In another thread I mentioned that back in 1986 I decided that it would be useful to have an analogue high impedance MOSFET Voltmeter (albeit at the time, I already had a Heathkit Valve Voltmeter). PW Mag in Dec 1986 featured such a meter, which has an inpuit impedance of 11 MegOhms, as compared for example to an AVO7 at just 1,000 Ohms per Volt, and an AVO 8 at 20,000 Ohms per Volt.
I've mentioned before that AVOs and similar low impedance meters when measuring low voltages will not produce accurate results and I'm sure most on the forum will know that anyway, without a lecture from me on Ohm's Law. True, the meters will measure the actual voltage that they see, but that voltage will be lower than we'd expect to see because the Avo will act like a resistor in parallel with the circuit under test. EG if we have a poptential divider using two 100k resistors across a 12 Volt suppy, we know from Ohms Law that the calculated voltage at the junction of the two resistors will be 6 Volts.
However, if we apply an Avo7 (or any 1,000 OPV meter) on the 10 Volt range across the junction it will be like putting a 10K resistor across the 100K one, so the reading will be 1 Volt - the actual voltage due to the loading of the AVO. An AVO 8 (or any 20,000 OPV meter) on the 10V range will be like plonking a 200K resistor in parallel with the 100K one in circuit, so will produce a reading of 4.8V. However, using an FET or Valve Voltmeter with an impedance of 11 Meg Ohm will show 5.97V, close to what we'd expect to see.
Not so important when taking readings of higher Voltages, and there is the merit of using the same meter that was used when measurements were taken on valve radios by the manufactuers and shown on service sheets, so I do understand why many restorers like to use AVOs on old radios and TVs, but the lower the voltage being measured, the more significant this loading effect becomes.
When peaking up tuned circuits in low power transmitters and such like, we may only be looking at a few millivolts of RF. The PW design has seven ranges from 100mV to 500V and will measure AC signals up to 28V P-P up to about 100MHz, with a usable response to 150 MHz, which covers alll of the LF, HF and VHF amateur radio frequencies. Thus, it is a very useful bit of cheaply made test gear, or at least vwas, when I was still interested in building low power amateur radio tranmitters.
I find it quite scary that I made the PCB for this project more than 35 years ago, and must have had the 100uA meter movement for it for at least 15 years. A year or so ago I added a second scale to the dial to read 10, 20, 30, 40, 50 as well as the original scale of 20, 40, 60, 80. 100, to suit the ranges of the Voltmeter. I've now built the PCB and made a little comb-jointed oak box in which to house the meter. The PCB is so small, and the meter so large, that I've mounted the PCB across the meter terminals. It needs to be wired up and the front panel lettered.
It's anyone's guess when I'll actually see the project through to fruition incuding making the AC and DC probes, but it's cost less than a fiver, and is quite good fun when/if I find the time.
Where there's life, there's hope!
Some pics attached of progress to date.
I've mentioned before that AVOs and similar low impedance meters when measuring low voltages will not produce accurate results and I'm sure most on the forum will know that anyway, without a lecture from me on Ohm's Law. True, the meters will measure the actual voltage that they see, but that voltage will be lower than we'd expect to see because the Avo will act like a resistor in parallel with the circuit under test. EG if we have a poptential divider using two 100k resistors across a 12 Volt suppy, we know from Ohms Law that the calculated voltage at the junction of the two resistors will be 6 Volts.
However, if we apply an Avo7 (or any 1,000 OPV meter) on the 10 Volt range across the junction it will be like putting a 10K resistor across the 100K one, so the reading will be 1 Volt - the actual voltage due to the loading of the AVO. An AVO 8 (or any 20,000 OPV meter) on the 10V range will be like plonking a 200K resistor in parallel with the 100K one in circuit, so will produce a reading of 4.8V. However, using an FET or Valve Voltmeter with an impedance of 11 Meg Ohm will show 5.97V, close to what we'd expect to see.
Not so important when taking readings of higher Voltages, and there is the merit of using the same meter that was used when measurements were taken on valve radios by the manufactuers and shown on service sheets, so I do understand why many restorers like to use AVOs on old radios and TVs, but the lower the voltage being measured, the more significant this loading effect becomes.
When peaking up tuned circuits in low power transmitters and such like, we may only be looking at a few millivolts of RF. The PW design has seven ranges from 100mV to 500V and will measure AC signals up to 28V P-P up to about 100MHz, with a usable response to 150 MHz, which covers alll of the LF, HF and VHF amateur radio frequencies. Thus, it is a very useful bit of cheaply made test gear, or at least vwas, when I was still interested in building low power amateur radio tranmitters.
I find it quite scary that I made the PCB for this project more than 35 years ago, and must have had the 100uA meter movement for it for at least 15 years. A year or so ago I added a second scale to the dial to read 10, 20, 30, 40, 50 as well as the original scale of 20, 40, 60, 80. 100, to suit the ranges of the Voltmeter. I've now built the PCB and made a little comb-jointed oak box in which to house the meter. The PCB is so small, and the meter so large, that I've mounted the PCB across the meter terminals. It needs to be wired up and the front panel lettered.
It's anyone's guess when I'll actually see the project through to fruition incuding making the AC and DC probes, but it's cost less than a fiver, and is quite good fun when/if I find the time.
Where there's life, there's hope!
Some pics attached of progress to date.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
