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

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

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Nick wrote: Wed Feb 13, 2019 11:45 pm These sort of isolated supplies can be great for providing a floating supply from a existing DC supply.

https://uk.rs-online.com/web/p/isolated ... s/3965174/
Isolated means the output is floating, right ? So I can ground to the top of the cathode resistor per the diagram. I know that's obvious, just checking my terminology.

To go this route I would need 6 isolated supplies, which I could feed from one transformer, but nevertheless. I'll pursue it a bit further for fun and may experiment as it's all good learning for me, but I'll hold off a decision whether to actually put into the amp.
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#62 Re: Warning LED when valve stops conducting - a little SS help needed

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Mike H wrote: Thu Feb 14, 2019 4:23 pm
How are the filaments heated?
Andrew's DC boards. At present there's 1 board per valve, 12 boards total. I did consider early on using those boards to drive the valve pairs in series, which would stop current if either fails (with series heaters there could be no virtual cathodes).

Yes it's getting complicated. lol. If I implement this I think it's looking like 1 transformer/BR/Cap into 6 isolated DC-DC converters driving 6 dual comparators. All to NOT light 12 LEDs, unless a valve stops working.

Or, I could just "keep an eye on the heaters". If only they glowed more.
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Nick
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#63 Re: Warning LED when valve stops conducting - a little SS help needed

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RhythMick wrote: Thu Feb 14, 2019 4:37 pm
Isolated means the output is floating, right ? So I can ground to the top of the cathode resistor per the diagram. I know that's obvious, just checking my terminology.
Well, you can connect to the cathode resistor without grounding anything which I think is what you are asking.
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#64 Re: Warning LED when valve stops conducting - a little SS help needed

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RhythMick wrote: Thu Feb 14, 2019 4:41 pm
Mike H wrote: Thu Feb 14, 2019 4:23 pm
How are the filaments heated?
Andrew's DC boards. At present there's 1 board per valve, 12 boards total. I did consider early on using those boards to drive the valve pairs in series, which would stop current if either fails (with series heaters there could be no virtual cathodes).
I don't know the 45 that well so I had to look it up. 2.5V heater! :shock:


OK, so all this time I've been expecting the "failure indicator" circuit to be "looking at" the heater supply curent, AND be powered by the same heater supply DC. Is just doable with 2.5V according to the LM193 datasheet. Screws up my component values tho (for some reason I was expecting 5V)

Back later.....
 
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#65 Re: Warning LED when valve stops conducting - a little SS help needed

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Mike H wrote: Thu Feb 14, 2019 5:12 pm
RhythMick wrote: Thu Feb 14, 2019 4:41 pm
Mike H wrote: Thu Feb 14, 2019 4:23 pm
How are the filaments heated?
Andrew's DC boards. At present there's 1 board per valve, 12 boards total. I did consider early on using those boards to drive the valve pairs in series, which would stop current if either fails (with series heaters there could be no virtual cathodes).
I don't know the 45 that well so I had to look it up. 2.5V heater! :shock:


OK, so all this time I've been expecting the "failure indicator" circuit to be "looking at" the heater supply curent, AND be powered by the same heater supply DC. Is just doable with 2.5V according to the LM193 datasheet. Screws up my component values tho (for some reason I was expecting 5V)

Back later.....
Yeah that was the plan Mike, until that is I realised that because I use Virtual Cathode resistors the D44H11 plan wouldn't work.
http://www.audio-talk.co.uk/phpBB3/view ... 45#p164218

I did acknowledge that your comparator plan would work, still looking at a sense resistor in the DC -ve leg. However I didn't realise you were thinking of powering it from the same heater supply DC. Minimum supply voltage is 2v. The 45 is 2.5v, but the 26 is 1.5v !
http://www.ti.com/lit/ds/symlink/lm293.pdf
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#66 Re: Warning LED when valve stops conducting - a little SS help needed

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Nick wrote: Thu Feb 14, 2019 5:00 pm
RhythMick wrote: Thu Feb 14, 2019 4:37 pm
Isolated means the output is floating, right ? So I can ground to the top of the cathode resistor per the diagram. I know that's obvious, just checking my terminology.
Well, you can connect to the cathode resistor without grounding anything which I think is what you are asking.
Yeah sorry my poor terminology. I mean the negative supply can be referenced to the top of the cathode resistor, so that the voltages being compared (across the 10R resistors) are within the input common mode for the LM293P.
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#67 Re: Warning LED when valve stops conducting - a little SS help needed

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OK second attempt - hopefully this is easier to understand - but all I've done is change values for a 2.5V supply. The crucial thing is tho, it is powered by the 45 heater supply module, so is very simple to do. No separate supply is necessary! :D
-
-
Heater LED Warning-2.jpg
 
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#68 Re: Warning LED when valve stops conducting - a little SS help needed

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PS: you will only be able to use one comparator in the chip, because the heater supplies are separate circuits, so you can't share two halves of the chip between both valves. Hope that makes sense.
 
