Homemade/DIY leaf switch failing at the contacts, need better contact material?

Thread Starter

Hamlet

Joined Jun 10, 2015
448
I'm experimenting with kinetic sculptures, using small motors and solenoids. I need switches with low friction, so
I'm attempting to make my own leaf switches. (Hall sensors or any silicon solutions are out, because of the required voltage,
reed switches can't handle the current, and mercury switches aren't fast enough.)

I'm switching DC at 2v - 3v, and about 2A to 5A. I'm using an R/C snubber 2ohm/4.7uF, but it doesn't seem to be
helping. I can faintly see an arc when they open.

For my leaf contacts, I'm using contact material that I salvaged from a relay rated for 120v AC, 20A. After about
1000 cycles, the contact becomes pitted and fails, with a reading of mega-ohms.

I don't know if a contact material found in an automotive relay could be better suited, or maybe I should try using
silver, graphite, or motor brushes for contact material? Perhaps my R/C snubber isn't large enough?
 

Thread Starter

Hamlet

Joined Jun 10, 2015
448
Contact materials: https://electronic-products-design.com/geek-area/electronics/relays/relay-contact-materials

For switching 2V to 3V, I don't see why you cannot use "silicon." Logic level N- and P-mosfets should be able to do that at the currents you mention with negligible voltage loss (several mV).
I didn't know Mosfets went that low? 2v??

I don't know why I forgot it, but I've now added a free-wheel diode to my test, and in a dark room, I can't see any arc.

I'll let it run for some time, and see if it holds up.
 

jpanhalt

Joined Jan 18, 2008
11,088
You do not want Vgs(th). That is when it starts to conduct a small current, e.g., 250 uA. You probably want something that turns on more at a lower voltage. There are lots of them today, here is just one example (SiR178DP):

1608490192793.png1608490361197.png

Vgs(th) is 0,6 to 1.5V. It is characterized to even lower voltage.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
448
Okay, if I'm not looking for Vgs(th), then what am I looking for? How do I find the full-on voltage?
 

ElectricSpidey

Joined Dec 2, 2017
1,761
The full on voltage of a MOSFET is known as "Drive Voltage".

But, you don't always need the full "Drive Voltage" to handle a given load.
 
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jpanhalt

Joined Jan 18, 2008
11,088
Unfortunately, I have never found a compilation of Vgs(on)*. Even if you look for "logic level" mosfets, that is not very rewarding. A lot of devices won't show.

It's really a matter of hunting. Parametric searches include Vgs(th). So, I would start with the low ones and see their transfer characteristics or Rds(on) v, Vgs.

*Probably because there is no consensus of what "on" is. It depends on what you need.
 
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Thread Starter

Hamlet

Joined Jun 10, 2015
448
So, for the device SIR178DP, we see:




Drive Voltage (Max Rds On, Min Rds On)2.5V, 10V

This always confused me, Max & Min Rds On??

Rds On (Max) @ Id, Vgs400µOhm @ 30A, 10V

I understand this to be a sampling of Rds On, basically, at the current and gate voltage, 30A & 10V...

Vgs(th) (Max) @ Id1.5V @ 250µA

Again, a sampling. But I'm getting lost... Is this the minimum current thru the device??
 

jpanhalt

Joined Jan 18, 2008
11,088
What you are interested in is the equivalent Rds at a particular Vgs*. Some datasheets actually show those curves; others show drain current vs. Vgs with a specific Vds. Ohm's Law allows you to calculate Rds from that. Remember, N-mosfets sink current; P-mosfets source current.

As for the maximum Vgs, that is like asking , what's the maximum speed of my car when you are driving in a school zone. It really doesn't matter, unless you have a drive circuit that may exceed it. The max is often something like 10V to 20V, which in your case doesn't matter. A lot of MCU's today are spec'd at 1.8V. That is, the mosfets ("CMOS") in them operate satisfactorily at that voltage. Of course, they do not provide much more that 25 mA at the pins either.

*For example, an Rds of 1Ω is quite high given that your current will be on the order of 2A, but it would not be too high, if your current were 2 mA. So, given the maximum current you anticipate and the voltage you need, you can calculate the maximum acceptable Rds at the voltage you have for Vgs.
 
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Thread Starter

Hamlet

Joined Jun 10, 2015
448
Thanks jpanhalt, that's some good 'spaining. I've always had it drilled into me that Rds On is everything,
then work backwards. Now, it's more like balancing needs vs optimums. The closer you get to a perfect mosfet, the sooner you get toward depletion-mode, haha.

As an update, my leaf switch has been working continuously with both an r/c snubber and flyback diode, and steady
down to 1.4v so far. That's good, because I'd perfer my kinetic sculptures to operate on a single "D" cell.
 
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