AC Power Meter
Cryptic letters are codes for Japanese/Chinese characters. English readers may skip them.

そこで安価なラジケータ（もっとも、最近は品薄で入手困難なようです）と電流トランスで同じ機能を持つ物を作ってみました。仕事で大阪に行ったときに、日本橋のジャンク屋さんで見つけたものです。ひとつ数百円だったと思います。がんばれば自作も可能かもしれません。

これで、家にあるいろんなものを測りました。たとえばパソコン本体は作動時約80W、待機（スタンバイ）時約35W、液晶ディスプレイは動作時約35W、待機時約5Wでした。これがCRTディスプレイになると、ぐっと消費電力は増えますが家にはないので測れません。それから、各種ACアダプタは物にもよるでしょうが、意外と損失は少ないようで、コンセントにささっていてDC側は何も使わなければ1W以下（というか、針はほとんどふれません）でした。

さて、我が家の場合パソコンを待機状態で夜中10時間つけっぱなしにすると、
(35W + 5W)×10h = 400Wh となります。1kWhがおよそ22円ですから、一晩当たり約9円で一か月で270円の無駄となります。最近は夜はパソコンの電源を切ることにしました。

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AC Power Meter

If you would like to save electric energy, you first of all need to measure the AC current flowing through your appliances. AC line voltage times the measured current value gives the power. Build an AC power meter with a current transformer as shown in the schematic given above.

The current transformer is not much different from an ordinary voltage transformer, but a primary wire is usually absent around the core. Either one of wires connecting an AC plug and an outlet wound around the core(two turns, in the present case) makes the primary coil. The current ratio flowing in the primary and the secondary wires is inversely proportional to the turns ratio of the transformer. The secondary AC current is rectified and smoothed by D1 and C. R1 and R2 are shunt resistors to give a correct full scale of the meter. You may need to combine a few standard value resistors to make the odd-valued R1 and R2. For the necessary calculations, refer to the equations given above. D2 is used to bypass the opposite-phase current.

I used the AC power meter to measure my PC and LCD display monitor. They are approximately 80W and 35W when in operation, and 35W and 5W in standby mode, respectively. I decided to switch off the both during the night for the energy saving.

N.B.: The current transformer may be broken if the secondary circuit is open with a high current on the primary wire. It is contrary to the common sense that a voltage transformer fires when the secondary is short-circuitted. Thanks to JA9CZJ for this comment.

MINOWA, Makoto
7N3WVM
Last revised 2001-01-02.