Re: [Drakelist] VTVM Probe with DMM
Good solution, Paul! Mentally dividing by ten is easy enough and you retain all of the advantages of using the VTVM probe. Dennis AE6C Well'sorta. The divide by ten works well and it's an easy mental exercise. But in creating a 1 meg/100K divider, the load across the voltage sampling point is reduced from 10 meg to 1 meg. That undermines the purpose of the resistive probe except that the effects from cable capacitance and lead inductance are swamped. But if the source Z providing grid voltage, for example, is greater than 100K big errors will start setting in. So...next I'm going to change the 1 meg resistor in VTVM probe to 10 meg (like the 100x probe), and then try a variable shunt at the DMM end to find the value where 10:1 is achieved. That 10 meg is a high value for low voltage readings and I'm not sure how stable readings will be. I think 10 meg is commonly available in carbon film, but tougher to find in metal film. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Carbon film 10M should be stable as long as it is not subjected to voltages near its maximum ratings for extended periods of time. Long term drift could occur if for example a 1/4 watt carbon film (typ 300V rated) is soldered into a circuit whereby it sees 200 to 300V continuously for many, many hours. Heat buildup will compound the drift. Though, in no way would this drift approach that of a carbon comp. But for a probe, I think you have no issue with stability. BTW, capacitance can be further reduced by putting resistors in series. It will already by low, but series resistors might help you with finding metal film of high enough value. It will also increase the voltage rating of your probe. The probe body is long so in-line series connections should fit. Dennis AE6C On 11/22/12, Paul Christensen w...@arrl.net wrote: Good solution, Paul! Mentally dividing by ten is easy enough and you retain all of the advantages of using the VTVM probe. Dennis AE6C Well'sorta. The divide by ten works well and it's an easy mental exercise. But in creating a 1 meg/100K divider, the load across the voltage sampling point is reduced from 10 meg to 1 meg. That undermines the purpose of the resistive probe except that the effects from cable capacitance and lead inductance are swamped. But if the source Z providing grid voltage, for example, is greater than 100K big errors will start setting in. So...next I'm going to change the 1 meg resistor in VTVM probe to 10 meg (like the 100x probe), and then try a variable shunt at the DMM end to find the value where 10:1 is achieved. That 10 meg is a high value for low voltage readings and I'm not sure how stable readings will be. I think 10 meg is commonly available in carbon film, but tougher to find in metal film. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Paul, The best probe I have found for the purpose are the old 100X scope probes. They have a very low capacitance on the order of 2.5pF in parallel with 10Meg so circuit loading is minimal. Most have extra voltage capability which is handy for working around transmitters. While designed for AC DC measurements via scopes, you can use them accurately with DMM's because the 100X divider swamps out the loading effect of the DMM. The numerical accuracy of the DMM compensates for the high voltage division ratio. Dennis AE6C On 11/17/12, Paul Christensen w...@arrl.net wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Dennis, Many thanks. I'll give that a try once I find a set of 100x probes. Darn, I didn't know 100x probes even existed for scopes as I've only worked with 10x types for the past 30 years! Paul, W9AC - Original Message - From: Dennis Monticelli dennis.montice...@gmail.com To: Paul Christensen w...@arrl.net Cc: drakelist@zerobeat.net Sent: Wednesday, November 21, 2012 3:38 AM Subject: Re: [Drakelist] VTVM Probe with DMM Paul, The best probe I have found for the purpose are the old 100X scope probes. They have a very low capacitance on the order of 2.5pF in parallel with 10Meg so circuit loading is minimal. Most have extra voltage capability which is handy for working around transmitters. While designed for AC DC measurements via scopes, you can use them accurately with DMM's because the 100X divider swamps out the loading effect of the DMM. The numerical accuracy of the DMM compensates for the high voltage division ratio. Dennis AE6C On 11/17/12, Paul Christensen w...@arrl.net wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
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 w...@arrl.net 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 Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] VTVM Probe with DMM
I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Hi Paul. Wouldn't it be the same thing if you set-up a reference voltage that is -1.07V or whatever the drop is and then pushed the relative button? If all you are trying to do is improve upon the input Z of the DMM, , my opinion is that I have never seen a measurement in troubleshooting any tube set that could not be done with a modern DMM. At 10M inputZ, it isn't going to do much in the way of load. Using one for an RF probe is slightly more of an issue but with a properly built RF probe, it still is no real issue. Perhaps I have misunderstood what and why you are trying to acheive. FWIW Curt KU8L On 11/17/2012 12:29 PM, Paul Christensen wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
- Original Message - From: Curt Nixon cptc...@flash.net To: drakelist@zerobeat.net Sent: Saturday, November 17, 2012 11:22 AM Subject: Re: [Drakelist] VTVM Probe with DMM Hi Paul. Wouldn't it be the same thing if you set-up a reference voltage that is -1.07V or whatever the drop is and then pushed the relative button? If all you are trying to do is improve upon the input Z of the DMM, , my opinion is that I have never seen a measurement in troubleshooting any tube set that could not be done with a modern DMM. At 10M inputZ, it isn't going to do much in the way of load. Using one for an RF probe is slightly more of an issue but with a properly built RF probe, it still is no real issue. Perhaps I have misunderstood what and why you are trying to acheive. FWIW Curt KU8L The idea of the 1 meg resistor at the probe tip is to reduce the effect of lead capacitance on the measurement. Evidently a lot of electronic voltmeters have them. In some cases a manufacturer will specify the use of such a resistor in making measurements. I think Collins does this in some older manuals. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Curt, Yes, that works but it will not save to memory. So, each time the DMM is powered up, I would need to set the Fluke 8060A against the 1.07V reference. And probably, the 189 may need a slightly different reference voltage for best accuracy. It's an inconvenience, but it may be worth building a precision voltage regulator in a small project box, powered from a 9V battery. I have not found a precision low-voltage regulator that will output below +1.24 V. I could use a pass transistor after the regulator, fed from a pot as a voltage divider to get it adjustable down below +1.24V. To invert the sign to achieve -1.07V, the leads would be reversed just for calibration. I've not had a good experience troubleshooting the grids of receiving tubes when using a DMM. There's often enough distributed lead C that creates measurement problems. I rarely run into this when using a VTVM and a switchable 1-meg probe. Nothing wrong with using a VTVM or an FET VOM with an isolated probe, but like my Keithley DMM, they're not as portable as I'd like. Paul, W9AC - Original Message - From: Curt Nixon cptc...@flash.net To: drakelist@zerobeat.net Sent: Saturday, November 17, 2012 2:22 PM Subject: Re: [Drakelist] VTVM Probe with DMM Hi Paul. Wouldn't it be the same thing if you set-up a reference voltage that is -1.07V or whatever the drop is and then pushed the relative button? If all you are trying to do is improve upon the input Z of the DMM, , my opinion is that I have never seen a measurement in troubleshooting any tube set that could not be done with a modern DMM. At 10M inputZ, it isn't going to do much in the way of load. Using one for an RF probe is slightly more of an issue but with a properly built RF probe, it still is no real issue. Perhaps I have misunderstood what and why you are trying to acheive. FWIW Curt KU8L On 11/17/2012 12:29 PM, Paul Christensen wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
