USB testers and load resistors. Why are they? USB testers and load resistors Load resistor for usb tester

How to find out the real output current I charger for a phone, tablet or other device. often our friends from the “heavenly Empire” like to overestimate the real parameters of devices, such as batteries, power banks and the output current of the charger (adapter). But devices such as those shown in the pictures below come to the rescue; they cost pennies. aliexpress.

But, if you urgently need to check the output current, but you don’t have this device, but you have a simple multimeter, but you also need to load the charger or battery itself with something as a load, then this circuit comes to the rescue.

The device consists of four resistors R1, R3, R5, R7 connected in parallel, type MLT-2, with a resistance of 56 Ohms. To indicate operation in parallel with each powerful resistor, an LED with an additional resistor is included. LEDs VD1, VD2 red, type L-7104HD, LEDs VD3, VD4 green, type AL307B, additional resistors R2, R4, R6, R8 type MLT-0.5, resistance 330 Ohms. The load is controlled using jumpers S1-S4, removed from the failed board. The resistances of the resistors are selected in such a way that turning on one of the branches of the circuit gives a load of about 100 mA.

I welcome everyone who stopped by. The review will focus, as you probably already guessed, on how to make a simple resistive USB load for long-term testing of the capacity of power banks (PBs), analyzing the quality of cables and network adapters. This is one of several possible articles about the independent manufacture of a resistive load (using ballast resistors); if everything goes well, you may even get your hands on an electronic load, with current regulation and stabilization. This load has been in service for quite a long time and constantly appears in my reviews, so if you are interested, please refer to the cat.

Lately, such homemade products are no longer very relevant, because... budget electronic loads have appeared, so it makes sense to pay extra and buy a ready-made one. I still bought at the old rate, and there weren’t really any electronic loads. Therefore, if you need a resistive one, then let’s get started...

Possible ways to purchase/manufacture a resistive load:
1) buy ready-made with resistors:


Pros:
+ ready-made working device (minimum body movements)
+ no plugs or wires needed (minimal losses)
+ switch for 1A/2A (indication)
+ small sizes
+ low cost

Minuses:
- it gets very hot (about 180°C at a current of 1A and about 230°C at a current of 2A) and starts to smell terribly (judging by the reviews, I don’t have one myself)
- does not have a housing, current-carrying/heating parts are open (you can get burned/burn something, short-circuit)
- difficult to attach the radiator

Since making a good load module takes time and effort, you can use this gadget, but you shouldn’t leave it unattended

2) find powerful resistors in the bins (Soviet PEV, PPB and the like), the power they dissipate for long-term operation must be at least 10 W


Pros:
+ less, but still quite high heating
+ no need to buy/average cost (owning a home/buying in a store)
+ resistance adjustment, i.e. you can smoothly change the current over a wide range (only some tweaks, or minor modifications)

Minuses:

- big sizes
- impossibility of attaching the radiator (on most)

- does not have a housing, current-carrying/heating parts are also open (you can get burned/burn something)

I don't have such resistors in stock, so the choice is yours.

3) purchase 25-100 W in a metal case for heat removal and assemble your own module with a casing


Pros:
+ average heating (can safely work without additional radiators)
+ average cost
+ possibility of mounting an additional radiator

Minuses:
- you need to solder the plug and wires
- big sizes
- no switch (can be remade, you need a second resistor)

At the same time, they can work without additional cooling, but at the same time they heat up well, within normal limits, of course. I turned on the 25W recharge to fully discharge my PB - they survived, but got very hot. I recommend buying 100W resistors, then an additional radiator may not be useful at all.

So, if you decide to assemble a homemade stand from similar resistors, then let’s get started. Required components:
1) two resistors 25-100W, 4.7 Ohms each. As luck would have it, prices rose and many denominations were no longer available for sale. But on a bike there is, . We are looking for “Power resistor”.


2) switch, I bought


3) a collapsible male USB plug, for example or


4) a small piece of copper stranded wire with a large cross-section, for example, speaker wire


