Devlog #02

— Power Supply —

I made schematic for power supply.
To make 15V for the zener, I decided to go with LT3045 and 7018 for a pre-regulator. 7818 will help with noise and ripple. Also LT3045 cannot handle input voltage so 7818 steps it down first, then it is regulated to 15V.

— Transformers —

I decided to go with 2 DP-241-4-20 20V transformers. The datasheet states that it has low capacitive coupling, which is crucial for such precision standard. Higher capacitance between windings means higher common mode currents, that could potentially make measurement inaccurate. This specific type of transformer has been tested by Marco Reps in his Extreme Isolation video, with quite good result. This transformer has lower leakage that Fluke 732A. which is much better than any standard based on LM399.
It is a 20V transformer, so after rectification there will no more than 28V - voltage drop on rectifiers without a load. That’s why pre-regulator is necessary, otherwise it would fry the main LDO.
Also I used low leakage diodes as the rectifier.
The heater supply isn’t too interesting - just a 7815.
I am wondering whether I should switch to 16V transformers or not. I am not concerned about ref supply, since it draws very little current. What may be a problem is the heater supply, but i will have to do some more calculations.