my new whats my transformer qiuz
- Thread starter aveatry
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the components label 9R10 are 9.1 ohm resistors which when in series with 3 LEDs ( volt drop of 9 volts ) would have 3 volts across them ( 12 volt supply ) giving a current of 330 mA
But there are more LEDs / 3 than ther eare resistors
.
Maybe there are two or more strings of 3 series connected LEDs fed by one resistor ( 166 mA or 110 mA per string of three LEDs ) and relies on the LEDs being similar to obtain ressonable current sharing.
But there are more LEDs / 3 than ther eare resistors
.
Maybe there are two or more strings of 3 series connected LEDs fed by one resistor ( 166 mA or 110 mA per string of three LEDs ) and relies on the LEDs being similar to obtain ressonable current sharing.
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LED's do vary in the voltage across them in respect of the current flowing so you could put two sets in parallel using a single resistor for 6 diodes. However a LED gives around 100 lumen per watt, but if some of that power is converted to heat with a resistor it will as a unit drop to around 75 lumen per watt, a fluorescent tube as around 90 lumen per watt so to compete with other forms of energy saving lamps resistors are really not good enough. My small LED lamps vary from 60 to 85 lumen per watt, but the large one a 24 watt tube designed to replace a fluorescent tube is the full 100 lumen per watt.
The tube of course is not dimmable which would clearly allow the use of a pulse width modulated (PWM) driver however also my R7S replacement for a quartz halogen lamp is only rated at 80 lumen per watt, but the voltage is 85 ~ 265 and I can't see how without a PWM driver one could get such a huge voltage range? it uses 5050 LED's.
Looking at some of the roles of LED's which you can stick on some with colour changing the efficiency can drop to 25 lumen per watt still better than tungsten but clearly one has to be careful in selecting LED lighting. With 230 volt versions clearly they can still use simple capacitors to limit the energy but in general they seem to be better made than those sold to run on 12 volt. OK there are exceptions found some for caravans rated 10 ~ 36 volt and 100 lumen per watt DC only, but in the main I am moving away from 12 volt types.
The tube of course is not dimmable which would clearly allow the use of a pulse width modulated (PWM) driver however also my R7S replacement for a quartz halogen lamp is only rated at 80 lumen per watt, but the voltage is 85 ~ 265 and I can't see how without a PWM driver one could get such a huge voltage range? it uses 5050 LED's.
Looking at some of the roles of LED's which you can stick on some with colour changing the efficiency can drop to 25 lumen per watt still better than tungsten but clearly one has to be careful in selecting LED lighting. With 230 volt versions clearly they can still use simple capacitors to limit the energy but in general they seem to be better made than those sold to run on 12 volt. OK there are exceptions found some for caravans rated 10 ~ 36 volt and 100 lumen per watt DC only, but in the main I am moving away from 12 volt types.
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I quite agree, there is a lot of confusion with regards to LED drivers.
Let us look into constant current source, irrespective of voltage input, a constant current source would draw or supply a constant amount of current.
So if you had 20v dc input and were drawing 300mA, the circuit would still draw 300ma even at 10v dc, or even at 5v dc input, so a constant current is a constant current and does not vary unless you want it to vary by altering the reference against which the amount of current is set.
A constant current passing through an LED will not dim, even if the input voltage has a wide variation in supply voltage , so if input voltage changes from say minimum of 5volts to all the way to say 20volts then an LED light on a constant current source should provide a constant light output as the current does not vary.
All LEDs are current devices, that means they emit light proportional to the amount of current flowing through them and are not voltage dependent, by their nature when current passes through them they will have a voltage across them, and it is called Forward Voltage, for white LED this in the region of 3volts, so even if you were passing 10mA there will be 3v across that LED, and conversely if you were passing 100mA they will still have around 3volts across them, in other words LEDs can also regulate voltage and can be used as voltage limiting devices or used as voltage reference devices similar to zener diodes. Except Zener diodes do not emit light when they pass current.
