150 W Simple Electronic Load Kit

Some time ago, a friend who works with various batteries asked me to come up with a test stand for battery packs. One of its key functions would be discharging them. At first, I planned to build everything myself, but then I found out that China sells an inexpensive, decent kit, and in some situations it makes more sense to use it than to start from scratch.


I’ve already reviewed quite a few different electronic loads, including a simple, fully DIY one. I’ll be referring to that review several times today, since this kit has a lot in common with it.

As I mentioned in the introduction, I need the load to discharge battery packs. I’m planning to build a device for testing and balancing packs up to 19S, so this certainly won’t be my last review on the subject.
I don’t need a high discharge current. The target power is around 120–130 W, which is within the manufacturer’s stated 150 W rating.

I’ll also explain how a simple electronic load works and why I liked this particular kit.

First, a word about the seller. The product page offers several options:
1. PCB only, about $1.60
2. PCB and component kit, without the power transistors — $3.80
3. PCB and all components, including the power transistors — $4.90
4. Ammeter-voltmeter — $2.20.

The specifications also list 150 W, 15 V, 0–10 A / 72 V, 0–2 A. In other words, there are supposedly two versions. I’ll talk about that, and about my mistake, later.

No complaints about the packaging, either from the store or the forwarding agent. The kit consists of a PCB and a bag of components.


I ordered the full kit: PCB, all the components, and the transistors. I didn’t order the ammeter-voltmeter because I don’t need it for my project.


The PCB measures 100 × 100 mm. There are extra holes for mounting the heatsink and securing the board inside the device enclosure.


Some of the high-current traces are on the underside of the PCB. I recommend reinforcing them with copper wire and solder, or at least adding more solder.


The PCB is very well made. It was easy to solder without any extra flux (I used only the flux in the solder), and the silkscreen is clear, with component locations, reference designators, and values marked. You don’t even need a schematic to assemble it.


Component list
Resistors
1 kΩ — 12 pcs
4.7 kΩ — 4 pcs.
10 kΩ — 1 pc
20 kΩ — 1 pc
220 kΩ — 4 pcs
0.22 Ω, 5 W — 4 pcs
4.7 kΩ potentiometer — 1 pc

Capacitors
1 nF — 4 pcs
100 nF — 1 pc
22 µF, 25 V — 1 pc
220 µF, 16 V — 1 pc
1000 µF, 16 V — 2 pcs

1N5408 diodes — 4 pcs
110N8F6 transistors — 4 pcs
TL431A IC
LM324N IC

Various connectors, a switch, and other odds and ends.

All the resistors except the power resistors are precision types, which is good. The electrolytic capacitors are the cheapest ones, but they don’t affect performance.


The product page included a schematic for this electronic load, but I don’t find it very informative. So below, I’ll break it down into sections to make it easier to understand. I’ll also explain how to increase the power or change the parameters, what each component does, and how the whole thing works.


A highly simplified schematic of a constant-current electronic load consists of just a few components:
1. A potentiometer
2. An operational amplifier
3. A transistor
4. A shunt.

The potentiometer applies a voltage to the op-amp input. The op-amp then applies a voltage to the MOSFET, allowing load current to flow through it and, at the same time, through the shunt. A voltage develops across the shunt and is fed to the op-amp’s second input. Once the voltages at the op-amp inputs are equal, it sets its output voltage so that the transistor is turned on just enough to keep the voltage across the shunt equal to the value set with the potentiometer.
Since the voltage drop across the shunt is directly proportional to the current through it, the circuit ends up regulating the current.

This means the load current depends on the input voltage.

For example, if the potentiometer supplies 0.4 V and the shunt resistance is 1 Ω, the current will be 0.4/1 = 0.4 A.
Let's make the example a little more complex: a 0.15 Ω shunt and 0.45 V from the potentiometer gives 0.45/0.15 = 3 A.

Two things mainly affect how accurately the current is maintained:
1. The stability of the reference voltage
2. The shunt's TCR (the way its resistance changes with temperature).


