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Led connected to AC power

Led connected to AC power

Led connected to AC power

(OP)
Hello friends

I have a project in mind time ago and I did it reciently, it is to connect 6 leds (19.6v Dc @ 700mA each) I connected them in series to avoid excessive currect consumption, I'm sending the schematic so you see how exactly I did it.
My question is I think still need something else, it works well but I do not know if it will support the peak currents or overvoltage (even though I put a fuse holder in series of 1 Amp).

Someone expert in this area could tell me what to do next please.

Thank you for your time and consideration.

RE: Led connected to AC power

For starters remember that rectified 117V with a fullwave bridge results in 117 x 1.4 - 1.4V = 162V

This means 162V - [6 x 19.6V] = 44.4V

44.4V/700mA = 63 ohms

Pres = I2 x R
Pres = 0.72 x 63
Pres = 31W

Keith Cress
kcress - http://www.flaminsystems.com

RE: Led connected to AC power

(OP)
Thank you itsmoked for your time to respond.
I have another question:

(According with the ohms law) if I put a resistor of 63 Ohms this is supouse to have 700mA through it right?
But in testing is not the same, I put 2 resistors in series of 10 ohms giving an equal to 20 ohms and this is giving at least 800ma (giving more brightness), when I put a resistor of 25 ohms the current drops until 560mA (giving less brightness), well if we multiply the voltage for the RMS (root square of 2= 1.4142)less the voltage on the diodes (1.4v) this should be equal to 162v - 165v right? and with this you can obtain the resistor for the current consumed, but again in testing is not the same, I measured the voltage on the socket and is 117vAC.

Am I doing something wrong? why is different according to the ohm's law?

Thank you again for your response.

RE: Led connected to AC power

Yes, 162V out of the rectifier is the peak voltage, not the RMS voltage. So, using a 63 ohms resistor would give a peak current of 700mA, not an RMS current of 700mA. The RMS current is what sets the brightness, more or less.

The 20 ohms resistor gives a peak current of about 2.2A. You need to see what the maximum current your LED is rated for from the datasheet.

Keith's answer is correct if you put a capacitor filter after the rectifier.

 

RE: Led connected to AC power

Actually, the OSTARs are rated for 700 mA, but tolerate up to 1 A peak current. Forward voltage according to datasheet is 20.8 V, not 19.6

Do the following (just to give yourself an image of what's happening:
Draw a sine half-wave with a peak of 164 V.
Draw a horisontal line at 6 x 20.8 V + 2 V = 126 V (appr.)
You'll see that your peak voltage over the LEDs is 38 V, but ONLY for a very short time!

This means that with this design you can NOT design to the average current without damaging the LEDs, you need to design to the peak current of 1 A. This gives 38 ohms.
Your average current will be well below 700 mA, so you should reconsider your design.

Best Regards,

Benta.
 

RE: Led connected to AC power

(OP)
ok guys I appreciate your help but I'm afraid to say that I'm a little confused here.

When I tested the led with my regulated power supply it gives me the maximum brightness at 19.6VDC and 700mA, but I will consider the datasheet standard for 20.8VDC.
This leds behave like the fluorescent lights after a few seconds the brightness increase trying to consume more current and obviously I have to limit and I limited to 700mA and gives me 19.6vDC on my power supply.

So do I need consider 1A or 700mA as given in datasheet?
What value of capacitor should I put after the bridge?

Now considering what my friend "benta" said:

(This means that with this design you can NOT design to the average current without damaging the LEDs, you need to design to the peak current of 1 A. This gives 38 ohms.
Your average current will be well below 700 mA, so you should reconsider your design.)

is this means that the peak current of 1A will go through the resistor? how the average current will be below 700mA?

Thank you again for your time

 

RE: Led connected to AC power

There is a major difference if you use a storage capacitor or not. In you schematic none is present.

Benta.

RE: Led connected to AC power

You should design currents below the ratings of the elements! Otherwise this gadget gets toast in minutes! We call it factor of safety. If I wanted bright lights, I would go for higher rating LEDs. Still I will make sure that the amps are lower than the rating of the LEDs!

RE: Led connected to AC power

Are you OK with using an ~60W resistor in the circuit?

If you use a capacitor filter then you end up with the 63ohm, 31W resistor requirement as Keith posted.

 

RE: Led connected to AC power

What I would do:
Get a 100W rheostat that is maybe 75, or a 100 ohms. Use it in the resistor location.

