Power supply voltage drop

[skmdmasud]

Hi..
i made a led driver which has a max input of 20VDC and output 2A. I connected 5 X 350ma led = 1.75A which requires 17.5VDC and + 2 VDC for the driver, total requirement = 1.75A and 19Vdc. I bought a 220VAC to 15VAC transformer rated 2A. Connected a full wave bridge with capacitor to the transformer and got around 20VDC.

THE PROBLEM

When i connect my driver with led to the power supply its voltage drops to 15VDC and the leds become very dim. With 1 led it works fine. After some reading i understood that its because of load the VDC drops.

Is there any solution without replacing the transformer?

I shorted the output of the transformer and found out that it gives 2A and voltage drops to around 5VDC, which is way below the mentioned rating.

Regards..

 

[audioguru]

A 15VAC transformer has a peak voltage of 21.2V which is loaded by the bridge rectifier and DC load. If the load current is 2A then the transformer must be rated at 21.2V x 2A= 42.4VA. But your transformer was overloaded because its maximum rating is 15V x 2A= only 30VA.

I agree that your transformer does not meet its 30VA rating since the output should be 15.0VAC when it has a load of 2A AC.
When shorted, the transformer should have no output voltage but produce more than 2A.

Maybe your filter capacitor is too small to keep ripple from reducing the DC output voltage?

 

[MikeMl]

Post the circuit of your diode and filter capacitor configuration.

 

[alec_t]

Regardless of capacitor value, trying to get 19V at 1.75A (33.25W, ignoring diode losses etc) from a 30VA transformer ain't going to work; at least, not for long (until the tranny gives up the ghost). Buy a bigger tranny.

 

[audioguru]

ignoring diode losses etc)

Do not ignore the power used by the rectifier diodes. The voltage drop is about 2V and at an average of 2A is 4W.

 

[skmdmasud]

Post the circuit of your diode and filter capacitor configuration.

the configuration is very simple its 4 diodes and a cap 2200uf 24V in the +- output.

My transformer is over rated, later i used a 30VDC 5A with a LM350 and it works fine the led's turned me blind.. voltage drop was only 2VDC before the regulator. Although i dont like the LM350 in between as it wasting at least 5 watts as heat ((30-20)X.5(my Amp meter shows .5A?? there is something that i dont understand about it, i will post pics of it soon.??)).

Next time i will just buy transformer with high Amp rating to be on the safe side...

Thanks and Regrds

 

[skmdmasud]

Regardless of capacitor value, trying to get 19V at 1.75A (33.25W, ignoring diode losses etc) from a 30VA transformer ain't going to work; at least, not for long (until the tranny gives up the ghost). Buy a bigger tranny.

I was just thinking and got confused...i have 5 X 1W led = 5 watts this is what it should consume it i am correct...but as calculation shown above 19V X 1.75A = 33.25W and seems correct as well..

so what am i missing

 

[kubeek]

Post the whole diagram.

 

[kinarfi]

are you connecting the LEDs in series or parallel?
5 LEDa X 350ma / led = 1.75A which requires 17.5VDC says they are in parallel and each LED runs at 17.5 v
maybe it should read 5 LEDs X 3.5V / LED = 17.5 v at 350ma for series
Kinarfi

 

[skmdmasud]

5 leds are in series using a led driver. The total forward V is 15V or 17.5V and the total A is 1.75A. which comes out to be 30W.

On the other hand each led is 1 watt therefor 5 leds = 5W.

What i am wondering is am i loosing 30W or 5W ??



if i am using 30W then i guess incandescent bulbs are more efficient than these 1W leds

 

[kubeek]

Post the whole diagram. Also, what do you use to measure the current? Do you have an oscilloscope?

 

[audioguru]

Your LEDs have a forward voltage range from 3.0V to 3.5V so the average is 3.25V.
17.5V/5= 3.5V which is the normal maximum forward voltage for a modern white LED. If its operating power is 1W then its operating current is 1W/3.5V= 286mA, not 1.75A. If the typical forward voltage is 3.0V then their operating current can be 1W/3.0V= 333mA.
An average LED with a forward voltage of 3.25V can operate at 1W/3.25V= 308mA.

In your first post you said the LEDs can operate at a current of 350mA then their power rating must be higher than 1W.
We are talking about having 5 LEDs in series so the current is typically 308mA, not 1.75A.
You are talking about 1.75A so maybe your LEDs are in parallel (350mA x 5= 1.75A).

If your LEDs are in parallel then the one with the lowest forward voltage will draw much more current than the others and will burn out soon. Then the remaining LEDs will burn out soon. The power supply voltage needs to be much lower, about 5.5VDC, not 17.5VDC.
Since your LEDs are in parallel then a linear driver wastes (17.5V - 3.25V) x 1.75A= 25W making heat.

