Operating Principal. Use in accordance with supplied product documentation.

The 686 DT Controller is designed to switch single Thyristors fitted in series with the output supply from a full wave bridge rectifier, or dual Thyristors replacing the two forward diodes in a full wave bridge rectifier when used in single phase constant potential battery chargers.


Mains synchronisation is achieved by monitoring the secondary output voltage of the charger mains isolating transformer via an op amp using components D1, D2, R13 and R14. This signal voltage is fed onto pin 5 (non-inverting input) and compared to a reference voltage on pin 6 (inverting input), derived by components R26, Zd2 and R24.

The square wave output from pin 7 of the op amp is used to discharge C5 through D6 whenever the mains supply approaches zero and this generates a saw tooth waveform called a Ramp. The ramp waveform is superimposed onto the 5.1V reference voltage fed onto pin 2 (inverting input) of the op amp via R22.


The signal voltage on pin 3 of the op amp is obtained from the voltage comparator output on the op amp pin 14. The signal voltage from this output varies as the differential between the battery voltage signal on pin 12, (non-inverting input) and the 5.1V reference on pin 13 (inverting input) alters.


When the ramp voltage on pin 2 (inverting input) rises above the voltage on pin 3 (non-inverting input), the output on pin 1 switches from high to low causing the led within the opto coupler IC1 to conduct. This triggers the small Thyristor within IC1 which delivers gate current pulses into the main charger Thyristors.

Once triggered the main Thyristors continue to conduct until they become reverse biased, caused by the input supply falling below the battery voltage.

A drawing is available showing the waveform interaction for this type of controller.


When setting the 686 DT controller, it is essential that the charging current be at the maintenance FLOAT level. Divide the amp-hour battery capacity by 600 to calculate the float current level.

Additionally, all external loads should be disconnected from the output supply.

Adjust the potentiometer VR3 until the voltage measured across the battery terminals is at the required level.

Adjust the potentiometer VR2 to adjust the boost voltage


Connect discharged batteries or a load (must draw 5A more than rated charger output to set current limit) to the charger output terminals.

Adjust the potentiometer VR5 fully anti-clockwise to reduce the output current to an absolute minimum. Slowly adjust the pot in the opposite direction until the output current is at the required level.

686 DT

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