Do-it-yourself electronic load circuit. Do-it-yourself electronic load. Devices based on KTC8550

The power controlled load is part of the test equipment required for various electronic projects. For example, when building a laboratory power supply, it can "simulate" a connected current sink to see how well your circuit performs not only at idle, but also at the load. Adding power resistors for the output can be done only as a last resort, but not everyone has them and they can’t be kept for a long time - they get very hot. This article will show you how you can build a variable electronic load box using inexpensive components available to radio amateurs.

Transistor electronic load circuit

In this design, the maximum current should be about 7 amps and is limited by the 5W resistor that was used and the relatively weak FET. Even higher load currents can be achieved with a 10W or 20W resistor. The input voltage must not exceed 60 volts (maximum for these FETs). The basis is the op-amp LM324 and 4 field-effect transistors.

The two "spare" LM324 op amps are used to protect and control the cooling fan. U2C forms a simple comparator between the voltage set by the thermistor and the voltage divider R5, R6. The hysteresis is controlled by the positive feedback received by R4. The thermistor is placed in direct contact with the transistors on the heatsinks and its resistance decreases as the temperature rises. When the temperature exceeds the set threshold, the U2C output will be high. You can replace R5 and R6 with an adjustable variable and manually adjust the threshold. When setting up, make sure that the protection works when the temperature of the MOSFET transistors is slightly below the maximum allowable value indicated in the datasheet. LED D2 signals when the overload protection function is activated - it is installed on the front panel.

The U2B element of the op-amp also has a voltage comparator hysteresis and is used to drive a 12V fan (can be used from older PCs). The 1N4001 diode protects the MOSFET BS170 from inductive voltage surges. The lower temperature threshold for fan activation is controlled by resistor RV2.

Device assembly

An old aluminum box from a switch was used for the case with plenty of internal space for components. The electronic load used old AC/DC adapters to supply 12V for the main circuit and 9V for the dashboard - it has a digital ammeter to immediately see the current consumption. You will already calculate the power yourself using a well-known formula.

Here is a photo of the test setup. The laboratory power supply is set to 5 V. The load shows 0.49A. A multimeter is also connected to the load, so that the load current and voltage are monitored simultaneously. You can see for yourself that the entire module works correctly.

I'll tell you about a useful device for radio amateurs - about a current electronic load with the ability to measure battery capacity. Why is this device needed?

Everyone has come across a situation where you need to find out the parameters of some kind of power source, for example, a laboratory PSU, an LED driver, or a charger. After all, practice shows that manufacturers do not always indicate the correct parameters. Of course, there is the simplest option - load it with a resistor calculated according to Ohm's law and measure the current with a multimeter. But for each case, you need to make your own calculations and it is not always possible to find a powerful resistor of the desired rating, they are quite expensive. It is more expedient to use an electronic or active load that allows you to load any PSU or battery, and regulate the load current with a conventional potentiometer.

And by including a multifunctional digital wattmeter in the circuit that shows the capacity, this load stand can discharge the battery and show its real power. By the way, unlike IMAX 6, our system can discharge batteries with a current of up to 40A. This is useful for car batteries.

The circuit is based on a dual operational amplifier (op-amp) LM358, although only 1 element is involved.

The current sensor is a powerful resistor R12, preferably 40W, although I set it to 20W. You can connect several resistors in parallel to obtain the desired power so that the final resistance is 0.1 ohm. R10 and R11 (0.22 Ohm / 10W) ​​are current leveling elements for power switches. I actually have 2 x 0.47 Ohm / 5W in parallel for each transistor.

The OU controls two composite KT827 transistors installed on separate radiators. Transistors are optimal for this circuit, although they are quite expensive.

Principle of operation.

When the device under test is connected, a voltage drop forms on a powerful current resistor R12, the voltage at the inputs of the op-amp changes accordingly, and therefore at its output. As a result, the signal supplied to the transistors depends on the voltage drop across the shunt. The current flowing through the transistors will change.

Using a potentiometer, we change the voltage at the non-inverting input of the op-amp and, as described above, the current through the transistors also changes. These transistors allow you to work with currents up to 40A, but require good cooling, because. they work in a linear fashion. Therefore, in addition to massive radiators, I installed a fan with speed control, which can be turned on with a separate button. The speed controller circuit is assembled on a small board.

Theoretically, the maximum input voltage can be up to 100V - the transistors will withstand, but the Chinese wattmeter is only rated up to 60V.

Button S1 changes the sensitivity of the OS, i.e. switches to low currents for accurate measurement of low power sources under test.

Important features of this scheme:

1. the presence of feedback for both transistors,
2. the ability to change the sensitivity of the OS.
3. coarse and fine current adjustment (R5 and R6).

