# Current Limiting Circuit Tips.

Discussion in 'Power Electronics' started by psrkallez06, Jul 26, 2016.

1. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4
Hello, I'm trying to make a current limiter which i can adjust from 10 mA up to 3A. I found this document:

http://fritzing.org/media/fritzing-...r_files/external current limiting circuit.pdf

The last circuit on that page (Current Limiting up to 8.4A) works fine except that i can't make the current go lower than around 300 mA, i can adjust R1 to change the max current but i don't know how to make it go lower. Tips? Thanks!

2. ### Sensacell Well-Known Member

Jun 19, 2012
1,184
277
Increase the value of R1

3. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4
If i increase the value of R1, the maximum current get's decreased. Like i said, i want a current from 10 mA to 3A. Thanks!

4. ### Sensacell Well-Known Member

Jun 19, 2012
1,184
277
Can be done, but you cannot do it with that simple circuit unless you range-switch R1 for different ranges.

Since it uses the transistors Vbe threshold to set the current, for 10 mA you are going to need ~600 mv to start limiting, at that point R1 needs to be 60 ohms- see the problem?

Last edited: Jul 26, 2016
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5. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4

I see, Using a switch to change ranges is a pretty good idea. Will try that out. Thank you!

6. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4
Oh and i might as well ask, in the circuit. the transistors used are different. How is that? The 2N3055 i can understand since it is a power transistor. But why use a BC546 and BD139 and not just two of the same?

7. ### Sensacell Well-Known Member

Jun 19, 2012
1,184
277
They are in a cascade, the collector current is much greater in each progressive device, they are scaled, beefy-er and beefy-er as you go down the line.

8. ### crutschow Expert

Mar 14, 2008
13,523
3,392
What is the voltage of the current you are trying to limit?

9. ### RichardO Well-Known Member

May 4, 2013
1,350
432
You can bias the current sense transistor by putting a high current diode in series with the current sense resistor. The diode must be sized for the maximum output current -- over 10 amps in this case. Make sure that you heat sink this diode since it can dissipate more than 10 watts.

You may also need resistors on both ends of the current limit adjust pot to get the range of current settings that you want.

10. ### atferrari AAC Fanatic!

Jan 6, 2004
2,674
785
Hola Richard,

Could you show the actual circuit? Gracias.

11. ### RichardO Well-Known Member

May 4, 2013
1,350
432
Here is an LTspice circuit. It only adjusts down to about 60 ma. I thought it would be lower. Maybe a different diode would work better.

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12. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
Continuously variable might be possible - but you'd probably need a low-drift precision op-amp to monitor the volt drop across the current sense resistor. If you use a bipolar pass transistor; you'll need a few driver stages to get up to 30A. MOSFETs are voltage driven, so can be easier - but there's a couple of gotcha's. If you can put the series pass device in the load negative lead; an efficient N-channel MOSFET is OK. If it must be in the positive lead; a P-channel MOSFET is easier - but they have inferior spec and usually cost more. You can put N-channel in the positive feed - but it requires an additional bootstrap supply for the gate.

13. ### RichardO Well-Known Member

May 4, 2013
1,350
432
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...

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14. ### crutschow Expert

Mar 14, 2008
13,523
3,392
Note that the current limit of that circuit is greatly affected by the output load and voltage, which you don't normally want.

15. ### crutschow Expert

Mar 14, 2008
13,523
3,392
Below is the LTspice simulation of a current-limit circuit using an op amp, for a current range of essentially zero to over 3A by adjusting pot U2.
The input voltage is limited by the op amp absolute maximum voltage of 32V.
Edit: Fixed error in schematic.

• ###### Current Limit PMOS Op Amp Adj.asc
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Last edited: Jul 26, 2016
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16. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4
I want to limit voltages between 1v up to 24v. Thanks for all the answers.

@crutschow. That circuit looks very good. I don't think i have a 5.2v zener diode avalible tho.. have to look!

17. ### #12 Expert

Nov 30, 2010
16,705
7,358
For a basic overview: The first thing to look at is that you want a range of control of 300 to 1. With a simple circuit, you will lose 300 times as much voltage at one end as you lose on the other end of the range. The cure for these types of challenge is always more gain. The op-amp has a voltage gain of somewhere near a million and the mosfet has a current gain that is nearly indescribable. Properly arranged, these can provide the gain of 300 you need without wrecking the available voltage by using it up in the sense resistor.

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18. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
5.2V isn't a standard voltage for Zeners - 5.1V is pretty common, 5.6V used to be abundant because they compensated for the 0.6V Vbe drop in a 5V emitter follower regulator.

Its worth keeping a small stash of TL431 "programmable Zeners" in the bits box, they can shunt 100mA and can be programmed for any voltage from 2.5V to somewhere around 35V. You have to watch the SOAR curve - they can't do 100mA and 35V both at once! In some applications you may have to take the programming resistors into account, they carry a tiny bypass current. The TL431 input resistance is something like 100k, the resistor divider should present about 1/10 of that, so the bypass current is pretty small.

19. ### crutschow Expert

Mar 14, 2008
13,523
3,392
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.

If you substitute a TL431 for the 5.6V zener and use it at its base voltage of 2.5V to avoid having to add two extra resistors, then you just reduce the 150k resistor, R5, to about 67k to give the same current limit range.

Note that I corrected an error in the schematic in post #15.

20. ### psrkallez06 Thread Starter Member

Dec 17, 2015
68
4
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.