Need some basic electronics help.

[im2smrt4u]

I want to have a reset switch for a project I'm doing. The device (a VFD) draws a maximum of 675 mAh @ 5VDC. This will be connected to my computer's power supply. I want a switch that will cut the power from the VFD in case there is an error so that the display will reset.

I have a pack of normally closed momentary buttons from Radio Shack (275-1548C) that are rated at 0.3A @ 50VDC. Doesn't the amperage go up as the voltage goes down? What amperage could it handle at 5VDC?

The bottom line is: will these switches work for me? Or do I need higher amperage switches? What would work that is available at a Radio Shack?

 

[im2smrt4u]

Please help!

 

[tontod]

Try posting in the Highly Technical forum, you might get quicker and better answers there.

 

[im2smrt4u]

<< Try posting in the Highly Technical forum, you might get quicker and better answers there. >>



Would this really be considered "Highly Technical"? I guess I'll give it a shot...

 

[NutBucket]

Should work fine. The key is wattage. So your swiches handle 15W, 50V * 0.3A. So at 5V, 15W/5V = 3A. So it should work fine since your VDF only draws 0.675mA @ 5V.

 

[im2smrt4u]

<< Should work fine. The key is wattage. So your swiches handle 15W, 50V * 0.3A. So at 5V, 15W/5V = 3A. So it should work fine since your VDF only draws 0.675mA @ 5V. >>



Okay. Sounds good. The only thing I was wondering was that when I press the button, it simply breaks the circuit and no power flows through the switch, right? Then, when the button is not depressed and the VFD is on, the most the VFD will ever "pull" is 675 mAh, and the power supply won't "push" to many amps through the button, correct? TIA!

 

[PowerEngineer]

Well, they'll probably work okay...

The 50V rating tells you how much voltage the open contacts can sustain without breaking down (i.e. if the voltage is too high, the air gap between the contacts will not be enough to stop curent flowing between the contacts). So your 5V should be a piece of cake.

The 0.3A rating tells you how much current the contacts can comfortably handle when closed. If you put too much current through the contacts, they may eventually melt and possibly fuse shut. So your 0.675A may be a bit ambitious. (The heat here is generated by current (squared) through the resistance in the contact leads and the resistance across the closed contacts, and is not really a function of the supply voltage.)

If this project is something temporary that will not actually be operated often or for long periods of time, then the switches you have will probably be okay. Otherwise I'd look for another switch with at least a 50V/1A rating (but then again I tend to be conservative).

Good luck!

 

[im2smrt4u]

<< If this project is something temporary that will not actually be operated often or for long periods of time, then the switches you have will probably be okay. Otherwise I'd look for another switch with at least a 50V/1A rating (but then again I tend to be conservative). >>



Well, this is meant to be a reset switch, so I shouldn't be using it often. I'll see if they have a larger button next time I'm at Radio Shack though.

 

[im2smrt4u]

BUMP!

 

[thedarkwolf]

<<

<< If this project is something temporary that will not actually be operated often or for long periods of time, then the switches you have will probably be okay. Otherwise I'd look for another switch with at least a 50V/1A rating (but then again I tend to be conservative). >>



Well, this is meant to be a reset switch, so I shouldn't be using it often. I'll see if they have a larger button next time I'm at Radio Shack though. >>



Yes but isn't a normally closed switch normally closed ? So that means the contacts are together untill you hit the switch therefore it will have .375ma more going through it all the time than it is rated for.

 

[im2smrt4u]

<< Yes but isn't a normally closed switch normally closed ? So that means the contacts are together untill you hit the switch therefore it will have .375ma more going through it all the time than it is rated for. >>



True, but the rating is at 50VDC, I'm only using 5VDC. I didn't remember the formulas, but someone came up with 3A @ 5VDC, which seems right to me. Three amps will give me plenty of room...

 

[thedarkwolf]

but I don't think thats how it works. Its been awhile since my electronics class but I believe PowerEngineer is correct and the ratings are seperate and its .3a no matter the voltage.

 

[im2smrt4u]

<< BUMP! >>

 

[etech]

Cat.#: 275-1548
Doesn't have the "C" on the end, but I did not find that model.

Anyway, you should be alright to use them.

The rating on the VFD you are stating is its maximum current draw. It won't draw that all of the time.

PowerEngineer was correct on the way to rate switches. The voltage rating is what voltage the contacts are rated for. Higher voltages require different materials to handle the arc. Lowers voltages require materials that will not corrode because there is no arc to "clean" the contacts.

The current rating is just that, the voltage is not related in this case.

If you are building this as a commercial project I would not use that switch. If you are just hooking up a short term project it should work.

Sometimes the easiest way to find out if it will work is just to hook it up and see if the smoke comes out.