3 LED's running of the Heater Supply?

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
686
Location
Orlando
i have these really cool control knobs w clear plexiglass around the edges w indicator numbers that i would like to use in my next build. what id like to do is put a 5mm LED under them so as you turn the knob, the number is lit up from underneath w/an LED. a total of 3 for the 3 control pots in my design. these will also serve as pilot lights letting me know the amp is on.

ive read up a little on how LEDS work and how to wire them and using a current limiting resistor, and from what i can tell, 3, 2v LEDS add up to 6v, @20mA total. (wired in series, the voltage goes up, the current draw stays the same. 2v/20mA each LED)
since the heaters are 6.3v (usually a little more) i figured this would be a perfect place to run the LEDs. ive got plenty of current available for another 20mA on my heater supply.
my quick schematic looks like this:

20221128_113156.jpg

note my heater supply doesnt have a C/T, so im using a humdinger/250 ohm trim pot for the heaters, and then AFTER the heaters, run the LEDs

i havent figured out the limiting resistor value yet, but im so close to the voltage on the supply, that resistor should be pretty low. a couple of ohms.

what concerns me is, will this affect my heaters? add hum? can i dial that out w/the humdinger? is there anything wrong with this circuit?
 

philosofriend

Tele-Afflicted
Joined
Oct 28, 2015
Posts
1,068
Location
Kalamazoo
I'm curious what someone who knows more than me has to say about this... but:
Your heaters are running on alternating current. Of course the LEDs are diodes and will only let current through in one direction. So the LEDs will be being powered only half the time. I have no idea if this will cause them to:
Not light up.
Be dim.
Blow up.
Put hum into the filaments by making the AC waveform be asymmetrical since they will only load one half of the waveform. But the hum trim pot should address this issue, if it is an issue.
Have strange effects because DC voltage is a steady voltage while AC is usually the RMS average of an ever changing voltage which peaks at a higher value than the RMS voltage.

Now that I am curious I might try it and see, knowing that I could blow the LEDs. At least I would start with a big limiting resistor.

The problem with powering them off the B+ (already DC) is you would be putting about a 6 watt increased load on the transformer. Since it is such a high voltage the limiting resistor would be a much higher value and in the end would draw the same 20 mA. 20mA x 300 volts = 6 watts. You would have a big hot resistor and the transformer itself would run a little hotter.

Being a total hick, I would just put a 9 volt battery in there to run the LEDs.

Good luck, your idea might work perfect!
 

Paul-T

Tele-Holic
Joined
Nov 23, 2018
Posts
745
Location
London
LEDs work fine off AC. They will be flickering at 60hz but that will look continuous to you - our brains fill in the gaps as they do for domestic LEDs. If they're 20mA than current/power draw is tiny. The only issue is if they breakdown when fed with reverse bias voltage (which I doubt they will, iirc it's not a problem below 20-30V or so).

You could make a little diode bridge to power them of course for smoother light.

I've never daisy chained three though. I think you'd be better having two in anti-parallel with a resistor to reduce reverse bias voltage - look at Merlin for ideas.
 
Last edited:

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
686
Location
Orlando
i did a search for LEDs on this forum, with limited results. @Jerry garrcia did a build w 1 LED fed from the 5v rectifier supply. big current limiting resistor from an already rectified supply using a 5v LED. another user @dougstrum said he used 2 coming off the heater supply, but didnt go into any details on how he did it. but it gives me confidence that it CAN be done.
fed off 1 leg of AC it will flicker at 60hz. same w your household christmas LEDs. from what ive read up on them, the more you have in the chain, (in series) the voltages add up, but the current draw for 1 is the same for all. so 3 at 1.7~2.1v each (lets say 2v to keep it simple) @ 20mA -in series- adds up to 6v @ 20mA. ill have to do the math for the limiting resistor, but it should be much at all. that part, im fairly confidant about.

what concerns me is what @philosofriend mentioned:
Put hum into the filaments by making the AC waveform be asymmetrical since they will only load one half of the waveform. But the hum trim pot should address this issue, if it is an issue.
Have strange effects because DC voltage is a steady voltage while AC is usually the RMS average of an ever changing voltage which peaks at a higher value than the RMS voltage.
 

