Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Mon May 27 2013, 02:25PMHi all ,
I’m planning to build a simple HV scope probe for measuring DC or low freq AC (50-60Hz) up to 20kV with a 1:1000 ratio
As I won’t deal with high frequencies my goal is to keep it as simple as possible, and my plan is to make a simple resistive divider
I found a 200MΩ 30kV 12W 1% resistor on Ebay that I would use as R1
Resistor datasheet (SSX78 model)
For R2 I would use a resistors arrangement for a few watts and 250k (which is 200k with the 1MΩ scope resistance in parallel)
Current at 20kV DC would be 0.1mA and dissipation 2W
And then put everything in a 1" sealed PVC pipe filled with olive oil with a tip at one end and the cable to the scope and the alligator ground clip at the other end
Do you think that would work, and what kind of coaxial cable I should use for this (considering the scope will be on 1MΩ impedance mode)?
Thanks for your advices :)
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
HV Enthusiast, Mon May 27 2013, 03:53PM
For low frequency use like that I think you can get scope probes for HV on ebay for very little.
HV Enthusiast, Mon May 27 2013, 03:53PM
For low frequency use like that I think you can get scope probes for HV on ebay for very little.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Mon May 27 2013, 04:23PM
Thanks EVR for the suggestion
But I can't find any like the one I want to build
The best match I found is a 20kV 20MHz probe, but it is $215
If you find a cheap one for mains frequency please link it...
My goal is to measure MOT's, NST's and other HV transformers eventually with rectification / voltage multipliers / smoothing
EDIT:
I found this intersting article:
In the example of a probe connected to a 1MΩ scope input, it says "The cable is a high-impedance cable in this case"
I am wondering what exact kind of cable must be used in such case....
PhilGood, Mon May 27 2013, 04:23PM
Thanks EVR for the suggestion
But I can't find any like the one I want to build
The best match I found is a 20kV 20MHz probe, but it is $215
If you find a cheap one for mains frequency please link it...
My goal is to measure MOT's, NST's and other HV transformers eventually with rectification / voltage multipliers / smoothing
EDIT:
I found this intersting article:
In the example of a probe connected to a 1MΩ scope input, it says "The cable is a high-impedance cable in this case"
I am wondering what exact kind of cable must be used in such case....
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Carl Pugh, Mon May 27 2013, 05:07PM
Get a resistor that you don't have to put in oil, or use resistors in series.
The oil expands and if there's not an air space, the PVC pipe is likely to burst. There may be considerable oil expansion. (From bitter experience)
Calibration at 60 hertz can be checked using a signal generator and oscilloscope and voltmeter.
It is probably advisable to place capacitors across all resistors in the divider. If capacitors are not used and there are high voltage transients, the end resistor will fail then the next resistor and so on.
Carl Pugh, Mon May 27 2013, 05:07PM
Get a resistor that you don't have to put in oil, or use resistors in series.
The oil expands and if there's not an air space, the PVC pipe is likely to burst. There may be considerable oil expansion. (From bitter experience)
Calibration at 60 hertz can be checked using a signal generator and oscilloscope and voltmeter.
It is probably advisable to place capacitors across all resistors in the divider. If capacitors are not used and there are high voltage transients, the end resistor will fail then the next resistor and so on.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Mon May 27 2013, 05:47PM
I thought I'd use oil for insulation, but maybe I don't need it, or I can use something else ?
It's not gonna heat a lot as I use a 12W resistor for a max dissipation of 2W
And I just thought, maybe the coaxial impedance doesn't really matters for such low frequency as 50Hz, still would like confirmation about this
PhilGood, Mon May 27 2013, 05:47PM
Carl Pugh wrote ...
The oil expands and if there's not an air space, the PVC pipe is likely to burst. There may be considerable oil expansion. (From bitter experience)
Good to know that !The oil expands and if there's not an air space, the PVC pipe is likely to burst. There may be considerable oil expansion. (From bitter experience)
I thought I'd use oil for insulation, but maybe I don't need it, or I can use something else ?
It's not gonna heat a lot as I use a 12W resistor for a max dissipation of 2W
And I just thought, maybe the coaxial impedance doesn't really matters for such low frequency as 50Hz, still would like confirmation about this
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Steve Conner, Tue May 28 2013, 10:17AM
There is no such thing as a simple HV scope probe.