5) small aluminum radiator (optional)
6) plastic box

Resistor values ​​are calculated using the familiar formula of Ohm's law - I=U/R or R=U/I, where R is resistance (Ohm), I is current (A) and U is voltage (V). For example, we need a current of 2A, so to load 5V adapters we need a 2.5 Ohm resistor, because 5/2=2.5 Ohm. For 1A we calculate in the same way - 5/1 = 5 Ohms. Since most adapters/power supplies reduce the voltage under load, it is necessary to make allowances for this and calculate an average of 4.8V. Then for a current of 2A you will need a reduction R= U/I=4.8V/2A=2.4 Ohm, and for 1A - R= U/I=4.8V/1A=4.8 Ohm. You also need to remember that the connecting wires, switch and USB plug also have some resistance. Let me remind you of one trick: when resistors are connected in series, the total resistance adds up, and when connected in parallel, it will be slightly less than the smallest resistor. The total resistance of several resistors can be calculated.
In order not to look for suitable ratings and not to be tricky with the circuit, I recommend doing it according to my version, albeit with different ratings - 2 resistors of 4.7 Ohms and a small switch. For 1A one resistor will be used, for 2A - two in parallel. At the same time, if the resistor power or resistance is not suitable, you can group several using the above formulas.
In my load module I used 2 resistors: 5.1 Ohm and 6 Ohm, because... I won them at an auction on Ebay for pennies; there were no auctions for other denominations at that time. When connected in parallel, I get 2.7 Ohms for a current of 2A (actually 1.75A), and for a current of 1A (0.95A) I use 1 5.1 Ohm resistor. They are a bit unsuitable; the ideal option would be to use two 4.7 Ohm leads, but there were no such lots at the auction.

Direct assembly:

Before this I used such a simple module, it was suitable even for long-term loads, although during prolonged use it became very hot, but it did not stink and did not burn out (it is, however, not convenient to take it out, you could get burned). As soon as the second 6 Ohm relay arrived, I began assembling the stand.


Here are the sizes of typical 25W resistors in an aluminum case:


The reverse side is uneven and varnished, in addition, the mounting lugs have burrs, so the resistors may not fit tightly to the radiator, I recommend using zero sandpaper:


I took the radiator itself from old stock. This is a radiator cut in half from budget GlacialTech coolers for Socket A processors. In service centers for repairing computers and household appliances, for 50-100 rubles they will give you a whole pack, for every taste and color. You can use a solid radiator, the heating temperature will be even lower. My load stand at 2A (more precisely 1.75A) does not heat up above 70g. In addition, a small fan can be attached to a solid radiator, then you can drive the module at high currents. When using 100W resistors, a heatsink may not be needed at all. Here is the same radiator:


The base of the radiator is uneven; it would be better to sand it down. You can leave it like this, the heat transfer will be a little worse.


Dimensions of my radiator:


Here's what we need to make the module (1000/2000 sandpaper/sandpaper, glass as a perfectly flat surface, drill, drill bits, taps for threading and machine oil):


There is no particular point in polishing perfectly with GOI paste; 2000 sandpaper is enough. Then we drill holes and cut the threads with a tap (I won’t tell you how to do this, see on the Internet). If you don’t have a suitable tool, then use hot glue/thermal tape/thermal pads (links below), you don’t have to drill anything. I’ll add on my own, so as not to break the tool, drip oil and after two full turns of the tap, make half a turn back. This way you will 100% not break the tap. If possible, use a finishing tap (look at the number of marks on it). You end up with something like this:


I used a protective screen from an old refrigerator as a casing. You can use anything from organics to any plastic gizmos. Plexiglas of small thickness bends easily when heated, I once bent it over the tip of a powerful soldering iron, only then the edges will have to be trimmed a little. In general, we use everything we have at hand.


Before final assembly, go through the holes with a drill of a larger diameter to remove burrs, otherwise the ribs will not fit tightly (countersink):


Next, spread a thin layer of thermal paste on the resistors; you can simply squeeze out a drop of paste; when tightened, it will spread out on its own. I used Russian “folk” thermal paste KPT-8 (bought in electrical stores):


It has average efficiency, it dries out over time, but it costs a penny and is sold in any radio electronics stores; it will be suitable for our module.


We tighten the screws and bend the terminals of the resistors (possibly before fastening):


As you can see, the excess thermal paste has come out and will not interfere:


We take a male USB plug, preferably with gold-plated contacts (see previous paragraphs) and an acoustic wire with thick copper (not copper-plated!) conductors. To protect against thermal and mechanical stress, I applied heat shrink. Since the wire is thick, use a knife to open the outlet hole:


We take the switch, it will turn on/off the “2A” mode. Any power will do. I used a simple KCD11, designed for 220V and 3A. I used an old cable duct as an edging, cutting off the edges a little. In one of them we cut out a window for the switch. Then solder the switch to the resistor terminals:


We solder the wire itself to a resistor, which will operate at 1A “by default”. In my case it is a 5.1 Ohm resistor. If you use two identical 4.7 Ohm leads, then solder to any:


One side of the pins will be connected through a switch, i.e. in the “off” position the current is 1A, in the “on” position - 2A, because the second cut is included in parallel.
It turns out this simple scheme:

Next, screw on the casing:




We install the top bar from the same cable channel or something similar to the location of the opening. It turns out pretty good:


Well, we glue the operating modes, paper and tape to help:




As a result, with a good adapter we have the following (0.95A and 1.75A):


The radiator temperature at a current of 2A (1.75A) never rose above 70°C, at 0.95A around 60°C:


Total: The device works, does not heat up too much, does not stink, and performs its functions 100%. Yes, we were a little unlucky with the denominations, but that’s okay. All my PB reviews are tested with this particular load; if desired, you can expand the current range, for example, by 0.5A/1A/1.5A/2A/2.5A...

