Electronic pulse load based on TL494
All electronic designers, involved in construction of power supply devices, sooner or later face the problem of absence of load equivalent or the functional limitations of available loads, as well as their dimensions. Fortunately, appearance of cheap and powerful field-effect transistors on Russian market has somewhat corrected the situation.
Amateur designs of electronic loads on the basis of field-effect transistors, more suitable for use as electronic resistance than their bipolar counterparts began to appear: better temperature stability, practically zero open channel resistance, small control currents – main advantages defining their preferability for use as a regulating component in powerful devices. Moreover, a wide variety of proposals appeared from the manufacturers of devices, the price lists of which are replete with a variety of models of electronic loads. But since manufacturers focus their very complex and multifunctional products called “electronic loads” mainly for production, the prices for these products are so high that only a very wealthy person can afford to buy. True, it is not quite clear – why a wealthy person needs an electronic load.
I have not seen any commercially made EN aimed at the amateur engineering sector. So, again I will have to do everything myself. Eh. Let’s begin.
Comrade, take a look at the Datagorian recommendations.
Useful and tested hardware, you can take it.
Tested in the editorial lab or by readers.
↑ Advantages of Electronic Load Equivalent
So what in principle are electronic load equivalents preferable to traditional means (powerful resistors, incandescent lamps, heaters and other devices), often used by designers when setting up various power devices?
Citizens of the portal, having to do with the design and repair of power supplies, undoubtedly know the answer to this question. Personally I see two factors sufficient to have in your “laboratory” electronic load: small size, the ability to control the load power within large limits by simple means (the way we control the volume of the sound or output voltage of the power supply – the usual variable resistor, not powerful switch contacts, rheostat slider, etc.).
In addition, the “action” of the electronic load can be easily automated, thus making it easier and more sophisticated to test the power device with an electronic load. In this case, of course, the engineer’s eyes and hands are freed, the work becomes more productive. But the charms of all the possible contrivances and improvements – not in this article, and maybe from another author. For now, – only about one more kind of electronic load – the pulse.
↑ Peculiarities of the pulsed version of the electronic load
Analog electronic loads are certainly good and many of those who have used ENs in power device tuning have appreciated its advantages. Pulsed loads have their own twist, making it possible to evaluate power supply performance under pulsed loads such as digital devices. Power amplifiers of sound frequencies also have a characteristic influence on power supply devices, so it would be good to know how the power supply designed and made for a particular amplifier will behave under a certain set character of load.
When diagnosing power supplies being repaired, the effect of using pulsed EN is also noticeable. So, for example, with the help of impulse EN the failure of a modern computer PSU was found. The declared malfunction of this 850 watt PSU was as follows: The computer while working with this PSU would shut down randomly at any time while working with any application, regardless of the power consumption at the moment of shutdown. When tested with the usual load (a lot of powerful resistors at +3V, +5V and halogen bulbs at +12V), the PSU worked smoothly for a few hours, but the load power was 2/3 of its nominal capacity. The problem manifested itself when I connected the pulse of the EN to the channel +3V and the PSU began to turn off, barely reaching the 1A division of the ammeter pointer. At the same time the load currents on each of the other channels of the positive voltage did not exceed 3A. The supervisor board turned out to be faulty and was replaced with a similar one (good thing there was the same PSU with a burned out power part), after which the PSU worked normally at the maximum current allowed for the used instance of pulsed loads (10A), which is the subject of this article.
The idea of creating a surge load appeared long ago and was first implemented in 2002, but not in its current form and on another element base and for somewhat different purposes and at that time there were no sufficient incentives and other grounds for me personally to develop this idea. Now the stars are different and something has come together for the next incarnation of this device. On the other hand, the device originally had a slightly different purpose – to check the parameters of pulse transformers and chokes. But one does not interfere with the other. By the way, if anyone wants to do research of inductive components with this or a similar device, you are welcome: below are archives of articles by venerable (in the field of power electronics) engineers devoted to this topic.
So, what is a “classic” (analog) EN in principle. A short circuit current regulator. Nothing more. And anyone would be right who, in a fit of passion, would short-circuit the output terminals of a battery charger or a welding machine and say: this is an electronic load! Not the fact, of course, that such short circuit will not have detrimental consequences, both for devices and for the operator, but both devices are really current sources and after some fine-tuning they could pretend to be electronic loads, as well as any other however primitive current source. Current in the analog EN will depend on the voltage at the output of the tested PSU, ohmic resistance of the field transistor channel, set by the value of the voltage on its gate.
