Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
|
|||
|
|||
HV Probe construction advice
A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into overload protection, I think as a result of leakage from the diodes (5647 tubes). Replacing them with solid-state diodes seems to have fixed that issue (I wish I could keep it in its original form, but I simply gave up after being unsuccessful for a few years.). Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. After reading specs on a 6kV Fluke HV probe, I realize they're really just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm resistor). So I figure I could save myself $75 by building my own. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) Any other warnings I should heed before doing this? I realize I'll lose some accuracy (they're only accurate to 5%). I guess I also have to worry about affecting the circuit when I draw .7mA out of it? I think my measurement error should be small, since my multimeter has a 10MOhm imput impedence. Is that good enough, or should I use a smaller final resistor in the voltage divider to reduce measurement error? Finally, my crazy idea of protecting myself from electrocution is to encase this thing in the plastic barrel of a Bic pen. They seem about the right size, plastic doesn't conduct, and as long as I hold it at the low-voltage end, it seems reasonably safe. Am I asking for trouble here, or does that seem reasonable for measuring only up to 7kV? I suppose I could build a safer handle onto it if needed. I'd appreciate any advice that could improve my life expectancy. Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=- -=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=- -=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=- -=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=- -=#| The above opinions are not necessarily those of my employers. |#=- |
#2
|
|||
|
|||
"Damian Menscher" wrote in message ... | A pet project for the past several years is repairing an ancient | Tektronix 674 oscilloscope. (The high voltage circuit was going into .... | One problem: Rat Shack says these have a "max working voltage" of | 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are .... | Finally, my crazy idea of protecting myself from electrocution is to | encase this thing in the plastic barrel of a Bic pen. They seem | about the right size, plastic doesn't conduct, and as long as I hold | it at the low-voltage end, it seems reasonably safe. Am I asking | for trouble here, or does that seem reasonable for measuring only up | to 7kV? I suppose I could build a safer handle onto it if needed. 1. I'd be reluctant to push the voltage too far. If one flashes over they all will. You're probably OK but be cautious. 2. Don't even THINK of using a hand held probe. Hook the divider up to the circuit, turn the scope on and check the voltage. Don't touch anything except the scope on/off switch or however you control this setup. 3. 5% is near enough. 4. If you want to own a probe, buy the real thing on eBay. Remember: "One flash and you're ash"! N |
#3
|
|||
|
|||
"Damian Menscher" wrote in message ... A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. (The high voltage circuit was going into overload protection, I think as a result of leakage from the diodes (5647 tubes). Replacing them with solid-state diodes seems to have fixed that issue (I wish I could keep it in its original form, but I simply gave up after being unsuccessful for a few years.). Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. After reading specs on a 6kV Fluke HV probe, I realize they're really just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm resistor). So I figure I could save myself $75 by building my own. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) They can flash over, that's your main worry. I've seen people make HV probes with simple resistors though, the best way seems to be to chain them together and then place them in a piece of plastic conduit then fill that with mineral oil. You can make a reasonably professional HV probe that way, and you're only measuring a few Kv so you should be fine. That said, I got a real Fluke 40 Kv HV probe on ebay for under $30 a couple years ago, I've seen the 6 Kv ones go for under $20 but with a 40 Kv you can measure CRT anodes. |
#4
|
|||
|
|||
"Damian Menscher" wrote in message ... After reading specs on a 6kV Fluke HV probe, I realize they're really just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm resistor). So I figure I could save myself $75 by building my own. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. In high voltage circuits like that there's not a whole lot current, and 10 Meg could be a significant load on it; however, that's probably no big deal. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) Not exactly. With voltages in that range theres's a possibility of a flash-over. You definitely want to make the divider so the resistors are phisically arranged in a series like: --####---####---####--- and don't use your HV probe when the humidity is too high. Also, I think those high value resistors are composite, so put one or two metal films in there -- if something bad happens, the metal film will evaporate, so in essence, it will act as a fuse. If you ever get hit by a CRT, 2.2kV isn't that high of a voltage, but it can be very inconvenient, or deadly if the PS can supply enough juice. |
