Surge Current
The cold resistance of a halogen studio lamp is approx. 1/17 of its value in normal operation. On switch on, theoretically a surge current of 17√ 2 x the normal current would flow and depending on the thermal mass of the filament* this will fall to the lamp normal current in approx. 1 sec. In practice this maximum theoretical current does not appear due to (a) switch on does not always occur at the peak of the AC voltage, (b) the supply has some impedance which is comparable with the cold resistance of high wattage lamps, i.e. maximum possible surge current where V is the applied voltage and Z is the sum of the lamp cold resistance and the supply impedance.
Typically supply impedance is the order of 0.3 ohm and lamp life is based on testing with such a supply. In the rare cases where the line impedance is lower than this figure, an adverse effect on life may be encountered particularly with high wattage types, due to the then extremely high surge current on switching.
lamp | type | cold | max. surge current (amps) |
|
|
| normal | |
|
| resistance | line impedance = |
|
|
| operating | |
|
| (ohms) | 0 ohms | 0.1 ohms | 0.3 ohms | 0.5 ohms | current | |
240V | 10kW | 0 | .34 | 1000 | 774 | 530 | 405 | 41.5 |
240V | 5kW | 0 | .7 | 486 | 424 | 340 | 283 | 20.8 |
115V | 5kW | 0 | .15 | 1085 | 650 | 360 | 250 | 43.5 |
240V | 2kW | 1 | .7 | 200 | 189 | 170 | 154 | 8.35 |
117V | 2kW | 0 | .41 | 404 | 324 | 233 | 182 | 17.1 |
240V | 1kW | 3 | .4 | 100 | 97 | 92 | 87 | 4.15 |
