Using digital output to operate a coil

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MechEdetour

MechEdetour

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Location
NY, USA
Occupation
Design Engineer
This really isn't code related, more of an application question. But I know there are a lot of people here that have experience with controls so I'm gonna give it a whirl.

Is there a component/device on the market that can take a digital output from a device to drive its contacts? I am talking something very simple that doesn't rely on software or programming or anything like that to get it done (such as PLCs). Maybe some sort of relay, but possibly it's called something else?

If I had to simplify things, it'd be something like this:

The device that provides the signal has two states. 0V, or it pulls high. Max sink current for the terminal that provides the signal is 50mA. I want to take this signal and use it to power a coil that would drive contacts rated for 3A at 24V.

There are relays on digikey that have contacts rated for what I need and the coil requires as little as 9mA to switch the contacts (only like $10). Since the digital realm is a bit new to me, can the max 50mA sink current be utilized in a way to drive the coil on a relay? Yes it is either zero or it pulls high, but is there a way to wire a 24V supply across "some coil" to drive some contacts?

The theory behind it tells me it should work. But then my doubt comes from the fact that I'm trying to use a 0V/float signal to power a coil. . .

I started with asking if there is something out there, and I ended with I think there is something will it work.

Thoughts? Will it work? If not, is there another way?
 
160217-2055 EST

MechEdetour:

What kind of engineering degree do you have, and from where? This would greatly assist one to answer your question.

Also I need to know what you mean by sink. I will assume that this means that your source has an NPN ouput transistor with its emitter tied to a common negative rail that is possibly also connected to chassis ground. Since you say it pulls high I will assume that internally there is a positive voltage source relative to the negative rail. We need to know this pull-up voltage and whether pull-up is via a simple resistor, or another transistor. Information on the high state source can determine the output device you use.

If we had an open collector output transistor, with no pull-up, the output transistor had a reasonable maximum collector voltage, like 40 V, then we could put a DC relay coil, 24 V and somewhat less than 50 mA, shunted by a reversed biased diode between your output terminal and a plus supply voltage of 24 V referenced to the common rail, and make a workable system.

More information on your digital output is required and what is the load to be switched?

.
 
ptonsparky said:
Will a solid state relay work for you?
Click to expand...

The type of relay doesn't really matter. So short answer yes.

As long as I can use that signal to operate contacts rated for 3A at 24 Vdc I'm good. The relays I was looking at were all mechanical. What exactly would be different with a solid-state relay that would change how it works (please excuse my "electronic-lessness").
 
gar said:
160217-2055 EST

MechEdetour:

What kind of engineering degree do you have, and from where? This would greatly assist one to answer your question.

Also I need to know what you mean by sink. I will assume that this means that your source has an NPN ouput transistor with its emitter tied to a common negative rail that is possibly also connected to chassis ground. Since you say it pulls high I will assume that internally there is a positive voltage source relative to the negative rail. We need to know this pull-up voltage and whether pull-up is via a simple resistor, or another transistor. Information on the high state source can determine the output device you use.

If we had an open collector output transistor, with no pull-up, the output transistor had a reasonable maximum collector voltage, like 40 V, then we could put a DC relay coil, 24 V and somewhat less than 50 mA, shunted by a reversed biased diode between your output terminal and a plus supply voltage of 24 V referenced to the common rail, and make a workable system.

More information on your digital output is required and what is the load to be switched?

.
Click to expand...

MET graduate. Career started in electrical, and has been so since. Started pulling wires for a panel shop while I was in school and it stuck. With that being said, majority of what I have done in the past has been more "power" as opposed to the "electronic" in terms of what is "electrical."

I miscommunicated one thing. It doesn't pull high, it is 0V or it floats. The pulls high was in conjunction with a pull up resistor to 24V. I didn't mention the resistor. However the solution I am looking for is in the context of using the 0V/float to operate contacts. I understand that a pull-up in conjunction with a FET will allow switching of another circuit. But I am looking for an off-the-shelf type component. But in my eyes, a pull up resistor, in conjunction with something that can switch a 3A circuit, is no longer the "simple" solution I'm looking for.

I am just wondering if it is possible to do it without that.

As mentioned, actually now corrected, digital output is 0V or float, and I need a 3A load switched. When digital state is 0V I need closed contacts. When it floats, I need them to open.
 
MET graduate

Sounds like they either left some important basic electrical concepts out of your classes ?

Or, possibly English is your second language ? Advise you to communicate a little better.

e.G. digital output from a device

What are the digital levels? What is the device?

If the 'digital... device' is an empty (0) or full (1) box of sawdust, then a spring loaded switch with a lever on it will work fine <G>
 
aka
extra credit:

Name 10 ways to find the height of a building with a barometer........
 
160217-25218 EST

MechEdetour:

You can simply do what I described in my previous post using a low coil power relay.

Your output is simply the equivalent to a normally open relay contact to a negative rail (ground if you wish). This is a special NO relay contact. For it to switch positive current flow (not electron flow) has to be present. This means you put a resistance (relay coil resistance or the equivalent) between a positive voltage source referenced to the negative rail (bus or whatever the output switch is connected to) and the output terminal of your digital output. You must stay within the limitations of the digital output. A diode is required across an electro-mechanical relay coil to prevent large switching transients on turn off of the coil.

I do think a DC SSR maybe be a better choice, but it depends upon on what you are switching. The coil side of the SSR should not require a shunt diode assuming it is not a relay coil. But you may need to protect the output switching element of the SSR.

.
 
gar said:
160217-25218 EST

MechEdetour:

You can simply do what I described in my previous post using a low coil power relay.

