On Sat, 7 Jan 2023 01:04:32 -0000 (UTC), antispam@math.uni.wroc.pl wrote:>Ed Lee <edward.ming.lee@gmail.com> wrote: >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > >High voltage can destroy resistors, but this seem to be quite fast. > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > >Lot of folks live in countries where line voltage is 230V. So 50 volts >makes a lot of difference. > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > >I know nothing about CenTech meters. But I have several "DT830B" >meters. Available schematics shows 3 resistors in series for 1000V. >My oldest one have 2 resistors. Newest one have single resistor. >Standard miniature resistors are rated for 250V, one can get >better ones, but I doubt that one can get cheaply 1000V capable >ones. Still, meter is marked as 1000V DC, 700V AC (the same >as old meters).They eliminated 0.2 cents worth of resistors. Ignore temperature and voltage coefficient effects. Maybe some of that is mathed out? Chinese product prices ratchet towards cheap, and the specs ratchet deep into the lies region. Chinese amps and volts and per cent are about 10:1 off from SI standards. It's our fault for buying the cheapest stuff on Amazon or ebay. Is any Chinese stuff really good? Rigol is. Extech is good but isn't all Chinese.
HV damages meter?
Started by ●December 31, 2022
Reply by ●January 7, 20232023-01-07
Reply by ●January 7, 20232023-01-07
On Saturday, January 7, 2023 at 11:44:42 AM UTC-5, John Larkin wrote:> On Sat, 7 Jan 2023 01:04:32 -0000 (UTC), anti...@math.uni.wroc.pl > wrote: > >Ed Lee <edward....@gmail.com> wrote: > >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > > > >High voltage can destroy resistors, but this seem to be quite fast. > > > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > > > >Lot of folks live in countries where line voltage is 230V. So 50 volts > >makes a lot of difference. > > > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > > >I know nothing about CenTech meters. But I have several "DT830B" > >meters. Available schematics shows 3 resistors in series for 1000V. > >My oldest one have 2 resistors. Newest one have single resistor. > >Standard miniature resistors are rated for 250V, one can get > >better ones, but I doubt that one can get cheaply 1000V capable > >ones. Still, meter is marked as 1000V DC, 700V AC (the same > >as old meters). > They eliminated 0.2 cents worth of resistors. Ignore temperature and > voltage coefficient effects. Maybe some of that is mathed out? > > Chinese product prices ratchet towards cheap, and the specs ratchet > deep into the lies region. Chinese amps and volts and per cent are > about 10:1 off from SI standards. > > It's our fault for buying the cheapest stuff on Amazon or ebay. > > Is any Chinese stuff really good? Rigol is. Extech is good but isn't > all Chinese.Made in China is good if supervised by western expertise. Made in China under Chinese supervision is bad. Designed in China is even worse.
Reply by ●January 7, 20232023-01-07
On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote:> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > > The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > > I am wondering if it's worth picking up some of the older 1000V models off ebay.You blew out some nominal protection like this: https://www.vishay.com/docs/29072/ptccl.pdf This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC. If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs.
Reply by ●January 7, 20232023-01-07
On a sunny day (Sat, 07 Jan 2023 08:44:30 -0800) it happened John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote in <st7jrh92oiv154ill1ve98mop0qpk4n8u1@4ax.com>:>Is any Chinese stuff really good? Rigol is. Extech is good but isn't >all Chinese.I have several of those Chinese 3$40 multimeters in use every day, and an analog one too. Most 'Chinese' stuff is OK, and much has US or Taiwan made chips in it. My Chinese AC / DC clamp on meter died, but more because it mechanically sort of fell apart, got a Voltcraft, not sure where that is made,,, works great. Have a Voltcraft soldering station too, now in use for 10 years or more. Lots of small modules from China from ebay.. My drone is super! China too. My Tecsun SW LW AM FM SSB radio, my Baofeng 2 way radio, my satellite receivers, my terrestrial TV receiver.. etc etc But from the US? Microchip PIC micros are OK... Oh and LCD modules, OLED modules... all from ebay Wow my Huawei 4 G USB modem sticks (have 2) SUPER SUPER US better get their trip together, just sanctioning the competition does not do it you guys need to produce cheaper faster better,, Like you hinted, why buy a Tek if a Rigol is cheaper and maybe better.
Reply by ●January 7, 20232023-01-07
On Sat, 7 Jan 2023 09:34:02 -0800 (PST), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote: >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. >> >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. >> >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > You blew out some nominal protection like this: > >https://www.vishay.com/docs/29072/ptccl.pdf > >This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC.PTC+TVS can help but isn't generally reliable. The combo still has failure modes. For signal-level stuff, depletion fets are good current limiters. I knew the guy who designed some nice Fluke benchtop DVMs and he used long strings of jfets as HV current limiters in the ohms sections. The world needs a really good 2-terminal bulletproof current limiter.> >If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs.
