Power indicator LED in High Voltage Circuit

[captainate]

Hey there helpful friends,

I'm still new to circuit design, although I've been building from schematics for years. I'm currently working on a microphone preamp project, and have recently built a high voltage power supply for the tube-based circuit (350V B+, tested with the help of a certified friend).

Please let me know if I am correct in assuming that I could drive a power indicator LED with my 350V power supply in series with a 22k resistor (R=350/.016). It seems like there would be high power dissipation through this current limiting resistor, but I don't have a lower voltage supply in this box. Is it ok just to use a large resistor here?

 

[audioguru]

The 22k resistor will dissipate 5.5W so use a 10W resistor that will get pretty darn hot.

 

[captainate]

Is this a normal situation for an indicator light? Obviously they would have used incandescent lamps in the days of this original equipment, but it still seems a little silly to have such great power inefficiency.

 

[ronsimpson]

Years ago you would have used a neon lamp. It takes 60 to 90 volts for then to work.

 

[ClydeCrashKop]

I use 100K resistors with 115 volts and the LEDs glow nicely with no problem.

Try a 300K resistor and see if it's bright enough.

 

[alec_t]

If you use a single resistor make sure it is rated for greater than 350V. It would be better to use two or more equal-valued resistors in series (e.g. 3 off 100k) to 'share' the voltage. And don't let all those Volts KILL you !

Edit:
Why don't you simply drive the LED more safely via a resistor from the tube heater supply (6.3V?)

 

[tvtech]

Why don't you simply drive the LED more safely via a resistor from the tube heater supply (6.3V?)

Perfectly logical question....why not?
Surely this should work...albeit you don't know if your 350V is there though..

Regards,
tvtech

 

[JimB]

Surely this should work...albeit you don't know if your 350V is there though..

And similarly, if the LED is driven from the 350v, you don't know if the 6.3v is there.
Oh, decisions, decisions!

Newer is not always better. (LED vs Neon)

JimB

 

[alec_t]

Would a neon be happy with a DC supply? I don't see why not, but I've never tried that.

 

[tvtech]

And similarly, if the LED is driven from the 350v, you don't know if the 6.3v is there.
Oh, decisions, decisions!

Newer is not always better. (LED vs Neon)

JimB

LOL.. You are supposed to be on holiday Jim...not here

So now that you are here....working as usual...I have ze kwestion for you..better heaters are working or maybe 350V is there???

I vonder vat is de ansa?? Both cannot happen at ze same time .

Don't you dare reply to this post...turn your Lappie off and enjoy your break...please. Forget about the Internet and Forums and all.

We are OK. Relax and enjoy your break and be you got one

All the best,
tvtech

 

[MrAl]

Hi,

The trick with using LED's as power line power indicators is to use a high brightness LED and keep the current down to an average of around 0.75ma and that keeps the dissipation under 1/4 watt on a 120vac line. For other power line voltages you would just adjust a little.

For a 120vac line using 1/2 wave rectification an 80k Ohm 1/2 watt resistor does not even get warm. The power is roughly 120*120/80000/2=0.09 watts, and this is more than 5 times less than the power rating of the 0.5 watt resistor so it stays cool. This is necessary when the LED and resistor must be mounted inside a closed space where it doesnt get much air flow. If you can mount it where it gets air flow then you can use a lower power rated resistor.

For 350vdc and the same current of 0.00075 amps, we get a total resistance of about 470k. The power dissipation would then be 350*350/470000=0.261 watts. Using two 220k resistors (in series) each 1/2 watt would do it nicely, as that would be enough overrated so the two resistors dont get too warm. The resistors have to be in series so their voltage rating does not come into question (175v each).

One thing to check is to make sure the voltage does not increase too much when the amp is not being used but still turned on.

So the key really is to use a high brightness LED so you dont have to pump too much current through the LED, and that keeps the resistor power dissipation down low enough to be a practical solution. Typical LED brightness ratings are 10000mcd or better. If you try to use a 'normal' LED you might find that it is not bright enough to run on very low current, it should be a high brightness type say around 10000mcd or better.

BTW neon bulbs do run on DC with the proper current limiting resistor. There are at least two sizes though each requiring a different value resistor for full brightness. The thing i dont like about them though is that they do burn out after some time. LED's run at very low current like 1/20 of their normal rating can last literally for 50 years, on for 24 hours a day 7 days a week.

 

[JimB]

LOL.. You are supposed to be on holiday Jim...

Do pay attention and try to keep up there that man! (Using silly Capain Mainwaring voice).
It is NEXT week when I am off on my travels.

JimB

 

[tvtech]

Do pay attention and try to keep up there that man! (Using silly Capain Mainwaring voice).
It is NEXT week when I am off on my travels.

JimB

Oh dwang..I make a ...........of myself....

LOL, oh well, Oh man it gets funnier the longer I think about it....here I am wondering why you are still here, commenting and all...and you are not even away yet...thinking why are you still Moderating when you are on holiday...laptop and all....

This is funny. And the worst part of all is...nobody warned me I was busy making an ass of myself

It's all fine though....at least it brought (confused) smiles to the Forum....LOL...oh heck.

I really need new glasses...

Sigh

 

[captainate]

I was thinking DC since my electronic experience has been limited primarily to guitar effect pedals, but of course I could use a neon lamp on AC. Something like this can run straight off the mains before it even gets to the power supply. No 350VDC necessary! Is this the right idea?

 

[audioguru]

There is no need to use a separate switch contact for the neon light. It can be connected directly to the input of the power supply.

 

[captainate]

Like this? Apologies for being a bit pedantic, but as I've mentioned, aspects of this are new to me. I appreciate the help!

 

[JimB]

Like this?

No, the neon goes in parallel with the power supply, not in series.

JimB

 

[audioguru]

Wire it like this:

 

[captainate]

Oof, that was a bit dumb. I blame it on lack of sleep from illness.

 

[MrAl]

Hi,

Actually that's still not exactly correct unless the indicator already has a built in current limiting resistor. The type of neon bulb dictates what size resistor to use. Too low a value with the wrong type will burn it out too soon, too high will not allow it to light up as much as it can. When you buy the bulb there is usually data that comes with it to indicate what kind of resistor to use with it.

I have had neon's that have a built in resistor but they are in a whole package not just the bulb itself. The package is stamped with the operating voltage like "120v". The bulb itself is inside along with the resistor. A lone bulb is a clear glass envelope with two thick wire things inside. That is what needs a resistor with it to operate properly because the true operating voltage is only around 60 volts or so.