Trefwoorden

 

relais

schakeling

berekening

werking

Bistable relay

 

 

 

Introduction

 

Sometimes we like to react to a mechanical action as brought about by an impulse switch and like to remember this action until a second impulse is resetting the situation. An example of such situation could be the mechanical end-switch at a moving action, a master switching clock etc. Usually we immediately are thinking in logic terms and design a circuit (or write SW-lines) to perform this action. This invariably also implies additional wiring for a power supply, a cabinet etc. and also additional facilities like power transistors to actually perform the switching action.

 

A simple bi-stable action however may also be accomplished without 'active elements' by utilizing the internal 'memory-effect' of a relay that is brought about by the different magnetic patch resistance in the open versus the closed condition of this relay. This will generate a (small) difference in the required switching energy. The switching 'hysteresis' will be utilized to our advantage in the following single relay, bi-stable switching circuit.

  

 

The relay

 

To start-off we first will determine the specific magnetic properties of the relay.

 

Switch-on voltage

First we will determine the switch-on voltage, by connecting to a power supply at the lowest voltage setting. Next we slowly turn up the voltage until the relay switches. This voltage we call Va

 

Switch-off voltage

Next we slowly turn down the voltage at the power supply, until the relay is falling-off again. This voltage is Vc

 

Holding voltage

We define holding voltage as in Vh = (Va + Vc) / 2 .

 

Nominal relay voltage

Nominal relay voltage Vs is usually provided by the manufacturer and is marked on the relay. When this specification is missing, we define nominal relay voltage as in:

Vs = 1,5 x Va

 

Relay resistance

Relay resistance R may be supplied by the manufacturer and/or may be measured with an Ohm-meter.

 

 

Circuit

 

The circuit of the bi-stable relay is presented in the following figure. Symbolic values will be calculated based on our first measurements, after the figure.

 

 

 

 

 

Calculations

 

 

Supply voltage for safe operation:

Series resistor R1 is equal to:

Power rating of R1 should be at least equal to: 

Series resistor R2 is equal to:

Power rating of R2 should be at least equal to 

Resistor R3 is not critical, but a fair value is:

Power rating of R3 is not critical and all (low) ratings will do

Capacitors C (electrolytic) are equal and should have a value:

Capacitor voltage rating should be at least equal to:

 

 

 

 

Vb = 2 x Vs

R1 = R

P1 = (Vh)2 / R

R2 = 2 x R x (Vs - Vh) / Vh

P2 = 2 x Vh x (Vs - Vh) / R

R3 = 200 x R

 

C = 14000 / R

V = 2 x Vb / 3

 

Operation

 

Circuit operation is straight forward.

At start-off with the power supply off, relay is of course not energized. At power supply switching on, the relay will remain de-energized. At an impulse of K1 (impulse switch e.g. push button, reed-switch), the relay energizes. At an impulse at K2, the relay de-energizes.

An impulse should last for at least 20 msec., witch is normally sufficient for even the slowest switching relay. Of course a longer impulse will do as well. Impulse K1 should be over when impulse K2 is switching, and vise versa.

 

 

Bob J. van Donselaar, on9cvd@veron.nl