Led indicator for Filament ON

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thebadtall

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Hello, I would like to put a LED inline with a tube/valve filament DC 12v supply so that when filament is ON the Led will be on, but
that would burn the led instantly (right ?).

Any suggestions ?
Maybe use a simple BC547 as switch ?

 

Wire an ordinary diode across the LED (A-K, K-A). then wire this ACROSS the filaments with a current limiting resistor in series. Value of resistor = Vfils/ Iled ohms.
Frank
 
Hello,

look for my a attached circuits.

If you use DC for the filament take the left circuit else if it is AC use the right circuit.

For AC you need the antiparallel diode D1, because the Vfr of the LED is very low an will distroy the LED.

Regards

Rainer


PS: Sorry, if my English is not so good.


Attach circuits

 
Vfils is 12V I led is 0.02A so R=600 ohm?

is the layout correct?

Dear Hans, in your circuit the led will be always on when supply is ON, but if for some reason filament fails, the led will be still on, I want the led to be corresponded to filament status (am I wrong ?)
 

Hello,

sorry, this was a misunderstanding, because a failure in filament is very rare.

Your circuit is not OK, because the valve need a filament current of 150mA. To control the heater current you must use a transistor.

Here my new circuit.

The resistor of 5.6 Ohms will give a voltage of 0,7V to the base of the transistor if current is flow. So the LED will light. If the current is less then 120mA the transistor will turn off.

I hope that will help.

Regards

Rainer


Attach circuit:

 
Last edited:
thank you again... I have BC547 NPN can I use same values?
 

Hello,

I have BC547 NPN can I use same values?
Click to expand...

It's easier to use a PNP - transistor, if you use as drawn. You can use every PNP you have. It must not be a BC557. in this case PNP is necessary, because you need the BE-voltage of 0,7V to open the transistor. This voltage is across the resistor.

If you want to use a NPN, you must put the resistor between heater and ground. Base at the haeter and Emitter to ground. LED and series resistor to Collector and +12V.

Regards

Rainer
 

until transistors arrive, can I use thyristor instead? a MAC97A8 triggers at max 2v
 

Hello.

No, you can't use a thyristor or, like you write a triac. Thyristors and triacs are use for AC - switching. If you use it in DC circuits it will switch on, after a trigger pulse arrive at the gate. After the trigger pulse is gone, the device will sty in "ON" - stage until the DC - supply is cut. Only in AC circuits the device will switch off alone after every alternation. Then you have to trigger again to switch on.

Also, if you need 2V to trigger the thyristor or triac the valve gets too low heater voltage. Look for this attached datasheet. . The heater voltage must be 12,6V +/-10%, else the data - specifiction is out of range and the life of the valve will be shorted.

Regards

Rainer

attached datasheet 12AX7.
 

i will keep that in mind.however , from personal experience,
for 12au7,ax7,bh7 etc series, the best filament voltage is below 11.5v and over 10.2 v

I will use your bc557 circuit as soon as my order arrives and will let you know!
 

If the filament is run from 12v DC you can put a resistor in series with the filament, that will give your desired 10.8v for the filament and leave 1.2v to operate the LED. The LED will still need it's own resistor for safety, and you need to choose the resistor value to give approx 1.2v (or a little more) so you have enough voltage to light the LED.
 

Hello Mr B,

that is possible, but your don't have any headroom, if the heater voltage drop down. Then your are out of specification. Also the led needs 10 to 20 mA to light, but the valve heater needs 150mA. So you have to calculate the parallel resisitant.

In my way, only you have to calculate a resistor, which regenerates a voltage of 0,7V across it. This voltage between Base and Emitter will open the transistor and you can use any device, like lamps, LED or relays, to show that heater current is flow.

regards

Rainer
 

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