Motive

Some wireless modules makers have not shown the actual transmitting peak current. So Enoki wants to know 3.3V DC-DC converter or regulator driving ability. We usually load some fixed resistors to test our designed circuit. With variable current loader, we can evaluate power supply easily.

Summary

Enoki made a simple active loader by a transistor and trimmer with ammeter and heat sink cutting from aluminum door frame.

経緯目的

市販の DC-DC コンバータの実力を知りたい。

要約

アルミ製ドアサッシから切り出したヒートシンク，トランジスタおよびトリマによる簡単負荷装置を製作した。

Power transistor

Bipolar transistor can dissipate power, when the base current increase. Enoki has a used power transistor NPN 2SD553 and Chinese universal connection board as shown in Photo1. The maximum ratings of the transistor are,

• Collector-emitter voltage: 50V
• Emitter-base voltage: 5V
• Collector current: 7A
• Base current: 1A
• Collector power dissipation (Ta=15°C): 1.5W

Transistor tester

A Chinese tester showed that h_FE and V_BE are 80 and 540 mV. The transistor works well. Enoki used it more than 15 years ago.

Photo1 Used 2SD553

Photo2 Transistor tester

Photo3 Channel

Heat sink

If 2SD553 drives at 3.3V, the allowable current will be 0.45 A. Considering margin and the actual ambient temperature, Enoki thinks 0.3 A reasonable. Enoki's room temperature is more than 30°C in summer.

Heat sink is expensive comparing with transistor itself. Enoki has a used aluminum door frame channel as shown in Photo3.

Base current

If base current increases, collector current increases by multiplying h_FE.

Base current i_B expresses of base supply voltage V, base resistor r_B and base emitter voltage V_BE.

I_C = h_FEi_B

i_B = (V-V_BE)/r_B

Differentiating,

di_B/dr_B = -V/r_B²

i_B changes non linear.

In case of base supply voltage 5V, the table shows base resistor values.

The changing rate of current ratio gets to 10. Enoki has a few trimmers and picked 5kΩ.

1kΩ was connected in series not to shorten.

ammeter

DMM ammeter internal resistor is high value. Enoki has an ammeter of which resistor is 50mΩ only.

Table 1 Base resistor

Ic [mA]	50	100	200	300
iB [mA]	0.62	1.13	2.27	3.75
rB [kΩ]	7.96	3.94	1.96	1.19
diB/drB	0.08	0.32	1.30	2.91

But its ground is common to detective ground not to measure high side current if Power supply will be supplied wiht the 5V line.

So Another 5V power supply supplies with the ammeter.

Work

Enoki cut the channel by disk cutter, grinded the line projection and worked thread by tapping M3.

Assemble

The wired board with 2SD553 was fastened with the heat sink by M3 brass screw as shown in Photo5.

The screw transfers heat well because brass contains copper a lot.

And blank hole without thread transfers heat less effective than thread fastening 2SD553, Enoki thinks.

Confirm

Power supply DM-310MV feeds 732 mA and the ammeter shows the value.

Photo4 Thread & grind

Photo5 Loader

Active loader

Passive resistor is as usual for current dissipated loader.

But power resistor is big and more expensive than common ones.

With active loader by transistor, you will adjust dissipated current easily.

Ammeter

With a small isolation DC-DC converter, the loader will be able to be driven by single power supply.

It works at 4V more, and dissipates 8mA only.

Battery driven

Loader itself dissipates 3mA only in case of 300 mA loading.

With another DC-DC converter, 2 AA batteries can drive 5V.

With smaller value trimmer, the loader may work even at 3.3V.

While the loader is apt to fall because of the channel shape as shown in Photo6.

Counter weight

The batteries also work as counter weight.

Photo6 Batteries