Warning LED when valve stops conducting - a little SS help needed

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RhythMick
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#31 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

I'm confused, or I may be the source of the confusion. The current proposal is that when there's no load this can be signalled from the base pin of the D44H11. The transistor is required as a reverse polarity switch, so the led lights when the base pin goes low.

I'm not sure where the resistor fits Mike? Is that for one of the earlier options?
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Mike H
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#32 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

OK, forgot about that, still tho I'm not convinced the Voltage will disappear from the D44H11 base like you think it would. A small value resistor can be inserted anywhere that's convenient in the current path, and if the current goes to zero, so will the Voltage.

Still don't need the BC547 transistor "inverter", just change over the input pins of the LM293. Or, use it's o/p transistor to source the LED current directly.

:D
 
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#33 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Appreciate the response Mike. So let me just follow through an example if I may, call it a lesson for me.

Let's say we use a 0R1 1% in the output. The 45 draws 1.5A, so that should give 0.15v across the resistor. Either side of the resistor connects to the IN+ & IN- of the comparator, with a separate supply, say 5v. That enough difference to switch the output? Presumably the comparator output connects between the current limiting resistor and the led.

I can see that would work. 2 resistors and half a dual comparator (plus led) per valve. The disadvantage of this method is that it requires an external supply. The component count is neither here nor there (2 BC547s per valve pair vs a dual comparator chip). The advantage of this method to me would be that it would alert if the transformer or board failed, whereas the D44H11 plan may not alert if the DC board itself failed (depending on the nature of the failure).

What an excellent forum this is. Multiple options to consider now. Many thanks all.
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Nick
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#34 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

If you use a comparator with a external supply you will steed need a connection between the grounds to ensure the comparator input voltage range is not exceeded.

If you are now starting to worry about the DC module failing you also have to worry about the detection circuit failing. That way lies madness (or at least redundant parallel safety systems).
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RhythMick
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#35 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Wed Feb 13, 2019 8:42 am If you use a comparator with a external supply you will steed need a connection between the grounds to ensure the comparator input voltage range is not exceeded.

If you are now starting to worry about the DC module failing you also have to worry about the detection circuit failing. That way lies madness (or at least redundant parallel safety systems).
Yep agreed on both points. Mike's contribution was valid though and it's in my nature to consider pros and cons of options. The original question was how to alert of one valve was not conducting. There are other causes for that beyond the heater.

I'm definitely going with Andrews suggestion though. Assuming it works, it may be worth considering for any future versions of the board.
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#36 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

why do we need the D44H11
If I understand, its because that's what is powering the heater.
The original question was how to alert of one valve was not conducting. There are other causes for that beyond the heater.
Yes and th way to do that is test the cathode or anode current, its just the heater is easier to measure, but if you are going to add extra checks, it may be better to go back to sensing the anode current.

Instead of using the LED across the NPN transistor, why not use a PNP transistor?
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#37 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Wed Feb 13, 2019 11:05 am Instead of using the LED across the NPN transistor, why not use a PNP transistor?
Need to better understand that Nick. To start, I redrew the NPN diagram with voltages, GREEN for voltage when the heaters are conducting and RED for when they are not.
Heater LED Warning-NPN.png
So when the valve heater isn't conducting the BC547 base falls to ground, turning it off and thus lighting the LED.

If we use a PNP instead how would we drive that ? My first attempt...
Heater LED Warning-PNP.png
whether the valve is conducting or not the base will be more than a diode drop below the emitter and thus the PNP will always conduct. what am I missing
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Nick
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#38 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

Put the LED in the collector.
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#39 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Wed Feb 13, 2019 1:45 pm Put the LED in the collector.
Sorry I'm being thick. The Resistor and LED are in the collector on that diagram ?
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#40 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Do you mean put the LED in the emitter ? Like this ?
Heater LED Warning-PNP (1).png
OK so the LED drop of 2.25v takes the emitter down to 5.75v.

When the valve is conducting D44H11 base is at 5.7v, PNP is switched off so no current flows.

When the valve is not conducting D44H11 base drops to 0v, PNP turns on and current flows, limited by the resistor.

OK I get it - thank you very much Nick.

STUPID QUESTION : if no current is flowing through the LED, does it still drop the voltage ?
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Nick
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#41 Re: Warning LED when valve stops conducting - a little SS help needed

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Transistors never fully turn off. I would consider the resistor on the same side as the LED.
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#42 Re: Warning LED when valve stops conducting - a little SS help needed

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Nick wrote: Wed Feb 13, 2019 2:24 pm Transistors never fully turn off. I would consider the resistor on the same side as the LED.
Ok thanks
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#43 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

RhythMick wrote: Wed Feb 13, 2019 2:11 pm When the valve is not conducting D44H11 base drops to 0v, PNP turns on and current flows, limited by the resistor.
I can't see an overall picture of what's going on because I don't know what this D44H11 circuit looks like. But [^ as Nick] the base Voltage of a bipolar transistor does not drop to zero just because the collector current stops. It will remain as whatever it is coming from the part of the circuit that is biasing the transistor.

It all looks terribly complicated to me, hence my preference for the resistor Voltage drop idea.
 
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Mike H
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#44 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

Nick wrote: Wed Feb 13, 2019 8:42 am If you use a comparator with a external supply you will steed need a connection between the grounds to ensure the comparator input voltage range is not exceeded.

If you are now starting to worry about the DC module failing you also have to worry about the detection circuit failing. That way lies madness (or at least redundant parallel safety systems).
I daren't mention what happens if a valve fails while you're out of the room for 20 minutes, or someone else is using the amp who doesn't know what the LED is for...

Oops too late I have :D
 
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#45 Re: Warning LED when valve stops conducting - a little SS help needed

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Yep fair points
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