Good solution, Paul! Mentally dividing by ten is easy enough and you retain all of the advantages of using the VTVM probe.
Dennis AE6C On 11/21/12, Paul Christensen <[email protected]> wrote: > Dennis' suggestion of using a 100x probe got me to thinking of an > alternative as the 100x scope probes are sorta' pricey, even on the used > market. As Dennis points out, the 100x scope probes are useful when > sampling high voltage, typically greater than 1KV. Since my focus has been > > with the grids of tube receivers, these measurements (even into the hundreds > > of volts) are adequate with the right 10x probe. > > The 100x probe uses a 10 meg sampling resistor with a 100K shunt at the > scope input. Most scopes have a ~ 1 meg input Z. The input Z of my Fluke > 8060A computes to 10.5 meg and is a value that is probably close to most > portable DMMs. > > Taking Dennis' suggestion, I used a 1 meg Simpson VTVM probe and shunted it > > with a value close to 100K to create the necessary 10:1 ratio. A 200K pot > was used to find a value that produced an exact 10:1 ratio. For the Fluke > 8060A and my Simpson VTVM probe, that value is 112.5K. I then created shunt > > consisting of two resistors in series whose total value is 112.5K and > inserted this combination into a Pomona dual banana jack. Different shunt > jacks can be created if one wants best accuracy over a choice of DMMs in the > > shack. > > In the links below, notice that the Simpson probe is connected to a > BNC-to-dual banana adapter with standard 3/4" pin spacing. The adapter has > > holes to allow shunting from a dual banana connector; the latter of which > has the 112.5K resistor combination. The black plug simply plugs into the > red plug. > > http://tinyurl.com/bc6ggo8 > > http://tinyurl.com/b56tfm5 > > Testing this idea in the real world shows that Dennis' idea works in a 10x > configuration. For example, when a +12V source is measured, the DMM reads > 1.2V. It's then just a simple matter of multiplying displayed results by a > > factor of 10 which is a whole lot easier than using the DMM's RELative > button and trying to find a 1.07V source as the calibrating reference. > > It appears good accuracy can be achieved when using a DMM with a traditional > > VTVM probe. The VTVM probe has the benefit of a 1 meg isolation resistor > which is highly useful for grid readings. The trade-off is the requirement > > of multiplying displayed readings by a factor of 10 -- the same as mentally > > moving the display's decimal place to the right by one digit. > > Paul, W9AC > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