Then I'd put a 'true RMS' digital multimeter (amps) in series with it all.  Turn down the rheostat until you achieve your 700mA.  This is with the cap!!

 Without the cap you need to instead monitor the current with a correctly utilized scope and a shunt resistor. (Note this can be very hazardous in your application.)

 Then again you adjust the rheostat but without the cap you must instead adjust the rheostat down until you reach the peak allowed PLED current.  The prudent person would then increase the rheostat position to be perhaps 10% (or more) below the peak allowed PLED current.

The RMS current would then be whatever it is. That is what you'd have to live with.

In all cases you will not have the brightness you could achieve with a proper PLED driver.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Led connected to AC power

I've checked my rectifier math on your first diagram:
Vpeak = sqrt(2) X 117 = 165.46 volts
Vrms = 0.637 X Vpeak = 105.4 volts (full-wave rectifier)
IMO, you won't get your LEDs very bright then!
Suggest to reduce the number of LEDs.

RE: Led connected to AC power

Hi burnt2x.

Please explain:

Vrms = 0.637 X Vpeak = 105.4 volts (full-wave rectifier)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Led connected to AC power

(OP)
I was thinking to use a capacitor in series with the bridge instead of the resistor but it turns it will be too expensive for this purpose.

I berely remember that equation burnt2x, we used in college in electromechanic time ago.

could you also explain what "IMO" means burnt2x?

Could anyone clarify and specify more the whole interpretation please?

what about when a rush current or overvoltage ocurr?
the capacitor in parallel will protect the circuit?

Thank you for your time and responses guys it is greatly appreciated.


 

RE: Led connected to AC power

The killer here is that the voltage has to be above 126V before you get any light output. Due to the nature of the sinewave, this only happens for a short time during each period. I haven't figured this out but it's likely amounts to the LED's being on < 50% of the time. Then, above this, the LED's are only on at full brightness for maybe something like 5% of the time, as the sinewave peaks and the current reaches 700mA.

In your first circuit, I was thinking the resistor has to be fairly high power but in reality it's only at full current and power for a short amount of time each cycle so something like a 10W resistor may even work.

If you want to continue persuing this, you need to look up capacitor filter supply theory and properly determine the capacitor size. Pay attention to the ripple current rating of your capacitor.

You are looking at providing approximately 85W of power to these LED's. A PWM switching circuit really the way to go.
 

RE: Led connected to AC power

(OP)
I'm using a multimeter Fluke III and a Greenlee clamp-ampmeter that measure peak current, min and max and relative current and according with this last diagram that I'm testing right now and the Greenlee multimeter read as follows;

In AC input
Relative current:  0.5 A
Min peak current:    -8A
Max peak current:     3A
Maximum  current:     1A

In DC output
Relative current:  0.5 A

As you see this diagram includes the capacitor of 47uf/250v.
The resitors are 20 ohms each connected in series in a TO247 package which is convenient for me.
What do you think about this?

I'm concern about the peak current probably my multimeter is bad?
I do not know but everybody is welcome to give an opinion on this project and thank you again.

RE: Led connected to AC power

(OP)
Sorry I forgot to upload the circuit.

And I forgot also to tell you that I put 2 stream of leds, though this will increase the current.
The capacitor according with spec sheet is 210mA rms at 105 degrees 120Hz. with a frequency coefficient of 0.90 @ 50Hz.

Here you go.

 

RE: Led connected to AC power

You completely lost me.

RMS voltage is calculated as the root of the mean of the squares. Squaring the samples removes the sign which means the RMS to Peak calculation for a sinewave or a rectified sinewave is the same. A sinewave with a RMS voltage of 1V is rectified to 1.41Vpeak and the RMS of this rectified unfiltered voltage is 1Vrms. If this isn't true then please explain why not?

I believe that the 0.637 multiplier is the peak to average calculation for a rectified sinewave. So, actually, you didn't completely lose me.

At any rate, last time I checked, you used the RMS voltage to calculate the equivalent DC voltage to get the same power in the circuit. Of course, with the LED's causing a non-linear operating characteristic it's not really true in this case.

 

RE: Led connected to AC power

(OP)
????????????????????????

LionelHutz, are you talking to me????

So far the last schematic with 2 rows of leds (6 each) is working, I left the circuit working all afternoon and all night yesterday and when I woke up still working, I measured it and I obtained the same readings than before, I'm a little concern about the capacitor, does anyone has a concern too? 210mA rms at 105²C ripple rate.