The circuit shown above will probably work, but it won't be stable. That's why the more complete version of the circuit is a little more involved.
A brief aside: the component reference designators don't match those on the circuit board; they're just shown as an example.

You can see the same components I showed above, with a few more added. Here's what they're for.
Resistor R1 isolates multiple channels and improves stability.
Resistor R2 limits the gate charging current of the MOSFET, protecting the op-amp.
Capacitor C1 and resistors R3 and R4 prevent the circuit from oscillating, so the load doesn't turn into an oscillator.
Resistor R5 turns the transistor off when the op-amp is unpowered. It also provides a small load current for the op-amp output and the oscillation protection circuit.


Since this is a four-channel load, we can simply "draw in" three more channels to get an almost complete view of the board being reviewed.
I've highlighted the four channels in blue. As you can see, they're identical, so you could add even more. The only component shared by all the channels is the quad op-amp.
The control circuit is highlighted in orange; the power circuits are red and black.


That's exactly how I implemented it in my high-power electronic load, except I used 8 channels and dual op-amps.


But as I mentioned above, current regulation accuracy depends on how accurately the reference voltage is regulated. That's why the circuit has a voltage reference based on an adjustable TL431 shunt regulator, whose reference voltage is 2.5 V.
Since 2.5 V is a bit too high, if that much voltage were dropped across the shunts:
1. The power dissipation would be high.
2. The minimum load voltage would be 2.5 V plus the voltage drop across the transistors.

A fixed 22 kΩ resistor, R8, is connected in series with the potentiometer. Together they form a divider of about 1:5.5, so the voltage after the potentiometer varies over a range of about 0–0.45 V.
Changing the value of the additional resistor lets you adjust the range without replacing the shunt. But this approach has a downside: op-amps running from a single supply work more easily with higher voltages, so it's best not to reduce the reference voltage too much.
Resistor R7 limits the current through the shunt regulator.

The power supply section contains a bridge rectifier and four capacitors. Apparently, a regulator for the op-amp supply was originally planned but later removed, while the 220 µF capacitor that was after it was left in place. I can't think of any other reason for connecting two 1000 µF capacitors and one 220 µF capacitor in parallel.

The bridge rectifier also supplies power to the fan and ammeter/voltmeter connectors.


I think the theory is clear, so let's move on to the practical part—or, more precisely, the assembly. I'll hide this section under a spoiler, since it's mainly of interest to beginners. Along the way, though, I'll explain some details about using particular components and possible replacements.


And that's pretty much it. Even a beginner can assemble the whole thing in about an hour without rushing. All the components fit where they should, everything is marked on the board, and assembly is intuitive.

I ended up with these components left over:
1. Switch
2. Power connector
3. Faulty capacitor.

The second power connector is probably included in case you want to mount it on the enclosure cover. The switch is most likely for turning the power on, but there’s nowhere to solder it in my setup. As for the faulty capacitor, well, the stars just aligned that way.


One side of the board has pads for connecting the input, ammeter, voltmeter, and power output.
You can install terminal blocks or simply solder the wires directly, whichever is more convenient, but there are a few connection details to keep in mind.


There are several ways to connect it; I’ll try to cover them all.
1. Just the board, without meters, connectors, or anything else.
2. Same thing, but if you want to install a connector, you’ll most likely need to fit a jumper in place of the ammeter output.
3. Connecting separate ammeter and voltmeter: positive is red, negative is blue.
4. If you want to install a digital ammeter-voltmeter, connect it like this:
Black - common power lead
Yellow - current measurement input
Red - voltage measurement input
Thin red - power for the ammeter-voltmeter.



The wire colors shown for connecting the digital ammeter-voltmeter match the one I’ve used before, and may differ on yours, so it’s best to check before connecting. Usually, black is negative, and the current measurement input has continuity with the black wire.


You could also use an ammeter-voltmeter like this one. It’s more “advanced,” but the connections are different.
Thick black - common power lead
Thick red - current measurement input
Yellow - voltage measurement input.
Thin red - power for the ammeter-voltmeter.

