Measuring >100Vac

[superjirka]

Hi,
I'm about to build a guitar valve preamp where supply voltage can be as high as 300Vdc, signals usually in tens of V with offset of ~150V. The supply is of course isolated by a transformer from mains. I wonder whether I can use standard 1:100 probe with PicoScope from 2000 Series or whether I need some kind of isolated probe.

Thank you in advance,
Jiri

[Gerry]

Hi Jiri,

As you are building it and presumably testing it, that would be classified as a CAT II source according to the IEC 61010 and UL 61010 standards for safety requirements of Test Equipment. So you would need a suitably rated Differential Probe, e.g. capable of measuring more than the maximum voltage (A.C. peak, RMS and also D.C. i.e. 300V if the A.C. maximum is not higher) at CAT II or higher. We have an Active Differential Probe rated at 700V for CAT III (see here: https://www.picotech.com/accessories/hi ... tial-probe) which would be suitable, if you can't find a lower voltage (if that is what you need), CAT II probe elsewhere.

Regards,

Gerry

[superjirka]

Hi Gerry,
Thank you for your answer.

If I understand measurement categories correctly, the measurements should fall into CAT I since I'm not going to measure anything connected to mains, i.e. I'm not going to measure primary side of the transformer. Even though it is not a "low voltage" source, the circuitry is isolated.
I may haven't looked deep enough into literature and there may be a voltage level requirement which I've omitted.

[Gerry]

Hi Jiri,

It all depends upon the voltages that you are measuring along with the potential impulse energy that you can get from any point in the circuit, under operating, or fault conditions (especially where you have built it and made a critical mistake that could expose you to a high impulse). So, for instance working on household appliances, portable tools, etc (as you can see from page 2 of this document: https://dam-assets.fluke.com/s3fs-publi ... NG_J_W.PDF) even though there may be isolation from the mains (via a transformer) there will still be relatively dangerous instantaneous currents at high enough voltage that could cause some damage, and so would not be safe to work on with CAT I rated equipment (in fact there is no CAT I rating anymore under the latest 61010 standards).

I know that you are working on a pre-amp as opposed to an amp, but there will still be valves that could potentially threaten your safety depending on how they are being used. My advice would be to either (a) establish that you won't get any impulse energy above 1.5kV across 30 ohms, (b) play it safe with CAT II ratings and find yourself a 300V Cat II Differential Probe, or (c) use the probe I mentioned previously if you can't do (b).

There's a useful post here about the dangers: https://www.valveheaven.com/2015/01/saf ... p-building (although admittedly it is referring to an amp as opposed to a pre-amp). I'm not necessarily suggesting that you drop everything that I mentioned previously and follow the guidelines of the post (as I can't vouch for the validity of a randomly found internet article) but I am pointing you at the depth of information, so that you can see that it is not so straightforward to make sure that you have covered all of the areas of safety that you need to consider.

Regards,

Gerry