Home Energy Monitor misreading SwitchedPowerSupply to Laptop

DVardysShadow

grahamc2003 wrote: »

When dealing with AC, multiplying voltage by currect gives VA (VoltAmps). If the current and voltage are in phase, you have a power factor of 1, if they are out of phase (due to mainly inductive circuits), then you have a power factor less than 1. The problem with this is that you pay for more power than you use, the difference (the reactive power, which really does exist) being pumped back into the mains.

Not true. The billing meter takes power factor into account

grahamc2003

DVardysShadow wrote: »

Not true. The billing meter takes power factor into account

Thanks for correcting me - looks like I've been mislled as to what component of power domestic meters measure, probably from reading guff the manufacturers of domestic power factor correction devices say.

I have several 12v reading lamps, with quite a heavy 12v tranformer built in. Each used to have a 20w mr11, now they have a 1w power led (a very good replacement in this instance). My plugin power meter (which, according to the specs, is quite accurate) tells me the 1w bulb (+transformer) takes 4.5w at a power factor of 0.45 (I guess the tranformer wasn't designed to drive leds).

I had assumed until now, that 4.5va was the apparent power, with the real power consumption of the device (trans+bulb) being about 2.2w, and about 2.3var, and that I was paying for 4.5w (being what I assumed the utiltiy company's meter measured). Great news if I'm only paying for 2.2w - is that correct?

DVardysShadow

grahamc2003 wrote: »

Thanks for correcting me - looks like I've been mislled as to what component of power domestic meters measure, probably from reading guff the manufacturers of domestic power factor correction devices say.

I have several 12v reading lamps, with quite a heavy 12v tranformer built in. Each used to have a 20w mr11, now they have a 1w power led (a very good replacement in this instance). My plugin power meter (which, according to the specs, is quite accurate) tells me the 1w bulb (+transformer) takes 4.5w at a power factor of 0.45 (I guess the tranformer wasn't designed to drive leds).

I had assumed until now, that 4.5va was the apparent power, with the real power consumption of the device (trans+bulb) being about 2.2w, and about 2.3var, and that I was paying for 4.5w (being what I assumed the utiltiy company's meter measured). Great news if I'm only paying for 2.2w - is that correct?

Correct in principle. Obviously with power this low, there will be inaccuracies. As for the VAr, on your figures, it is about 4VAr. Mostly the transformer, I imagine - try the transformer on its own with no bulbs

Whatdo

grahamc2003 wrote: »

(I guess the tranformer wasn't designed to drive leds).

Leds prefer to be PWM (pulse width modulation)driven as they have no inherent way of dissipating heat in the bowl. a simple divider circuit on dc will make them function but significantly reduces the lifespan of the component.

so if I drive an incandescant through a triac with PWM. what are the chances of my new service meter detecting the correct current usage. Sampling rate of the measuring equipment?

cheers for all the input


..