This version also has a temperature sensor, which can be very useful in a device that generates this much heat.


I used both of these ammeter-voltmeters in a small electronic load, although I still use the second one.


And of course, a few experiments.
I connected the board to an adjustable power supply, set it to 32 V for a test, and set the load current to 5 A. The board handled 160 W without any trouble, but that’s already its limit.


I checked how the current setting drifts as the board warms up. The drift isn’t very large, but it’s there: the current drops as the board heats up.


I hadn’t been experimenting for long, but I could already feel the heat. I measured the temperature and found that the transistors had warmed up to about 110 degrees. You absolutely must not let that happen. For example, the overheat protection on my high-power load is set to 90 degrees at the heatsink, or about 95 at the transistors.
Higher transistor temperatures sharply reduce reliability, especially in linear mode.


I often see people mention ordinary resistors as a load in the comments. Sure, you can use them too, but it’s harder to achieve results like the ones shown in these photos with resistors.
Here I lowered the voltage from 40 to 1 V and watched how the load current changed. Across that range, it varied from 2.017 to 2.026 A, which I think is pretty good given the simple circuit design.


Although I don’t plan to use a regular ammeter-voltmeter, I decided to test it anyway. First, it’s worth noting that the connection is slightly different from the one I used in my simple DIY load.


The ammeter-voltmeter wires are connected in this order:
Black
Yellow
Red.

Only one power wire is connected; I had to disconnect the black one because it affected the measurement results. The thing is, in my electronic load the ammeter-voltmeter was connected in series with the current-sense shunt, so the ammeter’s common wire was connected to the circuit’s common wire. Here, it is connected in series with the source under test, which I think is less correct.



While I was writing this review, someone asked about replacing the transistors, so I’ll explain that separately.
1. Choose transistors based on the package type and required power: 30 W for TO-220, 50 W for TO-247.
2. In principle, you can use almost any transistors, and even different ones at the same time.
3. The main thing to consider when choosing a transistor is its rated voltage. Ideally, it should be at least twice the input voltage.
4. On-resistance hardly matters, unless you use very high-voltage transistors with an on-resistance of several ohms.
5. It’s better to use transistors in non-insulated packages and not insulate them from the heatsink.

Why these power limitations? The vast majority of modern MOSFETs are designed for switching operation and perform poorly in linear mode. More precisely, they work fine, but with significant limitations on temperature, power, voltage, and current. There are MOSFETs that work well in this mode, but they’re so rare that there’s no point looking for them.

I was also asked what else they can be replaced with.
To start with, you can use IGBTs, which are essentially a hybrid of a MOSFET and a bipolar transistor, but they cost more. There’s also the reverse hybrid: a bipolar transistor combined with a MOSFET.

But there’s nothing stopping you from using bipolar transistors (which is precisely why IGBTs work well), whose maximum rated power dissipation is available even in linear mode.
In this circuit, you can simply use one in place of the MOSFET, but you need a transistor with a high current gain, since bipolar transistors are controlled by current, not voltage, unlike MOSFETs.
Alternatively, you can use a composite transistor configuration (a Darlington pair), in which case the overall gain is the product of the gains of the first and second transistors. For example, if the first has a gain of 200 and the second 70, the total will be about 14,000.

The only real drawback of bipolar transistors, especially Darlington pairs, is that they’re difficult to use at low voltages, below 1.5–2 V. If you don’t plan to test power supplies at those voltages, you can use bipolar transistors; otherwise, use MOSFETs.


The well-known KT827 transistors are a great fit, but I hardly have any, so the photo shows their complementary pair, the KT825. They can dissipate up to 125 W.


I even used them with heatsinks like these once—the size is just right :) Though they’re really more suited to passive cooling.


Inside, they contain two transistors connected as a Darlington pair, plus some additional components.

