Surge supressor diode

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Hello All,

I have a basic application where I am trying to understand the pros/cons of using a surge suppressor diode in parallel with my output solenoids.

I am using an AB compact Logix model (5069-L306ER) with a 24 VDC output model (5069-OB16).  The output I am dealing with is a numatics solenoid valve.  The solenoid draws very low current (less than 0.5 amps).

Over the years I have noticed that some people use a suppressor diode around the outputs and some do not. 

does anyone have any practical recommendations in this regard.  My figure below shows solenoids 1 and 2 without the diode and solenoid 3 with the diode. 





Edited by G_DeMark

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diode is only one of several  types of suppression devices.

need for suppression comes from operating principle of solenoid. basically is works like "flywheel" and opposes any current change. before solenoid is connected into circuit, current through it is zero amp. when solenoid is connected into circuit, it resists change and tries to maintain current of zero amp but it eventually gives in so current rises gradually. supposedly current settles at some value after while (say 0.5A). when output turns off, solenoid again resist change and tries to push that current (0.5A) through output. but since turned off output is high resistance (open circuit), this will result in very high transient voltage. this spike is brief but... it can damage output as well as cable etc. to put this into perspective, suppose output has off resistance of 100kOhm, using Ohms law we can see that untamed transient would generate voltage V=I*R=0.5A * 100000 Ohm = 50000V. in reality things are more complicated so there are usually other things that will dampen this transient to some degree. for example if wiring is using cable rated for 300V, it may break down at some 1500-1800V or so. this is of course bad for wiring (insulation deteriorates, lookup "lichtenberg") but... it may survive anywhere from few cycles to few hundred thousand cycles depending on case. this is also very bad for outputs - for example transistor outputs without protection will die instantly on a very first transient even if load was a tiny solenoid (drawing only few mA of current).

in short - concern with inductive loads is when they - turn off.  

so how to deal with this? as mentioned there are suppression devices such as diodes, varistors (MOVs), RC elements, zeners etc.

diodes are simplest and most common type of protection. manufacturers of PLCs are of course very well aware of this so they always add protection - for transistor outputs! without it, transistor outputs would be unable to do second cycle on inductive load. so if the protection is included, why add additional one? 

well it is not always included (they are added to transistor output cards such as OB16), for example there are other types of outputs such as relays (OX8 or OW16). another issue is that wiring may be long - wires themselves have inductive properties so diode in PLC will protect an output but it will do little or nothing to protect end of 10m long cable that is connected to inductive load such as solenoid or contactor coil (transient will be able to create large spike at the far end of the cable, away from suppressor).

how does it work?

diode is wired in parallel to a DC inductive load and - it must be reverse biased.

when load is turned on, current flows from output to load, no current goes through diode (except some really really small leakage current).

when load is turned off, inductive loads tries to keep pushing current in same direction. since output is off, no current will flow there, however now the current has closed path -  through diode. this will eliminate big voltage spike (well, it will reduce it to 0.7V above supply voltage so it will be 24.7V). this current will stop flowing once the energy stored in magnetic field is exhausted (usually this is fraction of a second).

relay contacts that are used to power inductive loads erode much faster when there is no suppression device across load. also transients can cause other issues (communication failures, "random" reboots etc). but adding diodes to each solenoid takes time (labour). 







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