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Old 21st May 2020, 10:51 pm   #1
60 oldjohn
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Default Low voltage indicator for 90v circuit

10 x PP3s HT supply for Battery valve radio. After being caught out when the voltage dropped below around 75v. (while trying to trouble shoot a radio) I have MA meters in the HT & LT leads and would like to use maybe a flashing LED to indicate the critical point of the battery voltage is being reached.
Circuit to be run at up to around 94v, LED to illuminate when down to 60v to 80v (settable). Low battery drain while sensing. minimum of components. hopefully transistor / Zener.

TIA, John.
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Old 22nd May 2020, 10:40 am   #2
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Default Re: Low voltage indicator for 90v circuit

Could something like this be adapted for 90v? This circuit is off the net, thanks to the original drawer.

John.
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Old 22nd May 2020, 2:09 pm   #3
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Default Re: Low voltage indicator for 90v circuit

How about the classic "flashing neon" circuit, takes very little current and the neon will flash until the battery goes down to the neons trigger voltage, about 70V approx. Select your neon to fit!
 
Old 22nd May 2020, 2:38 pm   #4
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Default Re: Low voltage indicator for 90v circuit

I'd agree with flashing neon. 3.9M series resistor, 0.1 uF cap in parallel with neon about as simple as it gets.
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Old 22nd May 2020, 3:41 pm   #5
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Default Re: Low voltage indicator for 90v circuit

Quote:
Originally Posted by merlinmaxwell View Post
How about the classic "flashing neon" circuit, takes very little current and the neon will flash until the battery goes down to the neons trigger voltage, about 70V approx. Select your neon to fit!
Wow that brings back memories, I had 10 neon's flashing randomly. Possibly circuit in PW around 1970ish.
Thank you Ian for the component values but I would prefer light to switch on to warn of low voltage. I think having it flash constantly be it very slow would be a bit distracting. The circuits I have seen only take around 1.5Ma a little more if using a chip.
I had a look at converting the above circuit from 12v to 90v, simple circuit but beyond me to re design that circuit. Is anyone able to design it for 90v?

John.
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Old 22nd May 2020, 4:08 pm   #6
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Default Re: Low voltage indicator for 90v circuit

Having a play, here we go, this turns on the LED at about 80V...
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Old 22nd May 2020, 6:48 pm   #7
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Default Re: Low voltage indicator for 90v circuit

That is great. Thank you very much for taking the trouble. What voltage is that Zener ?

John.
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Old 22nd May 2020, 7:47 pm   #8
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Default Re: Low voltage indicator for 90v circuit

Looks like 12V zener to me. Nice circuit, might have to build the original 12v one with a buzzer for messing round with auto electrics to warn me if I'm draining the battery too much.
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Old 22nd May 2020, 7:56 pm   #9
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Default Re: Low voltage indicator for 90v circuit

If you add a second transistor to drive the LED you can reduce the current drain when the LED is off. As it is, the current drain is much the same whether the LED is on or off, and is set by R3 to whatever current you need to light the LED (a few mA). With a second transistor the quiescent current can be a fraction of a mA, at the expense of somewhat greater complexity.
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Old 22nd May 2020, 8:17 pm   #10
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Default Re: Low voltage indicator for 90v circuit

Quote:
If you add a second transistor to drive the LED you can reduce the current drain when the LED is off
Indeed that is the case, as modern LEDs are so bloomin' efficient I didn't bother. It is only a milliamp or so and HT current for battery sets is 10mA so not much lost in the scheme of things.

The funny thing is this was the first hit, guessing values etc. I must have Spice* installed in my head.

*The simulator not curry, I do like a hot curry too.
 
Old 22nd May 2020, 8:19 pm   #11
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Default Re: Low voltage indicator for 90v circuit

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Originally Posted by ekjdm14 View Post
Looks like 12V zener to me.
That is what I thought, I was expecting maybe 84v if there is such a value Zener. I think I have some 12v Zener's I will have to give it a try.

John.
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Old 22nd May 2020, 8:20 pm   #12
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Thank you very much for taking the trouble
No trouble at all, it was fun. Some people like doing crosswords, why? The answers are already there to start with!
 
Old 22nd May 2020, 9:46 pm   #13
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Default Re: Low voltage indicator for 90v circuit

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Originally Posted by merlinmaxwell View Post
The funny thing is this was the first hit, guessing values etc. I must have Spice* installed in my head.
Funny thing is, I looked at the original circuit and couldn't see how it could work at all. The zener, if in breakdown has 10v across it, so the pnp is on. And if the supply drops so the zener comes out of breakdown, the pnp remains on.
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Old 23rd May 2020, 12:48 am   #14
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Default Re: Low voltage indicator for 90v circuit

The original circuit just gives a linearish increase in the LED current starting from a supply voltage somewhere between about 2 and 4 volts determined by the pot and going flat when the zener starts to conduct. Turn the pot down to zero ohms and the zener will go pop. It's not very useful!
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Old 23rd May 2020, 7:04 am   #15
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Default Re: Low voltage indicator for 90v circuit

It’s an npn.
 
Old 23rd May 2020, 7:12 am   #16
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Default Re: Low voltage indicator for 90v circuit

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It’s an npn.
I meant the circuit in post #2 - not yours which definitely does work!
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Old 23rd May 2020, 7:48 am   #17
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Default Re: Low voltage indicator for 90v circuit

That’s a relief, I can’t work that one out either!
 
Old 23rd May 2020, 9:22 am   #18
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Default Re: Low voltage indicator for 90v circuit

The circuit in #2 is a constant current supply to the led.
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Old 23rd May 2020, 11:04 am   #19
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Default Re: Low voltage indicator for 90v circuit

Just a poor design, though. There's scarcely enough supply volts for it to function as a current limiter at 12V and no " end stop" on the pot to stop the phantom twiddler destroying the Zener.

It's provoked more discussion than a circuit which did what the OP wanted would have, though.
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Old 23rd May 2020, 1:33 pm   #20
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Default Re: Low voltage indicator for 90v circuit

Quote:
Originally Posted by merlinmaxwell View Post
Having a play, here we go, this turns on the LED at about 80V...

Just to confirm the above. I have built the circuit using a BC547 that I had to hand, I suppose just about any Silicon general purpose NPN would be OK. LED lit at 78v. Circuit draws about 1.5ma. I will make it up on a piece of Veroboard.

Thank you again, John.
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