Your output is simply the equivalent to a normally open relay contact to a negative rail (ground if you wish). This is a special NO relay contact. For it to switch positive current flow (not electron flow) has to be present. This means you put a resistance (relay coil resistance or the equivalent) between a positive voltage source referenced to the negative rail (bus or whatever the output switch is connected to) and the output terminal of your digital output. You must stay within the limitations of the digital output. A diode is required across an electro-mechanical relay coil to prevent large switching transients on turn off of the coil.

I do think a DC SSR maybe be a better choice, but it depends upon on what you are switching. The coil side of the SSR should not require a shunt diode assuming it is not a relay coil. But you may need to protect the output switching element of the SSR.

.
Click to expand...

Thanks for the help I appreciate it. I knew it was going to be difficult to answer because I didn't provide all the info.

junkhound said:
MET graduate

Sounds like they either left some important basic electrical concepts out of your classes ?

Or, possibly English is your second language ? Advise you to communicate a little better.

e.G. digital output from a device

What are the digital levels? What is the device?

If the 'digital... device' is an empty (0) or full (1) box of sawdust, then a spring loaded switch with a lever on it will work fine <G>
Click to expand...

It was left out. Hence why I'm asking. And English is my second language. If I knew exactly how to ask the questions, I probably could have done my own research and came up with a solution eventually. But I didn't so I came here.

Anyways, I ended up speaking to a colleague and he explained to me what I needed to make it work (and why) so I am all set (no barometer req'd).

Edit: And the solution was what gar mentioned. A power relay that draws only 9mA.
 
junkhound said:
aka
extra credit:

Name 10 ways to find the height of a building with a barometer........
Click to expand...
Are there 10? I've never seen more than 7.

One I've never seen or heard:
Drop the barometer down the elevator shaft when the elevator is at the bottom. Using a sensitive sound transceiver, record the sound of it hitting the top of the elevator car and calculate the distance using the Doppler shift method. Add the height of the car to your findings.
 
Jraef said:
Ah, right. For the Doppler effect, you would break the barometer NEXT to the instrument and measure the time it takes for the ECHO to get back.
Click to expand...
Only if the elevator was moving and you cared about its speed and you already knew the characteristic frequency of a breaking barometer.
 
Come on, Use your imaginations

Come on, Use your imaginations

1) Measure the pressure at the bottom of the building, measure it at the top, repeat until you get two bottom readings and two top readings in a row the same (A B A B). Calculate the pressure difference according to altitude tables.
2) Measure the length of the barometer. Move the barometer up the outside of the building, marking the top each time and aligning the bottom with each succeeding mark. Count the number of "barometers" the building is high.
3) Wait for a sunny day, measure the length of the shadow of the building, measure the shadow of the barometer. Use proportions and the height of the barometer.
4) Tie a long string to the barometer, lower it off the roof of the building. Mark string when the barometer is completely on the ground. Measure string. Adjust for stretch and curvature from the wind.
5) Drop the barometer from the top of the building, time release to impact. Assume not reached terminal velocity. Use gravitational acceleration equation.
6) Disassemble barometer. Use mercury to construct a mercury-vapor lamp. Shine from top of building, measure the number of waves between top and bottom. Look up wavelength of mercury. Multiply.
7) Weigh barometer at top of building using spring scale, not balances. Weigh at bottom of building. Calculate differences and calculate difference in altitude from gravitational differences from center of the earth.
8) Offer the barometer to a rock climber in return for them rappelling down the building with a yard stick measuring height.
9) Offer the barometer to a surveyor to have him use a transit and chain to measure and calculate the height of the building.
10) Ascend the outside of the building (assume brick building) scratch a mark on the glass tube for every brick row up the building. Measure a brick and mortar height. Count scratches and multiply.
11) Get a roll of #12 wire at least 2 times the height of the building. Attach a pulley to the top of the barometer, and lower it from the roof retaining the standing end of the wire and the running end. When the barometer touches the ground, strip the running end at the parapet, measure resistance. Calculate length, divide by two, add height of barometer.
12) Get a really long guitar string. Lower it with the barometer attached to the bottom to the ground. Go to the middle floor (taking into account that floor 13 may be omitted). Pluck the string, record frequency. Take a known length of the string, pluck it, record frequency. Calculate lengths by ratios.
13) Get random string, attach to barometer. Lower barometer from roof until it is 1 inch from ground. Make it into pendulum offset from building enough so that it does not rub. Swing it. Time several swings, but finish before rotation of the earth causes it to rub building. Calculate length, taking into account 2nd and 3rd order (at least) parts of the equations.
14) Attach the barometer to a cable of known weight per foot. Lower cable and barometer until it touches ground. Measure weight of combination. Use adjustments as outlines in 4) above. Subtract weight of barometer, then divide by weight per foot, then add in height of barometer.
15) Offer the barometer to building superintendent in return for a look at the as-builts. Read or scale the height from them.
 
Last edited:
You forgot the OSHA/EPA answer: <G>

Measure a step in the stairwell. Put microphones at each floor.

Smash a mercury barometer on the roof and call 911 about a Hg spill.

Listen for the hazmat response team and count the mad scrabble up steps pounding up to the roof.

Multiply by step height. Divide by how many dozen responders there are.
 
fmtjfw said:
15) Offer the barometer to building superintendent in return for a look at the as-builts. Read or scale the height from them.
Click to expand...

He won't have any as-builts. And even if he did, the Contract Documents clearly state that the drawings shall not be scaled. :(
 
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