Coax has a capacitance of about 100pF per meter. The higher the impedance, the lower the capacitance. 75 ohm is the highest easily available. 92 ohm exists, but I've not seen it stocked.
So, if you use 2 meters of coax, your probe will be a low-pass RC filter with a time constant of 250k*200pF = 3.18kHz.
The usual solution is to introduce a small capacitance in parallel with the high voltage resistor. In this case, we need 200pF * (250k/200M) = 0.25pF.
Things now get complicated really fast, because the resistor would typically have at least 0.25pF capacitance between its ends already! But it also has capacitance to ground and to each end distributed along its length. The resulting frequency response can have a practically infinite number of poles, needing a complex compensation network. Tektronix's P6015 probe is a great example of this school of thought.
The brute force solution is to put a large capacitor across the 250k resistor, so you can put a bigger capacitor across the 200M resistor to swamp the stray capacitances. EasternVoltage designed a successful HV probe like this.
Another solution is to use a pure capacitive divider. Resistors have stray capacitance, but capacitors don't necessarily have stray resistance, so you can get a flat response to high frequencies without compensation. Jennings make some beautiful vacuum capacitor HV probes. They can't measure DC, but for many applications that isn't a problem.
Steve Conner, Tue May 28 2013, 10:17AM
There is no such thing as a simple HV scope probe.
Coax has a capacitance of about 100pF per meter. The higher the impedance, the lower the capacitance. 75 ohm is the highest easily available. 92 ohm exists, but I've not seen it stocked.
So, if you use 2 meters of coax, your probe will be a low-pass RC filter with a time constant of 250k*200pF = 3.18kHz.
The usual solution is to introduce a small capacitance in parallel with the high voltage resistor. In this case, we need 200pF * (250k/200M) = 0.25pF.
Things now get complicated really fast, because the resistor would typically have at least 0.25pF capacitance between its ends already! But it also has capacitance to ground and to each end distributed along its length. The resulting frequency response can have a practically infinite number of poles, needing a complex compensation network. Tektronix's P6015 probe is a great example of this school of thought.
The brute force solution is to put a large capacitor across the 250k resistor, so you can put a bigger capacitor across the 200M resistor to swamp the stray capacitances. EasternVoltage designed a successful HV probe like this.
Another solution is to use a pure capacitive divider. Resistors have stray capacitance, but capacitors don't necessarily have stray resistance, so you can get a flat response to high frequencies without compensation. Jennings make some beautiful vacuum capacitor HV probes. They can't measure DC, but for many applications that isn't a problem.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Tue May 28 2013, 01:10PM
Hi steve :)
Thanks for the detailed answer with values and calculations, this is quite instructive.
Would a 200pF of coax capacitance affect a lot a 50Hz signal ?
As for the capacitive divider, I do need to measure DC too
Is the EVR probe you mentionned this one ?
PhilGood, Tue May 28 2013, 01:10PM
Hi steve :)
Steve Conner wrote ...
There is no such thing as a simple HV scope probe.
Indeed I've seen that reading some threads here dedicated to this subject like Patrick's ProjectThere is no such thing as a simple HV scope probe.
Thanks for the detailed answer with values and calculations, this is quite instructive.
Steve Conner wrote ...
So, if you use 2 meters of coax, your probe will be a low-pass RC filter with a time constant of 250k*200pF = 3.18kHz.
Isn't that ok for DC and 50Hz AC measurements ?So, if you use 2 meters of coax, your probe will be a low-pass RC filter with a time constant of 250k*200pF = 3.18kHz.
Would a 200pF of coax capacitance affect a lot a 50Hz signal ?
As for the capacitive divider, I do need to measure DC too
Is the EVR probe you mentionned this one ?
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Steve Conner, Tue May 28 2013, 01:52PM
Hi Phil
Yes, that is the EVR probe.
An uncompensated probe may indeed be OK for measurements at 50Hz. It would be worth a try.
Steve Conner, Tue May 28 2013, 01:52PM
Hi Phil
Yes, that is the EVR probe.
An uncompensated probe may indeed be OK for measurements at 50Hz. It would be worth a try.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Tue May 28 2013, 03:14PM
Cool, so on an electrical point of view it seems ok
Thanks, I'll try that when I receive the resisitors and let you know how it goes...
