Resistive Current Limiter

Search high current resistors on Google

i may be misreading your post...please check :

1. You want to limit the current to 4.8A so R = 240/4.8 = 50Ω

2. You want to know the power which is I²R = (4.8)² x 50 = 1152 W so, to be safe you need say 1.25 kW ... _{gosh, that is high !}

So where does the 100Ω come in?

Since you say inrush, it may be a short-duration one, the wattage can be lower, depending on the duty cycle.

Hi,

The reference to the 100 Ohm resistor was an error on my behalf. It was meant to be 100 watts.

Ah...bad news then eh? 1.25kW would be massive. What is your duty? If it is 10s On, 30s Off, you may need only 100W (just a guess, don't hold me to it)

Perhaps you mistyped 30sec, where as you were intended to say somewhere near 100 sec.

Actually, i wasn't really meaning any real numbers....just giving the OP a feel of intermittent duty.

i have done some programming to calculate the intermittent ratings of resistors but that requires the time-constant for making any sense.(T = [Tm x e-(t/θ)])

Now, who would have that ? Very unlikely here, going by the posts so far.

You can use a 1 KwW Heating element used in domestic heater, or Buy a 50 Ohm 5 Amps Rheostat.

There are many ways to do this. A safe way would be to parallel multiple power resistors to split the power dissipation between them. For example if you purchased these 130 watt, 1500 ohm resistors, and wired 30 of them in parallel, you would have a 50 ohm, 3900 watt load. A very adequate safety margin, albeit a $300.00 solution.

Are you still using this basic schematic? Did you set up the shorting contactor like I suggested back in October?

If so, the current limiting resistor need only be in the circuit between the 240V input and the top terminal of Variac #2, basically just to limit the inrush through the portion of winding used in Variac #1 because #2 will already be inherently limited as well. So do you want to limit that inrush to 4.8A then? And if so, how often is it going to be energized? Seems to me this is for a test bench application. Even a moderate wattage resistor may be able to handle that once a day or even once an hour. I also like the idea of using a heating element as a resistor. A "strip heater" is a good choice because it has a bigger surface area which means your watts desity will be lower and safer if someone touches it.

Didn't want to go with the NTC thermistor eh?

The heating element sounds a good bet; cheers. There would be no duty cycle as such. The device would be swithed on and left on for a time period anywhere between 30 seconds and 5 minutes, however, the off period would be totally random.

BTW the load (output) is a 20kV transformer.

Yes, the basic layout of the two variacs is as in the drawing. The parellel switch is installed in parallel with the 240 volts input; just need this resistor I am questioning about now. I didn't go with the NTC Thermistors because they obviously require a cooling down period to restore their higher resistance and thus their function.