[volt-nuts] Traveling Standards

[Charles P. Steinmetz]

Bob wrote:

>Power transformer / rectifier / capacitor => ~35 Volts DC.
>LM7815A => 15 Volts (regulated).

With 20 volts of overhead, why not use two stages of pre-regulation 
(say, LM7824/LM7815) to improve the line regulation even 
further?  You might also use separate LM7815s for the reference and 
heater supplies, to minimize the reference supply changes due to 
varying heater current and the associated load regulation/wiring drop.

>The two largest errors (other than 20 ppm stability of the lm199ah) 
>are both under 1 ppm.  They could be reduce further but is it worth 
>it given the stability of the lm199a?

It is good design practice to reduce all sources of error to the 
degree that they can can be reduced for little cost.  In this 
context, I consider two additional three-terminal regulators "little 
cost."  (If two additional regulators are just too much to bear, the 
heater could be run from the 24 V supply.)

BTW, here is a standard similar to what you are 
proposing:  http://gellerlabs.com/LNVR%20Series.htm  It uses an LM399 
and the scaling amplifier of an AD587 to produce 10 V (I have 
previously criticized this design for using the 587 instead of a 
discrete op-amp that would give better performance, but the 587 used 
this way appears to meet your design goals).  Geller also offers a 
$40 bare board based on the 587 alone, for which he claims short-term 
transfer accuracy of +/- 5 
ppm:  http://gellerlabs.com/SVR%20Series.htm    Here is another one 
(+/- 25 ppm at 5 
V):  http://www.voltagestandard.com/New_Products.html    I believe 
that both vendors will recalibrate them for a nominal fee.


Best regards,

Charles