Test/review of Electronic load 35W 3A with OLed display J7-f

Electronic load 35W 3A with OLed display J7-f

Official specifications:

  • Industrial discharge load USB tester

  • 2016 new upgrade electronic load King

  • Intelligent temperature control Double adjustment design

  • USB Dual Adjustment accuracy Electronic Load discharger

  • Industrial grade long life 35W power continuous discharge

  • Voltage:3-21V Current:0-3A

  • Real-time monitoring

  • Voltage display Current display capacity display Electrical voltage display Power display Timing display

  • Power Down Data Retention Charging time protection

  • Support Qualcomm QC2.0 compatible QC3.0 fast charging voltage test

  • One screen multi group data Display

  • Measurement through kill

I bought it from ebay dealer kiss_buy

It arrived in a small box without any documentation or accessories.

The only paper included was the above notice.

On this side it has many different input connectors: Mini-usb, micro-usb, usb-C, Apple, screw terminals, they are all in parallel.

This side is the usb meter with a Oled display and a single button.

One end has usb input and usb output, the other end has coarse and fine control.
The current regulation is a bit slow, making it difficult to set a precise current, but with the low precision of the load it is not a problem.

The circuit board is fairly well labeled and there are a few interesting solder points on it.

The bottom is protected with a piece of plastic, due to the screws it is not a flat surface.

A closer look at the different connectors.

The bottom without plastic.

The display looks and function like a typical usb meter, it has one button.

  • The usb connector is reversible, this way it is always possible to connect the most convenient way.

  • Minimum setting about 30mA at 5V and 50mA at 21V when fan is off (Display shows 0)

  • Minimum setting is 0.18A when fan is on.

  • Maximum setting is slightly above 3.1A.

  • Coarse control regulates about a 3A range.

  • Fine control regulates about a 0.16A range.

  • Fan on/off is temperature controlled, it has no speed control.

  • At 12V and 3.00A the current was stable within 2.7%

  • At 12V and 1.00A the current was stable within 2.4%

  • Will remember settings after a power cycle.

  • Meter is 0.02V wrong around 5V and 0.2V wrong at 20V

  • Meter is about 0.1A wrong, it varies with temperature.

  • The circuits idle current consumption is not included in the meter reading.

  • The buzzer will sound when power is applied and when current/voltage is out of range.

The current varies with voltage. The load has a power limiter, this limiter will turn the load off for a short time if the power is too high as can be seen on the yellow, magenta and white curve.
Minimum voltage is about 4 volt.

At low current it will vary significantly with voltage.

I did two load tests, both at 11V, but at different currents.
At the 3A the build-in meter says: 59:01min, 2.941Ah and 31.573Wh
My power supply has delivered: 2,98546Ah, 32,840758Wh over 60:01min

At the 1A the build-in meter says: 59:01min, 0.918Ah and 10.051Wh
My power supply has delivered: 0,94121Ah, 10,353675Wh over 60:01min

Some reason for differences:
Meter on load need a few seconds to startup, but not one minute!
Some power is lost in the cable and connections.
The load do not include its own idle consumption in the calculations.
Loads current calibration was a bit low, it showed 2.94A and 0.94A (Values jumped about 0.02A up and down).

The precision is not very good, some of it can be calibrated out.

M1: 62,8°C, M2: 52,8°C, M3: 54,3°C, M4: 47,6°C, HS1: 79,1°C

M1: 54,8°C, M2: 39,7°C, HS1: 78,5°C

M1: 61,1°C, HS1: 63,3°C
The plastic prevents me from getting the real temperatures on the circuit board.

HS1: 98,2°C

HS1: 91,7°C
Here I tried to get a direct check of the transistor.

HS1: 150,8°C
The fan stopped working during my tests and the load got very hot.


This load has many input connectors, making it easy to use for just about any usb power device and also for some other stuff, but the fairly low precision and annoying current setting spoils it. That the fan quit working during my test is also a bad sign.


To test QuickChargers it is possible to use the QC2-3-MTK-PE Trigger J7-t with this load. For QC it can be used in the output connector and will not add any resistance to the circuit, with MTK-PE it must be used on the input and will add resistance.

Thanks! Will be sticking to my YZXStudio meters and loads for more critical measurements, even though this is very nice as an all-in-one package.

Thanks for the review! Good to see all those input ports including lightning.

Does your screen stay put?
The screen on my unit is separated from it’s backlight :confused:
It also looks like it was baerly finished before packing; crooked pots and more…

No problem with the screen and crooked pots looks to be the standard with these units.

We do seem to buy a lot of the same crap things :wink:

I just noticed this review.

I got this one from ETorch (now renamed HiDance) from AliExpress. It seems the previous model is slightly more functional. I think product now says 35-watt 0.1-4.5A adjustable USB load tester.

noted differences:

  • the one I got does NOT have a REVERSIBLE USB connector. It is still labeled as the same model “J7-F”, even with the slight changes.

- the screen on the new one is NOT OLED, it’s a bit difficult to read from an angle (OLED is like the J7-4T; non-OLED is like the J7-T —- the screen uses similar one as the J7-T instead of the OLED J7-4T)

- the fan always turns on when plugged to a power source, I don’t see a way of turning off the fan aside from pulling the 2-pin fan connector (in which case the power consumption reading goes down to 0.00A), with the fan always on, the display reading shows a minimum of around 0.08-0.11A (hovers around those readings).

(or maybe because our normal room temperature here is around 30-33 degrees Celsius, so the fan always turns on?)

- there is a second micro-USB input to independently power this load tester (I notice this is only used to power the display; not sure if it will also power the fan or not) if the USB main load goes below 3.0v; if this second micro-USB independent power supply is not plugged to a separate power source, and the main USB power source goes below around 3.0v, the display goes dim, then flickers and then shuts down. With the independent power source plugged to the second micro-USB input, the load tester can display as low as around 1.0v

(I don’t notice a second micro-USB input [independent power source for the display screen] for the above revision, is there one?)

- the maximum current according to the flyer included says it is 0.1-4.5A (I don’t have a strong enough power supply to test), but I know I could adjust to at least 4A momentarily
(on the display screen)

Nice Review!

“Measurement through kill. ”Can someone explain to me what this feature really means?

It’s a direct copy-and-paste from seller’s website:




they use the same term “measurement through kill”, which is likely a machine translation from Chinese to English.

Maybe it means to “measure” by direct actual load (I can’t figure why “kill” would be equivalent for the term “load”)