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Old 10-29-2018, 02:05 PM   #1
EasyGoing1
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Default I Don't Understand

Can someone explain to me why this circuit:



Would output voltage that looks like this? That was measured across all three LEDs

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File Type: png AR0nNDu.png (70.9 KB, 146 views)
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Old 10-29-2018, 03:21 PM   #2
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Default Re: I Don't Understand

not unless you upload it - i cant see anything.
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Old 10-29-2018, 03:53 PM   #3
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Default Re: I Don't Understand

If your input voltage is pulsating DC, like from a bridge rectifier output, then a ripple filter/capacitance multiplier will not work.

During the 120Hz pulsating DC low spots, something has to store energy to give to the LM317 and LED load.
This is usually a big filter capacitor like 1,000uF up. You would have to add one to get no flicker.

The little 47uF cap gives the transistor's base nice smooth DC but when the input voltage waveform goes to zero, there's nothing to power the LM317.

A capacitance multiplier/ripple filter works on the assumption the ripple never dips below the lowest system voltage.

Last edited by redwire; 10-29-2018 at 03:55 PM..
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Old 10-29-2018, 05:39 PM   #4
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Default Re: I Don't Understand

What is the purpose of the transister in this circuit with a LM317 in current mode
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Old 10-29-2018, 06:22 PM   #5
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Default Re: I Don't Understand

Quote:
Originally Posted by sam_sam_sam View Post
What is the purpose of the transister in this circuit with a LM317 in current mode
maybe supply too high voltage for the regulator ic ? can only guess .
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Old 10-29-2018, 09:27 PM   #6
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Default Re: I Don't Understand

Quote:
Originally Posted by stj View Post
not unless you upload it - i cant see anything.
Fixed that for you.
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Old 10-29-2018, 11:31 PM   #7
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Default Re: I Don't Understand

Quote:
Originally Posted by sam_sam_sam View Post
What is the purpose of the transister in this circuit with a LM317 in current mode
Capacitance multiplier as indicated by redwire: https://www.radio-electronics.com/in...er-circuit.php
https://www.electronics-notes.com/ar...er-circuit.php
OP did not show what kind of DC source is.
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Old 10-29-2018, 11:38 PM   #8
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Default Re: I Don't Understand

Quote:
Originally Posted by sam_sam_sam View Post
What is the purpose of the transister in this circuit with a LM317 in current mode
Take a look at this video (the link should start you at time 10:13)
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Old 10-30-2018, 12:30 AM   #9
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Default Re: I Don't Understand

Quote:
Originally Posted by budm View Post
OP did not show what kind of DC source is.
So I had assumed that my source was straight DC ... HOWEVER, here is the source voltage which I just now saw on the scope for the first time (and I'll explain why I didn't look at it until now)



I was hoping to avoid discussing the details of this circuit because I didn't want to waste anyones time, but apparently, I overlooked something critically important as can be seen in the above image.

Here is what I'm attempting to accomplish: I have this clamp on magnifying lamp that uses LEDs (90 of them) ... but as time has gone on (less than a year actually), the LEDs have been dropping out one by one, and it's to a point now where the brightness is no longer useful to me, so I intended on replacing the array of LEDs (which I mapped out the circuit board and discovered that the power supply built into the lamp I measured at 37 volts (with a straight meter so that would have been RMS) feeding 10 parallel LED arrays each consisting of 9 LEDs in series. The current for all 10 circuits was roughly 168ma bringing each branch down to about 16.8ma ...

Anyways, I want to replace the built in LED board with some super bright LEDs that only need 250ma to power (but are quite bright at 150 to 180) ... and now that I'm thinking this through while I type it out ... I think if I just put a bunch of those LEDs in series, I wonder if that built in power supply would just feed that series 168ma ... and if I used enough of them to consume the source voltage, I should be in good shape without using any additional circuitry.

Apparently what I've managed to accomplish with this circuit, is I slowed down the pulse width .....
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Old 10-30-2018, 06:38 AM   #10
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Default Re: I Don't Understand

your regulator has no ground reference - it wont work.
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Old 10-30-2018, 10:41 AM   #11
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Default Re: I Don't Understand

That LM317 is set up as 139mA (1.25Vref/9 Ohms) constant current source not as Voltage regulator.
The LED is current device, the Vf will vary with the If and temperature.
So what is the spec of the LED and how many LED are you going to put in series? Lets start with that first.
Then you also need to consider the power lost and power dissipation for the constant current source circuit.

