Enoki measured the efficiency of Holtek DC-DC converter before. But the efficiency curve was up and down. I used DMM to measure the input current. The current has complicated wave form. It is doubtful that DMM cannot respond varying input. So Enoki tried to measure DMM error at first.


It seems that the DMM has been accurate for 8 years. I had better use DSO measurement for true input current. Or I should use power meter.


以前 DC-DC コンバータの効率を測定した際,ガタピシした特性となった。入力電流は複雑な波形をしており,電流測定に 使用した DMM が応答していないのではないかと推測した。さらに DMM 自体の精度はどうなのだろうかと誤差の測定をした。


DMM は8年前の精度を概ね保持している。DSO の Measurement 機能を利用して真の電流を測定すべきだ。

DMM error test for measuring varying current 2020-01-17

Measurement method

Figure 1 Block diagram

Figure 1 shows the block diagram of the measurement system. Variable voltage power supply is Enoki's scratch to adjust voltage with 15 rotation trimmer more than 1.25V to 10V.

Load resistor was connected both PC510 and P-16, as the resistor voltage would drop cancel noise of them. DSO TDS2002 was used for monitoring the output voltage of the power supply.

I loaded fixed resistors of 1/4W (470k, 91k, 15k, 3.3k Ω), 2W 33 Ω, 10W 100 Ω.

Evaluation and P-16 review

Comparing relative error Now with 8-year-ago chart, the errors are the same as 1%. Error is less than 0.5 % in the less range of 1mA.

The power supply has 1Ω which is connected to output. So I observed the input current wave form as shown in Photo 1.

DSO showed 26.6 mV mean, 152 mV peak, 42.16 kHz at input voltage 1.82V, while PC510 showed 14.41 mA. PC510 also showed the same value, when measuring across the voltage in Photo 2. It is natural that DMM cannot respond to fast varying input. DMM shows half of the true value.

While input voltage showed sag at 43.4 kHz at the nearly input voltage, as GND is different from the above voltage drop.

To correct varying input power, it is necessary for measuring the dropping voltage and input voltage at the same time.

It needs 3CH DSO or real time power meter working at low input voltage. Connecting current detecting resistor to GND, I can use 2CH DSO. A tiny MPU has 8-bit A/D. I had better make a simple digital power meter to calculate DC-DC converter efficiency at real time.

Enoki reviews DMM P-16 a little. It takes some time to get a still value and it sets short auto power-off time 10 min. It is shorter than PC510.

And more it does not have power off contact of the rotary switch, as PC510 does.

So I have to be off the lead measuring mA range, when turning on every time.

© 2020 January 17