Silly Question on power switches for boards w/props
Tymkrs
Posts: 539
Hey all,
I'm still working on a board design involving a Prop (what else?!) and I'm trying to find a proper power switch. According to the PropBOE BOM, it's using a slide switch that's good for 0.1A at 12V - but should I be using a 0.3A at 12V instead given that the Prop can source 300mA at its max? (ideally maybe even 0.4A). Just wanted to see what others have been using. I've been looking for switches on digikey, mouser, and octopart, and as soon as you hit the 0.4A range the price goes way up, or if you're at the 0.3A mark and try to go from 5V to 12V, the price goes up and availability goes down....
Thanks for any suggestions!
Addie
I'm still working on a board design involving a Prop (what else?!) and I'm trying to find a proper power switch. According to the PropBOE BOM, it's using a slide switch that's good for 0.1A at 12V - but should I be using a 0.3A at 12V instead given that the Prop can source 300mA at its max? (ideally maybe even 0.4A). Just wanted to see what others have been using. I've been looking for switches on digikey, mouser, and octopart, and as soon as you hit the 0.4A range the price goes way up, or if you're at the 0.3A mark and try to go from 5V to 12V, the price goes up and availability goes down....
Thanks for any suggestions!
Addie
Comments
http://www.digikey.co.nz/product-detail/en/GF-123-0054/CWI333-ND/4089770
Heres a quick search with filters. Not sure if its exactly what you are after but its a start
http://www.digikey.co.nz/product-search/en?FV=fff40011%2Cfff80501%2C1140003%2C1140050%2C2080001%2C3ac0011%2C8780002%2C8780008%2C878000b%2C8780010%2C878001d%2C8780026%2C8780027%2C878002e%2C8780030%2C8780036%2C8780037%2C8780038%2C8780039%2C878003e%2C8780049%2C878006f%2C878007a%2C8780093%2C8780097%2C878009b%2C87800a5%2C87800a8%2C87800b1%2C87800b2%2C87800b6%2C87800b8%2C8780137%2C8780181%2C8780187%2C8780193%2C8780194%2C8780211%2C8780220&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25
EDIT: I applied way to many filters so this is a short list haha
Addie
After trying to find something that is both durable and small, I gave up and just use a toggle switch that is on the power cable and eliminate trying to put a tiny power switch on boards.
I know this is not exactly what you want, but it just might be the best way to build. I even distrust the longevity of the slide switches that Parallax puts on their Propeller Proto Board and expect that eventually I will have remove and bypass the devices when they fail. (We have a lot of corrosive air pollution in Kaohsiung due to a local steel industry and high humidity.)
I also have trouble with the tiny Reset buttons failing, and the DIP slide switches. On board I tend to stick with jumpers instead of switches.
-Phil
The problem with low current mechanical switches the same as with the contacts on a mechanical relay. The type of load might significantly accelerate their failure. Often a relay is de-rated as much as 60% or 80%, to assure the contacts will survive over the long-haul. So a 300ma load, might be better accommodated with at least a 1 amp mechanical switch. And a tiny low current switch has a much better chance at a long life if it is just toggling a MOSfet that handles the real power.
In a MOSfet situation, even a DIP slide switch might actually survive. Like these ===> http://www.goldmine-elec-products.com/prodinfo.asp?number=G17778
And if the switch is in a DIP socket, it is easily replaceable.
The DIP slide switches do come in a single switch unit, AND you can bend the tabs flat and outward to solder as surface mount. Try here +++> http://ex-en.alliedelec.com/search/productdetail.aspx?SKU=70128458
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There is the additional issue of miniaturization creating greater sensitivity to ageing and wear.
So I just prefer using a traditional switch with a proven track record that is very over-rated for both volts and amps. Not only is the longevity more assured, these devices are much cheaper. You can get them at places like Electronic Goldmine. And not everything has to be on the circuit board... a lot of creative and better solutions can be resolved with wire.
http://www.goldmine-elec-products.com/departments.asp?dept=1180
Everything that is new and miniature is not always the best solution for durable use.
This would mean there's always a little bit of current flowing through the pull-up right?
I don't suppose I could talk you into a link to an appropriate pMOSFET? I confess to not understanding many of the search parameter when attempting to find one at Digi-Key. I got the results down to 38 pages but I really have no idea (or very little idea) of which would work well for this application.