Paul-T

Tele-Holic
Joined
Nov 23, 2018
Posts
745
Location
London
I've done a couple and simply used around 1000 ohm for current limiting resistors as that's what I had in the stash. Mine were alibaba LEDs supposedly for car dash use at 12V so it's possible they also have an integral resistor. They're still plenty bright. No added hum, they don't seem to produce any switching noise.

If it were me I think I'd do them in parallel each with limiting resistors (and possibly merlin's anti-parallel LEDs or diodes) just because htey have very low forward bias resistance and I wonder if that might produce more of a 50Hz current surge for half the cycle (but I don't know that for a fact).
 

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
686
Location
Orlando
I've done a couple and simply used around 1000 ohm for current limiting resistors as that's what I had in the stash. Mine were alibaba LEDs supposedly for car dash use at 12V so it's possible they also have an integral resistor. They're still plenty bright. No added hum, they don't seem to produce any switching noise.
If it were me I think I'd do them in parallel each with limiting resistors (and possibly merlin's anti-parallel LEDs or diodes) just because they have very low forward bias resistance and I wonder if that might produce more of a 50Hz current surge for half the cycle (but I don't know that for a fact).
im pretty sure that wiring in parallel would triple my current draw to 60Ma, and that would over exceed whats left on my heater supply. but again, its encouraging to read that someone else has done this already
 

tubeToaster

Tele-Meister
Joined
Nov 9, 2007
Posts
349
Location
Brentwood,Tn.
I don’t know if this will help but check out this Fender Pro Junior schematic. I’ve used a single LED on a couple of amps using this circuit. The two 47 ohm resistors are the artificial center tap (normally 100 ohm)
1669664616476.png
 

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,432
Location
Central California
Three red LED's in series will light up fine. There is only 3.15Vrms of forward voltage or 4.45Vp. If all three LED's were identical, then each one would have 1.48Vp across it. This is plenty for a brightly lit LED.

The maximum reverse voltage is usually 5V. The series LED's can never get more than 5V across each one (or the whole group, for that matter), so diode protection isn't necessary.

EDIT: That ain't right. It would have 6.3Vrms and 8.9Vp of forward voltage.
 
Last edited:

corliss1

Poster Extraordinaire
Gold Supporter
Joined
Sep 13, 2008
Posts
6,037
Location
Lansing, MI
And let's be careful when we apply generalizations. It's true that some LEDs might work fine by themselves on AC, and your brain won't see the flicker. But...what happens if you're in another room where there's something else flickering and they aren't in sync? You get the weird camera shutter speed effect, like when tires start and turn backwards on film...flicker.

It would be best to just make some pretty DC as outlined by @Peegoo and not have to worry about anything weird happening.
 

dougstrum

Friend of Leo's
Joined
Oct 6, 2015
Posts
3,974
Location
blu ridge mtn cabin
It's been 10 yrs or more and they are still working. I didn't think about it too much, just joined 2 LEDs in series then joined them to the heater wires and it worked.
These are what I used~
IMG_20221128_184521251.jpg

Here's how they look when the amps on.
IMG_20221128_184438966.jpg
IMG_20221128_184451342.jpg
 

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
686
Location
Orlando
using @tubeToaster 's Fender Pro Junior schematic, heres what i came up with:

LED Heaters.jpg

seemed like a good idea to add the extra diode to keep things in check, and with the resistor where it is to keep the circuit balanced. the "C" off the trim pot goes to the Cathode of one of the preamp tubes. the 15 ohm resistor is based on a 6.3v supply, 6v total forward voltage @ 20mA. if its too bright i can go up to 18, 22, 27... see what looks best and still keep it "on". im using a Red LED and it can run from 1.7 to 2.2V. 2 seemed like a good place to be.

from what @Jerry garrcia and @dougstrum have said, no problems with any added noise with the heaters, and unless your staring at it out the corner of your eye, no noticeable flicker.

if i were to add another LED, id have to use an enormous dropping resistor off the end of the B+ (another node) to get the voltage and current where it needs to be. but as it is, this will suit my needs.

thanx everyone.
 

