High impedance adaptor AP89 for use with EIL CT54 voltmeter - UK Vintage Radio Repair and Restoration Discussion Forum

trobbins · 21st Nov 2021, 8:28 am

I'm just looking at ways to utilise a 200:1 voltage divider adaptor from 1958 that allowed an EIL valve voltmeter type CT54 with 40 Meg input resistance to be increased to 400 Meg probe resistance for up to 480Vdc range measurement.

The adaptor is hermitic sealed with a desiccant holder, and is rated to 480Vdc with a nice insulated probe and cable. A switch selects the FSD voltage range of an internal resistor divider when connected to the 2.4V range of the CT54 meter. The top arm (R1) of the divider is a potted Welwyn 39613, 400 Megohm resistor with taps at 200 and 300 Megohm with a +/-2% tolerance rating. A quick check with an out of cal Keithley 197 shows the Welwyn module is still likely within spec, but I'll have to do some more considered measurements to confirm that. The resistor block is quite large (83x35x67mm), and only dissipates 0.6mW with 480Vdc applied, but without appreciating what resistors are in it I'm unsure about whether I could repurpose it for higher voltage use (?) up to at least 1kVdc (ie. 2.4mW).

My initial thought is to use the adaptor with a typical 10Meg input DMM, and re-cal the divider, which shouldn't change too much as the 10Meg is in parallel with the bottom arm divider resistance of circa 2.5Meg (Morganite 1M trimpot and 1.8M Welwyn C21 series high stability resistor).

Any thoughts on other applications? The tolerance of the 100, 200, 400Megohm resistor module may well suit it as a reference resistor, especially if I can confirm the resistance levels and get some idea of value change with applied voltage and time.

Ciao, Tim

trobbins · 24th Nov 2021, 4:37 am

Welwyn have done a good job with resistance stability over the last 60+ years as the potted resistor shows measured values of about +0.8% +/-0.3% for the 100M, 200M and 400M settings at 21C, and what seems to be about -500ppm/C for a rise of 10C. So the adaptor makes a good reference resistor.

I haven't had a need for voltage measurements with high meter resistance loading above about 10V (when my DMM's kick down to 10-11M input resistance) but time will tell.

Now to find some fresh desiccant.

Herald1360 · 24th Nov 2021, 12:33 pm

If it's silica gel, can't you just cook it for a while to drive off the absorbed water?

In any case you can find it at about $30 per kilo or in various size individual sachets on Ebay and the like.

21st Nov 2021, 8:28 am	#1
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	High impedance adaptor AP89 for use with EIL CT54 voltmeter I'm just looking at ways to utilise a 200:1 voltage divider adaptor from 1958 that allowed an EIL valve voltmeter type CT54 with 40 Meg input resistance to be increased to 400 Meg probe resistance for up to 480Vdc range measurement. The adaptor is hermitic sealed with a desiccant holder, and is rated to 480Vdc with a nice insulated probe and cable. A switch selects the FSD voltage range of an internal resistor divider when connected to the 2.4V range of the CT54 meter. The top arm (R1) of the divider is a potted Welwyn 39613, 400 Megohm resistor with taps at 200 and 300 Megohm with a +/-2% tolerance rating. A quick check with an out of cal Keithley 197 shows the Welwyn module is still likely within spec, but I'll have to do some more considered measurements to confirm that. The resistor block is quite large (83x35x67mm), and only dissipates 0.6mW with 480Vdc applied, but without appreciating what resistors are in it I'm unsure about whether I could repurpose it for higher voltage use (?) up to at least 1kVdc (ie. 2.4mW). My initial thought is to use the adaptor with a typical 10Meg input DMM, and re-cal the divider, which shouldn't change too much as the 10Meg is in parallel with the bottom arm divider resistance of circa 2.5Meg (Morganite 1M trimpot and 1.8M Welwyn C21 series high stability resistor). Any thoughts on other applications? The tolerance of the 100, 200, 400Megohm resistor module may well suit it as a reference resistor, especially if I can confirm the resistance levels and get some idea of value change with applied voltage and time. Ciao, Tim Attached Thumbnails

24th Nov 2021, 4:37 am	#2
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: High impedance adaptor AP89 for use with EIL CT54 voltmeter Welwyn have done a good job with resistance stability over the last 60+ years as the potted resistor shows measured values of about +0.8% +/-0.3% for the 100M, 200M and 400M settings at 21C, and what seems to be about -500ppm/C for a rise of 10C. So the adaptor makes a good reference resistor. I haven't had a need for voltage measurements with high meter resistance loading above about 10V (when my DMM's kick down to 10-11M input resistance) but time will tell. Now to find some fresh desiccant.

24th Nov 2021, 12:33 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: High impedance adaptor AP89 for use with EIL CT54 voltmeter If it's silica gel, can't you just cook it for a while to drive off the absorbed water? In any case you can find it at about $30 per kilo or in various size individual sachets on Ebay and the like. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O