-Phil
What would you suggest for a good mosfet in this situation that can handle up to 1 amp?
And I was thinking a 10K or 50K ohm pullup resistor. Is that about right? Any smaller seems to always give me trouble with sensitivity from outside influences (touching the board, inductive loads nearby, and so on).
Is this good? http://www.onsemi.com/pub_link/Collateral/MMFT3055VL-D.PDF
-Phil
I think you started off on the wrong foot when you said that the Prop can source 300mA max, that is NOT an operating figure, these relate to ABSOLUTE MAXIMUM RATINGS, a bit like red-lining, something might happen (or stop happening). The datahsheet says in this section "Functional operation of the device is not implied at these or any other conditions in excess....".
Maximum current that the Prop itself would draw with all 8 cogs @80MHz running flat out would still be under 100ma. A power switch that's rated for 100ma is good for 100ma. The other thing is that if your power supply utilized a switching regulator then the actual current drawn at 12V would probably be more like 50ma.
BTW, a very compact and cheap switching regulator is this one here.
Peter J always brings something new to the table and sets me thinking.
One of the quirks of living in Kaohsiung is that the only 3.3VDC linear regulars that I can buy locally are a 4 wire device that has one extra pin used to Enable the device. I had never thought about it before, but the devices come in 5.0V as well as 3.3V. And they certainly can use a 100ma switch to toggle their 1 amp capacity.
http://www.onsemi.com/PowerSolutions/product.do?id=NCP691
Here are a couple of tiny ones .. 150ma and 350ma, but I have some 1.0 amp ones amongst my clutter.
http://www.onsemi.com/PowerSolutions/product.do?id=NCP600
http://www.onsemi.com/PowerSolutions/product.do?id=NCV8772
I would use a through hole version of a slide switch so customers don't complain about issues with it coming loose or off. Also, most switches are rated for higher voltages so they will handle more current at lower voltage. You may also see ratings for peak (highest for a moment), average, and continuous (same for a long time) voltage.
Many companies, such as Cherry, offer samples and will help to find you what meets your needs.
A switch is one of those items that gets a lot of use so spend a little more for a quality component.
I like is the e-switch 200 series toggle switches, and the 200U in particular. Not only is it small, it is sealed to IP67 (dustproof and immersible to 1m depth). That allows it to face up to a lot of environmental challenges. It is a dry-contact switch. That is the kind of low-current switch we have been talking about, and you will see a lot of them rated at 0.4VA. What that means is that you multiply the current the switch will be carrying (especially at the instant it needs to be turned off) times the voltage it will have to sustain as it opens.
Dry-contact switches have a thin gold layer over other metals, typically gold then nickel over a springy base such as beryllium copper. If the circuit stays within the 0.4VA rating, an arc will not form as the switch opens, and the thin gold layer will stay intact for the ~10000 activations typical of these switches. Switches rated for higher currents may have silver contacts, and oxidation is not so much of a problem due to the high conductivity of silver oxide. Or they use a wiping action to keep the contacts free of corrosion. Some even incorporate a magnet to help suppress the arc.
For a pmosfet, I like the FDN304, ID=2.4A and Vgs=0.8V.
Thanks for your various suggestions of wisdom! Sounds like a throughhole is the simplest solution, but I will carefully read through everything folks have been suggesting in terms of using pmosfets. That said, I may just need to hide the solderpoints with a lrf!
Addie
I like to clinch or bend the leads 90-degrees so they're parallel to the board and trim the lead where the pad ends. Removing the part later though can be a hassle.
I also prefer eutectic or 63/37 Sn-Pb solder because the joint hardens immediately after the iron is removed.
Some people bend the leads slightly to keep the part in place while they solder it and then trim the excess later, usually near the end of the solder cone that's formed.
Thanks. In any event, the MMFT3055VL is apparently discontinued. I forgot about high side switching versus low side switching.
I still think that just using a voltage regulator with Enable is a solid solution.
Yes it is, but from my experience solid or simple solutions (IOW best) are rarely followed or applied in this forum
BTW, many shutdown regs have fairly low absolute maximum input voltage ratings so this needs to be checked.