status LED and LM317, any suggestions?

[mr_doyle]

Hello,

i'm going to build a power supply for pedals: i'm using isolated outputs with an LM317 based regulator for each pedal.

Now, if i had to add a status LED for each output, and considering that voltage for each pedal could be regulated by a trimmer/pot (so it's not fixed), how would you get power to the LEDs without adding specific regulators for them?

At the moment i'm wondering if could i use the 1.25 reference volts beetween Out and Adj, but i'm not sure if:
- it's enough
- will affect normal functioning of the regulator

Any help appreciated!

D.

[R.G.]

Now, if i had to add a status LED for each output, and considering that voltage for each pedal could be regulated by a trimmer/pot (so it's not fixed), how would you get power to the LEDs without adding specific regulators for them?

As I read your note, I think you're worried about not having enough brightness from your LEDs or alternatively, having the LED brightness vary a lot as you turn voltage up and down. Is that right?

If it is two things suggest themselves.
(1) there really isn't all that big a range of useful voltages on effects circuits. There are a few circuits that will work over a wide  voltage range, but many get just about useless if you turn the voltage down to hald.
(2) you can make LEDs constant brightness regardless of the voltage available. How? Drive them with a constant current instead of a resistor/voltage.

LEDs are current sensitive devices. Their brightness is proportional to current. We use a series resistor to limit current simply because it's easy and cheap. But you can use a current source.

One good current source is an LM317. A 317 in the TO-92 package makes a dandy current source all by itself. The way a 317 works is to regulate the current through the device to force a 1.25V difference across the resistor from the output to adjust pins. That resistor then sets the current.

A TO-92 317 can provide up to 100ma. So if you want 20ma through your LED, you set the resistor to 1.25V/0.02A = 62 ohms. Connect the LED between the adjust pin and ground, and supply your variable voltage to the input pin of the 317.

The 317 needs about 1.5V to regulate, the LED needs about 2V to glow at all, so you will have constant brightness down to about 3.5 to 4V.

There are constant current source circuits with lower minimum voltage, but they are more complicated.

[mr_doyle]

thanks R.G.,

so - if i get it right - you suggest to use a LM317 configured as current regulator in parallel with the pedal itself after the other LM317 configured as voltage regulator?

D.

[R.G.]

I'm assuming that you want a status LED, not a power LED. So yes, kinda.

You'd use a second LM317 configured as a current source, in series with a status LED. That 317/LED would come on whenever you put more than about 4V across it. That means that if you switched voltage to it whenever you engaged the effect, it would conduct, and you could use the same less-than-9V that you're regulating to with the first voltage regulator to power it.

[mr_doyle]

Thank you R.G.,

i like your solution quite a lot.

On a further note, i think that outputs will be probably setted to 9, 12 and 18VDC.

D.

[toneman]

Mr_Doyle wrote:
"i'm using isolated outputs with an LM317 based regulator for each pedal. "

Well, since each xfmr out is isolated,
after the rectifiers, but B4 the reg is where to put the LED.
That way, if U vary the adjustable output, the LED brightness
stays the same.
Just pick the LED dropping resistor for the full-wave rectifier,
or fullwavebridge rectifier output V.
What could B simpler??
staysimple
tone

[spudulike]

This is ok if you just want to know each isolated supply is working. By status do you mean an LED that lights only when an fx box is taking power from that output ?

I think you need a current-sense resistor for that.

[H.Manback]

I'm in the exact same trouble as the last post describes. I need to find a way to sense any level of current above 0 as it were. Problem is I think you need a specific IC for that, a high-side current sense amplifier to be exact. The trouble with using a regular opamp difference amplifier is the very bad CMRR unless you use EXTREMELY exact matched components.

Anyone have another idea on that? I tried using a small sensing resistor on the output, and then connecting the + and - inputs of a opamp on it, figuring it in theory has a gain of infinity. Any current going through the resistor and it should output positive (or negative depending on the connection of the + and -).

Unfortunately this didn't work, at least it didn't with a 4558 or a 741 opamp. The output goes all goofy when the input is the same as the supply I guess. Am I thinking in the right direction? Would it work if the supply voltage was greater than the output voltage (since I could use the unregulated voltage before the 317 as supply)?

Also could a single side supply opamp cure my problems?

[Nasse]

??? Something like what I have in my junk box? Old vero prototype of circuit that lights the green led when more than 10 mA goes trough the  diode?

A power diode, pnp small transistor, two 1n4148 diodes, three resistors, the white wire goes to ground???

[toneman]

There was a thread about CurrentSensing a month or so ago.
Same application--indicating when 1 effect was on,
when several effects were powered by one multi-out xfmr.
MayB someone can find the thread(?)
There was talk of current-monitor ICs, and CurrentLimiters.
As4 CurrentLimiters, the LM317 is the simplist/fastest/easiest.
stayBrightlyLimited
tone

[H.Manback]

Something like what I have in my junk box? Old vero prototype of circuit that lights the green led when more than 10 mA goes trough the  diode?

A power diode, pnp small transistor, two 1n4148 diodes, three resistors, the white wire goes to ground???

Almost but not entirely. The 10mA minimum is a bit too high I think since average effects barely go over that if not under (:?


The current limiting isn't the problem in my case, got a nice circuit that does it very well (two transistors two resistors)..

Power box + leds here is that thread btw

[mr_doyle]

Well, since each xfmr out is isolated,
after the rectifiers, but B4 the reg is where to put the LED.
That way, if U vary the adjustable output, the LED brightness
stays the same.
Just pick the LED dropping resistor for the full-wave rectifier,
or fullwavebridge rectifier output V.
What could B simpler??
staysimple
tone

yes, i've thought the same, but then i discovered a major prob: if - say - the 317 fails, i have a bright LED but no volts feeding the pedal.
now, assuming that anything could stop working, i'd like to put the LED at the end of the regulation, just before the cable plug.

D.

[spudulike]

This is from another forum and not designed by me I hasten to add.

(Thanks Soeren in case youre browsing)

[R.G.]

yes, i've thought the same, but then i discovered a major prob: if - say - the 317 fails, i have a bright LED but no volts feeding the pedal.

That's why I suggested the LM317 in the TO-92 package which would run from as little as 4V. You hook it across the effect circuit power, which is varied by the primary regulator. No power to the pedal, no LED.