#5
|
|||
|
|||
Damian Menscher wrote:
A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. (The high voltage circuit was going into overload protection, I think as a result of leakage from the diodes (5647 tubes). Replacing them with solid-state diodes seems to have fixed that issue (I wish I could keep it in its original form, but I simply gave up after being unsuccessful for a few years.). Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. After reading specs on a 6kV Fluke HV probe, I realize they're really just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm resistor). So I figure I could save myself $75 by building my own. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) Any other warnings I should heed before doing this? I realize I'll lose some accuracy (they're only accurate to 5%). I guess I also have to worry about affecting the circuit when I draw .7mA out of it? I think my measurement error should be small, since my multimeter has a 10MOhm imput impedence. Is that good enough, or should I use a smaller final resistor in the voltage divider to reduce measurement error? Finally, my crazy idea of protecting myself from electrocution is to encase this thing in the plastic barrel of a Bic pen. They seem about the right size, plastic doesn't conduct, and as long as I hold it at the low-voltage end, it seems reasonably safe. Am I asking for trouble here, or does that seem reasonable for measuring only up to 7kV? I suppose I could build a safer handle onto it if needed. I'd appreciate any advice that could improve my life expectancy. Damian Menscher Your resistors are too small. Too much load on a typical high voltage circuit. You're putting too much voltage on each resistor. When you buy resistors from RS, you have no idea what you're getting. When you use a pen as a housing, you have no idea what impurities are in there and whether it will arc over or through. If you have regular need for a HV probe, put a watch on EBAY for one and snag one that goes at a low price. Whole bunch of 'em have sold near the $10 mark recently. IF you only need this one measurment, borrow a probe and be safe. I wouldn't risk my life on a probe that cost me $8 to build when I could have bought a safe one at a swapmeet for $10. mike -- Return address is VALID. Wanted, Slot 1 Motherboard 500MHz Tek DSOscilloscope TDS540 Make Offer http://nm7u.tripod.com/homepage/te.html Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#6
|
|||
|
|||
mike writes:
Damian Menscher wrote: A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. (The high voltage circuit was going into overload protection, I think as a result of leakage from the diodes (5647 tubes). Replacing them with solid-state diodes seems to have fixed that issue (I wish I could keep it in its original form, but I simply gave up after being unsuccessful for a few years.). Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. After reading specs on a 6kV Fluke HV probe, I realize they're really just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm resistor). So I figure I could save myself $75 by building my own. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) Any other warnings I should heed before doing this? I realize I'll lose some accuracy (they're only accurate to 5%). I guess I also have to worry about affecting the circuit when I draw .7mA out of it? I think my measurement error should be small, since my multimeter has a 10MOhm imput impedence. Is that good enough, or should I use a smaller final resistor in the voltage divider to reduce measurement error? Finally, my crazy idea of protecting myself from electrocution is to encase this thing in the plastic barrel of a Bic pen. They seem about the right size, plastic doesn't conduct, and as long as I hold it at the low-voltage end, it seems reasonably safe. Am I asking for trouble here, or does that seem reasonable for measuring only up to 7kV? I suppose I could build a safer handle onto it if needed. I'd appreciate any advice that could improve my life expectancy. Damian Menscher Your resistors are too small. Too much load on a typical high voltage circuit. You're putting too much voltage on each resistor. When you buy resistors from RS, you have no idea what you're getting. When you use a pen as a housing, you have no idea what impurities are in there and whether it will arc over or through. If you have regular need for a HV probe, put a watch on EBAY for one and snag one that goes at a low price. Whole bunch of 'em have sold near the $10 mark recently. IF you only need this one measurment, borrow a probe and be safe. I wouldn't risk my life on a probe that cost me $8 to build when I could have bought a safe one at a swapmeet for $10. mike He's talking about the miserable almost no current output of a scope high voltage multiplier! Everyone is going safety crazy. I'm all for safety - there are enough references to it in the FAQs but for this, a simple voltage divider really is sufficient and safe. At worst, it will fry his meter if something bad happens. This is NOT a microwave oven. This is NOT projection TV. It's only 2 or 3 kV at a few hundred microamps max. 2 or 3 kV isn't going to flash over 6 inches! See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm However, I do agree that if you plan to do more HV stuff, be on the lookout for a proper HV probe. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#7
