How to reverse the NO relay logic in a circuit without replacing the NO relay with an NC relay?

Thread Starter

wynna

Joined Jan 29, 2022
4
11122222.png
Sorry if the title doesn't make sense, I don't know how exactly to phrase my question.
I have this circuit and I am not allowed to change any of the components in black. Currently if I supply the 24V, the NO relay shorts and the circuit is able to flow.

I want to make it so that if I supply the 24V the circuit isn't able to flow, and if I don't supply the 24V the circuit is able to flow (basically electrically equivalent to replacing the NO relay with an NC relay)

Is there a way to do this without touching any of the components in black? (i.e. I can only add components to the red circuit portion)

Thank you!!
 

eetech00

Joined Jun 8, 2013
3,154
View attachment 259147
Sorry if the title doesn't make sense, I don't know how exactly to phrase my question.
I have this circuit and I am not allowed to change any of the components in black. Currently if I supply the 24V, the NO relay shorts and the circuit is able to flow.

I want to make it so that if I supply the 24V the circuit isn't able to flow, and if I don't supply the 24V the circuit is able to flow (basically electrically equivalent to replacing the NO relay with an NC relay)

Is there a way to do this without touching any of the components in black? (i.e. I can only add components to the red circuit portion)

Thank you!!
Can you add another 24 relay with a NC contact so that when you apply 24v to the existing relay, the new relay also energizes and opens the circuit?

Like this:

1643517640061.png
 
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Thread Starter

wynna

Joined Jan 29, 2022
4
Can you add another 24 relay with a NC contact so that when you apply 24v to the existing relay, the new relay also energizes and opens the circuit?

Like this:

View attachment 259158
This works but unfortunately I can only add components after the existing relay, I cannot touch the 24V, and am only allowed to add components to the area highlighted in red.

Appreciate you trying to help!! thankstest1.png
 

Thread Starter

wynna

Joined Jan 29, 2022
4
The end result I need is only an NC relay. The most simplest option would be to just swap out the NO relay with an NC relay as the below picture, but unfortunately I can't
test1.png
 

crutschow

Joined Mar 14, 2008
29,792
What are the relay contacts connected to exactly?
Can the load be driven by a transistor controlled by the relay contacts?
 

Ian0

Joined Aug 7, 2020
5,140
What drives the relay? If N can drive in both directions, connect the relay terminal that is currently at +24V to 0V (and reverse the diode that is across the coil)
 

sghioto

Joined Dec 31, 2017
3,288
The TS says he cannot touch the existing relay or the 24 volt connections so until we find out what the N.O. contacts are actually connected to I don't see any resolution.
Question, is this a real world circuit, an exercise or just homework?
 
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sghioto

Joined Dec 31, 2017
3,288
How would one use the contacts on the external relay to meet the requirements?
I suppose this would work assuming the red path is a voltage source that can operate a relay and is allowed.
1643560431849.png
 
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sghioto

Joined Dec 31, 2017
3,288
That is the same as what I showed but still depends on what the N.O. contacts of the existing relay are connected to. The TS has not elaborated on that.
But since it's probably just homework one could assume the existing contacts are connected to a voltage source of some type.
 
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MrAl

Joined Jun 17, 2014
8,990
A relay contact (or switch) in theory is zero Ohms when closed and Infinite Ohms when open.
The dual of that is just the opposite, and so the concepts of parallel and series are swapped because these are duals also.
This means if the relay contact set is in series to begin with, then it must be in parallel to reverse the logical action. If it is in parallel to begin with, then it must be in series to reverse the action. In practice however there are certain requirements and restrictions which need to be considered.

The dualism is entirely theoretical, and the extremes are not always possible due to the level of current or voltage that could be encountered.
For example, if you have a 12v power supply with current limit and a series relay contact, then to reverse the action in theory you would place the contact ACROSS the two power supply terminals. This would cause a lot of current to be drawn perhaps, depending on the current limit setting, but if the power supply was built for that then it should be able to take it. The power supply may even be able to sense the short and cut back power too so little energy is wasted.
If the power supply does not have a good current limit scheme then you can not do this without inserting a power resistor between the power supply and the relay contact. This will however waste some power depending on how much current the normal load needs and what size resistor is needed. This means the converted circuit has to be analyzed carefully. Perhaps you can use a current limit circuit to limit current when the contacts are closed. The current limit circuit can be made vary advanced so that it detects a short and shuts the power supply down to a low level (but not off). This would not be that hard to do because the contacts open and close via an external signal that the current detection circuit just has to sense. We could talk about that in more detail.

The only other way i think is to add an active device like another relay or transistor which others have suggested, but if you understand the above completely you should be able to come up with something.
If you cant use any active devices for the current limit circuit, then you will have to default to the series resistor and series/parallel swap concept. In your case, that would mean adding a series resistor of appropriate size and value and connecting the contacts in parallel after the resistor.
 
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