# Current Limiting on a AC current source

Discussion in 'General Electronics Chat' started by ashkarmalik, Jul 22, 2014.

1. ### ashkarmalik Thread Starter Member

Jul 1, 2012
56
1
Hello people,
. I need to build a current limiting source .
Specs are as follow
1. 5v AC
2. 5 amps
.
So how I can limit the current without using a power resistance.

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,098
1,478
Assuming 5VAC is the required circuit output, what is the input voltage (some headroom will be needed for controlling current)?
What frequency?

3. ### ashkarmalik Thread Starter Member

Jul 1, 2012
56
1
It is basically a transformer which is used in my power supply 220 vac to 5vac @5amp continous

4. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
Well it would seem to me that with a 5 amp transformer you will have natural magnetic limiting anyway.

What is the AC load that it can't take minor momentary overload, especially since the load determines the current.?

Would not a simple 5A resettable cutout do?

5. ### ashkarmalik Thread Starter Member

Jul 1, 2012
56
1
Load is a power resistor with rating 1 milli ohms.
A cut out cant be used.

6. ### SVS Member

Aug 16, 2012
89
4
1 milli ohm is almost like a short circuit........

What about external inductance as current limiter??
Or build a transformer with a big leakage inductance ??

7. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
5 milihoms with 5 vac applied will draw way more than 5 amps. there is no way to limit the current to 5 amps without reducing the voltage a lot. checkout ohms law, current equals voltage devided by resistance. 5 volts devided by .001 ohm equals 5000 amps. limiting the current to 5 amps means 5 milivolts to load.
if you put a .999 ohm resistor in series with the .001 ohm load, you would get p[retty near 5 amps through the circuit.

Last edited: Jul 22, 2014
8. ### studiot AAC Fanatic!

Nov 9, 2007
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A more complete explanation of why you want to wire up this resistor to a 5 amp transformer would help.

9. ### inwo Well-Known Member

Nov 7, 2013
2,441
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Magnetic shunts is the way micro wave oven and neon sign transformers do it.

For tesla coil use I've used inductive ballast or resistor in primary to have a similar effect.

Capacitor may also work.

10. ### crutschow Expert

Mar 14, 2008
16,509
4,442
The magnetic circuit does not provide current limiting in a normal power transformer. A transformer core does not saturate at high currents. Saturation is determined by the primary voltage, not current, since primary magnetic flux caused by primary current is balanced the the opposite direction flux from the secondary current. The main thing that limits transformer current is the winding resistance and the small leakage inductance.

Transformers, such as neon types, have a magnetic shunt that introduces high leakage inductance in series with the windings to limit short circuit current.

11. ### studiot AAC Fanatic!

Nov 9, 2007
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That would imply that it is impossible to saturate a mains transformer since the mains voltage is sensibly constant, regardless of the power in the circuit.

12. ### alfacliff Well-Known Member

Dec 13, 2013
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429
and would also imply that infinate current would not saturate a transformer.

13. ### ian field AAC Fanatic!

Oct 27, 2012
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One possibility is to put the basic current regulated linear series pass transistor inside a bridge rectifier - that is to say that the current limit transistor would be directly across the + & - terminals, so if you draw the symbols on paper, the transistor is inside the square of the 4 diodes - the AC terminals are on the outside of that box, which you put in series with the load.

14. ### ian field AAC Fanatic!

Oct 27, 2012
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Its more usual to use capacitance in the form of the so called "wattless dropper".

Basically Xc of the series capacitor is very large relative to the load resistance, so changes in the AC amplitude have little effect on load current.

Doesn't seem very likely for this application though - for one thing the capacitor would be pretty big.

15. ### ashkarmalik Thread Starter Member

Jul 1, 2012
56
1
I need to measure the resistance value of a unknown resistance using a Kelvin 4wire method.
So a current limiting circuit has to be used in that setup.

16. ### studiot AAC Fanatic!

Nov 9, 2007
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If you must measure at 5 amps, then replace your transformer with pulsed DC source operating at 1% duty cycle.

17. ### ashkarmalik Thread Starter Member

Jul 1, 2012
56
1
How is that done?

18. ### crutschow Expert

Mar 14, 2008
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4,442
True. I don't make the rules, just report them.

19. ### ian field AAC Fanatic!

Oct 27, 2012
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Probably the simplest way is a modified CMOS inverter oscillator.

Start with the timing capacitor from input to Vss, but your feedback resistor is split into 2 paths by a pair of diodes pointing opposite ways, your resistor is a preset pot that allows you to select the lowest resistance path to one or the other diodes.

20. ### inwo Well-Known Member

Nov 7, 2013
2,441
315
How much accuracy is required?

An off the shelf transformer with the primary under driven limits the short circuit current.