But again for those not into electronics will get badly confused, how come? and they will swear that they can alter an LED's brightness by reducing or increasing the applied voltage. That is correct, if the LED had a simple current limiting resistor in series with the LED, since the amount of current passing through a resistor will vary according to voltage across the resistor, so when you increase or decrease the voltage the resistor will vary the amount of current passing through it and so with this current being varied you get intensity control.
So in other words the intensity of an LED can only be varied with current passing through an LED. Constant current source will not alter LED intensity unless the constant current source can be made to vary the amount of current setting, so you could set a constant current source at say 100mA and an LED may give you 300 lumens of light and you reduce the current it may give you 200 lumens.
DRIVERS, it is a term used for controlling the amount of current to drive an LED light, the circuit used does this and may be part of the LED itself or a small chip build inside an LED, or even an external board with parts, but the source of power or transformer is not a Driver.
Let us look into constant current source, irrespective of voltage input, a constant current source would draw or supply a constant amount of current.
So if you had 20v dc input and were drawing 300mA, the circuit would still draw 300ma even at 10v dc, or even at 5v dc input, so a constant current is a constant current and does not vary unless you want it to vary by altering the reference against which the amount of current is set.
A constant current passing through an LED will not dim, even if the input voltage has a wide variation in supply voltage , so if input voltage changes from say minimum of 5volts to all the way to say 20volts then an LED light on a constant current source should provide a constant light output as the current does not vary.
All LEDs are current devices, that means they emit light proportional to the amount of current flowing through them and are not voltage dependent, by their nature when current passes through them they will have a voltage across them, and it is called Forward Voltage, for white LED this in the region of 3volts, so even if you were passing 10mA there will be 3v across that LED, and conversely if you were passing 100mA they will still have around 3volts across them, in other words LEDs can also regulate voltage and can be used as voltage limiting devices or used as voltage reference devices similar to zener diodes. Except Zener diodes do not emit light when they pass current.
But again for those not into electronics will get badly confused, how come? and they will swear that they can alter an LED's brightness by reducing or increasing the applied voltage. That is correct, if the LED had a simple current limiting resistor in series with the LED, since the amount of current passing through a resistor will vary according to voltage across the resistor, so when you increase or decrease the voltage the resistor will vary the amount of current passing through it and so with this current being varied you get intensity control.
So in other words the intensity of an LED can only be varied with current passing through an LED. Constant current source will not alter LED intensity unless the constant current source can be made to vary the amount of current setting, so you could set a constant current source at say 100mA and an LED may give you 300 lumens of light and you reduce the current it may give you 200 lumens.
DRIVERS, it is a term used for controlling the amount of current to drive an LED light, the circuit used does this and may be part of the LED itself or a small chip build inside an LED, or even an external board with parts, but the source of power or transformer is not a Driver.
Indeed so. It's hard to be sure from the photos, but there are probably around 40 LEDs.
Indeed. Assuming that we're still talking about a 2.5W 12V lamp, that equates to a total current of about 208mA. Assuming 4 series/parallel sub-circuits, each with a 9.1Ω resistor (hence each carrying about 52mA), that could be achieved by, for example, each sub-circuit consisting of 3 sets of 3-LEDs-in-series in parallel (hence ~17mA through each string of LEDs, if current were shared equally) - making a total of 36 LEDs, with ~17mA through each string of LEDs (if current were shared equally), and therefore about 36 x 3 x 17 = 1836 mW, the other 664 mW being 'wasted' in the resistors.
Kind Regards, John
More strictly "... can only be varied with average current passing through an LED". LED dimmers will often 'chop' the output so that the current (whether determined by a true 'constant-current' source, or a simple voltage-dependent resistor dropper) does not flow for 100% of the time ('pulse width modulation'). In that way, the average current through the LED (hence brightness of LED) can be varied, even though the current, when flowing (for only some of the time) is constant.
Kind Regards, John
A perfect LED element will have the same voltage across across it irrespective of th current passing through it. But perfect LEDs do not exist and all LEDs have a resistive component through which the current has to pass, ( the resistance of the layers of semi-conductor materials that make up the LED element and known as the parasitic resistance ).
