Consider the newer small Fluke multimeter input protection circuit, instead of desoldering things.
I was thinking I could maybe take the spring contact out of the range selector - it breaks the circuit, it's much easier!
Consider the newer small Fluke multimeter input protection circuit, instead of desoldering things.
I was thinking I could maybe take the spring contact out of the range selector - it breaks the circuit, it's much easier!
I would expect that my first Fluke DMM was highly susceptible to being damaged by RF (or looking at it sideways). I suspect as the front ends of these handhelds evolved, the need to derate them was less important. Of course, you still end up with meters like the UNI-T UT61E+ having some sort of frequency counter speced at 220MHz with a couple of PTCs in series, but I suspect most modern robust meters would not have a problem. Like I had shown with the BM78x, with the same setup that damaged the UT61e+, the PTCs had very little heat. As I demonstrated, that resistor was fairly stable to 50MHz and a 1kohm just doesn't present much of a load to the small amplifier.
I don't mind running other tests if you had something in mind. I could take that same resistor/PTC we just used and run them any way you like. The same with that Brymen BM78x prototype. Let me know.
I have 3 of the 8000A models in my junkpile, they're all toasted and they all have evidence that they've been soldered on during their lifetimes. But those were early days in the whole field, not just for Fluke. Meters may be more durable nowadays, but they still have limits and I, for one, would like to see those limits specified in detail--but they usually aren't.
I haven't see the video of you giving any other meters the jqs MeltCalTM treatment, I'll have to go look. I think it might be interesting to try to determine what parts are used in the meters and then characterize and test those--and look at datasheets if possible--to see who is using what and how the expected component performance matches up with the claimed performance of the meter.
...
After the attached comment, I demonstrated the BM78x at over 100MHz during Part 2 of the UT61E+ review. At these higher frequencies, the majority of the voltage drop will be across the resistors, not the PTCs. I'm guessing this would have been obvious to all the radio hobbyist.
You're firing on all eight cylinders today I see. Still, it was a good opportunity to try out that heat gun.
Just don't lose the bugger!
OK, I took out the spring from the selector switch.
Perhaps we need a MacroVNA, with a 1kW output.
For the F27, you could for example, try using the AC line to get the PTCs to switch, then quickly attach it to the calibrator. Maybe just a DPDT switch to prevent it from cooling.
I assume these are all after the PTCs have switched. Otherwise for LowZ, I would expect the current to be much higher. At a kHz, I doubt you will see much change.
QuoteI assume these are all after the PTCs have switched. Otherwise for LowZ, I would expect the current to be much higher. At a kHz, I doubt you will see much change.
Well, there's the problem--it does change, and a lot. I tested them at low and high voltages with different frequencies. I used 6V and 600V for the F116, then 15V and 1000V for the F27. I needed to use the higher voltage on the low end for the F27 because it doesn't have a Lo-Z mode, so I have to overwhelm the low voltage clamps in whatever circuit I'm using (OHMS in this case) so that I see mostly the PTC/resistor characteristics.
At 6VAC in 'AUTO V/Lo-Z', the F116 had a current of about 2.0mA @ 50Hz, and that didn't change much at 1kHz or even 10kHz. However, at 600VAC, it had about 1.9mA at 50Hz, but that rose to 5.1mA @ 1kHz.
The F27 at 15VAC stayed in the 2.1-2.3 range, not totally steady because the OHMs circuit was reacting a bit. At 1000VAC, it showed about 2.0mA @ 50Hz, but raising the frequency to 1kHz caused a surge to about 7mA and then it settled back down to 5.4mA.
Those are huge differences from 50Hz to 1kHz, but only at higher voltages. I have to wonder if the trend would continue if you kept going--the calibrator is maxed out.
I'm only at 106 V-Hz but some of these meters are supposedly good for 10X or 20X more. The next time I place an order maybe I'll get a few of those big dark grey monster PTCs that you find in the larger CAT III/1000 Flukes and try to characterize them. It looks like they pick up 500-1000pF when they warm up. Maybe they should be called thermisto-varactors.
"Not much change" meaning Amps vs mA. At these low frequencies, Xc is not going to be a problem. As you move up in frequency, Xc will dominate and we can damage the parts with a very low voltage, unless there is something else to limit the current. Perhaps a resistor...
"Not much change" meaning Amps vs mA. At these low frequencies, Xc is not going to be a problem. As you move up in frequency, Xc will dominate and we can damage the parts with a very low voltage, unless there is something else to limit the current. Perhaps a resistor...
Yes, the resistor is key at low voltage and much higher frequencies. But take the case of the F116 running at 600V and continuing to ramp up the frequencies. If you just assume that the PTC goes to a state of very high resistance but a capacitance of 0.5nF, the current will go high enough (~70mA) to exceed the rated power of the resistor (which I'm presuming is 5W for now) at around 40kHz. So unless there is some other change, by 100kHz it has likely unsoldered itself if the PTC hasn't come undone first. This is probably a bit of a far-fetched example, but perhaps not when it comes to VFD drives and such. In any case, what caught my eye was the reactance change at higher voltages that wasn't there at lower ones.
So use of the PTC thermistor in the AF and RF ranges is not possible, meaning that applications are restricted to DC and line frequency operation.
Section 6.2 Figure 7 provides some insight.
https://www.tdk-electronics.tdk.com/download/408374/d78540dfe0589d2bd90cabef477c90b9/pdf-general-technical-information.pdf
Where is the 500pF coming from?
Keep in mind, using your 500nF, 40kHz, 1k resistor that the power dissipation for the PTC is 36 Watts.
In part 4, the UT61E+ is temperature cycled, dropped and transient tested. I also use the NanoVNA to compare the 61E+'s input impedance with a few other meters.
Now these are some classic comments! There's just so many good points but that last sentence of Capture5 is priceless. I never knew I was a 121GW fanboy!
In part 4, the UT61E+ is temperature cycled, dropped and transient tested. I also use the NanoVNA to compare the 61E+'s input impedance with a few other meters.
it survived the low voltage zapper?
You know what that means, joe? You're going to have to buy another UT61E+ so you can repeat the tests on an "unmodified" one.
...
Also... parts operating from 0~50C?!? Perhaps in Shenzhen's specials.