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Help bypass switch with reverse current protection

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#1 mcinnes01


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Posted 26 August 2013 - 11:48 PM



I am trying to find a way to manually override the control of some relays.


The netduino uses mux to control 24v which switches the coil voltage of some relays. I want to manually control the relays in case something goes wrong on my control board or with my software.


The manual switches will carry 24v from the same supply that switches the relays on my control boards. Things that I am unsure about include, if the switch is closed supplying 24v to the the relay coil, but the control board is not is closed, how can I ensure current from the switch doesn't travel backward in to the control board mux circuit?


I guess this would be an easy answer with something like diode right? Any help on which one and where would be much appreciated?


Finally, what would happen in the switch was close supplying 24v and the control board was supplying 24v to the relay? Would this be fine as it is the same source or could this create some form of over voltage/over current?


Could this be resolved with a transistor and if so how would I wire it in.


Basically the ciruit has a common ground and the 24v is common to both the ULN2802 which will be used to switch the relay coil and to the switch which will directly supply the same 24v?


There is one final consideration, the relays have a 2.5v voltage drop and a 30ma current drop. How would I handle this in terms of resistors and the 2 possible sources of current (ULN2803 or Switch)?


I would also like a status LED to show which relays are on again 2.5v voltage drop, 30ma current drop.


Many thanks,



#2 carb


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Posted 27 August 2013 - 12:51 AM



You can use a single pole double pole switch. The center contact goes to the coil (other side to ground).


uC-----o o----- pwr supply


  spdt switch





#3 mcinnes01


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Posted 27 August 2013 - 02:17 AM

Hi Chuck


Thanks for the reply, I should elaborate a little further, the switches are the standard wall plates you have in your house to switch the lights on/off. My lights are going to be physically switched with a relay, this will be done using the netduino and my own boards, but I want a manual overide using the standard wall switch. This will have the same 24v power supply that the neduino uses to switch the relay.


This will allow me to overide what ever the netduino is doing in terms of switching on the lights, so when the wall switch is in the off position, the netduino is in control, but when I switch the wall switch on this will overide the netduino and turn the lights on.


Do you have any thoughts and sorry for not being clearer :)


Thanks again,



#4 Paul Newton

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Posted 27 August 2013 - 06:57 PM

Hi Andy,


You almost certainly want a transistor to control the current into the relay coil.

This could be directly connected to the Netduino or it could be an opto-isolator.


When working with higher voltages, it is usual to put the transistor between the ground and the -ve terminal of the load.

With this arrangement, the Netduino can pull the base of the transistor (e.g. an NPN) high enough to turn it on, and when the output is low or off, the transistor will turn off.


(If instead the transistor was at the top end connected to the 24V rail, the tranistor would need to be a PNP, which is turned on by pulling the base low. But, there is a problem, the Netduino is running at 3.3V - so what ever the output does the base will see a low voltage and turn on!)


Having a transistor placed between ground and the coil, you can safely place a switch accross the transistor (Collector to Emitter), this will then act in parallel to the transistor. If either the transistor or the switch is on, the coll will get current.


If you want to be fancy, you could wire an LED in series with the transistor (between the coil and the collector). Chose one that can take the coil's rated current - you mentioned 30mA, so chose a low intensity one. If you can't find a suitable one, then a reistor in parallel with the LED can be used to reduce the current in the LED. You could chose whether the LED is turned on by just the transistor, or by both the transistor and the switch....


With an opto-isolator, you can put the "transistor" half of the isolator any where you want it - at the ground or at the 24V rail. This might fit in better with your system.


I wrote a wiki page a while ago that describes using a darlington and an opto-isolator that both came from Maplin for not very much.


Hope this all helps - Paul

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