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

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

Post by RhythMick »

Mike's comments about low voltage supply set me thinking. The datasheet for the LM293P indicates only 0.4mA per comparator. So, how about a battery based circuit. A pair of rechargeable AA batteries should give 2.4v ish and last forever (4000 mAh, around 5000 hours at 0.8mA) if there are no valve failures. Even if 1 valve fails it would only draw 20.8mA, 190 hours and change. Could also easily build in a test switch (add a resistor in parallel to drop the IN-).

The following could be easily constructed and hooked up to a valve pair as a test. The advantage of monitoring the 10R is that I catch any scenario which stops the valve conducting. Battery -ve would connect to the top of the cathode resistor and both comparators in a single LM293P could be used.

To implement it in the amp would need space for 6 sets of battery pairs which would be an issue. Still I think this is worth building a test unit, even as a learning exercise.

As always, all input and comment welcomed and appreciated.
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Nick
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#77 Re: Warning LED when valve stops conducting - a little SS help needed

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It would light the LED's when you turn the amp off.
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RhythMick
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#78 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Fri Feb 15, 2019 8:56 am It would light the LED's when you turn the amp off.
Ha, yes I thought of that. Honestly. Would need an on/off.
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#79 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Or combine the outputs in a NOR. Single led lights if either of the comparator outputs goes low, but not both.
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Nick
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#80 Re: Warning LED when valve stops conducting - a little SS help needed

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RhythMick wrote: Fri Feb 15, 2019 9:22 am Or combine the outputs in a NOR. Single led lights if either of the comparator outputs goes low, but not both.
In that case you may as well just compare the voltages on the top of the to sense resistors. What is unknown is will it trigger with a audio signal.
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RhythMick
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#81 Re: Warning LED when valve stops conducting - a little SS help needed

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Nick wrote: Fri Feb 15, 2019 10:11 am
RhythMick wrote: Fri Feb 15, 2019 9:22 am Or combine the outputs in a NOR. Single led lights if either of the comparator outputs goes low, but not both.
In that case you may as well just compare the voltages on the top of the to sense resistors. What is unknown is will it trigger with a audio signal.
hmmm ... a hysteresis resistor should avoid triggering during signal fluctuations, but does that then introduce something into the signal path ? Will ponder.

your other point is interesting. Right now the objective (of this proposal) is to alert if either valve stops conducting. I'm not sure that's met by comparing the tops of the 10Rs. I'll need to think about that a bit.

The circuit can be simplified by having one reference voltage to both IN-, which could be adjustable using a pot. Also using a CR2032 battery gives 3v, with 220mAh lasting 275 hours or so. Plenty, though not rechargeable.
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#82 Re: Warning LED when valve stops conducting - a little SS help needed

Post by jack »

Been following this with interest.

There are isolated integrated current monitors that are not hall effect and thus good for low currents.

With an isolated monitor, one PSU is enough and even its isolation is not critical as the monitors are typically good for 1kv or so.

I'll look for suitable candidates...
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RhythMick
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#83 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Fri Feb 15, 2019 10:11 am
RhythMick wrote: Fri Feb 15, 2019 9:22 am Or combine the outputs in a NOR. Single led lights if either of the comparator outputs goes low, but not both.
In that case you may as well just compare the voltages on the top of the to sense resistors. What is unknown is will it trigger with a audio signal.
On 2nd thoughts you may well be right, with hysteresis to avoid trips with signal.
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#84 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

jack wrote: Fri Feb 15, 2019 11:30 am Been following this with interest.

There are isolated integrated current monitors that are not hall effect and thus good for low currents.

With an isolated monitor, one PSU is enough and even its isolation is not critical as the monitors are typically good for 1kv or so.

I'll look for suitable candidates...
Thanks Jack, will look into your suggested candidate.
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Nick
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#85 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

Yes, you are trying to detect when the two currents are different, you have two equal resistors, one in each current with a common tail, if the currents are different, the voltage at the top of each resistor will be different.
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#86 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Fri Feb 15, 2019 11:49 am Yes, you are trying to detect when the two currents are different, you have two equal resistors, one in each current with a common tail, if the currents are different, the voltage at the top of each resistor will be different.
Will need to use the two comparators, to catch either condition (A higher than B or B higher than A) and use hysteresis to smooth out the fluctuations caused by the signal. I think.
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#87 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

You could try a LED inside a bridge rectifier.
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#88 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Fri Feb 15, 2019 1:35 pm You could try a LED inside a bridge rectifier.
ok quick bit of googling and ...
Capture.PNG
Is that something like what you had in mind ?
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Mike H
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#89 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

Getting very complicated again. Batteries? Nah. :D

I like this:
RhythMick wrote: Thu Feb 14, 2019 10:56 pm
Something like this ?

1550185320490414.jpg
Comparator circuit current is VERY small so I can't see a problem with supply "loading". The biggest current will be the LED, and only when it's on.

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Mike H
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#90 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

So if supply is 8V, then in my last diagram:

R4 = 680R
R2 = 7.5k

Proper job. :D
 
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