Secure your bench

view of the control contactor and the cable fitted with an RJ11 plug ...Why do I find it necessary to want to secure his bench?
For several reasons, quite personal it is true, which I will detail:

  1. the primary interest is being able to activate / deactivate all the equipment ( measuring as well as models under development ) thanks to a simple command
  2. in the event of loss of mains power supply during work and / or measurement, the bench is not automatically re-supplied.
    In this case, the control system must be manually reset.
  3. having installed my little lab in my son's room which has become vacant, and often receiving visits from my grandchildren with more than agile fingers ...
    The protection system has a key to totally inhibit commissioning, but always allowing the assembly to be taken out of service for safety reasons
  4. The control system must be supplied with ELV ( voltage 12 V ) and the power stage isolated with an optocoupler
  5. I wanted a DIY system ( D o I t Y ourself, or do it yourself in French ) and of course electronic that takes into account my specifications

I could very well have used these small safety boxes fitted to machine tools, but as I indicated in my description, I wanted to design it myself, especially for integration issues, particularly in the electrical box ( Box to equip - 2 rows 18 modules ) which I installed especially in my lab.

executable  PROJECT INFO
  Component Types: Through
  Level: ◆ ◇◇◇◇
  Construction time: approx. 2:30
  Cost: approx. 60 €

view of the control box ...I therefore imagined an electronics that I wanted simple and using classic components. The control box receiving the two green and red pushbuttons for switching on and off as well as the SPDT ON- (ON) key switch are housed in a small BOPLA type M210 box. My choice fell on this box because I already had it, and what more it fits mechanically perfectly in my office. I don't have a reference for the key switch as it is from salvage. It must be of the "non-momentary" type and therefore lockable in both positions. You can use this PDF file to make your choice.
The power part is housed in a small boxto DIN Rail standards which is therefore fixed on the rails of the electrical panel. It is assisted by a contactor for off-peak hourly tariffs - two-pole 230 V / 20 A. The reason for this choice is that in the event of a control circuit failure (always possible, it should be provided ...) is that it is still possible to switch manually directly to the contactor. In addition I already had it too.

The function of memorizing the "ON" state when the green pushbutton is pressed is achieved through the use of a thyristor, which drives a transistor and the MOC3041M optocoupler . The pulse produced by the BP-ON through resistor R2 causes conduction of thyristor TRI1, which will remain in this state until the load current disappears. This is only possible on two conditions:

  1. application of a negative pulse on the trigger
  2. thyristor power cut

I use this second condition in my system, which is very simple to implement thanks to the BP-ARRÊT.

When the thyristor conducts, the base of transistor T1 brought to a potential close to 0 V causes it to conduction. Resistor R4 serves as the base bias of transistor T1 while R3 limits the current flowing through the thyristor. Resistor R5 initiates at this moment a current of approximately 13 mA which passes through the optocoupler OK1. This then causes conduction of the TRI2 triac and activates the power contactor.
The two resistors R6 / R7 have values ​​recommended in the datasheet of the manufacturer FAIRCHILD .
When the supply voltage disappears, the thyristor is instantly blocked which causes the blocking of transistor T1. In fact no current circulates in the optocoupler and the triac is blocked. The power contactor is deactivated.

This opto-coupler offers the particularity of controlling the triac during the passage through zero of the AC mains wave. The major interest is that at this precise moment of the sine wave the current is zero. It is this triac which controls the power contactor.

The control box being physically remote from the power contactor and its interface, I integrated in the small DIN box an RJ11 type plug which receives the cable in which the voltage passes in addition to the control info of the optocoupler. AC12V power supply.
This voltage is rectified and stabilized by the network consisting of diode D1 ( 1N4007 ) and its filter capacitor C2 ( 330µF / 25V ). A 15V Zener diode placed upstream of the filter protects against any possible overvoltage. Thyristor TRI1 ( BT169 ) drives a PNP transistor T1 ( BC516 ) which thus makes it possible to make the optocoupler OK1 ( MOC3041M). The output of the optocoupler supplies the triac TRI2 ( BTA08-400B ) which controls the power contactor.
The use of a triac with switched inductive loads such as motors or relays normally requires the use of a protection Snubber filter generally composed of a resistor and a capacitor of low values ​​( ex. 39 Ω / 10 nF ) placed between the two anodes of the triac. It is not useful here because the triac used already has it because of its internal composition.
The AC12V power supply comes from a recovery AC adapter whose rectification part was destroyed. It is therefore no more than a simple transformer making it possible to benefit from Class II insulation. A DC12V adapter would also be suitable of course, the diode D1 would then only serve as a simple protection against an unfortunate reversal of polarity ... The 330µF capacitor C2 allows, in addition to filtering after rectification, to damp micro-cuts, frequent on the network electricity from my home. Of course, nothing prevents you from reducing the value of this capacitor ( eg: 220µF or even 100µF ) to make the assembly more sensitive to micro-cuts.

Your warned eye will not have missed the absence of fuses ... in fact the power circuit input comes from a 30 mA differential switch ( DX³-ID differential switch high inlet and high outlet screw - 2P 230V ~ 40A type AC 30mA - 2 modules ) while the "use" output of the power contactor has a LEGRAND circuit breaker ref. 4067 74 ( Phase + Neutral DNX³4500 6kA inlet and outlet screw terminal - 1P + N 230V ~ 16A curve C - 1 module ). But maybe I was wrong to do this ... nothing prevents you from adding fuses, of course.


I also offer you a second version of my project ( v3.1 ), which this time no longer uses a thyristor, but a pair of PNP / NPN transistors of types BC557B and BC547B. These two transistors are accompanied by two 10kΩ bias resistors and form a pseudo thyristor ( I have spotted the ( A) Anode / ( G) Trigger / ( K) Cathode ). On power-up, the two transistors T2 ( BC557B ) and T3 ( BC547B ) are in the blocked state. They do not conduct and the transistor T1 ( BC557B ) is also blocked. No current is flowing through the OK1 optocoupler ( MOC3041M ).
During a brief press the push button Start , T3 becomes conductive and carries the base of T2 to a potential close to 0V through a junction V CE . Indeed T2 becomes in its turn conductive and applies a positive voltage at the base of T3 even upon release of the pushbutton Marche , the current limitation can be done via R2 ( 2,2KΩ ) and R5 ( 10K ). A current of around 10mA can now flow through the OK1 optocoupler via T1 which has also become a conductor. Stopping is done by pressing the Stop push button . You can see that the operation is strictly equivalent to the assembly built around a real thyristor.

I personally opted for the thyristor for reasons of simplicity of wiring, quite simply ...

On the other hand, there is no need to use a Darlington transistor for T1 ( BC516 ) that it is obviously possible to replace by a BC557B by modifying the value of its basic polarization resistance, however, which goes from 100KΩ at 22KΩ. I used a BC516 because I could no longer find the BC557s in my drawers, however well classified, quite simply ... The operation of the whole remains unchanged.


As I mentioned earlier, I used two boxes to make the assembly that I split into two parts:

I did not make any printed circuits for this project, I was content to use pelletized cards bought on the Chinese site ... It is only necessary to take some precautions on the entire surface used by the triac because the sector 230 V ~ is present there! I also did not use a heatsink for the triac given the low switched current.
The system is simple and very practical in use.


Benchtop Security - Diagram v3.0 ...Workbench security - Diagram v3.1 ...

Package comprena nt : save f2

◊ Block diagram ( without PCB ) under Eagle v7.7.0 ( version 3.0 and 3.1 )

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