The current in the pulsed EN will depend on the sum of parameters, which include the pulse width, the minimum resistance of the open channel of the output switch and the properties of the tested PSU (capacitance of capacitors, inductance of PSU chokes, output voltage). When the key is open the EN forms a short circuit, in which the capacitors of the tested PSU are discharged, and the chokes (if they are contained in the design of the PSU) tend to saturate. Classic short circuit, however, does not happen, because the width of the pulse is limited in time by microsecond values, determining the magnitude of the discharge current of the capacitors of the PSU. At the same time, testing of pulsed EN is more extreme for the tested PSU. But there are more pitfalls in such testing, right up to the quality of the supply conductors connected to the power supply device. So, when we connected the surge generator to a 12-volt PSU with copper wires, 0,8 mm core diameter, and 5A load current, the oscillogram on the generator showed pulsations, which were a sequence of square pulses with the amplitude up to 2V and sharp spikes with the amplitude equal to the supply voltage. At the terminals of the power supply itself the pulsations from the EN were practically absent. At the EN itself the pulsations were reduced to a minimum (less than 50mV) by increasing the number of cores of each of the conductors supplying the EN to 6. In the “two-core” version the minimum of pulsations, comparable to the “six-core” version, was achieved by installing an additional 4700mF electrolytic capacitor in the points of connection of power wires with the load. So, when building a PSU, the surge capacitor can really come in handy.
This circuit board is based on a popular (due to a lot of recycled computer PSU’s) components. This circuit includes a regulated clock and pulse width generator and thermal- current protection. The generator is made with TL494 PWM.
Frequency adjustment is done with variable resistor R1; duty cycle – R2; thermal sensitivity – R4; current limitation – R14. The oscillator output is reenergized by emitter repeaters (VT1, VT2) to operate on the gate capacitance of 4 or more field effect transistors.
Generator part of the circuit and buffer stage on transistors VT1, VT2 can be powered from a separate power supply with output voltage +12. 15V and current up to 2A or from +12V channel of the tested PSU.
The output of the FET is connected to the “+” of the power supply under test, the common wire of the FET is connected to the common wire of the power supply. Each of the gates of field-effect transistors (in case of their group use) should be connected to the output of the buffer stage with its own resistor, leveling the difference of gate parameters (capacitance, threshold voltage) and ensuring synchronous operation of the keys.
In the photos you can see that there is a pair of LEDs on the EN board: green one indicates the load power, and red one indicates the response of the microcircuit error amplifiers at critical temperature (constant glow) or at current limitation (barely visible flicker). Work red LED control switch on a transistor KT315, emitter of which is connected to a common wire, base (using a resistor of 5-15 kOhm) with pin 3 chip, collector – (via a resistor of 1.1 kOhm) with the cathode of the LED, the anode is connected to pins 8, 11, 12 chip DA1. This node is not shown on the schematic, because it is not unconditionally required.
The ratings of resistors and capacitors are not shown in the schematic:
About the resistor R16. With 10A current through it, the power dissipated from the resistor will be 5W (at the resistance shown in the diagram). The real design uses a 0.1 Ohm resistor (the correct rating was not available) and the power dissipated through it at the same current will be 10W. The resistor temperature is much higher than the temperature of the EN’s switches, which (when using the heat sink shown in the photo) are not very hot. Therefore it is better to install the temperature sensor on the resistor R16 (or in close proximity) rather than on the heatsink with the EN keys.
Electronic load for power supply with your own hands
During testing of another homemade or repaired power supply, to create a load you have to connect various light bulbs, powerful resistors and pieces of spiral from an electric stove. Selecting the right load in this way is a very time-consuming affair. So you don’t have to waste your precious time and nerves. It is easier to collect a simple electronic load with their own hands.
In fact this is a simple device consisting of powerful transistors that allow you to smoothly load the power supply with a stable regulated current.
In this figure shows the circuit of electronic load on high-power transistors to load any power supply up to 40A.
Electronic Power Supply Electronic Load Schematic
How does this circuit work? The voltage from the power supply under test goes to the base of transistor T1 through a voltage divider built with resistors R1, P1 and P2 and a limiting resistor R2. Transistor T1 controls four powerful transistors T2, T3, T4 and T5, acting as keys and creating a controlled load on the power supply. The circuit has two variable resistors P1 and P2 to set the load current more accurately and coarsely. The load current and voltage are measured by a Chinese electronic voltmeter ammeter. It is also possible to install a dial gauge in place of the electronic one.
This circuit is designed for an input voltage of up to 50V and a current of up to 40A. If you want to increase the current you must add the required number of TIP36C transistors and a 0.15 Ohm shunt resistor of 5W. Each added transistor increases the current by 10A.