The transformer in the circuit feeds only the op amp and the indicator block, any one with a current of 400mA and a voltage of 15-20V is suitable, anyway, the voltage is then stabilized to 12V by a linear stabilizer 7812. There is no need to put it on a radiator.

Eugene.A: Moreover, it is also meaningless. Modern electric meters do not spin in the opposite direction.

But there is almost nothing to warm up.

Eugene.A: About the transformation - some kind of rectal method. For lovers of perversion. Retired. Instead of watching porn.
...
You just need more nichrome, constantan, manganin and a switch to adjust the current, if there is such a need.

Or maybe I'm a pervert? True, there are no pensions, but it is not far off ... No, you can’t watch porn, it discourages you from doing it yourself - a scientifically proven fact!

And now let's compare the methods proposed by you and mine.

You offer the old fashioned way: more nichrome, constantan, manganin and a switch - this is rather cumbersome, not technologically advanced and not very accurate. I am already silent if a small step of adjusting the load current is required.

I suggest using one piece of nichrome, constantan or manganin and no switches at all.
Moreover, these pieces are not needed either. You can just take an iron, an electric heater, an electric stove ... whatever is at hand, and stick it with your own plug into a block called "electronic load". On the block there is a load current regulator in the form of a variable resistance, an encoder or buttons with a keyboard - according to taste and capabilities, and a display showing the current values ​​​​of voltage, current and power ...

Unlike your method, I will be able to regulate the load current not discretely
and pla-a-a-vnenko, and even stabilize the set value.

And the accuracy will not be much better than your method.
The load current is I=k*ktr*Rn, where:
k - duty cycle of PWM pulses,
ktr - transformation ratio of the used transformer,
Rн - resistance of the iron, electric heater or electric stove.

It is enough to accurately measure the resistance of the iron ...
Actually, why? It is enough to enter the calibration mode when working with the device - with an iron, electric heater or electric stove connected, apply (inside the device) a calibrated voltage to its input and set the maximum current value with a calibration trimmer at the maximum fill factor. You can even automate this operation if the MK is worth it.
Everything.
The adjustment turns out to be linear, therefore, by tying the maximum value of the load current of 20A to a duty cycle of 0.9 by calibration, with a coefficient of 0.1 we get a current of 2.2A.
To expand the limits, you can put a switch or relay and switch the taps of the converter transformer. We get several consistent subranges for adjusting the current (resistance) of the load.

I forgot to say - the transformer is better because of the easier coordination with calibrated loads such as an iron, electric heater or electric stove.
The transformer comes from a computer PSU (power). He has a lot of takeaways...

And now, Eugene.A, please explain to me - a pervert and almost a penisoner - why your method is not rectal, but mine is rectal, despite the fact that it is better, more technologically advanced, more versatile, more accurate and performs the same task?

First, let's take a look at the schema. I do not pretend to originality, as I spied on the constituent elements and adapted them to what I had from the details.

The protection circuit is made up of a fuse FU1 and a diode VD1 (perhaps it is superfluous). The load is made on four 818 transistors VT1…VT4. They have acceptable current and power dissipation characteristics, and they are not expensive and are not in short supply. VT5 control on the 815 transistor, and stabilization on the LM358 operational amplifier. Ammeter, showing the current passing through the load, I installed separately. Because if you replace the resistors R3 R4 with an ammeter (as in the circuit at the link above), then, in my opinion, part of the current that flows through VT5 will be lost and the readings will be underestimated. And judging by how the 815 heats up, a decent current flows through it. I even think that between the VT5 emitter and the ground it is necessary to put another Ohm resistance, so 50 ... 200.

Separately, it is necessary to talk about the circuit R10 ... R13. Since the adjustment is not linear, it is necessary to take one variable resistance of 200 ... 220 kOhm with a logarithmic scale, or set two variable resistors that provide smooth regulation over the entire range. Moreover, R10 (200 kOhm) regulates the current from 0 to 2.5A, and R11 (10 kOhm) with R10 turned to zero regulates the current from 2.5 to 8 A. The upper current limit is set by resistor R13. When setting up, be careful if the supply voltage accidentally falls on the third leg of the operational amplifier, the 815 opens completely, which will most likely lead to the failure of all 818 transistors.

Now a little about the power supplies for the load.

No, this is not perversion. I just didn’t have a small-sized 12-volt transformer at hand. I had to make a multiplier and increase the voltage from 6 volts to 12 for the fan and install a stabilizer to power the load itself and the alarm.

Yes, I inserted a simple temperature alarm into this device. I looked at the diagram. When the radiator heats up above 90 degrees, the red LED turns on and the buzzer with an integrated generator turns on, which makes a very unpleasant sound. This indicates that it is time to reduce the current in the load, otherwise you can lose the device due to overheating.

