I've just purchased a set of 3 outdoor low voltage halogen lights to provide some safety illumination for our somewhat gloomy path.
Obviously I don't want these to be lit all the time so I looked for a timer switch. I found one that worked with inductive loads as the low voltage halogens are run from a transformer.
This transformer has no useful information on it that I can see. It only says that its output is 12V, 30W which is exactly what I'd expect as each of the lights in the set of 3 is a 12V 10W halogen.
The timer switch says it will switch 13A resistive or 5A inductive loads. I'm pretty sure the transformer will present an inductive load but what about the total loading? What current is this transformer likely to draw from the mains in order that it can supply 30W at 12V (that would be 2.5A @
12V, no?).
Is this timer switch likely to be sufficient or should I look for another solution.
A loaded transformer isn't all that inductive. The warning is about motors and choke-driven fluorescents. There will be a switch-on surge, but largely due to the incandescent lamp filaments i.e. of the same order as mains halogens. There's an additional surge with toroidal transformers, but yours is unlikely to be one of those.
The problem with inductive loads is that turning them off in zero time while current is flowing will generate an infinite voltage, so switches will arc over to increase the turn-off time to the extent that the voltage is just high enough to maintain the arc, if you see what I mean.
A heavy resistive load on a transformer (i.e. watts of a substantial fraction of the rated VA for the transformer) will damp the process considerably, dissipating the small amount of stored energy without trying to keep it whizzing around an inductor.
Not really, the impedance of the filaments is small compared with the reactance of the choke, and it's the latter than determines the current in the circuit. The filaments only drop about 6 V each, IIRC (Andrew Gabriel will soon be along if I've got that wrong).
As with any circuit that involves a shunt winding across the supply - transformer primary, motor stator, fluorescent choke, etc. - there may be a transient at switch-on, as the magnetising current adjusts itself into the correct phase relationship with the supply voltage. What this does is to superimpose a unidirectional ("DC") transient on to the steady-state current waveform. If the circuit is linear and the inductance constant, the transient component of the current will decay exponentially with time constant equal to L/R. However in practice the circuit isn't linear because the transient current takes the iron core into saturation, lowering the inductance and greatly increasing the effect of the transient. Thus however resistive the circuit may look under steady state conditions, there is always the scope for burning switch or relay contacts at switch-on, or even welding them closed.
It's more prominent with toroidals, but affects any iron-cored transformer (see above). Also many 12 V lighting transformers are in fact toroidal - or used to be; most of them are electronic now. The electronic ones have their own inrush current transient, charging the reservoir capacitor.
The voltage would never be infinite, even without arcing, because stray capacitance is always present in a real circuit and will limit the rate of voltage rise.
Yes, the time constant for the switch-on transient is determined by the primary (magnetising) inductance and its winding resistance, whereas at switch-off its the leakage inductance and load resistance (as seen on the primary side) that matter.
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