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#69 Re: Warning LED when valve stops conducting - a little SS help needed

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Mike H wrote: Thu Feb 14, 2019 6:01 pm PS: you will only be able to use one comparator in the chip, because the heater supplies are separate circuits, so you can't share two halves of the chip between both valves. Hope that makes sense.
It does. Thanks. Would work for the 71A and 45, not for the 26. Clever though.
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#70 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Andrew »

I'm confused, to much digital stuff going on in my life at present, sorry. What does the virtual cathode circuit look like please, Mick?

A good understanding of Kirchhoff's Law doesn't do any harm here....

best regards,

Andrew
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#71 Re: Warning LED when valve stops conducting - a little SS help needed

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Andrew wrote: Thu Feb 14, 2019 8:28 pm I'm confused, to much digital stuff going on in my life at present, sorry. What does the virtual cathode circuit look like please, Mick?

A good understanding of Kirchhoff's Law doesn't do any harm here....

best regards,

Andrew
Here you go...
1550098328019861.jpg
The 10R are already in circuit, for measuring. All the comparator stuff is proposed.
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#72 Re: Warning LED when valve stops conducting - a little SS help needed

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RhythMick wrote: Thu Feb 14, 2019 6:06 pm It does. Thanks. Would work for the 71A and 45, not for the 26. Clever though.
I see what you mean. (Had to go look at 26 data.)

I don't know what the heater PSU circuit looks like, but would hazard a guess there is a higher Voltage available from there (e.g., before regulator), that could power the indicator circuit instead. In that case, and assuming all the heater PSU's are basically identical, then so also could be the indicator circuits. In terms of component values I mean. :?:
 
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#73 Re: Warning LED when valve stops conducting - a little SS help needed

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Andrew wrote: Thu Feb 14, 2019 8:28 pm I'm confused, to much digital stuff going on in my life at present, sorry. What does the virtual cathode circuit look like please, Mick?

A good understanding of Kirchhoff's Law doesn't do any harm here....

best regards,

Andrew
Actually you probably meant the cathode circuit as it stands. This is what the heater sees. For the 45, the cathode filament is a 1R67 and it's in parallel with the virtual cathode resistors , 400R total making 1R66. When the filament fails that becomes 400R, but it's not open circuit so the board just drops the current needed to generate 2.5v
IMG_20190214_224001.png
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#74 Re: Warning LED when valve stops conducting - a little SS help needed

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Mike H wrote: Thu Feb 14, 2019 9:54 pm
RhythMick wrote: Thu Feb 14, 2019 6:06 pm It does. Thanks. Would work for the 71A and 45, not for the 26. Clever though.
I see what you mean. (Had to go look at 26 data.)

I don't know what the heater PSU circuit looks like, but would hazard a guess there is a higher Voltage available from there (e.g., before regulator), that could power the indicator circuit instead. In that case, and assuming all the heater PSU's are basically identical, then so also could be the indicator circuits. In terms of component values I mean. :?:
From the measurements I took earlier (which were based on a board set to supply 5v) the D44H11 pin 2 is rock solid at 8.0v. That might be a good place to source the 20mA + change needed to power the LED and the comparator chip.

Andrew spent a lot of time designing this board and minimising ripple. I'd be reluctant to go there without his views on whether this might affect it.

Something like this ?
1550185320490414.jpg
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#75 Re: Warning LED when valve stops conducting - a little SS help needed

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Mike H wrote: Thu Feb 14, 2019 4:08 pm
RhythMick wrote: Wed Feb 13, 2019 3:39 pm If I understand it right, that circuit will light the LED when the valve is conducting and stop it when not. Simply reverse the IN+ and IN- to get the reverse effect ?
You might have already sussed, but no, other way round. If + input is higher than - input, then output is off (being a transistor switch), so LED is off. I.e. output is positive (aka, 'high').
Mike - I just checked this against the LM293P datasheet and this is what it says. So if IN+ > IN- (condition b below) the output transistor is off (not conducting). That's what you said, I just needed to check the datasheet and get my head straight. Thanks.

1. When both IN- and IN+ are both within the common-mode range:
a. If IN- is higher than IN+ and the offset voltage, the output is low and the output transistor is sinking
current
b. If IN- is lower than IN+ and the offset voltage, the output is high impedance and the output transistor is
not conducting
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