Precise voltages from imprecise parts..
2 Attachment(s)
Target: shunt regulator, 300V 0.1% from 20uA to 500uA.
Used LM285, J175, and trimmed divider for 25V 0.1% regulator. Used 1% resistors, added one calculated fixed trimming resistor to get 25.000V for 0.1% result at 20uA. Made twelve of these, in series to achieve 300V for presumed 0.1% precision. Well, the results are OK, but not quite "right". Firstly, it is high by 0.3%. Then there is a seeming discrepancy: a Tektronix 7CT1N curve tracer on six of the 25V trimmed regulators reads 150.24V (a bit above the 0.1%), but minimum current is 40uA (??). Direct measurement of the twelve is 300.9V from 30uA to 470uA which is high by 0.3%. I do not have an adjustable supply to try 20uA "zener" current so must assume the current displayed on the 7CT1N is in error, but that is a factor of two and not a mere 1% or so. Any idea as to why the error is so high (0.3%) and why the apparent minimum operating current is way above the individually measured 20uA used for setting each 25V regulator? |
Precise voltages from imprecise parts..UPDATE
2 Attachment(s)
Robert Baer wrote:
Target: shunt regulator, 300V 0.1% from 20uA to 500uA. Used LM285, J175, and trimmed divider for 25V 0.1% regulator. Used 1% resistors, added one calculated fixed trimming resistor to get 25.000V for 0.1% result at 20uA. Made twelve of these, in series to achieve 300V for presumed 0.1% precision. Well, the results are OK, but not quite "right". Firstly, it is high by 0.3%. Then there is a seeming discrepancy: a Tektronix 7CT1N curve tracer on six of the 25V trimmed regulators reads 150.24V (a bit above the 0.1%), but minimum current is 40uA (??). * Basically resolves with more careful measurement using no added metering. Direct measurement of the twelve is 300.9V from 30uA to 470uA which is high by 0.3%. I do not have an adjustable supply to try 20uA "zener" current so must assume the current displayed on the 7CT1N is in error, but that is a factor of two and not a mere 1% or so. * Roughly resolved, by separate, added voltage measurements with a forgotten 200K/V analog meter, at currents near the "knee". Any idea as to why the error is so high (0.3%)? |
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