Re: LF: RE: RF Current meter

["Dick Rollema" <d.w.rollema@freeler.nl>]

Dear Gary, LF-Group,

The coupling between the primary and secundary winding of the current
transformer must be purely inductive.  But even with a one-turn
primary there is also capacitive coupling. Especially when the voltage on
the primary is high, for instance when the meter is clamped over an 
LF aerial when transmitting, the capacitive coupling causes an extra
current through the meter circuit.  The circuit  for that
current is closed via the capacitance of the measuring instrument and its
connecting wires to earth.

To avoid this effect in my instrument I placed a small screen inside the
toroid that shields the secundary winding from the primary.
("Pliers-type RF current probe", Technical Topics, RadCom,
October 1992). 

It is easy to check whether capacitive coupling influences the
measurement.
1. Take a reading.
2. Turn the clamp  over 180 degrees.  This reverses the current
in the secunday winding by inductive coupling but does not effect the
current by capacitive coupling.
3. Read the instrument again.

If  readings 1 and 3 differ take the average of the two as the
correct value.

When measuring current in earth wires the problem does not arise because
voltage on the wire usually is very low. 

73, Dick,  PA0SE

At 11:45 19-2-04, you wrote:
> Dear Gary, LF Group
>
> >"When I have looked at designs in amateur radio magazines or
> internet
> for a
> >clip on RF Ammeter the designs seem to be limited to 2 ranges (100mA
> &
> 1A
> >FSD), is there a limit to the range of this type RF Ammeter or can
> the
> >ranges be extented to say 5 Amps maybe? is there a calculation
> to
> extend >the
> >range ?"
> >My LF antenna seems to be showing in excess of one Amp so I need an
> RF
> >Ammeter with a greater range than 1A.
>
> For ammeters using current transformers, you can increase the FSD 
> by
> INCREASING the number of turns on the secondary of the transformer
> (current transformation ratio for a transformer being the inverse of
> the
> voltage ratio). So if you want to increase meter FSD by a factor N,
> just
> increase the number of turns on the secondary (meter) side by N.
> This
> divides the current going through the rectifier and meter by a factor
> of
> N also, achieving the desired result. There are a number of
> second-order
> effects that may alter the calibration somewhat, but experience
> shows
> these are usually pretty small.
>
> Shunting a thermocouple meter can be problematic, partly because 
> the
> thermocouple resistance is usually pretty low to start with, and
> partly
> because it changes as the heater element heats up with increasing
> current. A more reliable way of extending the range up or down is to
> use
> a current transformer again. So using a 1:N transformer with the
> "N"
> winding connected to the ammeter will increase the FSD by a factor of
> N.
> Provided the impedance of the winding inductance is large compared
> to
> the meter impedance, and primary and secondary are closely coupled,
> this
> is an accurate way of changing the meter range - so don't reject
> thermocouple meters just because they don't have the range you
> require!
>
>
> Cheers, Jim Moritz
> 73 de M0BMU