Transistors T2, T3, T4 and T5 get very hot during operation, so good cooling is required. Put each transistor on a large 100x63x33 mm heatsink without insulating pads, because collectors of transistors in the circuit are connected together anyway.
The heatsinks are cooled by two powerful 120×120 mm fans. They are powered from a separate power supply via a L7812CV voltage regulator, and a Chinese ammeter voltmeter is also powered from here. Transistor T1 and the voltage regulator L7812CV are mounted on a separate small heatsink from the computer power supply so as not to interfere with the power transistors.
With this simple and reliable device it is easy to load and test any transformer and switching power supplies, as well as batteries and other power supplies.
I hope the electronic load for the power supply will be a useful homemade device for your home radio workshop.
Radio parts for assembling
- Transistor T1 TIP41, MJE13009, KT819
- Transistors T2, T3, T4, T5 TIP36C
- Voltage regulator L7812CV
- Capacitor C1 1000 uF 35V
- Diodes 1N4007
- Resistors R1, R2 1K, R3 2.2K, R4, R5, R6, R7 0.15 Ohm 5W, P1 10K, P2 1K
- Radiators 4 pcs. size 100x63x33 mm
- Fans 2 pcs. of 12V computer size 120×120 mm
- Chinese voltmeter ammeter for 50A with a shunt, you can put an arrow instrument, it will be much more accurate and reliable
Friends, I wish you good luck and good mood! See you in new articles!
I recommend to see a video how to make an electronic load for the power supply
How to Disassemble a Switching Transformer
Making Printed Circuit Boards at Home. LUT technology.
Heater from a microwave oven
Electronic load for power supply with your own hands
Short Circuit Protection for Power Supply with Your Hands
Charging device from a switching-mode power supply
141 comments on ” Electronic Load for Power Supply with Their Own Hands “
I want to be guided by this scheme to make a regulated load for the battery with the ability to regulate from 0 to 10 amperes and use it instead of a lamp for desulfation mode, along with a 12V battery charge controller. To discharge the battery with this load at times of discharge. Such a device will eventually be made by an understanding electrician, I just need to come to him with a specific idea. The voltage in the circuit is 12-14 volts, and the load needs up to 10 amps, I assume that you can get by with one revolving resistor, plus limit it so that at full discharging the consumption was about 10 amps. Question, is it possible to create a continuous discharge load on the battery with this device (instead of a light bulb), and whether in such conditions the resistors will be strongly heated, if for example to put them 2, 3, 4 pcs (can be by increasing the transistors do without cooling radiators, with a continuous load up to 10 amps)
Addendum to my post of 20.01.2022. In vain I got excited about the assembled design. When I tested the 13.8V PSU, the load worked fine, but when I connected a +44V source to tune the amp’s current protection unit, things didn’t go so smoothly. For a moment, some small current value came on, and then one of the KT825G transistors blew. Disassembled, replaced the faulty one. When I switched it on again, I managed to get the current up to 8.4A for a couple of seconds, then I had another breakdown and short circuit. I ordered TIP36C and I am going to change it to 6 or even 8 to keep it under the nameplate power that transistors can withstand. I need to adjust the protection tripping to 16-18A at 44V. In the meantime, I made a load from a nichrome spiral from an electric stove. At first I connected one end of it to the laboratory instrument with accuracy class 0,5, which was connected to + source with the other end. The minus of the source was connected to the crocodile to determine the length corresponding to the current of 4 A in the heated state. Then I screwed this piece onto the PEV-50 and 100W resistors and put the ends of the coil on the M3 screws in the holes of the resistor’s leads. I made 4 loads of 4 amps and another 1,4 amps from the rest. Thus, the load is switched in steps of 4+4+4+4+1,4=17,4A. 3 resistors soldered to constant, and 4 and 1,4 A also made pluggable crocodiles. Used 4 sq.mm wire everywhere. In the end I managed to adjust the protection unit, but I will still do the load, as the thing is useful, even for low voltages.
Hello. I used your circuit to assemble it with a hinged assembly. Everything is fine. Resistor P1 “coarse” regulates almost immediately, as well as P2 “fine”. On a 5 volt PSU, P2 adjusted within 500 mA. I decided to put everything on the board. Assembled on the breadboard (where the holes for soldering parts) and after the resistor P1 “roughly” began to regulate about 1/3 of its rotation, and resistor P2 – almost nothing regulates (voltage 5 volt PS – about 0,01-0,2 mA, and 20 volt PS maximum 0,2 mA). All parts were the same as for hinged mounting. Power wires have 1mm diameter wire. Please advise what can be the problem. And another question: what is the purpose of the 2.2 kOm resistor R3?
