RD Electronic USB load LD25

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Official specifications: I bought from Aliexpress dealer: RD official store

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I will take a closer look at the circuit further down.

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The user interface is two buttons, a multiturn potentiometer and a 4 digit display. The SET button is used to change between current (A), voltage (U) and power (P) display. The load is turned on/off with the SET button and adjusted with the potentiometer. The current display is flashing when load is off.

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Overload conditions will disable the load and show a warning on the display. Here over power protection (OPP) and over voltage protection (OUP)
Holding the SET button down will toggle automatic resume on/off.
Holding the On/Off button down will toggle automatic on when power is connected.



Load testing

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Both current and voltage readout has good precision.

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Due to the power limit of 25W it is not possible to test with high current and high voltage, here I had enabled automatic recovery, i.e. load would turn on when the voltage was low enough. On the yellow and green trace the fan did start and increased the current.

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Here I adjusted the display to 20mA, the real current was about 25mA and not completely stable.

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I tried two 1 hour test, one at 24V 1A and one at 6V 4A.

Temp5583

M1: 44.6C, M2: 46.1C, HS1: 57.4C
First set of thermo photos is from 24V 1A test

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HS1: 58.5C

Temp5585

M1: 72.2C, HS1: 79.1C
Both transistor and regulator warms up with high input voltage.

Temp5586

M1: 44.5C, M2: 50.8C, HS1: 59.8C
Next set of thermo photos is from 5V 4A test

Temp5587

HS1: 63.6C

Temp5588

M1: 84.9C, M2: 61.7C, HS1: 91.7C
With lower input voltage the transistor must handle all the power and gets a bit warmer, the shunt resistor do also get warm at 4A



A look at the circuit

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The transistor (TIP122) is the load element, it uses a resistor (R6: 0.025ohm) to sense the current. The fan is a 5V version and has its own regulator (LM317), at high input voltage it must handle some power. Turning the fan on/off is handled by a small transistor (Q2), the temperature sensor is very close to the TIP122 transistor and is a NTC (RT1) that is connected to the MPU.
The electronic has its own voltage regulators (U6: M5350B 5V, IC7:5333B 3.3V). The control of the load current is done with some OpAmps (U2 & U7: 358). The brain in the circuit is a 8051 microprocessor (U1: N76E003AT20 18KB Flash, 1k RAM, 12 bit ADC). For the display a 8 bit shift register (74HC595) is used to get more output pins.

Why 4 OpAmp (358 is a dual OpAmp), a load only need one OpAmp. A look at the circuit shows some filters connected to the MPU on U7. One filter is R11, R12, C8 and C9, the other filter is R15, R16, C14 and C15.

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Filter input from microprocessor at 0.1A

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Filter input from microprocessor at 0.2A

2A

Filter input from microprocessor at 1A
It looks like the R15 etc. filter is for fine adjustment and R11 etc. filter is for coarse adjustment.

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The above PWM frequency is 1kHz, is there any trace of it on the output? To test that I added a 1ohm resistor in series with the load and measured the voltage across the resistor with my oscilloscope. There is some noise from the load, but not the 1kHz PWM frequency.



Conclusion

The load works fine and with the multiturn adjustment and display it is easy to adjust. It has no problems handle the rated 25W power. It is not for really low loads, the display do not have resolution for it and it also need some minimum current to work.
It is a interesting design choice that the analog current setting is sampled by the microprocessor and then output as two PWM channels that is filtered and mixed, before being feed to the load regulation.
With USB-C I am missing the ability to turn outputs on (It is just a resistor), this will limit its usefulness for USB-C.