My last doubt is about dielectric filling of the probe, I could use olive oil and leave some air to allow oil expansion ? Or use something solid like parafin ? Any better suggestion ?
PhilGood, Tue May 28 2013, 03:14PM
Cool, so on an electrical point of view it seems ok
Thanks, I'll try that when I receive the resisitors and let you know how it goes...
My last doubt is about dielectric filling of the probe, I could use olive oil and leave some air to allow oil expansion ? Or use something solid like parafin ? Any better suggestion ?
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Tony Matt, Wed May 29 2013, 03:26AM
Hi Phil !
Your thread make me think about the influence of the cable capacitance, mainly when you are using to test hv at 60Hz or lower frequencies.
It is clear that the cable capacitance presents a capacitive reactance to the 60Hz. And this reactance produce a " load" to the voltage that is going to the meter/oscilocope.
The question is find out HOW MUCH is this load,
The cable capacitance of 200pF running at 60Hz will give a reactance of :
Xc = 1/2*pi*F*C
Xc = 1/(2*pi*60 *200* 10^12)
Xc = 1.33 *10^7
Xc= 13,5megohm.
The capacitive reactance in parallel with the 250Kohm resistor (0.25Mohm)
R = 0.25Mohm >> 1/R = 4 *10^(-6) mho
Xc = 13.5 Mohm >> 1/Xc = 0.074 *10^(-6)mho
1/Z= (4 + 0.074j) *10^(-6) mho
Z= 1/(4+0.074j) *10^(-6)= (0.2499 - 0.00462j) Mohm
|Z| = 0.24996Mohms
ou 249.96Kohms
Conclusion:
The cable capacitive reactance at 60Hz is so high that, in practical way, the mesurement error will be minimum.
And your schematic using only a 200Mohm and 250Kohm resistor will be good for DC and 60Hz with reasonable precision
I think no oil feeling will be better.
Tony Matt, Wed May 29 2013, 03:26AM
Hi Phil !
Your thread make me think about the influence of the cable capacitance, mainly when you are using to test hv at 60Hz or lower frequencies.
It is clear that the cable capacitance presents a capacitive reactance to the 60Hz. And this reactance produce a " load" to the voltage that is going to the meter/oscilocope.
The question is find out HOW MUCH is this load,
The cable capacitance of 200pF running at 60Hz will give a reactance of :
Xc = 1/2*pi*F*C
Xc = 1/(2*pi*60 *200* 10^12)
Xc = 1.33 *10^7
Xc= 13,5megohm.
The capacitive reactance in parallel with the 250Kohm resistor (0.25Mohm)
R = 0.25Mohm >> 1/R = 4 *10^(-6) mho
Xc = 13.5 Mohm >> 1/Xc = 0.074 *10^(-6)mho
1/Z= (4 + 0.074j) *10^(-6) mho
Z= 1/(4+0.074j) *10^(-6)= (0.2499 - 0.00462j) Mohm
|Z| = 0.24996Mohms
ou 249.96Kohms
Conclusion:
The cable capacitive reactance at 60Hz is so high that, in practical way, the mesurement error will be minimum.
And your schematic using only a 200Mohm and 250Kohm resistor will be good for DC and 60Hz with reasonable precision
I think no oil feeling will be better.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Wed May 29 2013, 08:17AM
Thanks Tony for these calculations, it is really interesting
I have a new idea for insulation: I could use these laser transparencies people use for DIY HV capacitors. These transparencies are said to stand 10+kV:
So I should be fine with 10 layers. I would just roll them inside the PVC pipe. And doing this I can even use a smaller (1/2 ") PVC pipe for the front part of the probe, this would give the whole thing a better look.
Voltcraft makes a probe like the one I want to build, retail price is around 90€
Datasheet:
PhilGood, Wed May 29 2013, 08:17AM
Thanks Tony for these calculations, it is really interesting
I have a new idea for insulation: I could use these laser transparencies people use for DIY HV capacitors. These transparencies are said to stand 10+kV:
So I should be fine with 10 layers. I would just roll them inside the PVC pipe. And doing this I can even use a smaller (1/2 ") PVC pipe for the front part of the probe, this would give the whole thing a better look.
Steve Conner wrote ...
The resulting frequency response can have a practically infinite number of poles, needing a complex compensation network. Tektronix's P6015 probe is a great example of this school of thought.
