function of the Zener in this SMPS circuit

[papunblg]

Recently I purchase a china made Rechargeable LED emergency light which contains 1 6V5Ah battery. Upon inspection I find that the circuit board contains an SMPS + a transistor switch to control the light. The SMPS it contains is exactly the same as this .



The output voltage to charge the 6V battery is showing around 14 Volt with no battery connected.

All the component values are same as mentioned in the picture. Only two component values I do not know are the zener (can't read) and the transformer(no marks).

I changed the unknown zener with a 7.5 volt to check the effect and no load voltage is around 19 volt this time. Can anyone please explain function of the zener in the circuit?

 

[D.A.(Tony)Stewart]

The zener clips the + base drive pulse to bypass the base current and drain the 0.004 cap to reduce the output voltage. Raising the Zener voltage increases the duration of pulse and saturate the core more.