Need ideas for a LED Pilot light for a tube amp

  • Thread starter Planobilly
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  • #1
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Hi,
Is there any reason I should not use the 5VAC or the 6.3VAC taps on the power transformer and design a rectifier circuit to power a LED?

Thanks,

Billy

Be back in a bit
 

Answers and Replies

  • #2
berkeman
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Hi,
Is there any reason I should not use the 5VAC or the 6.3VAC taps on the power transformer and design a rectifier circuit to power a LED?

Thanks,

Billy

Be back in a bit
Seems okay to me. The lower the starting voltage the better, to avoid wasting too much power in the current-limiting resistor. I'd suggest a DC-DC converter to be more efficient, but I know that you want to keep noise out of the amp as much as possible. Maybe power a string of LEDs to get more useful light out instead of just heating up a resistor with the extra voltage drop... :smile:
 
  • #3
jim hardy
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two diode drops plus LED voltage
ought to leave room for a current setting resistor
there exist bicolor led's, red one polrity green the other
sans rectifier might make a nice incandescent looking orange ?
 
  • #4
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Thanks guys,

The B+ voltage on this amp is a bit lower than I would like to begin with. The new PT that was sent to me puts out 362VAC CT to either side. I did not want to add any more resistance to the primary. I also wanted to use a LED for a pilot because they last longer than other types and are less subject to vibration. A bit more resistance in the heater circuit will be OK as I see it.

Thanks for the feedback. Perhaps one of these days I will get smart enough to wire up a light bulb!!..lol

Billy
 
  • #5
jim hardy
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I will get smart enough to wire up a light bulb!!..lol
We'll find out how many engineers that takes !:smile:
 
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  • #7
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I ran 6.4 VAC out of the variac into a 1N4003 into a 200 ohm resistor into a unknown white DC LED.

It lit up in a normal way and nothing got hot. The current was 340 ua and I don't remember what the DC (no filter) voltage was. Half wave and ugly I am sure.

The tap on the transformer in the amp is right next to the power switch and next to the hole in the chassis where the LED will go so I assume it will not cause any issues. I had in mind to put the diode and the resistor in some shrink tube. Or course I will need to change up some values for a red LED.

See any issues??

Billy

BTW...any issues doing this kind of testing in general with a variac? I understand the scope issues with the grounding.
 
  • #9
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Hi Jim,

Yes it is bright enough and I just looked over in the corner and I have a clear red plastic piece of round rod big enough to turn a fitting to hold the white LED.

I will take a look at the link...may be too lazy to crank up the machine shop...lol...then again....if I don't use the machine tools my wife will be asking why did you buy all that stuff...lol

Billy
 
  • #10
berkeman
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The current was 340 ua
That number looks wrong. I've believe anything between 10mA-50mA if you are getting good brightness...
 
  • #11
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Hi Berkeman,

I also was perplexed as to the readings. Perhaps I have something hooked up incorrectly. Here are photos of the test. One thing for sure is that the LED is lit and bright and nothing is even warm to the touch.
xLlsPId.jpg

szNjexu.jpg

xETzPHA.jpg

PtgQgQy.jpg

eF2Kvnl.jpg


Do you see anything that is hooked up in a manner that cause these results?

Billy
 
  • #12
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The photos are not 100% clear.

There is the wood terminal block connected to a variac set to 6.357 VAC as indicated on the Triplett DMM.

From the terminal block via red jumper wire there is a 1N4003 diode connected to a green jumper to a 200 ohm resistor.

From the other end of the resistor a white jumper is connected to the LED.

From the other lead of the LED a Radio Shack DMM is connected in series and set to the ua scale and reading is around 340 ua then connected back to the neutral terminal of the wood terminal block. The reading is moving around a small amount due to the fact of the half wave pulsating DC current. NOTE: When I first hooked up the DMM I set it to the ma scale because I was expecting a current in the ma range but no current flowed and the LED was not lit.

The Simpson is connected to the diode and back to neutral on the block which is reading 2.7 VDC.

It is obvious that there is some resistance in the jumper wires and the test equipment but removing the test equipment produces the same results. That is, the LED is bright and no heat to the touch.

I need to find some way to draw schematics...lol

Billy
 
  • #13
berkeman
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What is the voltage drop across the 200 Ohm resistor?
 
  • #14
jim hardy
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From the other lead of the LED a Radio Shack DMM is connected in series and set to the ua scale and reading is around 340 ua

here's my guess

average responding, RMS indicating

planobillsLED.jpg


persistence of eye makes it look brighter than it really is
 
Last edited:
  • #15
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I think you are correct Jim. When I hooked up my scope it indicated a DC voltage of around 2.5 VDC and a pulse wave more or less like your diagram.

In any event the circuit works like I have it designed. The less current draw the better.

Billy
 
  • #16
jim hardy
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DC voltage of around 2.5 VDC and a pulse wave more or less like your diagram.
presumably you saw the voltage across LED
should resemble the yellow line
current would resemble the blue one , methinks

glad to hear you're getting handy with the 'scope !
 
  • #17
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Sorry Berkeman...I took everything apart before I saw your post ask for the voltage drop.
 
  • #18
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This brings up another question!!

If the current is not steady, which it is not, (half wave) how would a digital meter (DMM) display that sort of current? What would it read if it was pure DC, in other words I assume the DMM would show a higher current reading with pure DC. Also if I built a full wave rectifier would the current not be twice a much and or display differently?

All this is starting to look like the Texas A&M jokes we use to tell in school. How many Aggies does it take to screw in a light bulb? Answer: 100...99 to turn the house and one to hold the bulb.....lol....well, if you go to Texas A&M, there are certain things you just have to put up with...lol

Billy
 
  • #19
rbelli1
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If using a Red filter you will want to use a red LED. The white LED is not really white. It emits a spectrum that your visual system interprets as white. The true spectrum is mostly blue and yellow.

BoB
 
  • #20
jim hardy
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Also if I built a full wave rectifier would the current not be twice a much and or display differently?
well sure it would be twice as much, you'd get twice as many current pulses 120 per second instead of just 60. Think simple.

What would it read if it was pure DC, in other words I assume the DMM would show a higher current reading with pure DC.

You already know that answer - if you added a big filter capacitor , what would that do to voltage waveform ?
 
  • #21
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Thanks Jim,

I thought I had a good understanding of the above. I guess I also need to study a bit about how a typical DMM arrives at current measurements.

Thanks reblli1 for the LED info.

Billy
 
  • #22
jim hardy
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I also need to study a bit about how a typical DMM arrives at current measurements.
measures the voltage across its internal shunt, likely on its lowest voltage scale 200 mv or so

If selected to DC it just reports the average so you'll get whatever is the DC component , the area under the curve,
if selected to AC it somehow rectifies before reporting the result
if a "true RMS" meter it actually squares , averages and square roots,
but most are "average responding" which means they just rectify then average then multiply by .707/.636 to convert from average to RMS on assumption it was a sinewave.
 

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