Indeed, seven adjustments The resulting frequency response can have a practically infinite number of poles, needing a complex compensation network. Tektronix's P6015 probe is a great example of this school of thought.
Voltcraft makes a probe like the one I want to build, retail price is around 90€
Datasheet:
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Tony Matt, Thu May 30 2013, 03:24AM
what could be the damage if the capacitor get a short circuit??
EDIT :
I'm refering to the DIY capacitors that you want to use .
If this cap is connected in PARALLEL with the 200megohm resistor, and if this cap get a arc or fail, the 50 kV will be connected to your scope or vtvm...
The same with the oil, we do not know the olive oil beavior under so much voltage... Leave the prober dry and avoid a lot of trouble ...
Tony Matt, Thu May 30 2013, 03:24AM
what could be the damage if the capacitor get a short circuit??
EDIT :
I'm refering to the DIY capacitors that you want to use .
If this cap is connected in PARALLEL with the 200megohm resistor, and if this cap get a arc or fail, the 50 kV will be connected to your scope or vtvm...
The same with the oil, we do not know the olive oil beavior under so much voltage... Leave the prober dry and avoid a lot of trouble ...
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Thu May 30 2013, 06:41AM
EDIT: I wasn't planning to add any capacitor to my design, my idea is just to roll one of these transperencies sheet inside my PVC pipe for insulation
PhilGood, Thu May 30 2013, 06:41AM
Tony Matt wrote ...
what could be the damage if the capacitor get a short circuit??
Which capacitor ? I don't get your question ... srywhat could be the damage if the capacitor get a short circuit??
EDIT: I wasn't planning to add any capacitor to my design, my idea is just to roll one of these transperencies sheet inside my PVC pipe for insulation
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Steve Conner, Thu May 30 2013, 09:54AM
I imagine it would cause an "earth-shattering Kaboom".
Steve Conner, Thu May 30 2013, 09:54AM
I imagine it would cause an "earth-shattering Kaboom".
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
HV Enthusiast, Thu May 30 2013, 12:28PM
Regarding high voltage probes. Just recognize that layout and the types of oil or other potting material will greatly affect the overall frequency response of the hv divider.
Getting high bandwidth isn't too difficult, but getting high bandwidth with a flat response across the entire band, is EXTREMELY DIFFICULT.
HV Enthusiast, Thu May 30 2013, 12:28PM
Regarding high voltage probes. Just recognize that layout and the types of oil or other potting material will greatly affect the overall frequency response of the hv divider.
Getting high bandwidth isn't too difficult, but getting high bandwidth with a flat response across the entire band, is EXTREMELY DIFFICULT.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Thu May 30 2013, 11:58PM
Hi EVR,
I understand all the difficulties one may face when trying to build an HV probe with a high bandwith.
But my project is for a DC and AC 50Hz probe so I should be fine
As I said above I already gave up with using oil for insulation, I'll use a few layers of laser transparencies instead
Btw, do you have any pic you could share of your HV Probe described here
(finished probe, and construction pics showing components)
Would be much appreciated
PhilGood, Thu May 30 2013, 11:58PM
Hi EVR,
I understand all the difficulties one may face when trying to build an HV probe with a high bandwith.
But my project is for a DC and AC 50Hz probe so I should be fine
As I said above I already gave up with using oil for insulation, I'll use a few layers of laser transparencies instead
Btw, do you have any pic you could share of your HV Probe described here
(finished probe, and construction pics showing components)
Would be much appreciated
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Shrad, Fri May 31 2013, 07:24AM
If you wish I have for sale a high voltage probe which would suit your needs (vintage 50's with original box and manual)
Shrad, Fri May 31 2013, 07:24AM
If you wish I have for sale a high voltage probe which would suit your needs (vintage 50's with original box and manual)
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Patrick, Mon Jun 03 2013, 07:52PM
im not sure I did get a totally flat frequency response, even with my final best probe. But its a difficult task at MhZ periods.
for your dc and lo freq app, ill remind us all of Proud Mary and my own embarrassing oversight. using very high value (1Gohm) resistors and very low input current DMM, we had inexplicable slow oscillations even at 20Hz, where it would oscillate from a 20 kv known source, down to 7kv, the slowly to 80kv over 20-40 seconds. this was a coupled tiny leak capacity and very high Ohm impedance causing a RC with current drain to yield a totally bogus measurement
these devices a hard to get right, even at low freq. though DC is easier.
