stopped working then a product worth hundreds or thousands of dollars might not be able to turn it on all because of a switch. If you read switch datasheets they will most likely give a reliability number for actuation or lifetime.

The problem is the current running through them affects the reliability, the more current there is, the more likely there will be problems with arcing and heating, both of which reduce the lifetime of the switch. (or if there is many times the rated current, it could heat the contact and melt materials or blow out the contact by vaporizing it!).

Switches are drawn in schematics but they don't usually show the resistance associated with the contacts (and other metal in the switch)

Typically the two worst enemies for switches are heating and arcing. AC is much easier on contacts because it extinguishes arcs (it crosses 0V many times a second) and because of RMS values (10Vpk AC has less power than 10V DC because it is not constant\sine wave). This means AC will heat less and have less problems with arcing VS DC. Less arcing and heating also means less wear on the contacts and longer lifetime.

What are some other options I should consider?

Use the smaller switch to drive a larger relay if you need higher loads. In product's I've designed that are 1kW or more, I've used a smaller switch to drive a relay and then that turns the product on.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here Source: Using a momentary push button as a latching on-off toggle switch

Typically the smaller switch is used to actuate something larger (like the circuits above). Typically a relay, SSR, or an electronic switch like a mosfet or BJT. Surprisingly enough, electronic switches are more reliable than mechanical ones because they don't have contacts that can break down (and oxidize). These days the latest transistors can switch hundreds of volts.

Or is it not a big problem to use a switch rated for 250 V AC in a DC circuit, especially if the number of times the switch will be actuated with an energized circuit is low (few dozen)?

The ratings are designed for lifetime and reliability for millions of actuations, the datasheet says if you keep the switch within the given ratings (voltage, current, temperature, humidity) it will last for tens or hundreds of thousands of actuations (50k for the D2S-10 and 200K for the D2S-01). If the switch only needs to last for hundreds of actuations, it could probably be derated a bit.

If the application is for a lab or hobby project, the ratings could be considered differently than a certified product (RMA's and service calls are very expensive to companies). If your in a lab and the switch breaks, buy a new one, or get two when you order them in case you burn out the first one.

enter image description here

Only the D2S-01 is technically rated (if the product needed to be tested at an ETL like UL) for DC and only 0.1A, Why is that? Because the switches were tested at those levels and ratings. Since most of the lifetime reduction in a switch is from either heating or arcing, these switches must be more susceptible to arcing from inrush current and opening the switch as it mentions that as being a leading factor in modeling reliability (pg 4 of the datasheet)

Looking at the datasheet for the D2S-10L13D shows that it has roughly 50mΩ of resistance, so I don't think heating is a leading factor as the D2S-5 and D2S-10 have current ratings of at least 5A, at 50mΩ this equates to 5W to 1W of power (P=I^2*R), which the DC rating would only have 5mW of heating with it's rating of current. enter image description here

Also, will two switches in series allow for double the switching voltage capacity?

No, because one switch will always close faster than the other one, and you'll still have roughly the same voltage across the other switch as it closes. The current will remain the same.

\$\endgroup\$
2
\$\begingroup\$

DC is much harder to switch at high voltage than AC. The reason for this is arcing. AC current goes to zero 100-120 times per second, allowing an arc to extinguish. DC does not have this luxury.

To use a single switch, you would have to find one with a DC rating high enough. They wont be cheap. Here is an example of one.

An alternative approach that you will see used often is to use a small switch to activate a relay that switches the high voltage. This can be done quite affordably because you can use a small value switch and relays can be very cheap.

\$\endgroup\$
  • \$\begingroup\$ The reason I am using mechanical switches is because they are meant to discharge high voltage capacitors when the enclosure is opened. These are NC switches, the enclosure cover will physically actuate these so they are open. When the cover is removed the switches close a path through power resistors to discharge 400vdc to 2000vdc down. Also, why are cheap relays rated for higher voltage? Why can't the same switch be actuated mechanically instead of by a relay coil? \$\endgroup\$ – A.S. Feb 26 at 18:35
  • \$\begingroup\$ What types of mechanical switches are appropriate for a 200 V DC 20 mA circuit? Definitely not a single switch if you actually want it to discharge 400vdc to 2000vdc down. Please add the relevant information to your original question. Please also describe the purpose. It looks to me this is more a protection measure (single event) than opening and closing the box 100 times a day. \$\endgroup\$ – Huisman Mar 1 at 15:46
0
\$\begingroup\$

These switches in the <2A category are all gold plated to permit the low current non-wetting requirement and rated for non-reactive loads (resistive).

As the open switch contact gap gives good margin for 1kV/mm breakdown from arcing, the reactive surge to a cap or open surge from an inductive wire can create and small arc that burns the micro" gold plating on the contacts and wears out, then you have oxidation problems over time with rising contact resistance or in other words the contacts become an insulator.

For this reason, DC contact switches are heavily de-rated for even small inductive loads or else you suffer long term reliability issues. Thus semiconductor switches can be more reliable or suitable.

The alternative is a vacuum glass encapsulated tin over nickel-plated steel contact reed relay

\$\endgroup\$
  • \$\begingroup\$ WIth so-called "dry switching", my understanding is that it only applies to low currents and low voltages. With higher voltages and lwo currents, won't the high voltage be able to just punch through the oxide or corrosion deposits on the contacts? \$\endgroup\$ – Toor Feb 28 at 19:51
  • \$\begingroup\$ Yes typ. wetting current are spec'd at 10% of rated and this can be done on non-gold plated sense contacts as I once did with an R pullup Cap. discharge to gnd. But AFAIK for gold plated, it depends on the frequency of use as oxidation rises with temp if left ON or OFF at room temp for a long time between arcs and thickness of gold plating critical for telephony relay switches. I once had 96 of these 30A power relays with 1A sense contacts by P&B that got oxidized and failed to work for TTL on line lines which I fixed by using 10k up and 22uF to Gnd \$\endgroup\$ – Sunnyskyguy EE75 Feb 28 at 19:56
  • \$\begingroup\$ Well yeah, if you have gold contacts it'd just be a waste just because it'd foul up or burn off the gold plating and there wasn't much point for the gold to be there to begin with. \$\endgroup\$ – Toor Feb 28 at 19:58
  • \$\begingroup\$ Yes so the fine print must say to be aware of the impact of reactive loads without explicit MTBF curves. However Omron as I recall offered the best MTBF curves vs load types. \$\endgroup\$ – Sunnyskyguy EE75 Feb 28 at 19:59
0
\$\begingroup\$

Putting two relays together will NOT solve your problems, theory says if you have similar impedances the current will split equally, however, the differences in manufacturing make impossible to guarantee that (thinking in mechanical devices), also, always one will connect first, and in consideration of the fast constants involved, one relay will take all the wear making the problem bigger.

If the power is too much for a switch, maybe a strategy with PWM to slowly show the load to the power supply/switch is a good idea.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.