Oscilloscope

[woody1]

I want to get one to view the pulse waveform from an electric fence unit. I'll use a resistor divider to reduce the voltage from 10kV to say 50V.

.............10kV
................|
.............10kΩ (high voltage resistor)
................|
................|----------•
..............50Ω..............Oscilloscope
................|----------•
................|
..............Gnd

The unit produces a pulse every couple of seconds. The duration of the the pulse is around a millisecond.

I can pick up a cheap handheld 200KHz bandwidth scope on eBay for around 25 quid, or 50 quid for a 10MHz one.

Any pointers?

[Protagoras]

For general purpose and portable use, I use a basic low cost USB oscilloscope (mine was made by Pico).

I noticed that there are some cheap USB scopes on eBay so one of them might be worth the risk, but I've not tried any of these.

For capturing the electric fence pulse waveform, I suppose it's just a matter of getting the scope triggering set up properly given the pulse repetition rate.

That potential divider will give c.50V for 10kV input. That might be a bit high for USB type scopes, so maybe need to drop that further.

It should be obvious, but another point is that the high value resistor in the divider needs to have a suitable voltage rating and not be inductive!

[woody1]

It should be obvious, but another point is that the high value resistor in the divider needs to have a suitable voltage rating and not be inductive!

Finding a 10kΩ resistor, rated at 10kV, is not that easy.

I did some (destructive) testing a while back, and found that ten 1kΩ (¼W 250V) metal oxide film resistors, in series, could handle the output from the fence unit without breaking down. 10kΩ (10W 750V) wirewound also worked fine. I think it's the length of the resistor body (40mm) which is what stops the high voltage jumping across the windings.

I hadn't considered inductive reactance. I imagine wirewound might be more prone to this than the metal film ones.

[Protagoras]

High voltage high ohmic value resistors are available.
e.g. https://uk.farnell.com/tt-electronic...age%20resistor

By way of example, a divider of 300kΩ / 300Ω would give c.10V output for 10kV input.
If the 300kΩ is constructed as three 100kΩ in series that would avoid stressing the resistors.
Resistor power rating is also a consideration, so it would help to understand the duty cycle of the electric fence supply.

If you want accurately to display the waveform, then adding reactance to the measurement system won't be helpful. I'd be avoiding wire-wound resistors.

[woody1]

Resistor power rating is also a consideration, so it would help to understand the duty cycle of the electric fence supply.

The only spec I've got for the unit is that it's 0.6 Joule. From what I've read, typically the pulses will be of the order of a few mS and a few mA.

The wattage is probably quite high but since the pulses are only a few mS, every couple of seconds, I doubt it would generate any heat in a resistor. After all, it's designed to shock, not fry/kill.

[Protagoras]

The only spec I've got for the unit is that it's 0.6 Joule.

Indeed, 0.6J (Watt-seconds) isn't much energy.

[DoctorStrangelove]

https://uk.rs-online.com/web/p/oscil...E&gclsrc=aw.ds

I used to frighten the esteemed customers with one of these.

More sensibly, Mouser stock 10kV resistors in a range of values up to 100G ohms.

Whether they are available in quantities of less than a lot is debatable.

Dunno what loading an electric fence with 10k is likely to do.