# Series current limiter

Discussion in 'General Electronics Chat' started by kubeek, Oct 24, 2008.

1. ### kubeek Thread Starter Expert

Sep 20, 2005
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Hi,

I am often repairing solid-state audio amplifiers, and would like to build some kind of current limiting circuit.
Ideal would be some "black box" similar to limiting current with a light bulb, because it would be great if it didn't need a ground connection.
Is that possible?

If not, could someone help me design a current limiter, which won't be too sensitive to input voltage variation?

Another thing that concerns me is, that if I use some current limiting devices on both power rails, is it possible that the two limiters would fight against each other or oscillate?

Many thanks

2. ### mik3 Senior Member

Feb 4, 2008
4,846
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Unless you have two power supplies, you just need a single current limiter. You can put a 1 ohm resistor or less and a transistor in series with the whole circuit, monitor the voltage across the resistor (which equals I*R so you monitor the current through it because R is fixed) and use a comparator to sense when your current reaches the limit. When the comparator will sense the current limit, it will activate an op amp which will decrease the voltage across the base-emitter junction of the transistor (the transistor has to be fully turned on if the current is below the limit) and thus increase its resistance and keep the current fixed at the desired value.

3. ### kubeek Thread Starter Expert

Sep 20, 2005
5,584
1,093
I need two of them, because there can be faults like transistor's case shroted to ground, which affects only one of the rails.

I am aware of the op amp current limiter, but I was hoping that there exists some device circuit that doesn't need external power supply at all (which I know is highly unlikely), or something that could operate from up to +/-100V rails, the maximal current can be cca 1-2A for short time (10-15 sec).
100V is quite impractical to regulate, but it can be done. The problem is that I wish to make some universal kind of limiter, so the voltage can range from say +/-25V to +/-100V.

Any suggestions?

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
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One handy item for decreasing the voltage on amplifier supply rails is the variac. Lower AC in means lower DC out.

5. ### kubeek Thread Starter Expert

Sep 20, 2005
5,584
1,093
You mean I should attach servo and operate it based on secondary current?

Seriously, I don't want to lower the rails because that would change the quiescent current, and also could create DC shift of output, which really won't help too much with finding possible faults.
I would like to have a simple once-for-all solution, in the way that I connect it to he main rails of the amp, and the amp either triggers one protection, both, or stays with some reasonable current.

Nov 9, 2007
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Sep 20, 2005
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8. ### studiot AAC Fanatic!

Nov 9, 2007
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Its too old, but see your PM.

May 16, 2005
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10. ### SgtWookie Expert

Jul 17, 2007
22,201
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Well, I suppose you could use a TL783 high voltage linear regulator with a resistor, similar to how you'd use an LM317 in a lower voltage situation. However, the dropout voltage is pretty large at higher current levels (10v to 18v depending upon temp and current).

But, if you're trying to allow even a moderate current flow with a high voltage differential across it, you'll have a bit of excitement with the noise and smoke generated by it's sudden demise. 250mA with a 100v differential is 25W. Even a hefty copper heatsink with a fan wouldn't likely be enough.

Sep 20, 2005
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12. ### kubeek Thread Starter Expert

Sep 20, 2005
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And one more question, does anybody know what is a typical inductance of wirewound ceramci 5W resistor, in my case 0.1ohm? Thanks

13. ### SgtWookie Expert

Jul 17, 2007
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Ohmite makes non-inductive wirewound resistors. Digikey and Mouser are two distributors that carry Ohmite products.

14. ### kubeek Thread Starter Expert

Sep 20, 2005
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Thanks, but I don´t need non-inductive resistor, I would just like to know what is the typical inductance.

15. ### SgtWookie Expert

Jul 17, 2007
22,201
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It will vary widely between manufacturers, resistance, and wattage rating.
Different gauge wire, different diameters of winding, different spacing of the coils, and number of turns will all make a big difference in the inductance.

Wheeler's Formula will get you within about 3%, if you know all of the above parameters.

16. ### kubeek Thread Starter Expert

Sep 20, 2005
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Ok, that sounds reasonable, I will have to try it with my meter, but I suppose it is too small to measure.

You also forgot the permeability of the surrounding material

17. ### SgtWookie Expert

Jul 17, 2007
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I would think that sand/ceramic would have a pretty low permeability

18. ### Bailey45 Member

Oct 27, 2008
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Heres a simple solution, however, it is a little old school. The attached circuit will limit at a value of .7 volts across either emitter resistor. Select transistor with appropriate Vce max and power dissapation for your application.

Raychem poly fuses might also solve your problem

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