Last edited by budm; 10-30-2018 at 10:52 AM..
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Old 10-30-2018, 11:00 AM   #12
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Default Re: I Don't Understand

why not just use a real led driver from LT ??
https://www.analog.com/en/products/p...driver-ic.html
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Old 10-30-2018, 12:02 PM   #13
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Default Re: I Don't Understand

Speaking more generally, oscilloscope pictures are easier to interpret if the amplitude and time scales are included, and also where the zero volts reference is.

The LM317 is being used as a constant current source. It will source whatever current will cause a 1.25V difference between pins 2 and 1 in the OP diagram. It is still a dissipative method, so the IC may need a heatsink, and if the voltage across pins 2 and 3 is high enough, the IC may power limit.
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Old 11-03-2018, 06:31 AM   #14
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Default Re: I Don't Understand

Quote:
Originally Posted by budm View Post
That LM317 is set up as 139mA (1.25Vref/9 Ohms) constant current source not as Voltage regulator.
The LED is current device, the Vf will vary with the If and temperature.
So what is the spec of the LED and how many LED are you going to put in series? Lets start with that first.
Then you also need to consider the power lost and power dissipation for the constant current source circuit.
150ma with 9 of them in series ... my voltage source is AC 120 with not much space to mount a circuit in so I need the circuit to be as small as possible ... the circuit that is in the lamp now has a small transformer and a bunch of surface mount components ... OH the LEDs are rated at 3 Volts.
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Old 11-03-2018, 06:51 AM   #15
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Default Re: I Don't Understand

capacitive dropper??
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Old 11-03-2018, 08:18 AM   #16
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capacitive dropper??
Never heard of that till now ... looks simple enough...
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Old 11-03-2018, 03:16 PM   #17
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150ma with 9 of them in series ... my voltage source is AC 120 with not much space to mount a circuit in so I need the circuit to be as small as possible ... the circuit that is in the lamp now has a small transformer and a bunch of surface mount components ... OH the LEDs are rated at 3 Volts.
So you are forcing 150mA through the LED.
But what is the current rating? The Vf rating of 3V is at what current being forced through the LED?
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Old 11-03-2018, 08:19 PM   #18
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So you are forcing 150mA through the LED.
But what is the current rating? The Vf rating of 3V is at what current being forced through the LED?
The LEDs are rated at 3V, 150ma - per the datasheet.
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Old 11-04-2018, 10:21 PM   #19
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Default Re: I Don't Understand

It is rated at 150mA max with proper thermal management that you can force through it before it is damaged but how much current are you forcing through it, the current source will force what ever current it can force through the device, the device will not limit the current, so when you force more current than the LED can handle that is when you blow them up. You can have 150mA LED that you can force less than 150mA or more than 150mA through it.
So did you actually measure the current flowing through the LED. You need to understand how the device functions. Look at the graph of Vf VS If to see what the effect of current it has on the Vf.

Last edited by budm; 11-04-2018 at 10:29 PM..
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Old 11-05-2018, 09:35 AM   #20
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It is rated at 150mA max with proper thermal management that you can force through it before it is damaged but how much current are you forcing through it, the current source will force what ever current it can force through the device, the device will not limit the current, so when you force more current than the LED can handle that is when you blow them up. You can have 150mA LED that you can force less than 150mA or more than 150mA through it.
So did you actually measure the current flowing through the LED. You need to understand how the device functions. Look at the graph of Vf VS If to see what the effect of current it has on the Vf.
So of the 9 series LEDs that I put in this lamp after removing the stock LEDs ... only 4 work ... but after reading your message, I threw an ammeter on it and it's drawing 170ma ... so I obviously fried the other LEDs and I'm over driving these ... of course that 170 is basically an RMS because we already know that the signal pulses so they are seeing much higher current at any given time ...

Here is a pic of the circuit that came with the lamp this is what Im using to drive these LEDs ... by the way I have a bunch of these LEDs as a got pack of 100 for $7 on Amazon...

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