Hello, I’m new to this forum so please excuse me if I screw up some valued protocol or something.


A little about myself…


I’m 61 years old, self-employed, and have been enslaving electrons to do my bidding since I was a teenager, have some formal training in electronics, but mostly self-taught doing hobby work.


Anyway, my question is which method would be better for a LM317 regulator.


What I have is 10 white LEDs being driven by a LM317 thru dropping resistors, which limit the current to approx. 100ma each @ 10 volts. (12 volts minus the dropout and emitter-collector drop of the drivers) More or less…


These LEDs are set to a dim level most of the time by R2 (regulator)


What I want to do is flash these LEDs to a brighter level every so often at something like 3 Hz give or take, so the two methods I can use are. (I’m sure there are more)

1. Bypass the regulator with the higher voltage. (directly to the output IE: the anodes of the LEDs)
   
   
2. Tie the 555 output directly to the adjust pin using a diode. (not an issue if the LEDs don’t receive full voltage)
   

Concerns are abusing the regulator with either one of these methods, I have built many linear power supplies during my years, but never used one quite like this.


I know bypassing the regulator is probably not an issue, but would require using a transistor with at least one amp current rating…not a big concern, but the other method is simpler.


Thoughts?

 

First thing to consider is that descriptions referring to something like "R2" are meaningless to anyone unless you show us the schematic that has R2 in it.

Post your schematic and also a bit more detail about what you are trying to achieve. Perhaps the most important piece of additional information is how much current you want to put through the LEDs when they are being driven to a higher output.

What is your current set up? Do you have ten parallel loads each consisting of an LED and current-limiting resistor?

Are you talking about driving the adjust pin on the regulator with the output of the 555? I would strongly recommend against doing that unless you very carefully take into account how the regulator works. There are certainly ways to incorporate the 555 into that part of the circuit, but it must be done knowledgeably.

Notice how most of these questions become unnecessary if you show the schematic of what you have and preferably also for the two options you are considering.

 

Hello, I’m new to this forum so please excuse me if I screw up some valued protocol or something.

Welcome to AAC!

Your post would be easier to follow if you included a schematic.

LM317 is rated for about 1A, which your 10 LEDs at 100mA would draw. To determine if power dissipation is an issue, we need to know the input voltage to the regulator.

To know whether your LEDs are capable of being pulsed at a higher current, we need to know the specs for the LEDs and the dim current .

 

Ok, sorry about not including a schematic.

I don’t really need any advice on whether the regulator can handle the load or resistor values etc…I just need to know will either one of these configurations can damage the regulator.

The 555 is set to go low every 30 seconds and flash the LEDs @ about 3 HZ for 3 seconds or so.

 

Both proposed methods have issues.

Method #1 will just stress the regulator and transistor. Ideally, the regulator will maintain it's output at the set voltage (this isn't shown in your schematic, but was stated as 10V your first post), but LED brightness will decrease when the regulator enters safe mode. The transistor would dissipate a couple watts when initially turned on and dissipation would increase when the regulator enters safe mode.

Method #2 will reduce regulator voltage to about 2V when the timer is off and have no affect when it's on.

Dropout voltage for the regulator will likely be more than 2V at 1A. This is one of the reasons I asked for the input voltage.

A schematic without component designators and values for every component is incomplete.

 

Ok, sorry about not including a schematic.

I don’t really need any advice on whether the regulator can handle the load or resistor values etc…I just need to know will either one of these configurations can damage the regulator.

The 555 is set to go low every 30 seconds and flash the LEDs @ about 3 HZ for 3 seconds or so.

View attachment 140751

Arranging the 317 for current regulation would probably be better for LEDs, an extra resistor from the control input to GND and switched in by a transistor would change the set current value.

 

Using the regulator in CC mode is not an option, but thanks for the suggestion.


So method #1 will cause the regulator into safe mode…I don’t understand why, can you explain please?


If this is so, that option is off the table.


