Small passive DC constant current electronic load

Introduction

A small passive DC adjustable constant current (CC) electronic load with a temperature-controlled fan for testing constant voltage DC power supplies.

Features and Parameters:

• Passive design
• Input voltage: 5~18V
• Constant current: 0~3A adjustable (set the maximum value through R8, use R1 to adjust the constant current value)
• Maximum design power: 25W, measured power: 10W in fanless mode, 35W in fan mode

Precautions:

• When using the USB interface as input, the current should not exceed 1.5A
• It is recommended to use a low-power model for the fan, otherwise it may be difficult or impossible to start.
• The two LDOs that power the fan can be selectively welded to one or two groups according to the actual power consumption of the fan.
• The temperature-controlled fan starts at about 65 degrees Celsius, be careful of burns! The starting temperature can be fine-tuned by changing the value of R10.
• It is strictly prohibited to reverse the power supply, otherwise it will cause irreparable damage.
• The heat sink is connected to the ground, be careful not to touch other electrodes when using it.
• When the input voltage is less than 6V, the fan cannot start normally due to the LDO voltage drop, and the power should be reduced.
• The power tube can be any type of TO-220 Darlington transistor, as long as it is NPN, but pay attention to the upper voltage limit and maximum power of the triode.

image display

Proofing finished product display

Physical picture

Work pictures (U-list project address: link )

Making tutorial

First prepare a heat sink, the size is 4x4cm, the height is arbitrary

Use a board to compare it on the top and mark the location of the screw holes.

Drill and tap holes

First use screws to temporarily fix the heat sink to the PCB, and then trace the position of the power tube.

Still drilling and tapping

Then remove the heat sink and solder all components except the power tube (if you don’t want a fan, the related circuits can be omitted)

After welding is completed, apply silicone grease to the front radiator part of the board, and seal the pad of the power tube with high temperature resistant tape to prevent short circuit.

Install the radiator and solder the power tube (don't forget the silicone grease)

Done

What it looks like after welding all the components