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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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21st May 2020, 10:51 pm | #1 |
Dekatron
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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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22nd May 2020, 10:40 am | #2 |
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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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22nd May 2020, 2:09 pm | #3 |
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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!
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22nd May 2020, 2:38 pm | #4 |
Pentode
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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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22nd May 2020, 3:41 pm | #5 | |
Dekatron
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Re: Low voltage indicator for 90v circuit
Quote:
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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22nd May 2020, 4:08 pm | #6 |
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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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22nd May 2020, 6:48 pm | #7 |
Dekatron
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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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22nd May 2020, 7:47 pm | #8 |
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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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22nd May 2020, 7:56 pm | #9 |
Heptode
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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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22nd May 2020, 8:17 pm | #10 | |
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Re: Low voltage indicator for 90v circuit
Quote:
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. |
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22nd May 2020, 8:19 pm | #11 |
Dekatron
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Re: Low voltage indicator for 90v circuit
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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22nd May 2020, 8:20 pm | #12 |
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Re: Low voltage indicator for 90v circuit
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22nd May 2020, 9:46 pm | #13 |
Heptode
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Re: Low voltage indicator for 90v circuit
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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23rd May 2020, 12:48 am | #14 |
Dekatron
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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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23rd May 2020, 7:04 am | #15 |
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Re: Low voltage indicator for 90v circuit
It’s an npn.
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23rd May 2020, 7:12 am | #16 |
Heptode
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Re: Low voltage indicator for 90v circuit
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23rd May 2020, 7:48 am | #17 |
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Re: Low voltage indicator for 90v circuit
That’s a relief, I can’t work that one out either!
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23rd May 2020, 9:22 am | #18 |
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Re: Low voltage indicator for 90v circuit
The circuit in #2 is a constant current supply to the led.
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23rd May 2020, 11:04 am | #19 |
Dekatron
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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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23rd May 2020, 1:33 pm | #20 | |
Dekatron
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Re: Low voltage indicator for 90v circuit
Quote:
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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