Thursday, December 25, 2014

Converting a BK Precision 2831A bench multimeter to 220V supply

I just came across a BK Precision 2831A bench multimeter (for free). I do not know the age or the general reputation of the unit, but it seems to be a rather low-end 3-1/2 digit multimeter. Nevertheless, another multimeter might come in handy.

An old BK Precision 2831A (on top of something else)

The device is supposedly only meant for the US marked and is labeled 120V, so I could not test it right away. First, I thought of buying a 120V/240V transformer, but good quality ones does not come for free.

It is labeled 120V on the back side

Based on the labels on the circuit board (GDM-558D), it seems like this unit is a relabeled GW Instek multimeter of some kind. It is probably ins the same family as the GW Instek GDM-8034 (or the GDM-8135, although it has a circuit board marked GDM-625A). The accuracy of those two devices (DC volt) are 0.5% and 0.1% respectively, whereas the BK is reportedly 0.1%.

The circuit board also has some markings indicating that the transformer has two primary windings that can be coupled in parallel for 120V usage (marked as 114V on the circuit board) or in series for 240V usage (marked as 234V on the circuit board).

Typical configuration for switching between 110V and 220V with two primary windings.

Normally, units with such a transformer has a switch on the back, making it usable for both 120V and 240V mains supply, but this multimeter has no such switch. However, there are some resistors on the board that can be replaced to allow for the higher voltage setting.

Zero-ohm resistors configured for 120V (117V)  mains supply

Zero-ohm resistor configured for 240V (234V) mains supply

All there is to do is to remove the two resistors (and resolder one of them), and voila, the multimeter is ready for european voltages. In addition, the main fuse should be reduced to about 2/3 of its original size.

As indicated on the first image, the multimeter now works on 230V (or 220V/234V/240V or whatever). I cannot report on the quality on the device itself, such as the accuracy in taking measurements, but it seems to be fairly close to my Fluke 8050A so at least it is not damaged or anything. Hurray!

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