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Do you need a constant current load? Why do you need it for? when you say adjust then what is the limit? You should provide all the necessary information.
As far as i am guessing you probably need it to measure battery capacity.
The power consumed will be as much as 600 watts (ballpark figure).
It's possible to parallel enough mosfets/transistors to handle such a load. Massive heatsinking will be needed. Besides needing attention in regard to balancing load across all devices, lest one oddball device hog too much current and burn up.
As an alternative, I once saw a project to make a test load for car batteries, using steel wrapping band.
At 12V and 50A, the resistance will be about 1/4 ohm.
At 12A you'll want 1 ohm.
Several feet of steel band is sufficient to yield this amount of resistance. Choose a long enough amount that will carry the smallest amp load you desire.
Nails are pounded partway into a wood board, forming rows, so that the band can be wrapped back and forth on them.
Because it's painted steel, it will be tricky to make secure electrical connections. Scraping, applying flux, heating sufficiently, etc. Or tack welding could work. Maybe simple clips can do, the kind that come attached to car battery chargers.
Once you establish the resistance of the entire length, it will be easy to attach clips at intermediate points to obtain various current draws.
I don't think so. MJ11033 can handle 50 Amps (which is expensive) but come on its rating is for the ideal condition. many factor involved are power across collector-emitter which inturns generate heat so roughly you have to design your device to work at 100 degree C. such restriction would never allow you to use that darlington transistor for more that 30 Amps or may be less. And another problem is that you can not make them use to work in parallel because of their slightly different base-emitter potential drop and you need a series resistor with every BJT to turn on another BJT and that resistor goes up to many Watts and is expensive.
Fets or Mosfets can be easily parallel above all they will allow you to give flexibility to adjust your constant current and they will keep your design simple.
Hi, What about a simple 555 pulse width modulator driving driving some mos-fets and using stainless mig welding wire for the load resistor?
I built a resistor by twisting strands of the wire with a battery drill and terminated the ss wire with earth bolt eyes that crimp on. To check the resistance/length use a constant current and ohms law. I wound mine on some cement fibre sheet for support.
Another approach could be to use a dc/ac inverter and vary the load?
If he needs to measure battery capacity then he needs a constant current load. In inverter do note that as battery voltage drops, current to the input of inverter increases. so its not a constant current load, though output load is regulated to some extent.
You can make dummy load with power resistors, and with switch or relay to regulate current needed for testing or something else. Its cheapest way.
You can put more resistors in parallel to get higher power of resistor, but you need to do calculation for current.
I made few years ago, dummy for lead acid 100A with 17W resistors. Very important thing is right dimensioning of this device. How long this device will work ? Few sec or few minutes, hours..... ? If few sec then you can go under needed wattage.
Example :
30 resistors in parallel
Resistor 17W 7R
Power of all resistors 510W
Battery Voltage 12,73V
Total needed power 695W
Resistance 0,23R
Amperage 55A
One resistor dissipate 23W with current 1,8A
Make in Excel spreadsheet calculation and change value and look what happen. ;-)
If you put some cooler, and device oeprate few sec or some short time no problem. This is cheapest way.
Only thing what can beat this, is resistive wire, if you find some resistive wire you can build some resistors and make very very cheap device even for higher amperage testing. Resistive wire called "Kanthal" made from kanthal, before kanthal "Cecas" is used (I think on english is also "cecas").
In attachment I put Excel calc.
If You need testing internal resistance with AC current and other material about batteries just say.
I mean no disrespect but this is a very poor example of dummy load, no precision at all.Its ok if you just have to drain batteries. Else its of no good. I think you don't know the prices of these power resistors. these are going to be way more expensive then bunch of FET's or MOsfets. Besides i think he needs a portable load.
Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. Kanthal FeCrAl alloys consist of mainly iron, chromium (20–30 %) and aluminium (4–7.5 %). The first Kanthal FeCrAl alloy was developed by Hans von Kantzow in Hallstahammar, Sweden. Its name is derived from Kantzow and Hallstahammar. The alloys are known for their ability to withstand high temperatures and having intermediate electric resistance. As such, it is frequently used in heating elements. The trademark Kanthal is owned by Sandvik Intellectual Property.
For heating, resistance wire must be stable in air when hot. Kanthal FeCrAl alloy forms a protective layer of aluminum oxide (alumina).[1] Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity; special techniques may be required to make good electrical connections.
Ordinary Kanthal has a melting point of 1,200 °C (2,190 °F). Special grades have a melting point as high as 1,425 °C (2,597 °F).[2] Kanthal is the name not only of a family of alloys, it is also the brand name for Sandvik's heating technology products. Another trademark of Kanthal is MAXthal, which uses ternary carbides such as Ti2AlC that are known as MAX Phases have good oxidation resistance at high temperatures. Kanthal used to be a name of a company AB Kanthal that was founded 1931. In the late 1990s Sandvik became majority shareholder in Kanthal after its purchase linked to the Trustor affair
I mean no disrespect but this is a very poor example of dummy load, no precision at all.Its ok if you just have to drain batteries. Else its of no good. I think you don't know the prices of these power resistors. these are going to be way more expensive then bunch of FET's or MOsfets. Besides i think he needs a portable load.
Yes its just example and pictures of power resistors. You can just use google search like images and type power resistor. Price in my country is about 0,3eur for 20W power resistor.
For mosfet you need more, you need heatsink. But I say what is duty cycle?
I think you are wrong about precizion. This should not be professional device. One mosfet or few used like dummy load not better then this. All commercial low cost and some higher priced dummy load for lead acid are based on some kind of resistive wire or on parallel power resistors.
Size of 20W resistor :
Should I say no need for reinvention of hot water, all he need is resistance, and what is best for that - resistor! ;-)
Solution should be cheapest as possible, safest as possible, to work, and to be simple solution.
We should give some idea to our friend, I give mine. I think You should give some idea with mosfet for high current dummy load and whos knows, I'm always open for conversation and consideration of ideas, maybe Your idea be better. More ideas better solution.
Ok, no prob, if You need something say. Of course and other ideas should be considered and take in consideration, I give one easy to build and cheap idea.
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