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Where to place a fuse in a circuit?

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I want to make a zener diode tester

There is no fuse in this schematic,But It use 3 Pin Plug which contains a fuse like below screenshot.

Unfortunately,There is no 3 Pin plug in the market of my nation ,Instead There is 2 Pin plug in the market which doesn't contains any type of fuse.

So I want to place a fuse in the schematic just in case,But Where to place a fuse in a circuit?
And what Ampere of fuse should I use in the schematic?

S. Petrukhin:
In the primary coil of the transformer line.

if the transformer were perfect then the primary current would be about 5% of the sum of the zener test current and the LED current.  You probably cannot obtain such a low current fuse.  I would use a 200mA one.  Allows for non perfect transformer and initial surge as capacitor on bridge output charges.

That may be an old circuit from a time when LEDs were not as efficient as modern ones.  You may find that you can increase the 1K resistor and still get acceptable brightness with a modern LED.

It is difficult to protect small line frequency transformers with just a primary side fuse, as the normal operating primary current is so small compared to typical readily available 250V fuse ratings.  It is often worth adding an extra fuse on the secondary side, between the secondary and the rectifier, chosen to have a rating less than or equal to the secondary's RMS current rating but greater than the expected RMS load current (which for a bridge rectifier directly feeding a reservoir capacitor will be approx 1.5 x the DC current).  Due to the reservoir capacitor inrush (surge) current, it will need to be a slow blow (T) fuse.   

Good Morning Bshi02,   In similar situations, I have put an amp meter in the hot leg, feeding the xfmr., (a meter with mA capabilities).  My go-to meter in this application is one that has Min/Max capability, and then apply a maximum load on the circuit. :popcorn:


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