I put up several threads over 3-4 years here as my need was great, and there were no or few instruments described online, and fewer still that average tinkerer could build on there own.
id like steve Connors point on complex mulit-pole compensation elaborated on please...
Patrick, Mon Jun 03 2013, 07:52PM
wrote ...
Hi steve :)
Thanks for the detailed answer with values and calculations, this is quite instructive.
Yay! my work has been quoted, that will float my ego for a while.Hi steve :)
Steve Conner wrote ...
There is no such thing as a simple HV scope probe.
Indeed I've seen that reading some threads here dedicated to this subject like Patrick's ProjectThere is no such thing as a simple HV scope probe.
Thanks for the detailed answer with values and calculations, this is quite instructive.
im not sure I did get a totally flat frequency response, even with my final best probe. But its a difficult task at MhZ periods.
for your dc and lo freq app, ill remind us all of Proud Mary and my own embarrassing oversight. using very high value (1Gohm) resistors and very low input current DMM, we had inexplicable slow oscillations even at 20Hz, where it would oscillate from a 20 kv known source, down to 7kv, the slowly to 80kv over 20-40 seconds. this was a coupled tiny leak capacity and very high Ohm impedance causing a RC with current drain to yield a totally bogus measurement
these devices a hard to get right, even at low freq. though DC is easier.
I put up several threads over 3-4 years here as my need was great, and there were no or few instruments described online, and fewer still that average tinkerer could build on there own.
id like steve Connors point on complex mulit-pole compensation elaborated on please...
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
The Lightning Stalker, Mon Jun 03 2013, 10:13PM
DO NOT use olive oil! It will go rancid and its properties will change. If you must use oil, use minerail oil. It is better though to pot it in silicone or fiberglass resin.
The whole thing should be shielded with some type of foil as best you can.
The Lightning Stalker, Mon Jun 03 2013, 10:13PM
DO NOT use olive oil! It will go rancid and its properties will change. If you must use oil, use minerail oil. It is better though to pot it in silicone or fiberglass resin.
The whole thing should be shielded with some type of foil as best you can.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Patrick, Wed Jun 05 2013, 04:36PM
several of us have used olive oil for precision instruments, and canola for common instruments, and in 3+ years the device oil is clear and normal in color... the problem with ridged potting like epoxy, is that if it isn't done right you can have age change the surface bond on the HV device body to epoxy surface, and have a void which fills with air, then supports corona.
soft silicone and oil, don't have that potential problem. and besides oil cools way better than epoxy or silicone. Non-detergent engine oil, and mineral oil are exceptable. though if you can get real electrical oil from shell or exxon that would be best.
olive oil has excellent electrical properties for fast HV waves and such, with excellent age stability. However corn oil is just worthless.
Patrick, Wed Jun 05 2013, 04:36PM
several of us have used olive oil for precision instruments, and canola for common instruments, and in 3+ years the device oil is clear and normal in color... the problem with ridged potting like epoxy, is that if it isn't done right you can have age change the surface bond on the HV device body to epoxy surface, and have a void which fills with air, then supports corona.
soft silicone and oil, don't have that potential problem. and besides oil cools way better than epoxy or silicone. Non-detergent engine oil, and mineral oil are exceptable. though if you can get real electrical oil from shell or exxon that would be best.
olive oil has excellent electrical properties for fast HV waves and such, with excellent age stability. However corn oil is just worthless.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
HV Enthusiast, Wed Jun 05 2013, 05:31PM
Regarding oil, be sure to check out the dielectric properties. Some oils can have much higher significant dielectric constants than others which means they will introduce much more stray capacitance into your divider and affect the frequency response.
This is especially important in high bandwidth applications. An oil with high dielectric will really slow down your response due to the high stray capacitance.
HV Enthusiast, Wed Jun 05 2013, 05:31PM
Regarding oil, be sure to check out the dielectric properties. Some oils can have much higher significant dielectric constants than others which means they will introduce much more stray capacitance into your divider and affect the frequency response.
This is especially important in high bandwidth applications. An oil with high dielectric will really slow down your response due to the high stray capacitance.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
Patrick, Thu Jun 06 2013, 11:21PM
my previous threads :
High voltage probe for O-scope, 2 of 2, ( FABRICATION ).