By the way, transistors like these are also great for linear power supplies, and I’m often asked about imported replacements. I did a bit of searching online and put together a list of alternatives.
Near-direct equivalents for the KT827 transistor: 2N6057, BDX87
KT827A: BDX65A, BDX67, BDX87C, MJ3521, MJ4035
KT827B 2N6058, 2N6283, BDX63, BDX65, BDX67, BDX85B, BDX87B, MJ3001, MJ4034
KT827V: 2N6057, 2N6282, BDX85, BDX85A, BDX87, BDX87A, MJ3000, MJ3520, MJ4033



By the way, the seller may ship a version with different transistors. There’s also an option with transistors in TO-247 packages (IRFP250, IRFP260), though it will probably cost extra. I think that would be a much more interesting option.


And while we’re at it, here’s a quick note on replacing the other components.
The small resistors aren’t critical; just get reasonably close to the specified value. The same goes for the capacitors.
For the low-value resistors used as current-sense shunts, it’s best to choose ones with a higher power rating. They’ll run cooler, so their resistance will change less as they heat up, and the current won’t “run away.” You can use more accurate, higher-quality resistors, but they’re expensive.
The op-amp can also be replaced with almost any inexpensive one. For example, I’ve used the LM358, but it’s dual-channel, so you need twice as many. It’s easier to use the LM324 instead, which costs next to nothing.

The load can be powered from either DC or AC, but make sure the op-amp supply voltage doesn’t exceed 20 V. The op-amp itself can easily handle up to 30–35 V, but the voltage at the MOSFET gate must not exceed 18–20 V, and preferably should stay below 15 V, so I’d limit it to that. One option is to power this circuit from a small regulator.



During testing, I did end up burning out one of the transistors. It happened by accident: I applied 40 V and set the load current to 5 A, giving a power of 200 W. The power supply simply went into short-circuit protection, and that was the end of it.
I also found that interference picked up by the potentiometer can noticeably affect the set current. For example, when I held the potentiometer in my hand, the current was 4.1 A; when I put it on the table, it dropped to 4 A. This issue may or may not occur, depending on the power supply, the load itself, and the device under test.

The load under review compared with my old DIY unit, which is built around the same TL431, LM358, and a single transistor. I do have a power supply regulator for the op-amp, though.


Possible improvements:
1. Control the fans based on temperature, at least with a simple thermal switch.
2. Halve the shunt resistance and change the values in the potentiometer divider so the voltage varies from 0 to 0.25 V. The shunts will run half as hot.
3. Add a CV mode using a second op-amp, although overload protection is an issue here.
4. Add a voltage regulator for the op-amp, such as a 7808 or 7809.
5. In my electronic load, there is a buffer op-amp after the TL431. I think this is a better solution, but it would be very awkward to add it here.
6. Combine two or three similar boards to increase the power. One board would be the master, with the others as slaves.

I can't show it in an enclosure yet, since I don't have one. I'll choose one based on the overall dimensions of the complete setup. I'll also be using it with a more advanced voltmeter-ammeter that monitors upper and lower voltage limits, and so on.
Video review



That should cover everything. I've tried not to leave anything out, so now it's time to sum up.
I think this is a pretty good option: it's inexpensive, easy to assemble, and most importantly, it works once assembled :)
There are a few caveats, such as a damaged capacitor and the awkward placement of the heatsink mounting holes.
The specs claim 150 W. In my view, that's the maximum power. I wouldn't recommend running it continuously above 120 W without replacing the transistors with some in TO-247 packages, and good cooling is essential.

If you plan to use it at voltages up to 30–40 V, you can buy the kit with the transistors included. But if you plan to test power supplies with higher voltages, I'd buy everything except the transistors and replace the supplied ones with higher-voltage parts.
Alternatively, you can just buy the PCB. The components aren't hard to find, and you may even have some at home.
Ordering just one kit probably won't be very cost-effective; it's better to buy several at once to get a lower overall price.


The same kit on AliExpress - link

That's all for now. Questions, comments, and suggestions are welcome.