I just want this circuit works with no problem (brightness I do not care too much).

Thank you again
 

RE: Led connected to AC power

No, the guys above explaining that Vrms = 0.637 x Vpeak

But, did you measure the ripple current in your capacitor? To me, a 470uF capacitor seems too small for the power level you're handling but I'm not spending the time to actually calculate anything. Consider this though. When the output of the rectifier is 162V, the rectifier will be supplying all the LED current. When the output is at 0V, the capacitor will be supplying all the LED current. So, I expect the capacitor ripple current to be at least 700mA. Add the capacitor re-charging current and I'd expect even more ripple current.

 

RE: Led connected to AC power

(OP)
Thank you LionelHutz for your info, do not worry about making calculus after all we want to know how to fish right?
 
I search for a bigger capacitor and I found this one in Digikey which the ripple current is 900mA @120Hz and 250v and the value is 470 uF. Little by little I'm understanding this situation, still the last diagram is working and is about 4 full days working fine with no overheating.

I'm very thankful for your help guys, keep the good tracking.
God bless.


 

RE: Led connected to AC power

(OP)
Hello everybody again.

Well the circuit is running so far very well for about 3 weeks continously with out problems. Now my concern is about the surge protection, overvoltage and current limitance.

I made this diagram according to the datasheet and I hope you can tell me if this is ok or if Im wrong.

The first diagram is with surge protection and the second one is with surge protection and current control.

I hope you can tell me if this is ok or if Im wrong.

Thank you for your time.  

RE: Led connected to AC power

Well now only the first one shows.  Your file names are too long/weird.  I think that's the problem because the only one that shows up is the only one that actually shows a file extension.(jpg)

Well anyway the one with the added MOV will work.  Pointless but it will work.

Put the zener after the resistor and it will be much more robust.

Move the fuse to the legal location of right after the actual plug.  Where it is now won't stop the capacitor from burning the joint down.

 

Keith Cress
kcress - http://www.flaminsystems.com

RE: Led connected to AC power

(OP)
you right about the fuse, probably I was thinking that the MOV will protect the circuit.

The reason why Im putting a MOV is because I want to put outside my house as a light for my patio and just for protect from surges or lightning or overvoltages.

I hope this time the schematic can show, I change the fuse in this second schematic.

Thanks Keith for your evaluation.

RE: Led connected to AC power

I hope that ground symbol doesn't represent earth ground.

There should be a X2 cap each line to earth ground and an X1 cap between lines.

The zener voltage looks to low. Is that device actually rated for 220VDC or 160VDC?

What is the MOV rated voltage. I believe for 120VAC a rating around 180V is the minimum.

It might be a good idea to put some protection on that MOSFET. Maybe a gate resistor of around 50 ohms and a zener between the gate and source to keep that voltage within the device ratings.

 

RE: Led connected to AC power

(OP)
Hello Mr Hutz.

The zener is an unidirectional Diode TVS (transient voltage supressor) which has a 1500watts max and 231v breakdown

The voltage for the MOV is 224vDc max and 130V ACrms 175Vdc.

About the capacitors X1 and X2 I just copied the circuit from the datasheet of the MOV and for using a rectifier complete wave they suggest this same circuit. This capacitors appears without a rectifier in a AC line.

Thank you for the suggestion about the zener in Mosfet gate it will be good a 24v zener I think.

Here is the updated circuit thanks to you all.

RE: Led connected to AC power

(OP)
Keith, why you said the MOV is pointless? how you will protect the circuit?

I have another question, the TVS diode number is bi-directional, it should be unidirectional for DC? or both will work?

Thank you.

RE: Led connected to AC power

I said pointless because your whole "front end" is already doing what a MOV would do for you.  Your TVS on the other side of the diodes is plenty.

Putting a MOV in front of all impedances, but the power cord, also reduces its effectiveness greatly.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Led connected to AC power

(OP)
I see your point.

I get very nervous when a AC circuit comes to realize, I'm not accustomed to do this kind of circuits, my strong part is in electronics.

Still, the TVS diode should be uni-directional or bi-directional?

Thanks.

RE: Led connected to AC power

(OP)
Thank you Keith

By the way apologizes to everyone for my english, I'm still learning and I hope I did not make any stupid sentence.


Greetings.

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