calculating power

[JonoPashley]

why even justify him with a response BAS, he is obviously trolling at this point.

 

[ban-all-sheds]

Could be - I must admit I put his behaviour down to him being a tw@.

 

[davelx]

Some people should be left to wallow in their own ignorance. If they decide to try for a Darwin Award and remove themselves from the gene pool, so much the better for the rest of the species.

 

[ban-all-sheds]

Most electronic 'transformers' used for ELV lighting are more akin to SMPSs, capable of delivering a stable output over a range of input voltages, so the simple model of a fixed resistance load goes out the window.

I don't know why this has only just occurred to me - if they are SMPSs, capable of delivering a stable output over a range of input voltages, how could you dim them?

 

[Stoday]

if they are SMPSs, capable of delivering a stable output over a range of input voltages, how could you dim them?

Output stability and target output voltage are independent, as any fule versed in automatic control noze.

 

[ban-all-sheds]

They are indeed independent, but surely such independence would require a separate control signal if the output voltage were to be variable, otherwise the PSU would react to variations in supply voltage by maintaining its stable output voltage.

No?

 

[Stoday]

They are indeed independent, but surely such independence would require a separate control signal if the output voltage were to be variable, otherwise the PSU would react to variations in supply voltage by maintaining its stable output voltage.

Indeed so. There must, of course, be two controls. One is a voltage standard against which the output voltage is scaled. That could be a zener diode. The other is a voltage proportional to the output voltage required. That could be from a variable resistor.

You could, of course, build two circuits in series, one to control the maximum output voltage and the other to scale it according to the setting of a knob. Practically, both would be combined in one integrated circuit. Nevertheless, buried in that circuit would be two feedback control systems.

 

[Pensdown]

You've been doing 2330 for 2 years and you don't know the relationship between volts, amps and watts?

Thank you Bas. You would love to be a fly on the wall when our JTL training adviser pops in

 

[ban-all-sheds]

Indeed so. There must, of course, be two controls. One is a voltage standard against which the output voltage is scaled. That could be a zener diode. The other is a voltage proportional to the output voltage required. That could be from a variable resistor.

You could, of course, build two circuits in series, one to control the maximum output voltage and the other to scale it according to the setting of a knob. Practically, both would be combined in one integrated circuit. Nevertheless, buried in that circuit would be two feedback control systems.

That's my point - ELV lighting supplies don't have such a control. If using an external dimmer to lower the RMS voltage causes them to lower their output voltage then they aren't proper SMPSUs....

 

[Stoday]

That's my point - ELV lighting supplies don't have such a control. If using an external dimmer to lower the RMS voltage causes them to lower their output voltage then they aren't proper SMPSUs....

You're past your sell-by date BAS. Stuck in the last century.

Integrated circuit controllers for ELV convertors for TH lamps now have these characteristics and far more! Soft start to limit the inrush current of cold lamps, short circuit shutdown too.

Have a look at this typical Halogen Convertor Control IC Data Sheet

You can't, of course, save money by using a simple chip; once developed, complex chips cost the same as earlier simple ones.

Friends of BAS should observe the note on the bottom of page 1 of the linked pdf. It shows that BAS must have a brother in the electronics industry.

 

[electronicsuk]

That's my point - ELV lighting supplies don't have such a control. If using an external dimmer to lower the RMS voltage causes them to lower their output voltage then they aren't proper SMPSUs....

I wasn't aware that a stable output voltage from a varying input voltage was a requirement of a switch mode PSU. Anyway, a modern dimmer doesn't vary the voltage to a load (be it a lamp or transformer), as that would more or less make it a variac. Instead, PWM is used, so a SMPSU would be able to look at the mark/space ratio of the incoming PWM modulated AC mains, and approximate this to the required output on the ELV side for the lamp.

I suppose you could argue that if you approximate the output of a PWM dimmer to an RMS voltage, you could say that the output voltage is being varied. However, as the dimmer can only turn it's output on and off at varying speeds and durations, the peak voltage at the output will still remain the same, and the dimming is achieved by only passing current through the load for a set period of time every 1/50th of a second.

 

[Jaymack]

liampope:
1300 watt divide 230 = 5.7A
1300 watt divide 240 = 5.41A
the more voltage with the same current is LESS amps as there are more volts to divide the amps by!

Wrong!, the resistance or the impedance for a transformer, is fixed in this example, voltage up - current up etc.

Regards

 

[ban-all-sheds]

I wasn't aware that a stable output voltage from a varying input voltage was a requirement of a switch mode PSU.

Most electronic 'transformers' used for ELV lighting are more akin to SMPSs, capable of delivering a stable output over a range of input voltages, so the simple model of a fixed resistance load goes out the window.

Anyway, a modern dimmer doesn't vary the voltage to a load (be it a lamp or transformer),

Of course it does - if it didn't then a directly connected incandescent lamp would not dim.

Instead, PWM is used,

It could be, but is it?

Do mass-market consumer dimmers use PWM, or just a simple thyristor circuit which fires at a given point in the cycle?

so a SMPSU would be able to look at the mark/space ratio of the incoming PWM modulated AC mains, and approximate this to the required output on the ELV side for the lamp.

.
It could, but does it?

Do mass-market consumer ELV supplies do that?

I suppose you could argue that if you approximate the output of a PWM dimmer to an RMS voltage, you could say that the output voltage is being varied. However, as the dimmer can only turn it's output on and off at varying speeds and durations, the peak voltage at the output will still remain the same, and the dimming is achieved by only passing current through the load for a set period of time every 1/50th of a second.

In other words the voltage is reduced.

The only voltage we ever talk about is the RMS one. The value is actually the integral of the waveform - if parts of it are chopped off the area under the curve is reduced, i.e. the RMS voltage is reduced, even though the peak stays the same.

I suppose you could argue that if you approximate the output of a PWM dimmer to an RMS voltage, you could say that the output voltage is being varied.

It's not some semi-specious argument you could make - the RMS voltage is reduced, and that is an indisputable fact.

 

[ban-all-sheds]

Integrated circuit controllers for ELV convertors for TH lamps now have these characteristics and far more! Soft start to limit the inrush current of cold lamps, short circuit shutdown too.

Have a look at this typical Halogen Convertor Control IC Data Sheet

Cool - so it could be done - you could build an ELV supply which offered both stability of output in the face of variations in input peak voltage, and the detection of the waveform being chopped to signal dimming.

So the mass-market consumer ELV supplies made in China and sold in the sheds do this?

 

[SUNRAY]

Surely it must be obvious to any one with an understanding of electricery that to only way to control the brightness of a lamp is to change its current, ie the higher the current the hotter (and hence brighter) it burns. In turn the voltage across the lamp is directly related to the current and its resistance.

Therefore of course the RMS voltage across a lamp varies with its brightness and dimmer.