# modifying a current limiting circuit

Discussion in 'The Projects Forum' started by lokeycmos, Sep 12, 2009.

Apr 3, 2009
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2. ### Audioguru New Member

Dec 20, 2007
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You could add many parts to perform current-limiting. Sense the current with a low-value resistor then use an opamp to amplify the voltage across it that is made by the current in it.

May 29, 2008
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4. ### lokeycmos Thread Starter Active Member

Apr 3, 2009
432
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i meant just modifying values on the exisiting circuit not adding stuff to it.

5. ### CDRIVE Senior Member

Jul 1, 2008
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If you don't want to modify this circuit appreciably you may be forced to add a range switch to select 3 different values of R1. You didn't say what the min and max input voltages will be. This which will affect power dissipation on Q1. For the kind of current and probably voltage range that you probably want you will need to parallel (Emitter bal resistors required) some more 2N3055s. Don't forget that the 15A current rating on that transistor is given in a saturated state and with serious heat sinking!

6. ### lokeycmos Thread Starter Active Member

Apr 3, 2009
432
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can anyone help me calculate the values?

7. ### hgmjr Moderator

Jan 28, 2005
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By reducing R4 from 1K to 820 ohms, you can achieve very close to 10A at the upper limit. This change will not effect the low end. It will remain at the present setpoint of 1.4A.

$I_{\small MAX}\,=\,{(\frac{5000}{820}+1)}*{\frac{1}{0.5}}*0.7$

hgmjr

Last edited: Sep 13, 2009
8. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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According to the author; doubling the value of R1 (.5R x 2) will double the lower limit and half the upper limit, giving a lower limit of .7A and an upper limit of 4.2A.
Therefor it stands to reason that tripling the value of R1 (R1 x 3) to 1.5R should return:
R1= 1.5R
Lower Limit=.47A
Upper Limit= 2.8A

9. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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This is certainly a better solution than what I suggested.

10. ### lokeycmos Thread Starter Active Member

Apr 3, 2009
432
7
thanks hgmjr, thats what im looking for. can you tell me the formula for current minimum. between the 2 formulas ill be able to come up with a happy medium.

11. ### hgmjr Moderator

Jan 28, 2005
9,030
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Formula for the minimum is

$I_{\small MIN}\,=\,{0.7*\frac{1}{0.5}}\,=\,1.4\,Amps$

hgmjr

12. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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That circuit is a controlled current source which will have several voltas of drop across it in operation. If you hang it on the output of a lab supply, you no longer have a regulated output voltage.

You need to build the current control circuits into the lab supply's control circuitry, you don't hang it on the regulated output in series with the load.

Last edited: Sep 13, 2009
13. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Upon further review I have determined that if you change the current sense resistor from 0.5 ohms to 1.0 ohms and then drop R4 to 560 ohms then you will have adjustable range from 0.7 Amps to 9.9 Amps.

You should keep in mind that 10 amps across 1 ohm is 10 Volts so the input voltage will need to be at least 10 Volts greater than the voltage you to which you are regulating.

Adequate heatsinking is going to be critical to the reliable operation of this circuit. The sense resistor will have to be able to dissipate at least 10 watts. You will need to size it accordingly. You will probably want to use a 20W or 25W rated sense resistor.

hgmjr

14. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Thats mostly true. The current sense resistor is still the last inactive component in the chain though and has one terminal connected to the output terminal. Current sense is picked off the more positive side of the sense resistor and the Voltage sense is taken off the more negative (Output Terminal) of the sense resistor.

Here's a link from the same site that the lokeycmos found his current limiter. Please note that the author has a typo there. He states that the current limit adj. is R3. That should read R13.

BTW, I have two Lambda rack mounted supplies. Both are Vregulated and current (adjustable) limited. Both have a jumper bar on a rear terminal block. When the jumper is connected the voltage sense is measured directly at the pos output terminal on the front panel. If the load is remote and long line voltage drop can't be tolerated you can remove the jumper and connect a third wire to the Vsens terminal. The other end is connected to the remote load. This setup regulates the voltage at the load instead of the output terminal.

After examining the circuit in the link it should become apparent that the same pass transistor is used for both voltage regulation and current limiting, which is what lokeycmos is ultimately going to have to do. I predict this is going to be a lengthy thread!

15. ### bountyhunter Well-Known Member

Sep 7, 2009
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All commercial lab supplies have remote sensing capability for voltage. I designed a number of them when I worked at Power 10. Lambda was one of our competitors. That doesn't have any connection to the "series current limiter" shown in the figure in the third post of the thread which the OP wanted to "modify". That is a "hang on the end" limiter not a limiter built into the control circuitry, which is what every lab supply builder does.

I guess you are saying it would be possible to build a supply with enough "remote sense headroom" to be able to use this kluge current limiter? maybe possible, sure throws away a lot of power and voltage.

16. ### CDRIVE Senior Member

Jul 1, 2008
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No, that's not what I'm saying. I have no argument with your premise that that limiter (as is) is a hang on circuit. The OP has some work ahead of him. That's why I said this is going to be a long thread. I think he'd be well advised to work with the circuit in my link and scale it up, or better yet, maybe you can design it for him.
BTW, I asked but the OP never stated what voltage range his supply will deliver. For all we know it's a fixed 5V supply. Doubt it but ....????

17. ### bountyhunter Well-Known Member

Sep 7, 2009
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I definitely can, and my consultation rates are very reasonable.