High Voltage resistors, Measuring M Ohms to high accuracy.
High Voltage measurment with an O-scope....
High voltage probe for O-scope, 1 of 2, ( DEVELOPMENT ).
About High Voltage Insulating oils ....
Protecting O-scope inputs from HV transients...
Learning from North Star HV probes...
Capacitive Voltage divider, for O-scopes (10,000:1)
Making liquid tight or silicone resistor containers....
Potential Transformers, HV measurement.
HV Resistors for HV measurement.... (more important, shows methods known to work)
HV Flyback Bench Top Power Supply for Lab. (more important, exactly what your trying to do.)
i quote myself:
"
Patrick, Thu Jun 06 2013, 11:21PM
my previous threads :
High voltage probe for O-scope, 2 of 2, ( FABRICATION ). High Voltage resistors, Measuring M Ohms to high accuracy. High Voltage measurment with an O-scope.... High voltage probe for O-scope, 1 of 2, ( DEVELOPMENT ). About High Voltage Insulating oils .... Protecting O-scope inputs from HV transients... Learning from North Star HV probes... Capacitive Voltage divider, for O-scopes (10,000:1) Making liquid tight or silicone resistor containers.... Potential Transformers, HV measurement. HV Resistors for HV measurement.... (more important, shows methods known to work) HV Flyback Bench Top Power Supply for Lab. (more important, exactly what your trying to do.)i quote myself:
"
Patrick wrote ...
im thinking of making a tray of hard epoxy, filling it with Silicone tub and tile caulk, then pressing the resistors in to the wet silicone. if neccasary the silicone can be removed, and the resistors reused, this removes the hazard of springing a oil leak. since the silicone and epoxy are both solids.
i was advised by senior app engineer Richard Drawz, from Caddock not to hard encapsulate the resistors, as a difference in TCo would cause gaps between the resistor body and insulator, where corona will surely pop up. if expansion doesnt cause gaps, then shrinkage may cause cracking of the resistor Al2O body/core. so he advised only oil be used. He speculated that silicone would reduce heat dissipation over what is normally air, oil would increase effective wattage.
i wonder if silicone would conform to any dimensional changes while not being a liquid.
for background see these previous threads:
this is a great pic, from LutzH, his FuG.
"im thinking of making a tray of hard epoxy, filling it with Silicone tub and tile caulk, then pressing the resistors in to the wet silicone. if neccasary the silicone can be removed, and the resistors reused, this removes the hazard of springing a oil leak. since the silicone and epoxy are both solids.
i was advised by senior app engineer Richard Drawz, from Caddock not to hard encapsulate the resistors, as a difference in TCo would cause gaps between the resistor body and insulator, where corona will surely pop up. if expansion doesnt cause gaps, then shrinkage may cause cracking of the resistor Al2O body/core. so he advised only oil be used. He speculated that silicone would reduce heat dissipation over what is normally air, oil would increase effective wattage.
i wonder if silicone would conform to any dimensional changes while not being a liquid.
for background see these previous threads:
this is a great pic, from LutzH, his FuG.
Re: Questions about a simple HV scope probe (20kV - DC and AC 50Hz)
PhilGood, Tue Jun 11 2013, 08:12PM
Thanks guys for your replies and thanks Patrick for all the links !
I found a very clever idea on Sam's repair faq HV
I received the HV resistors and will start first tests soon...
PhilGood, Tue Jun 11 2013, 08:12PM
Thanks guys for your replies and thanks Patrick for all the links !
I found a very clever idea on Sam's repair faq HV
'Samuel M. Goldwasser ' wrote ...
A variety of devices could be placed across R2 to limit the maximum voltage present in the event of a breakdown. Suitable devices include neon light bulbs (NE2s without resistors)… The neon bulb is good since its impedance is essentially infinite until its breakdown of 90 volts or so is reached.
I really like the idea of this neon bulb across R2 !A variety of devices could be placed across R2 to limit the maximum voltage present in the event of a breakdown. Suitable devices include neon light bulbs (NE2s without resistors)… The neon bulb is good since its impedance is essentially infinite until its breakdown of 90 volts or so is reached.
I received the HV resistors and will start first tests soon...
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