Digital switch IC vs. relay
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Digital switch IC vs. relay

by joshuabardwell on Fri Aug 23, 2013 6:10 pm

In my project, I need to switch a signal at 3v or less, 4 mA or less. Is there any reason to chose a relay over a digital switch IC like, for example, this: http://www.mouser.com/ProductDetail/Vis ... 7AodUncAfw

My understanding of relays is that they're usually used when you need to 1) switch a much higher voltage than the logic circuit, or 2) switch a high amperage. Since neither of these hold in this case, it seems like the digital IC would be more compact and cheaper than a large relay, but being very new to ICs, I'd like to confirm.
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Re: Digital switch IC vs. relay

by franklin97355 on Fri Aug 23, 2013 8:22 pm

Depending on what you are switching you may be able to use a transistor or mosfet.
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Re: Digital switch IC vs. relay

by joshuabardwell on Fri Aug 23, 2013 10:02 pm

franklin97355 wrote:Depending on what you are switching you may be able to use a transistor or mosfet.


Can you help me understand why that might be preferable?
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Re: Digital switch IC vs. relay

by adafruit_support_bill on Sat Aug 24, 2013 5:20 am

The switch linked is designed for switching analog signals. If you are switching a digital signal, a transistor or mosfet would likely be cheaper.
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Re: Digital switch IC vs. relay

by joshuabardwell on Sat Aug 24, 2013 8:51 am

adafruit_support_bill wrote:The switch linked is designed for switching analog signals. If you are switching a digital signal, a transistor or mosfet would likely be cheaper.


I see what you mean regarding cost. Adafruit sells packs of ten transistors for something like a dollar. I guess the reason it'd be more appropriate for a digital signal is that it's not going to pass an analog voltage through unchanged?
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Re: Digital switch IC vs. relay

by adafruit_support_mike on Sat Aug 24, 2013 9:13 am

Generally speaking, yes.

By themselves, transistors aren't linear.. changing the input by X doesn't change the output by kX where 'k' is some constant. You can build circuits that force them to be linear, but it takes a bit of work.

What kind of signal are you trying to control?
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Re: Digital switch IC vs. relay

by adafruit_support_bill on Sat Aug 24, 2013 9:27 am

It would help if you could tell us more about what you are trying to do. The term 'switching' is somewhat ambiguous.

On the one hand, it can refer to simply turning something on and off - usually with a digital signal. The load can be anything from an LED to an Arc-furnace. And the choice of transistor, mosfet, relay, scr, contactor etc depends a lot on the nature of the load that you are switching.

On the other hand, it can refer to switching between two or more signal streams. These could be digital signals (RS-232, USB, HDMI Video, etc.) or analog signals (audio, NTSC video, etc). There are many specialized chips for different signal types.
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Re: Digital switch IC vs. relay

by joshuabardwell on Sat Aug 24, 2013 6:42 pm

Sorry for the ambiguity. I try to describe the problem in as abstract a term as possible, to avoid boring people with unnecessary details, but I see that's usually not helpful. The signal is from a welder, through a switch on the welder's torch. The switch is used to activate the arc. The welder is putting 2.7 volts on one pin, the switch is normally-open. Presumably, the welder has a voltage threshold over which it will consider the pin to be "high", but I don't know exactly what that is. I don't have a proper DC power source to put direct, controlled voltage on the pin to test it out. Maybe I could put a 10k pot on a 5 volt digital output set to HIGH and raise the pot until the welder trips, then measure the voltage on the line.

The intent of the device that I'm building is to act as an intermediary between the welder's remote inputs (torch and pedal) to cause the welder to act in more sophisticated ways. But the simplest thing the device has to do is to be able to pass inputs through transparently. So when the Arduino detects that the torch switch is pressed, it needs to connect the equivalent two pins on the welder's input.

EDIT to add: just to be clear, the switch is used to activate the arc, but the switch is NOT carrying the full welding current. The switch circuit is something like 2.7 volts at 4 mA.
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Re: Digital switch IC vs. relay

by adafruit_support_mike on Sun Aug 25, 2013 6:42 pm

Okay, that's a 'digital switching' application. You can use a bipolar junction transistor like the 2N2222 in place of the switch.
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Re: Digital switch IC vs. relay

by joshuabardwell on Mon Aug 26, 2013 2:57 am

Thanks very much. I'll pick up a batch of them on my next Adafruit order.

On a related note: I really need something like Adafruit Prime. I'm spending probably 1/5 as much on shipping as I am on ordering items, and that's even WITH me trying to hold out on placing orders until I have saved up a bunch of things to buy.
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