|
|||
|
|||
See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm
Building a voltage divider out of 10, 10M ohm resistors (and suitable math!) is sufficient and safe. However, I do agree with the other posts that if you plan to do more HV stuff, be on the lookout for a proper HV probe. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#8
|
|||
|
|||
Sam Goldwasser wrote:
See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm Building a voltage divider out of 10, 10M ohm resistors (and suitable math!) is sufficient and safe. And if that's what he'd proposed, I probably wouldn't have chimed in at all. Reread the original post. He said "10 'one meg' resistors in series". I maintain that the value is too low. I err on the side of safety simply because there are so many people here recommending generally unsafe behavior without sufficient explanation of why it might be OK in THIS case. Note that people (even famous repair gurus) often fail to read what's written, as suggested by your own comment above. Remember that these posts hang around in the archive for a LONG time. It's easy to imagine some kid reading a random post out of context, generalizing the snippet of information and killing himself off. Anybody who has to ask how to build a voltage divider shouldn't be using their first attempt on a 2KV measurement inside an instrument that has other sources of lethal voltages...especially when there are a bunch of commercial HV probes on ebay for cheap. Save your death wish for something that can save you some real money. While I'm on a rant, I'll suggest a general process to be used when posting advice to the web. You're familiar with the game "telephone". Tell someone what you're about to post. Tell them to pass it on. Come back a day later, pick someone 3 layers down the chain and see if what they heard bears any resemblance to what you said. If it's still safe, post it to the web. Soon you'll get to where you can do this in your head quickly by reviewing your proposed post with a blank mind. (I pride myself on my blank mind) The reader has no idea what you meant. All they know is what you wrote...then interpreted in their own unique way. Just cause I got away with it for 30 years, doesn't mean It's safe to recommend it to a newbie. mike However, I do agree with the other posts that if you plan to do more HV stuff, be on the lookout for a proper HV probe. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. -- Return address is VALID. Wanted, Slot 1 Motherboard 500MHz Tek DSOscilloscope TDS540 Make Offer http://nm7u.tripod.com/homepage/te.html Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#9
|
|||
|
|||
mike wrote in :
Sam Goldwasser wrote: See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm Building a voltage divider out of 10, 10M ohm resistors (and suitable math!) is sufficient and safe. And if that's what he'd proposed, I probably wouldn't have chimed in at all. Reread the original post. He said "10 'one meg' resistors in series". I maintain that the value is too low. I agree,and it WILL load the HV circuit.Even a 100 Mohm probe does. I used 4 22.5Mohm or 9 10Mohm 1/2w precision metal film resistors in series for a simple probe for a 10 Mohm input Z DMM. It works great. HV ------[90meg]------[DMM 10meg]-----/gnd If you do the true divider way,you have to compensate for the DMM input Z,if you want the probe to be reasonably accurate. -- Jim Yanik jyanik-at-kua.net |
#10
|
|||
|
|||
"Sam Goldwasser" bravely wrote to "All" (16 Dec 04 08:57:46)
--- on the heady topic of " HV Probe construction advice" SG From: Sam Goldwasser SG See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm SG Building a voltage divider out of 10, 10M ohm resistors (and suitable SG math!) is sufficient and safe. SG However, I do agree with the other posts that if you plan to do more SG HV stuff, be on the lookout for a proper HV probe. The high voltage divider in a broken flyback can be used in a pinch. There are high value resistors in it along with high value adjustable pots for the screen and focus. It may be hard to get the potted stuff out though... A*s*i*m*o*v .... You mean 15" sparks are SUPPOSED to come out of this thing?!? |
#11
|
|||
|
|||
"Asimov" writes:
"Sam Goldwasser" bravely wrote to "All" (16 Dec 04 08:57:46) --- on the heady topic of " HV Probe construction advice" SG From: Sam Goldwasser SG See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm SG Building a voltage divider out of 10, 10M ohm resistors (and suitable SG math!) is sufficient and safe. SG However, I do agree with the other posts that if you plan to do more SG HV stuff, be on the lookout for a proper HV probe. The high voltage divider in a broken flyback can be used in a pinch. There are high value resistors in it along with high value adjustable pots for the screen and focus. It may be hard to get the potted stuff out though... If it's a one time measurement, no real need to disassemble it. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#12
