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A generic Chinese meter reads

  • 577 VDC LV probe
  • 631 VDC HV probe
Keithley 2015 audio bench top DMM
  • 590 VDC LV probe
  • 645 VDC HV probe
I haven't run a calibration on the Keithley. I could take it into work to get cal'd but probably not necessary, I think the measurement can be trusted, and it does correlate with the cheap Chinese.

Taking the Keithley data the difference is 55VDC which is 9% of the LV value, but Kevin meant +- 10% of the HV probe resistance. According to the data sheet for mine the spec is +-10%, interestingly the data sheet for the 40kV version has a spec at +- 1%. Further, the data sheet for the HV probe specifies it needs to be used with a meter with a 10 Mohm, but the input impedance of the Keithley is 1MOhm. Not sure about the Chinese meter.

Have to think about this

Doesn't your Keithley have 10Meg input impedance at the 1000V range? And >10G at the 1V range? ....

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Doesn't your Keithley have 10Meg input impedance at the 1000V range? And >10G at the 1V range? ....

 

 Good catch! I mistakenly used the AC input impedance spec. 

 

I'll check again tomorrow with the Keithley and check the range I'm using, but given my equipment and this spec the Keithley looks like the best measurement, with the variation probably attributable to errors in the HV probe and handheld meter. Shows the importance of a good measurement or you might have a bias significantly off. 

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Good catch! I mistakenly used the AC input impedance spec.

I'll check again tomorrow with the Keithley and check the range I'm using, but given my equipment and this spec the Keithley looks like the best measurement, with the variation probably attributable to errors in the HV probe and handheld meter. Shows the importance of a good measurement or you might have a bias significantly off.

The Fluke datasheet says you need a shunt resistor if you DMM has more than 10Meg input impedance .... I guess you are using the 1V range when probing with the HV probe?

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Seems like Kevin's assumption about the input impedance of your DMMs is off spec might be right ...

 

if the DMM reads 590V, and has 2Meg input impedance and the output impedance at the testpoint is 200k the actual voltage will be 59V higher. Using the HV Probe that has 75Meg input impedance will give a reading much closer to the 590V+59V ... 

Edited by sorenb

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Seems like Kevin's assumption about the input impedance of your DMMs is off spec might be right ...

 

if the DMM reads 590V, and has 2Meg input impedance and the output impedance at the testpoint is 200k the actual voltage will be 59V higher. Using the HV Probe that has 75Meg input impedance will give a reading much closer to the 590V+59V ... 

 

Seems more than likely that the probe is off, though by appearance it looks like a precision resistor. 

 

The probe looks like simply a resistor with HV standoff, is it a simple resistance or a reactive impedance? I did try measuring its resistance and didn't see anything useful. 

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82k between positive and negative terminals, nothing between the tip and the ground clip or other two terminals. Probably a simple explanation but I'm missing it. 

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82k between positive and negative terminals, nothing between the tip and the ground clip or other two terminals. Probably a simple explanation but I'm missing it. 

probably the shunt resistor in the probe you are measuring ..the probes are three terminal dividers .... and I guess you measured the 80k-40 probe tip-ground? ....your Keithley only goes to 100Meg, so measuring something at 1G will put it out of range....

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Assuming the shunt resistance is 82k, and that it's a 1000:1 HV probe, the series resistance (between positive terminal and probe tip) should be somewhere around 82 megohms - most standard ohm-meters will show "infinite" resistance.

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Here's a good link I found on the design of these. 

Here is the basic circuit:
   High Voltage     ------/\/\/\/\/\---------+-------------> + to DMM/VOM
                           R1            |                      |
                                         \                      \
                                      R2 /                   R3 /
                                         \                      \
                                         /                      /
                                         |                      |
  Ground Clip      -------------------------+-------------> - to DMM/VOM 


 

R1 together with R2||R3 form a voltage divider where R3 is the internal resistance of the DMM or VOM on the scale for which the probe is designed. While R2 is not strictly needed, it is recommended that it be included and approximately equal to the Z-in of the meter on the scale you will be using. The reason to include R2 is to insure that high voltage never can reach the meter. The ground clip should be securely connected to the metal chassis of the device being tested - the frame of a microwave oven or CRT grounding/mounting strap of a TV or monitor - before it is powered up. Both R1 and R2 should be located in the probe head. The only difficult part is locating a suitable resistor for R1 that has high enough resistance and physically is long enough such that arc-over is avoided. Caddock, OhmCraft, Victoreen, and Vishay are among the major companies that manufacture suitable resstors.

Edited by Earspeakers

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Here's a good link I found on the design of these. 