Reply by ●January 7, 20232023-01-07
On Saturday, January 7, 2023 at 10:14:06 AM UTC-8, John Larkin wrote:> On Sat, 7 Jan 2023 09:34:02 -0800 (PST), Fred Bloggs > <bloggs.fred...@gmail.com> wrote: > > >On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote: > >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > >> > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > >> > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > > > You blew out some nominal protection like this: > > > >https://www.vishay.com/docs/29072/ptccl.pdf > > > >This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC. > PTC+TVS can help but isn't generally reliable. The combo still has > failure modes. > > For signal-level stuff, depletion fets are good current limiters. I > knew the guy who designed some nice Fluke benchtop DVMs and he used > long strings of jfets as HV current limiters in the ohms sections. > > The world needs a really good 2-terminal bulletproof current limiter. > > > >If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs.The failed cenTech meters come on with display but no meaningful readings. I don't see any PTC. There are glass fuses. One failed open but another one intact. I got an auto ranging meter. I will measure 400V with a 4:1 resister divider. I only need to know between 90% to 100% of 400V anyway. https://www.ebay.com/itm/295076537933
Reply by ●January 7, 20232023-01-07
On Saturday, January 7, 2023 at 1:42:05 PM UTC-5, Ed Lee wrote:> On Saturday, January 7, 2023 at 10:14:06 AM UTC-8, John Larkin wrote: > > On Sat, 7 Jan 2023 09:34:02 -0800 (PST), Fred Bloggs > > <bloggs.fred...@gmail.com> wrote: > > > > >On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote: > > >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > > >> > > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > > >> > > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > > > > > You blew out some nominal protection like this: > > > > > >https://www.vishay.com/docs/29072/ptccl.pdf > > > > > >This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC. > > PTC+TVS can help but isn't generally reliable. The combo still has > > failure modes. > > > > For signal-level stuff, depletion fets are good current limiters. I > > knew the guy who designed some nice Fluke benchtop DVMs and he used > > long strings of jfets as HV current limiters in the ohms sections. > > > > The world needs a really good 2-terminal bulletproof current limiter. > > > > > >If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs. > The failed cenTech meters come on with display but no meaningful readings. I don't see any PTC. There are glass fuses. One failed open but another one intact. > > I got an auto ranging meter. I will measure 400V with a 4:1 resister divider. I only need to know between 90% to 100% of 400V anyway. > > https://www.ebay.com/itm/295076537933Does that even measure current? Can you read that display in bright sunlight? This one looks more standard and is a lot of meter for the money: https://www.protechtrader.com/Digital-Multimeter-Autoranging-DMM-with-Test-Leads---Checks-Voltage-ACDC-Volts-ResistanceOhms-Current--10-Amps-15s500-MA-Diode-Transistor-NPNPNP-Battery-Tester-Meter-with-ONOFF Then there's this, same as the above: https://www.harborfreight.com/7-function-digital-multimeter-63759.html These meters are not bad for field work convenience.
Reply by ●January 7, 20232023-01-07
On Saturday, January 7, 2023 at 11:10:02 AM UTC-8, Fred Bloggs wrote:> On Saturday, January 7, 2023 at 1:42:05 PM UTC-5, Ed Lee wrote: > > On Saturday, January 7, 2023 at 10:14:06 AM UTC-8, John Larkin wrote: > > > On Sat, 7 Jan 2023 09:34:02 -0800 (PST), Fred Bloggs > > > <bloggs.fred...@gmail.com> wrote: > > > > > > >On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote: > > > >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > > > >> > > > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > > > >> > > > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > > > > > > > You blew out some nominal protection like this: > > > > > > > >https://www.vishay.com/docs/29072/ptccl.pdf > > > > > > > >This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC. > > > PTC+TVS can help but isn't generally reliable. The combo still has > > > failure modes. > > > > > > For signal-level stuff, depletion fets are good current limiters. I > > > knew the guy who designed some nice Fluke benchtop DVMs and he used > > > long strings of jfets as HV current limiters in the ohms sections. > > > > > > The world needs a really good 2-terminal bulletproof current limiter. > > > > > > > >If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs. > > The failed cenTech meters come on with display but no meaningful readings. I don't see any PTC. There are glass fuses. One failed open but another one intact. > > > > I got an auto ranging meter. I will measure 400V with a 4:1 resister divider. I only need to know between 90% to 100% of 400V anyway. > > > > https://www.ebay.com/itm/295076537933 > Does that even measure current?No, voltage and resistance only. I can probably calculate I=V/R. I still have a few cenTechs for measuring current. BTW, the failed meters still work with High Amp (HA) mode, with a separate lead connector.> Can you read that display in bright sunlight?Yes, display is large and good, even has back light at night.> This one looks more standard and is a lot of meter for the money: > https://www.protechtrader.com/Digital-Multimeter-Autoranging-DMM-with-Test-Leads---Checks-Voltage-ACDC-Volts-ResistanceOhms-Current--10-Amps-15s500-MA-Diode-Transistor-NPNPNP-Battery-Tester-Meter-with-ONOFF > > Then there's this, same as the above: > https://www.harborfreight.com/7-function-digital-multimeter-63759.html > > These meters are not bad for field work convenience.