|
|||
|
|||
On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher
wrote: A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. 674? What's that? Is there a typo there? Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. That's a pretty low voltage for a Tek scope, so my guess is that this has already been divided down. This makes it more important that you make a pretty high impedance divider. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter as the last resistor in the chain? That would reduce the load on your test point by a factor of 10, which would be a good thing. One problem: Rat Shack says these have a "max working voltage" of 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are they just arbitrarily throwing a fudge factor of 2 away, or are they mislabeling 1/4W resistors as 1/2W resistors, or what? (They list 10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe to ignore.) That's just the voltage where they are likely to flash over. Since you're proposing to work with just 200 V on each one you should be fine. It's probably still a good idea to solder these together first and then wipe off the resistor bodys with alcohol to remove finger oils which can make surface leakage or flashover problems. Any other warnings I should heed before doing this? I realize I'll lose some accuracy (they're only accurate to 5%). I guess I also have to worry about affecting the circuit when I draw .7mA out of it? I think my measurement error should be small, since my multimeter has a 10MOhm imput impedence. Is that good enough, or should I use a smaller final resistor in the voltage divider to reduce measurement error? Don't touch any of this while you're doing the measurement. .7mA is probably too much for this circuit; .07mA is likely to be fine. Finally, my crazy idea of protecting myself from electrocution is to encase this thing in the plastic barrel of a Bic pen. They seem about the right size, plastic doesn't conduct, and as long as I hold it at the low-voltage end, it seems reasonably safe. It's safe only until they flash over. Then it will destroy your meter and would probably not kill you but really make you wish you'd used a real HV probe. Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign There are probably 50 HV probes within a quarter mile of where you work. Ask around. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#13
|
|||
|
|||
Jim Adney wrote:
On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher wrote: A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. 674? What's that? Is there a typo there? Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently it doesn't work to remember numbers after it's been 2-3 years. Anyway, with the reduced load for lighting the tubes, I think this needs to be recalibrated (intensity suggests it's got too much HV now). The service manual indicates a HV test point, at which I should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe. That's a pretty low voltage for a Tek scope, so my guess is that this has already been divided down. This makes it more important that you make a pretty high impedance divider. This is the "HV test point" (-2.2kV). The anode of the CRT gets the higher voltage you're expecting: +11.8kV. Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W resistors for $1. Recalling high school physics, I work out that one of these should be able to drop 707V at .707mA. So if I put 10 in series, I should be able to measure up to 7kV, and simply measure off the last one and multiply the voltmeter reading by 10. Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter as the last resistor in the chain? That would reduce the load on your test point by a factor of 10, which would be a good thing. Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea to use so many of them. I suppose I could add more to the chain, though. Or I could just get a real HV probe.... Finally, my crazy idea of protecting myself from electrocution is to encase this thing in the plastic barrel of a Bic pen. They seem about the right size, plastic doesn't conduct, and as long as I hold it at the low-voltage end, it seems reasonably safe. It's safe only until they flash over. Then it will destroy your meter and would probably not kill you but really make you wish you'd used a real HV probe. Thanks to you and everyone else for the advice. I'll certainly think more carefully about this (and probably just use a real HV probe). Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=- -=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=- -=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=- -=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=- -=#| The above opinions are not necessarily those of my employers. |#=- |
#14
|
|||
|
|||