Based on recommendations from local users, I bought this device for testing :)
The device has already been described and tested, this is just an addition...

I immediately ordered a kit with a load module.
Sent as usual - in a bag










Model: KCX-017
The tester is assembled neatly, there is a protective film on the screen.
USB cable length 15cm.
The screen simultaneously displays: voltage (V), current (A), passing energy of electric charge (mAh), memory cell number (0-9).
The device has already been described in detail several times, so I will try to write only additional information.
The case is held together with 4 latches and can be easily disassembled.

















The installation quality is normal, the flux has not been washed off.
The meter board contains the following components:
- Inverse LCD display with white backlight
- LCD controller HT1621B

- PIC controller PIC16F1933-I/SS

- Operational amplifier LM358 (Ku=21)

- Voltage stabilizer 3V
- Control micro button
- Current shunt 0.025 Ohm
- Voltage divider (K=7.66)
- Connectors and connection cable

The circuit itself is powered before the current meter, the voltage is also controlled before it.
Internal current consumption 6.6 mA (33 mW)
LCD USB tester circuit copied from the board


To simplify the circuit design, the measuring shunt was placed in the common wire circuit.
The op-amp amplifies the small voltage drop across the 0.025 ohm current shunt to improve the conversion accuracy of the on-chip ADC.
The connection wire is very thin (appears to be 28AWG) and causes additional voltage drop under heavy load. The total pass resistance of the LCD USB tester is 0.115 Ohm, that is, with a current of 2A, the output voltage will be approximately 0.25V less than the input:(

The indicator slightly underestimates the displayed voltage (by 2%) and current (by 3%).
Real voltage - Displayed voltage relationship:
2.60 – indicator does not light up
2,70 – 2,64
2,80 – 2,76
3,00 – 2,95
3,50 – 3,44
4,00 – 3,94
4,50 – 4,44
5,00 – 4,93
5,50 – 5,43
6,00 – 5,91
7,00 – 6,90
8,00 – 7,88
9,00 – 8,86
10,00 – 9,85

The ability of the device to carry out measurements with acceptable accuracy in a wide range of voltages (from 2.70V to 10.0V) and currents (from 0.05A to 3.50A) allows you to measure not only USB parameters, but also directly the parameters of lithium batteries.
The current meter has a deadband of 50mA, i.e. current less than this value will be displayed as zero.
When the voltage is less than 4.6V and more than 5.35V, the indicator backlight begins to blink, indicating an unacceptable input voltage value.
The maximum measured current is 3.67A, the readings do not increase further, because The op-amp enters output saturation. As the supply voltage decreases, the current limit measurement limit decreases.
The maximum measured voltage is 9.99V; with a further increase in voltage, the indicator loses the value of ten volts, but the remaining values ​​are displayed correctly - I checked up to 12V.
Interface conductors D+ D- pass through the tester in transit.
You should also keep in mind that it is mAh that is measured without taking into account the voltage value. Those. indicator readings I(mA)*T(h), there is current - there is also an increase in the accumulated mAh readings.
A very big advantage of the tester is that it stores the accumulated values ​​in memory when the device’s power is turned off. This happens at intervals of about 10 mAh - the accumulated values ​​are stored in memory.

The load module has such a simple circuit.

In 1A load mode, the green indicator is on.
In 2A load mode, the red indicator lights up.
There is an unpleasant feature - the resistors heat up very strongly, so place the load module so that it does not touch flammable substances and plastics.
Also, you should not touch the resistors of a working module with your hands - you will instantly get burned.
In 1A mode, the measured maximum heating of the resistors is 183ºC.
In 2A mode, the measured maximum heating of the resistors is 235ºC. In this mode, the board smells quite strongly of burning - the enamel of the resistors and the overheated board itself burns.
Load resistance at a temperature of 20ºC: 5.141 Ohm/2.587 Ohm and, accordingly, the calculated current at a voltage of 5V - 0.972A/1.932A




Load resistance at a temperature of about 180ºC: 5.119 Ohm/2.576 Ohm and, accordingly, the calculated current at a voltage of 5V - 0.977A/1.941A




With heating, the load resistance decreases by only 0.5%, so the change in current can be ignored.
It is noteworthy that the resistance of the resistors does not increase with heating - this means that the wire material of the resistors is temperature compensated (most likely constantan or manganin).
To reduce the heating of the board, I glued a couple of radiators to the board with thermally conductive glue






The temperature of the board has dropped significantly, the load has become less stinky, and removing a hot board has become safer.

Final conclusions: both devices are suitable, but have the features and minor disadvantages described above.