Dranetz-BMI Dranetz 658
Converting Current Inputs
to Industry Standard 1V

The Dranetz 658 Power Analyser current clamp inputs are designed to convert current supplied from a current clamp on the input to voltage. This is done with the aid of a 120W resistor.

Popular current clamps intended for use with standard multimeters (e.g. Chauvin Arnoux) have the current to voltage conversion built in with typical full scale voltages of 1V. These are thus unsuited to the 658. Converting the current clamp is sometimes possible, but this would render them only useful for the 658.

Also, with the 658, the input resistor is often reported to be damaged with over-current on the current clamp. This renders the input channel useless until the unit is repaired.

The following is a modification to bring the 658 in line with the industry standard 1V making it suitable for use with a range of clamps within the PQ industry (with the added advantage that should a current clamp be damaged by over current, the 658 is still functional). The converted input suits my favourite, the Chauvin Arnoux 173, a switchable 1-10-100-1000A:1V clamp.

There are two notes:
  1. Dranetz-BMI have offered no support with this conversion. Sadly, when requesting the circuits of the input channels all that returned was a mouthful from a salesman. It must therefore be assumed that you are likely to have all support for your 658 cease with this conversion.

  2. The 5A range becomes a strange range and is effectively made redundant. If Dranetz-BMI wake up to this terrible reputation they have gained and let me have the circuits, we may be able to resurrect the 5A range in due course (how nice it would be to have a 1A range for earth leakage issues) - but don't hold your breath.

The Input Circuit

Current Input

Above is the circuit of the unmodified current input amplifier/buffer. The current-to-voltage conversion centres around R82, a 120W resistor. The full scale input current is 12.2mA which translates to 1.465Vrms (as tested on all 3 channels of my 658).

The Conversion

Modifying the 658 is extremely simple and centres around R82 only. The input impedance decided on was the industry standard of 1MW.

While consulting the inset photo, the steps are:
  1. Remove R82
  2. Solder a 22k resistor in R82 hole closest to U23 with the resistor perpendicular to the circuit board.
  3. Solder one of a pair of matched 470k resistors in the R82 hole closest to the input connector with the resistor perpendicular to the circuit board.
  4. Solder the remaining 470k resistor across the open legs of the two previous resistors to form a 'bridge'.
  5. Unsolder only the leg of R75 closest to the input connector.
  6. Solder this leg to the join of the 470k resistors.

This completes the circuit seen above, a 2.048:1 voltage divider and creates an input full scale of 1.465 x 2.048 = 3Vrms.


The 5A scale is unused from this point on (unless you are prepared to permanently divide the result by 1.66!).

The input full scale is now 3V for 30, 300, or 3000A as set on the Main Channel Setups, which represents 1V for 10, 100, and 1000A and thereby matches a majority of industry available PQ clamps.

Set-Up of 658
Clamp 658
10A:1V(100mV/A) I30
100A:1V(10mV/A) I300
1kA:1V(1mV/A) I3k

If there are any questions please ask.

Note: This web page does not form part of any official documentation.
Any information contained herein is used at own risk.

©  M.T.P. - 25.02.04