Reply by ●January 7, 20232023-01-07
> This one looks more standard and is a lot of meter for the money: > https://www.protechtrader.com/Digital-Multimeter-Autoranging-DMM-with-Test-Leads---Checks-Voltage-ACDC-Volts-ResistanceOhms-Current--10-Amps-15s500-MA-Diode-Transistor-NPNPNP-Battery-Tester-Meter-with-ONOFFThat's the CenTech i am talking about. I got 1 partial (bad LCD), 2 fail (bad LV and LA) and 4 good one. Got load of them at Harbour Freight Tool between $0 and $4. They used to give it away with purchase, but cost $4 to $6 now.
Reply by ●January 7, 20232023-01-07
On Saturday, January 7, 2023 at 2:16:50 PM UTC-5, Ed Lee wrote:> On Saturday, January 7, 2023 at 11:10:02 AM UTC-8, Fred Bloggs wrote: > > On Saturday, January 7, 2023 at 1:42:05 PM UTC-5, Ed Lee wrote: > > > On Saturday, January 7, 2023 at 10:14:06 AM UTC-8, John Larkin wrote: > > > > On Sat, 7 Jan 2023 09:34:02 -0800 (PST), Fred Bloggs > > > > <bloggs.fred...@gmail.com> wrote: > > > > > > > > >On Saturday, December 31, 2022 at 1:19:58 PM UTC-5, Ed Lee wrote: > > > > >> But not immediately. I tested 400V occasionally, but couple of them died while testing 12V. I am wondering it 400V weaken the meter. > > > > >> > > > > >> The old CenTech meters are 1000V, but the new models are 250V. Why even bother to have 50V more than the next range of 200V. Perhaps it's just same design with new label, when they got enough reports/complaints. > > > > >> > > > > >> I am wondering if it's worth picking up some of the older 1000V models off ebay. > > > > > > > > > > You blew out some nominal protection like this: > > > > > > > > > >https://www.vishay.com/docs/29072/ptccl.pdf > > > > > > > > > >This is a quiescently low ohm resistor in series with measurement input. Schematics show a voltage clamp shunt to COM after the PTC. > > > > PTC+TVS can help but isn't generally reliable. The combo still has > > > > failure modes. > > > > > > > > For signal-level stuff, depletion fets are good current limiters. I > > > > knew the guy who designed some nice Fluke benchtop DVMs and he used > > > > long strings of jfets as HV current limiters in the ohms sections. > > > > > > > > The world needs a really good 2-terminal bulletproof current limiter. > > > > > > > > > >If the meter comes on and presents a normal display but doesn't produce any readings, your PTC is open. If the meter is dead and produces a haywire or no display, you've blown their processor chip, which means the voltage clamp failed, was missing, or never worked. Apparently the PTC will fail from overvoltage. It should fail open. Hard to imagine the voltage clamp being overstressed. The PTC is easy to spot. You might take a look at it. And if an issue with the CenTech brand, you might buy a handful and replace the garbage in your working meters with better PTCs. > > > The failed cenTech meters come on with display but no meaningful readings. I don't see any PTC. There are glass fuses. One failed open but another one intact. > > > > > > I got an auto ranging meter. I will measure 400V with a 4:1 resister divider. I only need to know between 90% to 100% of 400V anyway. > > > > > > https://www.ebay.com/itm/295076537933 > > Does that even measure current? > No, voltage and resistance only. I can probably calculate I=V/R. I still have a few cenTechs for measuring current. BTW, the failed meters still work with High Amp (HA) mode, with a separate lead connector. > > Can you read that display in bright sunlight? > Yes, display is large and good, even has back light at night. > > This one looks more standard and is a lot of meter for the money: > > https://www.protechtrader.com/Digital-Multimeter-Autoranging-DMM-with-Test-Leads---Checks-Voltage-ACDC-Volts-ResistanceOhms-Current--10-Amps-15s500-MA-Diode-Transistor-NPNPNP-Battery-Tester-Meter-with-ONOFF > > > > Then there's this, same as the above: > > https://www.harborfreight.com/7-function-digital-multimeter-63759.html > > > > These meters are not bad for field work convenience.Check to be sure the display is not some pain in the ass that you have to activate and it only stays on for 3-seconds or something. Dunno what kind of display it is but some of them draw a LOT of current, closer to 100mA than 10mA, hence the quick timeout.