It would seem that with such powerful transistors that can withstand up to 80 volts and 10 A, the total power should be at least 3 kW. But, since we are making a “boiler” and all the power of the source goes into heat, the limitation is imposed by the dissipated power of the transistors. According to the datasheet, it is only 60 W per transistor, and given that the thermal conductivity between the transistor and the heatsink is not ideal, the actual power dissipation is even less. And therefore, in order to somehow improve the heat dissipation, I screwed the VT1 ... VT4 transistors directly to the radiator without gaskets for heat-conducting paste. At the same time, I had to organize special linings for the radiator so that it would not close to the case.

Unfortunately, I did not have the opportunity to test the operation of the device in the entire voltage range, but at 22V 5A the load works without overheating stably. But as always, there is a fly in the ointment in a barrel of honey. Due to the insufficient area of ​​​​the radiator I took, with a load of more than 130 watts, after some time (3 ... 5 minutes), the transistors begin to overheat. What does the alarm indicate? Hence the conclusion. If you do a load, take the radiator as large as possible and provide it with reliable forced cooling.

Also, a small drift in the direction of reducing the load current by 100 ... 200 mA can be considered a fly in the ointment. I think this drift is due to the heating of resistors R3, R4. So, if you can find 0.15 ohm resistors for 20 watts or more, then it is better to use them.

In general, the scheme, as far as I understand, is not critical to the replacement of parts. Four 818 transistors can be replaced by two kt896a, kt815g can, and perhaps should be, replaced by kt817g. I think you can also take another operational amplifier.

I want to emphasize that it is imperative to set the R13 resistor at least 10 kOhm during setup, then, as you understand what current you need, reduce this resistance. I do not lay out the printed circuit board, because the installation of the main part of the load is made hinged.

Addition.

As it turned out, I have to use the load regularly, and in the process of using it, I realized that, in addition to the ammeter, I also need a voltmeter to control the source voltage. On Ali, I came across a small device that combines a voltmeter and an ammeter. Priborchik 100 V / 10 And it cost me 150 rubles with shipping. As for me, this is a penny. half a beer costs about the same. Without thinking twice, I ordered two.

From time to time, radio amateurs need an electronic load. What is an electronic load? Well, in simple terms, this is a device that allows you to load the power supply (or other source) with a stable current, which is naturally regulated. The respected Kirich already wrote about this, but I decided to try the “proprietary” device in the case, stuffing it into some case and attaching a device for indication to it. As you can see, they are perfectly combined according to the declared parameters.

So, the load. A handkerchief with a size of 59x55mm, a pair of 6.5mm terminals is included (very tight, and even with a latch - you can’t just remove it, you need to press a special tongue. Excellent terminals), 3-wire cable with a connector for connecting a potentiometer, a two-wire cable with a connector for connecting power, an M3 screw for screwing the transistor to the radiator.

The scarf is beautiful, the edges are milled, the soldering is even, the flux is washed.

The board has two power connectors for connecting the actual load, connectors for connecting a potentiometer (3-pin), power (2-pin), fan (3-pin) and three pins for connecting the device. Here I want to draw your attention to the fact that usually the black thin wire from the meter will not be used! In particular, in my case, with the device described above (see the link to the review) - it is NOT NEEDED to connect a thin black wire, because both the load and the device are powered from the same PSU.

Power element - transistor (200V, 30A)

Well, from the microcircuits on the board there are an LM393 comparator, an LM258 opamp and an adjustable zener diode TL431.

Found on the Internet:

To be honest, I didn’t thoroughly double-check the entire circuit, but a quick comparison of the circuit with the board showed that everything seems to fit together.

Actually, there is nothing more to tell about the load itself. The scheme is quite simple and generally speaking it cannot fail. And in this case, the interest in this case is rather its work under load as part of the finished device, in particular, the temperature of the radiator.

For a long time I thought of what to make the case. there was an idea to bend it from stainless steel, glue it from plastic ... And then I thought - so here it is, the most accessible and repeatable solution - the “button post” KP-102, for two buttons. I found a radiator in a box, a fan in the same place, bought terminals and a switch offline, and dug out bananas and a network connector from something old in the attic;)

Looking ahead, I’ll say that I screwed up, and the transformer that I used (complete with a rectifier bridge, of course) didn’t pull this device due to the high current consumed by the fan. Alas. I will order, should just fit in the dimensions. As an option, you can also use an external 12V power supply, of which there are also plenty of them both on the bang and in the arsenal of any radio amateur. It is highly undesirable to power the load from the power supply under study, not to mention the voltage range.

