Frequency Counter Probe - Measuring High Voltage

[nebulix]

I just purchased one of those Victor 3165 frequency counters from China.
For the most part, it works well for my purposes.

I'm concerned about measuring high voltages directly to the BNC inputs. (120 - 250V AC)

The specs say the A channel ( 1Hz to 50 MHz) is 1M impedance with a maximum AC/DC peak input of 30V.

The B channel (50 Mhz to 2.4 GHz) is 50R with maximum of 3V input.

Most of my measurements will be on the A channel.

The manual is not very clear, but somewhat addresses the measurement of higher voltages:

"When measuring high voltage or strong RF signal by cable input, cables should be
in series with large resistance to prevent damage. Wireless input of very strong signals should be measured in coarse mode to avoid damage."

So my questions are,

1. What value of resistor(s) would I need to measure 60 Hz AC line frequency? Would it only be a single resistor in series, or do I also need to add a second resistor to ground?

2. Not sure what they mean by "coarse mode", but I want to be able to measure the direct frequency output of my portable ham radios (2m, 70cm, 6m, etc.), which would be in the range of 5-10W RF output.

Currently I have a BNC coax to dual alligator clips, and my 100 MHz scope probes.

Any advice would be greatly appreciated. Thank you kindly in advance.

Duncan, VE3DMC

[tggzzz]

If you are thinking of connecting it directly to the mains to measure the mains frequency - don't. Just don't. There are too many ways you can damage the equipment or yourself.

If that's what you want to measure, then get a mains-rated transformer with isolated windings, and connect the counter to the low voltage winding. Choose the LV winding to be directly compatible with the counter input.

Simple, effective, safe.

[bdunham7]

Mains direct--don't do that.  There's no reason to and your counter is a simple gate type that will only give you 0.1Hz resolution at best.

RF--simplest method and one that won't load your RF output is a simple wire antenna stuck in the input to pick up the signal. 

[nebulix]

Thank you all for the quick replies and the safety warnings.

I now recall building an Arduino device to log the AC mains voltage throughout the day.  I ran that through a step-down transformer to a safe voltage of about 6V or so.

So for the RF, it should only need like a paperclip antenna on the BNC and just transmit with the antenna close but not touching?  I'll try that.

[David Hess]

The specs say the A channel ( 1Hz to 50 MHz) is 1M impedance with a maximum AC/DC peak input of 30V.

1 megohm frequency counter inputs are essentially the same as 1 megohm oscilloscope inputs so should support up to 400 volts peak, which means Victor cheaped out on the design.

1. What value of resistor(s) would I need to measure 60 Hz AC line frequency? Would it only be a single resistor in series, or do I also need to add a second resistor to ground?

Build an RF sampler.

2. Not sure what they mean by "coarse mode", but I want to be able to measure the direct frequency output of my portable ham radios (2m, 70cm, 6m, etc.), which would be in the range of 5-10W RF output.

Some x10 oscilloscope probes can make that measurement directly and they can be used with 1 megohm frequency counter inputs.