 

[skmdmasud]

Your LEDs have a forward voltage range from 3.0V to 3.5V so the average is 3.25V.
17.5V/5= 3.5V which is the normal maximum forward voltage for a modern white LED. If its operating power is 1W then its operating current is 1W/3.5V= 286mA, not 1.75A. If the typical forward voltage is 3.0V then their operating current can be 1W/3.0V= 333mA.
An average LED with a forward voltage of 3.25V can operate at 1W/3.25V= 308mA.

In your first post you said the LEDs can operate at a current of 350mA then their power rating must be higher than 1W.
We are talking about having 5 LEDs in series so the current is typically 308mA, not 1.75A.
You are talking about 1.75A so maybe your LEDs are in parallel (350mA x 5= 1.75A).

If your LEDs are in parallel then the one with the lowest forward voltage will draw much more current than the others and will burn out soon. Then the remaining LEDs will burn out soon. The power supply voltage needs to be much lower, about 5.5VDC, not 17.5VDC.
Since your LEDs are in parallel then a linear driver wastes (17.5V - 3.25V) x 1.75A= 25W making heat.

thanks, my leds are 1 watt so its V X I = 1 watt may be voltage is low or amp is low, which ever way its consumption is 1 watt.

Lets forget my circuit, leds my drivers etc and solve the problem below:

Lets say
Led forward voltage is 2.8v and amp is 350ma, which makes up = 1watt

5 led in series
Total Forward Voltage = 5 X 2.8 = 14V
Total Amp = 5 X 350ma = 1.75A
total Watt consumption = 14 X 1.75 = 24.5W DONE....


BUT since each led = 1 watt therefor 5 leds should consume up to 5 watts not 24.5w.

Hope my question is clear.

Regards

 

[audioguru]

5 led in series
Total Forward Voltage = 5 X 2.8 = 14V
Total Amp = 5 X 350ma = 1.75A
total Watt consumption = 14 X 1.75 = 24.5W DONE....

Absolutely NOT!
The LEDs are in series so they all pass the same 0.35A. Only if they are in parallel then their individual currents add.
The LEDs are 2.8V each (this time and there are 5 in series so they dissipate 14V x 0.35A= 4.9W.

Your supply voltage is too high to have the LEDs in parallel.

 

[kubeek]

Only if they are in parallel then their individual currents add.

And then the voltage across the parallel diodes is 2.8V.

 

[skmdmasud]

Absolutely NOT!
The LEDs are in series so they all pass the same 0.35A. Only if they are in parallel then their individual currents add.
The LEDs are 2.8V each (this time and there are 5 in series so they dissipate 14V x 0.35A= 4.9W.

Your supply voltage is too high to have the LEDs in parallel.

OK so now it makes sense. Thanks Audioguru.

 

[skmdmasud]

This morning i was thinking about the above and another thing popped in my head

Audioguru as you explained 5 leds should consume 4.9W and lets say my driver is 90% efficient. With 10% loss which rounds about 5.5W total consumption. Can i use a 20V .5amp powersupply to run my leds because 20V X .5ma = 10W ???

 

[ronsimpson]

Audioguru as you explained 5 leds should consume 4.9W and lets say my driver is 90% efficient. With 10% loss which rounds about 5.5W total consumption. Can i use a 20V .5amp powersupply to run my leds because 20V X .5ma = 10W ???

The power supply is capable of delivering 10 watts. The power supply does not have to deliver 10 watts. It is only able to. With 0 LEDs there will be 0 watts taken from the 10 watt power supply.

The 5 LEDs can only handle 5 watts total.

Example: It takes 50 horse power to move your car up a hill at 50mph. You have a 100 horse power motor. Even sitting at a red light you still have a 100 horse power motor doing nothing. Burning very little fuel.

 

[ChrisP58]

The 1 watt rating of the LEDs is just that, a rating. The maximum power dissipation that the manufacturer recommends that they be operated at. Though, to keep them happy at that much power, they probably need to be heatsinked.

They do not control or regulate their own power. You need to control the power by controlling the current you allow to flow through them. Allow to much current, and they will dissipate more than 1 watt, though maybe not for very long.

 

[skmdmasud]

Well theoretically it is working out. I must say the transformer i have is totally worthless.

My amp meter reading and everything makes sense now...

If you are interested here is what i made.

View attachment 68199
the circuit on the left and right are the 2 led drivers each runs 5 leds. the heatsink in the centre is my voltage regulator using lm350 cuz i am using 30VAC 5A transformer

View attachment 68200
the whole thing with leds showing off


View attachment 68201
the leds in heatsink on top of the assembled box (metal box is from a old tv modulator)