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Old 20th Dec 2007, 11:42 pm   #1
Neil Purling
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Default Oscilloscope probes

Is it possible to make usable X10 & X100 'scope probes and avoid paying out too much?
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Old 21st Dec 2007, 8:47 am   #2
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Default Re: Oscilloscope probes

Yes, but only for low bandwidths. If you want decent waveform fidelity for a 50MHz+ 'scope then you really need to buy them. For 200MHz+ decent probes will be expensive - the cheap ones are not very good.

The one main exception is for probing low impedance circuits such as logic. A simple 470R or 4K7 resistor connected to a length of 50R co-ax and terminated in 50R at the 'scope will be better than most very expensive 1GHz+ probes. And dirt cheap too.
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Old 21st Dec 2007, 6:40 pm   #3
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Default Re: Oscilloscope probes

RS do some reasonable ones for about £10, a little more and you can have x1 and x10 switched.
I would echo what Jeffery says; anything much above audio and you need proper compensated probes.
Note that these RS ones, and other similar ones need to be set up to your scope with whatever the adjustment is on the probes you get.
It's simply a matter of connecting the probe to the square wave cal point and adjusting for a good square wave without ringing or rounding of the waveform

Ed
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Old 21st Dec 2007, 7:19 pm   #4
Brian R Pateman
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Default Re: Oscilloscope probes

Quote:
Originally Posted by Ed_Dinning View Post
Note that these RS ones, and other similar ones need to be set up to your scope with whatever the adjustment is on the probes you get.
And not only that, you need to check and adjust the compensation regularly.

I do it each time I set up for a session of work with the scope.

I use the RS ones as a rule and they are fine for the sort of work we do unless you are looking at frequencies over about 150 MHz or very fast pulses with short rise times.

Do check the probes from time to time. Any halfway decent ones will have a maximum working voltage - usually 500 or 1000 V. If the outer surface is dirty or has deep scratches the insulation will be compromised.

That can be a significant hazard in some valve and TV work.

Regards,
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Old 21st Dec 2007, 8:00 pm   #5
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Exclamation Re: Oscilloscope probes

Yes - do watch the voltage rating of the probes - for some of the cheaper ones, it isn't very high. By 'very high', I mean relative to the magnitude of ac voltages that you are likely to find in valve equipment.

Here's a little anecdote - - -

Several years ago - when I was regularly working on low-voltage stuff on a day-to-day basis, a big mono. valve amp. came my way for a repair. I hadn't worked on big valve amps. for a long time - prior to this walking in. Anyway, fixed it & final testing time arose. Input signal from AF gen.; dummy load on O/P: getting many tens of watts out on power meter. Let's have a look 'round to see what's going on. Connect new 'scope probe to anode of EL34s (parallel connected). Bang! Whoops! A few hundred volts peak-to-peak at 1 Kc/s. One melted 'scope probe.

You only learn that lesson once.

Al / Skywave

Last edited by Skywave; 21st Dec 2007 at 8:04 pm. Reason: Typo + additions
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Old 21st Dec 2007, 11:18 pm   #6
Neil Purling
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Default Re: Oscilloscope probes

I am getting this scope (out of a school physics lab after they upgraded) and wanted to make up a probe. The scope may not be that brilliant so not spending too much on a probe sounds sensible.
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Old 22nd Dec 2007, 10:59 am   #7
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Arrow Re: Oscilloscope probes

In that case, have a look at the max. speed of the time-base. If it is, say, 1 uS, then the 'scope would be OK for audio and low RF work - so one of the cheaper X10 probes would seem to be appropriate.

Al / Skywave.
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Old 24th Dec 2007, 8:33 pm   #8
Paul Stenning
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Default Re: Oscilloscope probes

I bought two cheap X10 probes on eBay which were claimed to be suitable for up to 50MHz. Since the 'scope is only 20MHz that's good enough for me. They came from China and cost about £15 total for the two and postage. They seem to be reasonably well made, and include the usual clip-on tips etc. I'm not sure I'd trust them on very high voltages, though they are fine at valve radio HT voltages.
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Old 24th Dec 2007, 11:48 pm   #9
Brian R Pateman
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Default Re: Oscilloscope probes

I know the ones you mean - OK for the price but I'd tend to be wary at voltages over about 350.

Still at that price they are fine. I think that they are the same as the cheap Maplins ones which are about £16.00 each from memory.

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