Here is the basic circuit:
   High Voltage     ------/\/\/\/\/\---------+-------------> + to DMM/VOM
                           R1            |                      |
                                         \                      \
                                      R2 /                   R3 /
                                         \                      \
                                         /                      /
                                         |                      |
  Ground Clip      -------------------------+-------------> - to DMM/VOM 


But doesn't it already state that in the manual for the probe's?

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I've been playing with my KGST and noticed something: when I power it on, the offsets measure crazy high. Like ~350V. In about 3-5 seconds, they drop to the expected 1-2V and stay there. I guess it has to do with the way the tubes turn on, maybe the heaters?

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I've been playing with my KGST and noticed something: when I power it on, the offsets measure crazy high. Like ~350V. In about 3-5 seconds, they drop to the expected 1-2V and stay there. I guess it has to do with the way the tubes turn on, maybe the heaters?

Exactly right. The heaters take a while to warm up for the tubes to start conducting.

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The KGST is easy to live with while putting out quite a bit of mojo (400v swing) in a relatively small package.  Runs pretty darn cool too...thanks to Geoff.  I haven't run it any longer than 12 or so hours straight, but I'm pretty confident it could wrong longer w/o drama given how well the temps are managed.

At the moment I'm listening to Duane Allman - an Anthology....Loan Me a Dime is the tune which he plays with Boz Scaggs.  All kinds of goodness.  

Song just rolled and now it's Dave Meniketti's - On the Blue Side; song:  Until the Next Time.

I'm sippin' rum and smiling after a looooong day...

HS

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Hi guys,

I'm newbie here. I've already built a KGST PSU. I hooked up 2X 335VAC and 18-0-18 at the same time for provisional.

However, the 150-150-100 zener string for +400V exploded. Yep, exploded.

After that, I was trying to figure out what's wrong about the zener string. I only hooked up the 335VAC for the -400V and found the zener string ran extremely hot. Even the solder melts! I turned off the power immediately, and luckily they looked fine.(hope so)

The voltage should be 150-150-100. But actually they ran at about 180-180-110.

So, could any one tell me what to check and how to make it right?

 

Plus, I had run the PSU once about a weeks ago. It was fine at that moment( not sure if it's me that didn't notice some thing was going wrong)

All the voltage include +-400V and +-15V was fine then.

 

It was kinda weird.......

Edited by joehpj

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Never had that happen after building at least 100 of those PSU's over the years.  It only gets that hot if massive amounts of current are flowing though it so something somewhere is very wrong. 

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Never had that happen after building at least 100 of those PSU's over the years.  It only gets that hot if massive amounts of current are flowing though it so something somewhere is very wrong. 

Any suggestion for where to start checking from?  I know little about schematic and electric things. What I could do is put the components on where it should be and solder it. 

BTW, the the 335VAC from the transformer reads about 350V when unloaded. I thought it was fine for an unloaded transformer.

The zener besides the 8N80C was bought from an local store and I can't really remember if I bought the right spec as 24V/1W. Will it matter if the actual spec is 24v/0.5W?

All other things are bought from mouser and the same as BOM. Double checked for the direction. I use metal screw for the 10M90S but I had them isolated with plastic washer extend in to ceramic thermal pad. I thought it should be right.

I was thinking if I mistakenly put one thing in wrong direction, the problem should  happen in one direction but not  both +-400V supply.

Thanks for replying!

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I'd check if the diodes didn't have a high enough wattage rating. The vishays I use are rated for 1.3W (some are 1.5W).

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Post some pictures to start with.  Anyhing from local stores worries me... 

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ZcKhHsh.jpg

hQ2oaHy.jpg

Wn7MmVF.jpg

7SHdZXK.jpg

QsRhT0B.jpg

the solder is noclean type so I didn't clean them up. Maybe it's because of flux?

please don't hesitate to ask for more detailed pics. many thanks!

Edited by joehpj

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Pretty damn scary !

But start with serious soldering. Like cutting the sand's legs. Like not putting that much solder (2nd picture, at the 10M90S).

And source genuine components from reliable sources, even if it can be expensive.

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my guess is that there is a short on the 10m90 that i cannot see, or that part is dead resulting

in way to much current to the zener string

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I agree that the 10M90 is the most likely culprit here.  I would strip it out and clean the boards well.  No clean flux does not apply at these voltages!!!  :)

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I had a similar problem, but with my kgsshv psu resulting from 8n80c bought from a local store. After replacing them with the mouser ones, and replacing the zener strings but leaving the resistors alone, the problem was solved.

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