Yes, that's what I was wondering - just remembering my schoolboy electronics from a VERY long time ago... and that's about as far as it goes...
Do these litle lights need a "starter"?
- Thread starter Roger465
- Start date
Yes, that's what I was wondering - just remembering my schoolboy electronics from a VERY long time ago... and that's about as far as it goes...
Hmmm. I don't know about 'rated', but I seriously doubt that you'd be able to find a bridge rectifier which would not work happily and satisfactorily at a few dozen kHz, probably an awful lot higher.
If "overheat" means heat production, is this some sort of new physics? If the amount of light and heat being produced are the same, where is the additional 50% of energy usage going?
... and were you actually measuring the power going to the LED (rather than going to the transformer/SMPS)? If you measured the input power to a wirewound transformer (for the 50 Hz) and a SMPS (for the 40 Hz), then the difference (for same light/heat output from LED amp) could probably be explained by the SMPS being less efficient. ... and I also wonder how one accurately measures power with the rather odd waveform associated with an SMPS.
Kind Regards, John
I used a plug in power meter. It plugs into the mains and has a socket for the load. It will measure voltage, current, power, VA, and power factor.
It measures the input of the transformer or SMPS. WIth a 20 watt halogen lamp it reads 20 watts on both a transformer or SMPS. With 4 5 watt LEDs (so as to get the minimum SMPS load) it reads 20 watts on a transformer and 30 watts on the SMPS.
The extra power dissipates as heat in the lamps.
I've not deconstructed one but I imagine they have a bridge rectifier and a small SMPS inside, not a simple dropper resistor. If the bridge rectifier does not use switching diodes the switch off time will be too slow to operate efficiently at 40kHz.
It measures the input of the transformer or SMPS. WIth a 20 watt halogen lamp it reads 20 watts on both a transformer or SMPS. With 4 5 watt LEDs (so as to get the minimum SMPS load) it reads 20 watts on a transformer and 30 watts on the SMPS.
The extra power dissipates as heat in the lamps.
I've not deconstructed one but I imagine they have a bridge rectifier and a small SMPS inside, not a simple dropper resistor. If the bridge rectifier does not use switching diodes the switch off time will be too slow to operate efficiently at 40kHz.
What's its frequency response?
Don't know, but my supply is 50 Hz. If you are suggesting the current is 40kHz, forget it, the 40kHz is generated inside the SMPT.
Even if the current was 40kHz it would apply to both halogen lamps and LEDs anyway.
Unless it's a clever/expensive one which looks at instantaneous voltage and and current very frequently and then integrates the vector VA product to get power, it will be assuming a perfect sine wave current, which I imagine is probably not the case with and SMPS
Well, for a start, I thought you said that the 'heat' was the same. Whatever, how do you know that the heat is being dissipated in the lamps, rather than in the SMPS - did you perhaps measure the lamp temperature?
I have, in the past, deconstructed some LED lamps, and cheap ones often just have the rectifier and a resistor (and often a pathetically small reservoir capacitor). I also suspect that 'dimmable' ones may be the same (regardless of price), since that is presumably the easiest way to facilitate dimming.
It's true that the 'efficiency' of a bridge rectifier (indeed, any semiconductor diode rectifier) will decrease (i.e.'losses' will increase) with increasing frequency, so that if one wanted one to work at high frequencies, one would use fast-recovery (e.g. Shotkky) diodes. However, I have often used bog standard cheapo bridge rectifiers ('optimised' for 50-60Hz) at frequencies up to 100 kHz and beyond, with only modest losses, and would therefore not expect a lot in the way of losses at 40 kHz (or whatever). In any event, when there are losses, they result in additional heat being dissipated in the rectifier diodes, not in the load as you suggest (or is that perhaps what you meant when you said "in the lamp"?).
Kind Regards, John
...backs way slowly... 
But it is the case with the supply to the electronic transformer, which is what the meter is looking at.
Is it? I would have thought that the 'switching' undertaken in the early stages of an SMPS would result in the input current being far from a pure sine wave.
Kind Regards, John
Ah - I see what you mean.
But it shouldn't happen like that? Doesn't the 230V AC input get rectified and smoothed, and then that is used to supply an oscillator?
So the input current would be a sine wave?
But it shouldn't happen like that? Doesn't the 230V AC input get rectified and smoothed, and then that is used to supply an oscillator?
So the input current would be a sine wave?
Depends on the SMPS BAS. Some just chop the incoming supply and rectify the chopped waveform. Even the others will have a spiky current waveform, since current will only flow when the incoming voltage is greater than that in the smoothing capacitor.
Current will only flow when the incoming voltage is greater than that in the smoothing capacitor with a linear PSU as well.
Yes.
In that case, it would presumably be pretty close. However, as stillp has said, I think that a true SMPS simply chops the supply (the clue is in the first 'S' of SMPS), which will certainly result in a supply current which is not a pure sine wave.
A lot of the devices are physically pretty small and I doubt there would be enough space inside them for a capacitor large enough to do an appreciable amount of 'smoothing' at 50 Hz, and an oscillator would probably not be very happy running off a very-poorly-smoothed supply. I therefore suspect that the small ones usually are true ('chopping') SMPSs.
Since it's not using any chopping/'switching', I suspect that a device such as you've described is not correctly called an 'SMPS'. Edit: I think I would be inclined to call it an 'inverter', or something like that.
Kind Regards, John
Last edited:
In many cases nothing like one.
A lot of electronic items have hideously distorted current curves for the input.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
