Need High Voltage Coaxial Cable 75 Ohm

[takachan]

Hi,

I have to connect a pulse of 500V peak 50uS to an oscilloscope using a coaxial cable. What type of coaxial should I use?

The type of coaxial cables available here are the types used in cable TV: RG58 and RG11, but in cable TV signals, I think, the voltages are small.

I tried with the RG11(75 ohms) , but according to the specifications of the Belden RG11, the maximun voltage is 300Vrms.

The pulse is derived from a capacitive divisor (K=800) in a high voltage impulse test of transformers.

Thanks

 

[Nigel Goodwin]

Use a different capacitive divider and make the pulse smaller.

However, 50uS 500V peaks should be perfectly fine on 300V RMS coax.

I'm a bit sceptical over the losses (and changes) in the cable though?.

 

[ronsimpson]

What oscilloscope can handle 500 volts on its input?

 

[takachan]

Hi Nigel,

I can't change the capacitive divider because of its high cost. To connect the oscilloscope I have a 100X probe.

Thanks for your reply.

 

[MikeMl]

Is the cable properly terminated with a 75Ω termination?

 

[spamme0]

metrology

You give no details, but It's likely that your problem is MUCH more complicated than which coax to use.

Unless your divider is buffered, the considerable capacitance of the coax may or
may not significantly alter the divider ratio...depending on which coax,
how long, the cap of the scope probe attached to the other end.
Terminating the coax in 75 ohms likely renders your divider useless...maybe.
Not terminating leaves you open to significant transient response fidelity issues.

Coax has both RMS and Peak ratings. While it's possible to exceed ratings and
have it work, it's not recommended.

You don't disclose, but one might infer that you're in an industrial environment
with considerable risk of DEATH!!
While it might work, several classes of people salivate when you
exceed specs, including
Safety inspectors
Personal injury lawyers
Your boss's boss
Your heirs...

Some other things to think about:
Scope probes have peak and rms voltage ratings that are VERY
frequency dependent. Your short pulse may exceed the probe
spec by a large margin, check it out.

Certainly depends on other unspecified constraints, but you're
more likely gonna have to put a voltage divider right at the cap divider
and run terminated 50-75 ohm coax to the scope. Make the division
ratio as big as the scope sensitivity will allow to reduce loading
on the cap divider. Once you get the DC part sorted out, you still have to
worry about frequency compensation/transient response.
You may have to correct the transient response of one or both dividers.
The proper way to compensate is to put a resistor over the high voltage
side of the cap divider. But this may not be possible because of the voltages.
There's relatively simple calculus to determine what to do. Problem is that
there are parasitic elements that you don't know. And that's what
you're trying to compensate.

But even after all this, your scope trace may not accurately show the pulse.
There's a lot of ART involved in measuring transients accurately.
You need a way to confirm that the measurement is accurate.
Something as simple as transient ground potential differences between the scope
and the measurement apparatus. If I'm interpreting you correctly, you've
got 400KV stuff going on.

The customers for your products expect you to have the required
experience to get the right answer.

This does not appear to be a "which cable" issue...

 

[KeepItSimpleStupid]

Why 75 ohm cables? How long of a length of cable do you need? How do you expect to calibrate the scope probe? What's the input Z of the scope? 1M or 50 ohm? Usually 50 ohm cables are used, such as RG174, RG58 or RG-6. RG-6 has no issues operating at 1000W into 50 ohms at 13.56 MHz, for instance. Connectors are typically N for the RG-6.

 

[takachan]

Thanks for your reply.

The equipment is working now, the original coaxial was broken in 2009 and was replaced by a RG11 75 Ohms. Now we have to change the location of the equipment and we need a longer coaxial cable, but the operating voltage of the Belden RG11 is only 300V RMS. I think this operating voltage seems not suitable for the application(500V pulse 50uS).

I agree with spamme0 about the "ART involved in measuring transients accurately", and I'm taking note of your suggestions. The system is used as Impulse Voltage Test of Power Transformers and generates a pulse of 1.2uS - 50uS of amplitude variable up to 400KV.