In my previous blog post, I did a teardown of the EDC/Krohn-Hite MV216A DC voltage standard and measured the temperature coefficient of its reference Zener diode. And as I promised, this time I will explore its internal and external battery options.
As I mentioned in my previous post, everything of MV216A seemed to be identical to that in MV206A except for the internal battery and external power options. Because of this, the power supply portion of these two models are a little bit different. In 206A, a 7815 and 7915 are used to produce the ±15V from a center tapped transformer. Whereas in MV206A, the transformer has only one winding. The rectified and filtered voltage is around 15V, which is then fed into a TO3 LM309K 5V regulator. The regulated voltage is in turn used to drive a DC/DC converter that produces the ±15V used by the reset of the system.
While there is no information on the battery voltage used in the 216A as I was told by Krohn-Hite that some of the EDC product manuals were lost after Krohn-Hite acquired EDC in 1998, we can safely assume though that the battery voltage is in the neighborhood between 9V and 12V (for the internal battery option). The reason is that LM309K has a dropout voltage of at least 2 volts and the battery pack could have an additional 2 volts voltage drop when it reaches the terminal voltage, so 9 volts seems to be a reasonable guess for the battery voltage at the lower end. For the higher end battery voltage estimate, since the DC supply voltage is 15V and there need to be some headroom for the battery to charge so 12 volts is likely the highest battery voltage you can use and still be able to fully charge the battery. For external battery, the voltage will need to be at least 1.4 V higher due to the diode drop from the bridge rectifier as I mentioned before.
Here is a video of me exploring both the internal and external battery options.