He electrician spent 30 yuan and made a mini load! And it has gained 1W popularity on the open source website!
Why is a load so popular? And how did he make such a load for only 30 yuan?
Let's take a look at his design!
Why do you want to do this project?
Because my two electronic loads, when the discharge of a small current (less than 100mA) will exist: the control is not accurate, and the discharge current fluctuates greatly.
This makes it impossible for me to know the actual discharge curve of the battery, and I can only calculate the corresponding amount of power from the theoretical value, which leads to inaccurate fuel detection.
With all of this in mind, I decided to make one based on a more accurate analog circuit that would be more realistic! Mini loads at a lower cost!
What are the features/highlights?
- It supports the discharge of small pouch lithium batteries at the level of 100 milliamperes within 100 milliamperes.
- Support battery discharge curve recording: use IOTpowerCC meter head with host computer recording. Benefits: When making products, the amount of power can be reversed through voltage more accurately, and it can also avoid the situation that merchants falsely mark the battery capacity and discharge curve.
- With constant current/constant power load mode, the maximum load current can be switched by toggle switch: the maximum load current of constant current mode depends on the current carrying capacity of the triode you choose
- The structure is flexible and has a laminated design that retains the heatsink and cooling fan. It is connected to the IOTpowerCC meter head with a banana socket + pogo pin, and the meter head can be removed when not in use
How do I design the circuit? What about the test results of the circuit?
Let's analyze my plan!
Design Principles
Schematic diagram of the load board
Load board PCB diagram
Fan plate
Heatsink bracket
Physical drawing
The circuit of this project is relatively simple, and it is described in detail in 5 modules below.
1. Current direction
The principle of this part of the circuit is as follows:
- Plug the battery into the terminal block
- Enter the meter head through the copper pillar + banana plug
- It flows out of the meter head and into the load
- In this way, the discharge curve is recorded by the IOTpowerCC meter header
The switch is responsible for the control of the two paths of meter power supply and battery input load at the same time.
2. System power
The system power comes from 5V from the IOTpowerCC header.
When using this load, it is necessary to use an external power supply to power the meter head and toggle the toggle switch on the meter head.
The fan board was also designed for this project, why am I mentioning it here?
This board not only plays the role of a fan bracket, but also provides a pad with a pogo pin, which can be used to take power and send it to the control board. One board is multi-purpose~
The power supply part uses MT3608 as the main control, and the SW switch node is used to achieve back-voltage output to obtain ± 15V, and then the LDO is used to obtain a more stable ± 12V power supply.
The power supply part is shown in the figure above
3. Parameter setting circuit
As mentioned earlier, the constant/constant-resistance load mode is switched by a toggle switch.
- In constant resistance mode, RP1 is used to divide the voltage to output the reference value, and R17 is used to limit the upper limit of the reference voltage.
- The principle of constant current mode is similar, with the R19 being used to limit the maximum load current, which is currently limited to 200mA.
It's that the sources of feedback are different.
Since the load itself is a current feedback, giving a fixed reference voltage is the constant current mode, and the linear change of the reference voltage with the change of the input voltage is the constant resistance mode.
For the same reason, we can also design a constant power mode.
4. Load circuit
The load part uses 741 op amps to build negative feedback. The R0 resistor not only serves as a load function, but also serves as a current sensing resistor for feedback, so it adopts a high-power package of RX24-5W.
TIP41C is a high-power triode, which is inconvenient to use op amp for direct control, so 3904 is used to form a Darlington circuit with it.
Red and warm
In the working process of the load, the power resistance and TIP41C will heat up severely, so you need the cooling fan mentioned above to actively dissipate heat!
5. Fan temperature control circuit
The open-drain output feature of the AP331 comparator allows for easy control of the AO3401 MOS transistor, thereby reducing the number of peripheral circuits.
Voltage selection can be performed via the J1 patch pad (0 Ω resistors in the 0805 package are recommended for jumpers).
Use a resistor with a resistance value as a jumper to limit the problem of fan speed being too high. It is generally not recommended to exceed 5Ω, please test after setting the resistance to avoid the fan not being able to start.
Circuit function testing
The following is the actual test shot, in order to facilitate the test under different voltage conditions, the CNC power supply is used instead of the battery input load.
1. Constant resistance mode
Change the input voltage, and the load resistance remains the same
2. Constant current mode
Change the input voltage, and the load current remains unchanged
3. Battery discharge curve test
Use the IOTpowerCC meter head to record the battery discharge curve with the host computer.
This is also the main function of my design of this module - to achieve accurate small current discharge, so as to obtain the accurate discharge curve of small pouch lithium battery, so that the product can be more accurately reversed through the voltage when making the product.
The discharge curve of the sample battery is shown in the figure (the discharge current is set to 100mA)
Cost description
This project is designed at a low cost.
The overall cost without counting the header is only less than 30 yuan.
It can also be cheaper to replace some components, and I made the selection based on my own design habits.
The project is based on Jialichuang free PCB + free panel proofing
Do not use this load in an unattended environment, especially when the battery is discharged, the equipment and battery should be kept under observation throughout the whole process to avoid accidents!
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