And…oh my gosh…I was thinking totally ass backwards on method #2…I was thinking bypassing R2 would put out full voltage (minus dropout) Thanks for that info…


So if I keep a resistor bypassed with a transistor, then turn that off and on…that will work? See my original concept was to use the adjust pin as the voltage control, but I started to worry about strobing that pin. (I know it can be controlled, maybe just not at that rate.


I will get a revised schematic and post it later…thanks so far.

 

There is a reason I asked for the desired current levels and component values.

Let's say that your white LEDs drop 3 V at 100 mA. That means that you have 7 V dropped across the current limiting resistor. If you succeed in getting to 12 V, you would have 9 V instead, meaning that your current would increase by less than 30% and, given the logarithmic nature of the response of the human eye, would not represent much in the way of perceived increased brightness.

Have you determined the current levels you need for each brightness level experimentally? If not, I'd suggest that you do that as your next step.

Your first approach won't even increase the output to 12 V, but rather to about 11.8 V or something near there. Your output will also be unregulated. You must be sure that your 555 output goes up to near 12 V in order to turn the transistor off, also. I don't think that you will damage the regulator by bypassing it this way since I don't think it has the ability to sink current from the output if the output voltage is too high, it will just refuse to source any current. I could be wrong on that.

The second approach will reduce the output voltage to about 1.25 V.

A better approach would be to use a transistor to switch a second lower-valued resistor in parallel with the current limiting resistor in each leg. This can be done with a single transistor.

 

No the idea is to set the LEDs to a very low brightness using a pot on the regulator, then letting them see the 11.8 volts thru the transistor.

No need for regulation at this point, only the dimmed state.

But dl324 seems to believe the regulator will go into safe mode.

Revised method #2 will work in theory, but I was worried about the rapid change.

Placing parallel resistors is not an option because the board has no room, but bypassing a single resistor in series with the array could be an option.

 

Hello,

You might want to have a look at the schematic fom the LM317 datasheet:



Bertus

 

Yes, I understand how a 317 works, I was only worried about the RAPID switching...

Thanks anyway.

 

No the idea is to set the LEDs to a very low brightness using a pot on the regulator, then letting them see the 11.8 volts thru the transistor.

No need for regulation at this point, only the dimmed state.

But dl324 seems to believe the regulator will go into safe mode.View attachment 140773

Revised method #2 will work in theory, but I was worried about the rapid change.

Placing parallel resistors is not an option because the board has no room, but bypassing a single resistor in series with the array could be an option.

You would need a separate resistor for each string, otherwise you invite thermal runaway. Though there's a way that you could put a single resistor and transistor in series with your strings that should work.

But a better way would be to adjust your bright current limiting resistance to something that can be provided by the 10 V regulated output and then use a transistor to switch in another resistor into the regulator's feedback circuit to reduce the voltage to what you need for the dimmed state. This is essentially what your new method #2 is doing.

 

No, it would look something like this, I have done this in the past to tone down green LEDs.

 

LEDs, 555s, Flashers, and Light Chasers

 

So method #1 will cause the regulator into safe mode…I don’t understand why, can you explain please?

On second thought, it could work. I looked at the schematic for LM317 and it won't sink any current. So pulling the regulator output to 12V looks like it could work without stressing the regulator.

 

No, it would look something like this, I have done this in the past to tone down green LEDs.

View attachment 140781

That's the basic idea. Now put a transistor in parallel with that single resistor and when you turn on the transistor it effectively shorts out the resistor making all of the LEDs brighter.

If you put the single resistor between the LEDs and ground it will make it easier to control the transistor with a logic signal.

 

Ok, so I was just going over some of my old project notes, and I found the schematics for a TEC project where I used a 117 to control fan speed, and sure enough…I used a SPST switch to bypass the regulator to get full speed on the fans when needed…so yea, I don’t think there will be a problem using a transistor to do a bypass.

(I can’t for the life of me figure out why I didn’t remember that, considering the cooler is sitting right next to me all day long at work…must be the age)

I’m going to go that route.

So thanks for the replies….