Many cheap LED assemblies be they strings of decorative ( Christmas ) lights or lamps with several LED elements rely on the LED elements being ( far ) less than perfect and use the parasitic resistance to evenly distribute current between paralleled elements. Some LED elements are probably designed tpo have a high parasitic resistance to ensure they work "properly" when paralleled.
OK, in that case I assume that each 9.1Ω resistor must serve four parallel strings of three LEDs in series, with ~13mA going through each string of 3 LEDs. Hence (assuming 3V per LED) ~ 48 x 3 x 13 = 1872 mW into the LEDs, and ~628 mW 'wasted' in the resistors.
Kind Regards, John
Totally agree John. Yes intensity of LEDs and many other things in electrics can be varied by pulse width modulation, meaning how long they are allowed to remain on and then off for a period in time so on off cycling can be used to control intensity, speed of electric motors, heat of electrical elements. etc etc.
So is that why it is 2.5w but only giving 15olm which is 60 lm per W
I have also just brought some gu10 (240v) which work out at 74 lm per W Im not sure how much the lenses come into play with that but the little g4 are bare leds without any help from a lens and giving 60lm per w. I have yet to compare these with the halogen 20w they are replacing but they are supposed to be the equivalent of a 20W. When I get the driver/transformer/smps/WW/magnetic/witch craft /magic/lord of the rings/Harry Potter/ Doctor Who sonic screwdriver power supply I will take some before and after pictures.
@ John, have you noticed how my mate's Enlite LED bulb provides constant light output from about 6v dc (using a bench power supply) to all the way to 16v dc,
and if that LED was running on a constant current source its current would not have gone up to almost 1100mA at 5volts from 420mA at 12v.
This means that Enlite LEDs are using an altogether different technique to drive their LED lamps, and as I said it radiates some noise on MW radio receiver so that would indicate some kind of switch mode operation to regulate the output power (voltage and current) from the input power applied, so if the input voltage is high, its converter will use less current and produce constant power to drive the LED, at 16volts it was drawing 300mA from the source.
Of course this can be achieved quite easily by applying a constant voltage to the current source driver for the LEDs, such that secondary voltage must remain constant, at a predetermined value, so take an example say there are 4 banks of LEDs and each bank has 4 LED s in series, 16 total LEDs, each bank will require a minimum of 12volts as each LED needs 3v forward voltage and each bank set to pass 100mA maximum at 12V, so 4 banks would need a total of 400mA at 12v.
Now to get this 12v constant, you would need a converter circuit that converts any input voltage from say minimum of 5 volts to all the way to 20volts and regulates it to 12v before sending it to the LED current driver.
and if that LED was running on a constant current source its current would not have gone up to almost 1100mA at 5volts from 420mA at 12v.
This means that Enlite LEDs are using an altogether different technique to drive their LED lamps, and as I said it radiates some noise on MW radio receiver so that would indicate some kind of switch mode operation to regulate the output power (voltage and current) from the input power applied, so if the input voltage is high, its converter will use less current and produce constant power to drive the LED, at 16volts it was drawing 300mA from the source.
Of course this can be achieved quite easily by applying a constant voltage to the current source driver for the LEDs, such that secondary voltage must remain constant, at a predetermined value, so take an example say there are 4 banks of LEDs and each bank has 4 LED s in series, 16 total LEDs, each bank will require a minimum of 12volts as each LED needs 3v forward voltage and each bank set to pass 100mA maximum at 12V, so 4 banks would need a total of 400mA at 12v.
Now to get this 12v constant, you would need a converter circuit that converts any input voltage from say minimum of 5 volts to all the way to 20volts and regulates it to 12v before sending it to the LED current driver.
That's obviously part of it. If my guesstimates are roughly correct, then only 1.872W is actually 'getting to' the LEDs (the rest of the 2.5W being 'wasted' in the resistors), so that would be about 80 lm per W in terms of the LEDs themselves. Using resistors as the current-controlling method inevitably results in such wastage/inefficiency, reflected in an appreciable deterioration in the overall lm per W (that being supplied Watts) figure. Other means of current control can theoretically be much more efficient.
Kind Regards, John
Is the problem with these tiny tiny units that a better means of control would be too big to fit
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