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,432
Location
Central California
using @tubeToaster 's Fender Pro Junior schematic, heres what i came up with:

View attachment 1056104

seemed like a good idea to add the extra diode to keep things in check, and with the resistor where it is to keep the circuit balanced. the "C" off the trim pot goes to the Cathode of one of the preamp tubes. the 15 ohm resistor is based on a 6.3v supply, 6v total forward voltage @ 20mA. if its too bright i can go up to 18, 22, 27... see what looks best and still keep it "on". im using a Red LED and it can run from 1.7 to 2.2V. 2 seemed like a good place to be.

from what @Jerry garrcia and @dougstrum have said, no problems with any added noise with the heaters, and unless your staring at it out the corner of your eye, no noticeable flicker.

if i were to add another LED, id have to use an enormous dropping resistor off the end of the B+ (another node) to get the voltage and current where it needs to be. but as it is, this will suit my needs.

thanx everyone.
I don't like the regular diode and the 15R resistor going across the heater winding. That's going to take a big gulp of current every cycle. How about putting the regular diode in series with the LED's? That will still protect against reverse voltage and it won't add to the current.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,987
Age
63
Location
Finland
If you need only three LEDs you could "waste" fourth somewhere and install two series LEDs parallel with other two LEDs opposite direction and of course voltage dropper resistor.
 

Bill Moore

Tele-Afflicted
Joined
Feb 2, 2012
Posts
1,437
Location
Silver City, New Mexico
While working for our local power company, I built a low voltage lineman training aid, 120-208V from an automotive alternator. After the "pole" the voltage is reduced to 12-20.8 through transformers, and phase rotation is important! I couldn't find anything commercially available, so decided to build one from LED's adjusting the circuit from a neon unit. After bread boarding the circuit I tried it out, ABC LED was bright, but glowed dimly when connected CBA. There was a young engineer who beside his EE degree, had several more minors including electronics. I asked his to look at the circuit and see if he could see what I was missing. I didn't even get back to the shop and he called, he said, "what you're missing is the voltage is AC". "Stick a couple of diodes in the circuit, and I'll bet it works"! Sure enough, it worked fine after adding the diodes.
 

dougstrum

Friend of Leo's
Joined
Oct 6, 2015
Posts
3,974
Location
blu ridge mtn cabin
I had the back cover off yesterday and looked at how I wired the LEDs.
First I ran the heater wires to a terminal strip, used two diodes to build a rectifier for the LEDs and wired them in parallel.
I just continued the ac heater wiring as usual.
No noise no flicker just a simple way to get dc for the lights.
 

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
686
Location
Orlando
back to this again...
ive been thinking about this, and now im wondering which is better?

A)
20221205_094450.jpg

i was thinking about a bridge rectifier and using the LEDs as the diodes, but this has problems for me (i think?).
in each pulse, it goes through 2 diodes in series, with the 15ohm resistor between them. the 1st diode (or LED) has no current protection, but the 2nd does. that doesnt seem optimal.
also, the voltage and current draw confuses me. again, im powering this off the 6.3v heaters, so i need to be at 6v. the positive pulse has 2 LEDs for 4 volts @ 20mA. the negative has 1 LED and 1 diode. the LED needs 2V, but the regular diode throws me off. I have a bunch of 1N4007's. if im reading the specs correctly, it takes 1.1v, and 10mA. if thats true, then the negative pulse will be 3.1v @ how many mA? -then whatever that is, is in parallel w/ the positive pulse which adds up to 7.1v and "more than 20" mA? im over-reaching on my voltage, and not sure about how many mA it will draw.

B)
20221205_094456.jpg

this looks better to me. all 3 LEDs in series for 6v, and a total of 20mA. the regular diode keeps the electron flow in the correct direction, and both +/- have the current limiting 15ohm resistor to keep them balanced.
my question about this circuit is where to put the CT trim pot. after the heaters or before? after is more in line w/what Fender is doing on the Pro Jr (schematic above). is this critical?

thoughts??
 

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,432
Location
Central California
B)
View attachment 1058476

this looks better to me. all 3 LEDs in series for 6v, and a total of 20mA. the regular diode keeps the electron flow in the correct direction, and both +/- have the current limiting 15ohm resistor to keep them balanced.
my question about this circuit is where to put the CT trim pot. after the heaters or before? after is more in line w/what Fender is doing on the Pro Jr (schematic above). is this critical?

thoughts??
B) is the same as the schematic in post #14.

The CT pot is in parallel with the heaters, so there is no such thing as before or after.
 




Top