Good evening! Somewhere you have a mistake in the assembly or more likely there is a problem with the variable resistor. Sometimes when you solder the wires to the variable resistor the contact is broken by overheating on the rivet, which riveted the legs to the resistor and then such a lambada. Check the function of the variable resistors with a multimeter on all three legs. When you rotate the knob between the center leg and any extreme leg, the resistance should adjust smoothly, and there should be constant resistance between the extreme legs. Resistor R3 is the collector load of the stage. With this resistor one tries to make the collector voltage to be Epit/2 in which case the gain of the stage is maximal. But that’s if it’s the fancy way. Even if R3 is removed the circuit will work fine.
Hi, can you make me a file of the circuit ready for the printed circuit board?
You don’t need a printed circuit board for an electronic load, it’s easier to assemble the circuit by hanging up.
Can you make me a schematic for a circuit board
I made such a load, but with NPN transistors 2SC4110, 2 pcs. It works fine. Thanks to the author. In future I will add overheat and overcurrent protection. I have TIP42C T1. So far I loaded it with 15.1 amps, 260W, since there are no 0.15 Ohm resistors, then I plan to put in 300W. I don’t think I need more than that yet. I already checked one PSU from a laptop with unknown specifications.
The resistors came and I took 50 Watt, 0.1 Ohm. Loaded it up to 335 watts without looking, 320 watts of rated power, radiator was cold, all intact. Replaced 2 variable resistors with one multi-turn resistor, works great. Now designing a new board, abandoned the light signaling polarity reversal, left only the sound. I also made an automatic switch on the cooler over the temperature. I also added a paired Schottky diodes for protection against polarity reversal on the input.
I assembled this design to set up the current protection unit operation of a 500 Watt 144 MHz power amplifier, and later on to 432 MHz. It is required to provide triggering of protection at 44V and 15-16A. As a P-N-P transistor I used an existing KT825G and as a regulating transistor I used a TESLA KU607. I had no problems with the power part, but the adjustment with the potentiometer “Grubo” was very sharp. Finally I changed a little bit the rating of the resistors and now the load current setting was more or less acceptable. R1- 15 kOhm, R2- 4,7 kOhm. I changed potentiometer P2 to 470 ohms, but it wasn’t necessary. Checking current and voltage with the same device as the author’s. In the process of adjustment, an M253 ammeter of accuracy class 0.5 was included in the load circuit to monitor the current. The discrepancy in the readings was about 0.6A, so I had to tweak the “Chinese” a bit. For cooling we used SUNON 48V fans. Separate power supply was used for fans and digital device. As a test source used switching power supply of the transceiver 13,8V 23A. When configuring the protection unit with power from the Flatpack S 48/1800 HE may have to increase the nominal value of R1. On the whole I liked the design. Many thanks to the author.
Thank you for the very detailed review .
My pleasure. Already had to try this load, though for other purposes. I converted the motorcycle battery charger 6MTS9, made back in the Soviet days, to a more powerful current to charge a car battery with a capacity of 70 Ah. I replaced the transformer and the shunt in the ammeter. For testing just this load came in handy. It is a very useful device. I used to make it out of PEV resistors and car light bulbs, but it had not smoothly regulated the load current.
Yes, it is a very useful tool especially for testing power supplies.
Greetings. I ordered tip35c by mistake , is it possible to remake this circuit to NPN and how to change it?
Good evening! No, the NPN transistors will not work.
Well it is clear that under this scheme will not work, and in principle possible to change the circuit so that it could work on NPN transistors, or it is impossible?
If you change the circuit, then it will. You need to reverse polarity at the input where you connect the power supply under test, T1 should be replaced by a PNP transistor such as TIP42, KT818, T2-T5 put NPN transistors. Voltmeter will have to be connected differently, yellow wire to the bottom. Swap the red thick one and the black thick one on the shunt. It should work.
Works great on TIP42 and NPN. Thanks for the circuit. Above is my detailed review.
If you still need help, we can get in touch on Whatsapp. I converted it to NPN. 9081302122
I wonder if you change the 0.15 resistors to 0.1.
It will also work. You can put resistors from 0.1 to 0.15 ohms. Everything will work just fine.
Hello. Will my device be suitable as a load for discharging a 48 volt 200 Ah battery with a current of 20A?
Good evening! One transistor at 12V can handle 10A for a long time. Short term up to 20A at 12V. A higher current will destroy the case. Your voltage is 48V and the current is 20A. 48V/12V=4 transistors at 10A. To withstand 48V 20A you would have to put 8 TIP36C transistors. The higher the voltage, the less current the transistors can handle.