"James Sweet" wrote in message news:Ll7xd.258$h.247@trnddc04... | | "NSM" wrote in message news:F37xd.5807$KO5.1825@clgrps13... | If you ever have any doubts about the power of electricity, watch the | episode of "The Beverly Hillbillies" where the bear eats granny's love | letter and Jethro comes up with a plan to X-ray the bear. | I would hardly think Beverly Hillbillies to be a good source of information | on electricity safety, anything in a TV show like that is pure fabrication, | it may coincide with real life but no more likely than what any random | person would assume to be true. After you see Jethro stick his tongue in the electric lamp socket (to show what the "dumb old bear" will do) you'll understand why the prospect of the electric chair fills him with dread. N |
#15
|
|||
|
|||
On Sun, 19 Dec 2004 05:06:25 GMT, "NSM" wrote:
"James Sweet" wrote in message news:Ll7xd.258$h.247@trnddc04... | | "NSM" wrote in message news:F37xd.5807$KO5.1825@clgrps13... | If you ever have any doubts about the power of electricity, watch the | episode of "The Beverly Hillbillies" where the bear eats granny's love | letter and Jethro comes up with a plan to X-ray the bear. | I would hardly think Beverly Hillbillies to be a good source of information | on electricity safety, anything in a TV show like that is pure fabrication, | it may coincide with real life but no more likely than what any random | person would assume to be true. After you see Jethro stick his tongue in the electric lamp socket (to show what the "dumb old bear" will do) you'll understand why the prospect of the electric chair fills him with dread. N Sheesh...it was bad enough when I was stupid and drunk enough to tongue-test a 10.6 VDC, 850 mA cellphone adapter... Tom |
#16
|
|||
|
|||
On 16 Dec 2004 08:57:46 -0500, Sam Goldwasser
wrote: See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm Building a voltage divider out of 10, 10M ohm resistors (and suitable math!) is sufficient and safe. I've heard, though, that it isn't a good idea to handle resistors that high with bare hands, due to the possibility of hand oils conducting current across the resistors. Tom However, I do agree with the other posts that if you plan to do more HV stuff, be on the lookout for a proper HV probe. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#17
|
|||
|
|||
Tom MacIntyre writes:
On 16 Dec 2004 08:57:46 -0500, Sam Goldwasser wrote: See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm Building a voltage divider out of 10, 10M ohm resistors (and suitable math!) is sufficient and safe. I've heard, though, that it isn't a good idea to handle resistors that high with bare hands, due to the possibility of hand oils conducting current across the resistors. Yes, once everything is soldered, they should be cleaned. Although, at 100M total, it probably isn't that significant. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#18
|
|||
|
|||
On Sun, 19 Dec 2004 03:38:11 +0000 (UTC) Damian Menscher
wrote: Jim Adney wrote: On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher wrote: A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. 674? What's that? Is there a typo there? Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently it doesn't work to remember numbers after it's been 2-3 years. How about 547? Does that sound any more familiar? ;-) Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter as the last resistor in the chain? That would reduce the load on your test point by a factor of 10, which would be a good thing. Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea to use so many of them. I suppose I could add more to the chain, though. Or I could just get a real HV probe.... If you really want to do this, let me know and I'll see if I can come up with 90 MOhms of resistors to send you. (In the suggestion above, I meant to suggest that you use nine rather than ten 10 MOhm resistors in series with your 10 MOhm voltmeter. Does your voltmeter have a 10 MOhm input impedance? What model is it? - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#19
|
|||
|
|||
Jim Adney wrote:
On Sun, 19 Dec 2004 Damian Menscher wrote: Jim Adney wrote: On Thu, 16 Dec 2004 Damian Menscher wrote: A pet project for the past several years is repairing an ancient Tektronix 674 oscilloscope. 674? What's that? Is there a typo there? Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently it doesn't work to remember numbers after it's been 2-3 years. How about 547? Does that sound any more familiar? ;-) No, it's definitely a 647. Got it right here in front of me. Looks to be one of the earlier ones, judging by the serial number. It has a strange malfunction which I've been trying (unsuccessfully) to correct for the past 10 years or so. I'll start a new thread about that, though, since it has little to do with this thread. Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter as the last resistor in the chain? That would reduce the load on your test point by a factor of 10, which would be a good thing. Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea to use so many of them. I suppose I could add more to the chain, though. Or I could just get a real HV probe.... If you really want to do this, let me know and I'll see if I can come up with 90 MOhms of resistors to send you. (In the suggestion above, I meant to suggest that you use nine rather than ten 10 MOhm resistors in series with your 10 MOhm voltmeter. You'll have to explain to me how to use the meter as part of the chain... that doesn't really make sense to me. Also, doesn't that impact the safety of the measurement? Does your voltmeter have a 10 MOhm input impedance? What model is it? Yes, it does. It's a BK Precision 2703B. Spec sheet is available online at: http://www.bkprecision.com/www/np_pdf.asp?m=2703B and shows a 10 MOhm resistance for DC measurements (which is what I'm planning to do with it. Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=- -=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=- -=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=- -=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=- -=#| The above opinions are not necessarily those of my employers. |#=- |
#20
|
|||
|
|||
On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher
wrote: Jim Adney wrote: How about 547? Does that sound any more familiar? ;-) No, it's definitely a 647. Got it right here in front of me. Looks to be one of the earlier ones, judging by the serial number. It has a strange malfunction which I've been trying (unsuccessfully) to correct for the past 10 years or so. I'll start a new thread about that, though, since it has little to do with this thread. Is this a solid state scope? You'll have to explain to me how to use the meter as part of the chain... that doesn't really make sense to me. Also, doesn't that impact the safety of the measurement? Think of ten 10 MOhm resistors in series. Each one sees 1/10th of the total voltage. Now replace the bottom resistor with your meter, which just looks like a 10 MOhm resistor when you put it in a circuit. Now your meter reads out the voltage it sees across itself, which is 1/10th of the total. So the total voltage is 10 times the meter reading. As long as you keep your fingers away from the upper resistors, I don't think you're in any real danger at the voltage levels you're talking about here. Of course you need to make sure that the low side test lead of your meter is securely grounded before you start this. Otherwise, the meter itself will float up to the full voltage. A better probe for high voltages would actually have a 990 MOhm resistor. Then you get to multiply by 100, and there is even less loading of the test point. But resistors for this will be much harder to find. A probe made to work to 40 kV would be made like this. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#21
|
|||
|
|||
Jim Adney wrote:
On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher wrote: Jim Adney wrote: How about 547? Does that sound any more familiar? ;-) No, it's definitely a 647. Got it right here in front of me. Looks to be one of the earlier ones, judging by the serial number. It has a strange malfunction which I've been trying (unsuccessfully) to correct for the past 10 years or so. I'll start a new thread about that, though, since it has little to do with this thread. Is this a solid state scope? Yup, but it still uses tubes as rectifiers in the CRT supply. It's kind of a weird hybrid, since everything else is solid state. I'm guessing they either didn't have solid state diodes that could handle the HV, or they thought the tubes would give a smoother output. I should take some pictures of it... I get the feeling this was a rare one. Even Google the Omniscient doesn't have pics of it. Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=- -=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=- -=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=- -=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=- -=#| The above opinions are not necessarily those of my employers. |#=- |
#22
|
|||
|
|||
Damian Menscher wrote:
Jim Adney wrote: On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher wrote: Jim Adney wrote: How about 547? Does that sound any more familiar? ;-) No, it's definitely a 647. Got it right here in front of me. Looks to be one of the earlier ones, judging by the serial number. Is this a solid state scope? Yup, but it still uses tubes as rectifiers in the CRT supply. It's kind of a weird hybrid, since everything else is solid state. I'm guessing they either didn't have solid state diodes that could handle the HV, or they thought the tubes would give a smoother output. I should take some pictures of it... I get the feeling this was a rare one. Even Google the Omniscient doesn't have pics of it. Here are pictures, for anyone interested: http://www.uiuc.edu/~menscher/Tek647/ Now that it's working, I'll have to think about what to do with it! Damian Menscher -- -=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=- -=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=- -=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=- -=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=- -=#| The above opinions are not necessarily those of my employers. |#=- |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Risk Management/Shop Safety and Advice (long) | Woodworking | |||
Newbie to Veneering Needs Advice | Woodworking | |||
Appraisal, refinance, and "under construction" question | Home Ownership | |||
Single Wall Construction Extension - Problems?? | UK diy |