Update: Checking the 2N3055 datasheet i saw: Base−Emitter On Voltage (IC = 4.0 Adc, VCE = 4.0 Vdc) VBE(on) − 1.5 Vdc ÎÎ

Does that mean that the base voltage cannot exceed 4v?

 

You need to in series with a bjt between the output of op amp and 2N3055, and if you wish to have a big range to adjust variable current then you will need a 10 turns VR, the adjust range will be expanding to 10 times of original range and the adjust scale will be smaller.

 

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I only use 10 turn pots so that's good!

THank you for that circuit, gonna try to simulate it now!

 

You need to in series with a bjt between the output of op amp and 2N3055, and if you wish to have a big range to adjust variable current then you will need a 10 turns VR, the adjust range will be expanding to 10 times of original range and the adjust scale will be smaller.

Could you maybe explain a bit more/draw a schematic on how to meant with a bjt in series? Thanks!

Update: Solved it!!!

 

I'am a beginner in electronics. I've recently learned about op amps and only worked with LM317/the easy stuff before. So this is my design as of now for a current limiter:

Some parts are taken online from various websites, and some i've come up with myself. What do you guys think? It's adjustable from 10 mA up to 3A and i can use most of the pots range. Not sure how this will work outside of simulation tho, so it doesn't hurt to ask you experts! You can ignore the voltage regulator in the end since that is just for testing, i will use another regulator that is variable like the L200 or similar when i build this thing!. Thanks again.

 

Could you maybe explain a bit more/draw a schematic on how to meant with a bjt in series? Thanks!

Update: Solved it!!!

You already did the bjt in series as I said in #26, if you just for test is ok, but if you want to use it then use the VR like this is better and the adjust will be smoothly, but you have to choose the values of VR to match what you want.

 

Thank you folks! I have a few TL431 which i can use. Another quick question. Testing a few different circuits. And i found this on the web:

How come this circuit won't go any higher than 2 Amps? It can go down to 1 mA by adjusting the input voltage to the opamp. But when i change the voltage from 2V to 3V it should show 3 Amps but shows 2.06 Amps instead. Why is that?

My guess is that it has to do with the base of the transistor but can't figure out what.

That circuit has a floating load (the resistor in the transistor collector). You want a current limit into a grounded load.

The current is limited by how much base current the op amp can deliver.
You need to add another transistor to make a Darlington stage.

 

Your circuit is post #26 is rather a hodge-podge.

What is the purpose of the upper op amp connected to the input voltage and configured as a follower?

What's the purpose of the top 2Ω resistor.

It looks like the limit will vary with the input voltage which is not desirable.

The output regulation won't be good since you've added an emitter follower output to the 7805 regulator.
That will reduce the output by about 0.7V to 4.3V and the voltage will vary with output current and the transistor temperature.

Next time you post a schematic, include the part reference designations (R1, C1, etc.).
It's a schematic option.

 

Your circuit is post #26 is rather a hodge-podge.

What is the purpose of the upper op amp connected to the input voltage and configured as a follower?

What's the purpose of the top 2Ω resistor.

It looks like the limit will vary with the input voltage which is not desirable.

The output regulation won't be good since you've added an emitter follower output to the 7805 regulator.
That will reduce the output by about 0.7V to 4.3V and the voltage will vary with output current and the transistor temperature.

Next time you post a schematic, include the part reference designations (R1, C1, etc.).
It's a schematic option.

Sorry about that, i will add the part reference in the future. The upper opamp i use because if i skip it and connect to a voltage regulator. The voltage won't be enough for any regulator to turn on. If i use the upper opamp the voltage stays at 18 volts all the time (minus the dropout voltages etc). The top 2 Ohm resistor i have no idea.I took that part from here: http://www.ecircuitcenter.com/Circuits/curr_src1/curr_src1.htm

He explains why it is used i think. But maybe i don't need it for my circuit.

And what if i used at external voltage source for the op amp set pin instead of the primary supply voltage. Then the limit won't change with the input voltage?

 

Wow. I sure had a bad day when I did the LTspice drawing. Now I can't get corrected files to upload. I will try later...

Here is what I really had in mind. Note that the load is now between the sense circuit and ground.

 

I'm rather fond of the TL431 also. It can also be used as a comparator or an op amp with a built-in 2.5V reference for certain applications.

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It *IS* a comparator with a built in 2.5V reference.

 

It *IS* a comparator with a built in 2.5V reference.

Since it is stable with negative feedback I would say it "IS" an opamp with a 2.5V reference that can be used as a comparator.

 

Here is what I really had in mind. Note that the load is now between the sense circuit and ground.
View attachment 109607

Thank you! Gonna simulate this one also

 

If we ignore all the "problems", and that the current will change with the input voltage. What is the best way to drive the non-inverting input of the op amp without PWM? I need a small power supply that can supply 0.01V - 3V. It doesn't need to be able to handle any current (maybe 50 mA max). And i won't use this for the final design, i will prolly use one of the circuits you guys posted. It's just fun to test different circuits and learning new things

 

The circuit there is no Load, the 2N3055 bjt will be the load.

 

The circuit there is no Load, the 2N3055 bjt will be the load.

What do you mean? R2 is the "sense" resistor and the load would come after that, in this case i have it shorted to ground.

Still, i was just wondering if you have any idea how to easiest get a precise 0.01 - 3 voltage source. most voltage regulators like the lm317 has a 1.25v reference. To drive that down to 0 volts require diodes/negative voltage on the adjust pin, seems like a lot of trouble just to change the current

 

You may try the below circuit, and choosing the low offset voltage op amp < 10mV.

 

What do you mean? R2 is the "sense" resistor and the load would come after that, in this case i have it shorted to ground.
.............

Which means the current limit will depend upon the load resistance, since it is series with the sense resistor,
not something you normally want.

 

You may try the below circuit, and choosing the low offset voltage op amp < 10mV.
View attachment 109662

Great thank you!!

Which means the current limit will depend upon the load resistance, since it is series with the sense resistor,
not something you normally want.

I see, that must be the problem i had with the voltage not being enough for powering a regulator (since the output voltage will be the set voltage of the opamp, in this case 0 - 3v. What do you recommend i do to make it not depend on the load resistance? Or so that i get enough voltage to power a regulator. Regards.