In addition, we need a 10kΩ potentiometer to adjust the current. I recommend using multi-turn potentiometers such as or . And there and there there are nuances. the first type - by 10 turns, the second by 5. the second type has a very thin shaft, about 4 mm, it seems, and standard handles do not fit - I pulled two layers of heat shrink. the first type has a thicker shaft, but IMHO also falls short of standard sizes, so problems are possible - however, I didn’t hold them in my hands, so I can’t say 100%. Well, the diameter / length, as we see, is noticeably different, so you need to figure it out in place. I had the second type pots available, so I didn’t worry about it, although I should have bought the first ones for the collection. The potentiometer needs a knob - for aesthetics and convenience. It seems like handles should be suitable for potentiometers of the first type, in any case, they are with a fixing screw and will normally stay on a smooth shaft. I used what was available, pulling on a couple of layers of heat shrink and dropping superglue to fix the heat shrink on the shaft. The method is proven - I use it for the power supply, while everything works, for a couple of years.

Then there were the agony of layout, which showed that in fact the only possible solution is what I will give below. Unfortunately, this solution requires trimming the case, because the board is not included due to the stiffening ribs, and the switch and regulator are not included due to the fact that I tried to place them in the center of the recesses on the case, but they eventually rested against a thick wall inside. I would have known - I would have turned the front panel over.

So, we mark up and make holes for the network connector, transistor and radiator on the back wall:

Now the front panel. The hole for the device is simple (although, as I wrote in the previous review, its latches are stupid, and out of harm's way, I preferred to first snap the device case into the device case, and then snap the device insides into it). The holes for the switch and the regulator are also relatively simple, although I had to select the grooves on the walls on the milling machine. But how to arrange the nests in order to “bypass” the hole on the front panel is a task. But I glued a piece of black plastic and drilled holes right into it. It turned out nice and neat.

Now the nuance. in the device we have a temperature sensor. But why measure the temperature in a case when you can lean it against a heatsink? This is much more useful information! And since the device is disassembled anyway, nothing prevents you from soldering the temperature sensor and lengthening the wires.

To press the sensor to the radiator, I glued a piece of plastic to the case in such a way that, by releasing the radiator mounting screws, you can slip the temperature sensor under the plastic, and tightening these screws securely fix it there. The hole around the transistor was made a few mm larger in advance.

Well, we push all this “explosion at the pasta factory” into the case:

Result:

Radiator temperature check:

As you can see, at about 55W, after 20 minutes, the temperature of the radiator in the immediate vicinity of the power transistor stabilized at 58 degrees.

Here is the temperature of the radiator itself outside:

Here, I repeat, there are nuances: at the time of the check, the device worked from a frail transformer, and not only did the voltage drop to 9 volts under load (that is, with normal power, the cooling will be SIGNIFICANTLY better), but also because of poor-quality power, the current cannot really be stabilized succeeded, so in different photos it is a little different.

When powered from the crown and, accordingly, with the fan turned off, we have this:

The wires from the PSU are thin, so the voltage drop here turned out to be quite significant, well, if you wish, you can still reduce the number of transient resistances by soldering wherever possible and removing the terminals. I am quite satisfied with such accuracy - however, they spoke about accuracy in the last review. ;)

Conclusions: quite a working thing that allows you to save time on developing your own solution. As a "serious" and "professional" workload, it is probably not worth perceiving it, but IMHO it's a great thing for beginners, well, or when you rarely need it.

Of the pluses, I can note the good workmanship, and perhaps the only minus is the lack of a potentiometer and a heatsink in the kit, and this must be borne in mind - the device will have to be understaffed in order for it to start working. The second minus is the lack of thermal control of the fan. Despite the fact that the “unnecessary” half of the comparator is just there. But this had to be introduced at the stage of development and manufacture of the board, because if you hang the thermostat "from above", then it is more reasonable to assemble it on a separate board;)

According to my finished design, there are also nuances, in particular, it will be necessary to change the power supply, and generally speaking, it would be nice to put some kind of fuse. But the fuse is extra contacts and extra resistance in the circuit, so here I'm not completely sure yet. You can also move the shunt from the device to the board and use it for both the device and the load electronics, removing the “extra” shunt from the circuit.

Undoubtedly, there are "more different" electronic loads that cost comparable. For example . The difference between the monitored one is in the declared input voltage, up to 100V, while in general the loads are designed to work up to 30V. Well, in this case, we have a modular design, which personally suits me very much. Tired of the device? They put it more precisely or larger, or something else. Not satisfied with the power? They changed the transistor or radiator, etc.

In a word - I am quite pleased with the result (well, just screw the power supply to another - but I myself am a fool, and you are warned), and I highly recommend it for purchase.

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