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Old 20th June 2016, 07:56 PM   #1
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Default Bias Supply

Greetings,

I've drawn a simple Bias supply with a simple program, Fritzing. The caps are in series (MKP 0.1F, 250VAC) because I have a lot of them left and I'm using a neon blink charger at the end, I just don't know what value the cap has to be that's parallel with the neon bulb ? Does anyone have a suggestion or is this value not that critical ?

Many thanks guys.
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Old 20th June 2016, 09:24 PM   #2
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The value of the capacitor in parallel with the neon bulb isn't critical, but it does determine the blink rate. For a given leakage or charge current, higher values of capacitance will blink more slowly then lower values. Typically, values between 0.1uF and 0.01uF are used. For reference, Quad uses 0.047uF.
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Old 20th June 2016, 10:22 PM   #3
tyu is offline tyu  United States
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I have to ask......what am I missing with out the neon bulb in my bias? .......dose this neon bulb do anything for the sound or bias...are just tell us that we have leakage.... lot are littel..... an as we know we all have leakage.... what well most do to fix ...if you get more blinks...
I can see this as a test setup maybe ...for the panels at first ck...or time to time..
I have had..... quad 57s an they played fine with minny bliks....thay were 50 years old stock panels......thanks for any an all info on getting better sound out of ESLs
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Old 20th June 2016, 10:22 PM   #4
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Thanks for the reply, I've put a 0.1 uF Cap in that position because I have a lot of them in different sizes, is there a way to know or to calculate what the blinking rate should be ? Will a 0.1uF, 600V AC cap suffice ?

Also, the resistor value in series with this cap, let's say 24Mohm, is this too high or too Low if the coating is Licron Crystal ? Maybe That's a bit off topic but I was just wondering.

Thanks for the help Bolserst
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Old 20th June 2016, 10:25 PM   #5
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As far as I know it is indeed only to see if there is any leakage on your panel, it's a quick and easy way to see it ...
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Old 21st June 2016, 05:09 AM   #6
Calvin is offline Calvin  Germany
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Hi,

the cap across the blinker/flashlight is charged up by the supply.
When the voltage across its pins reaches the ignition voltage of the blinker, the blinker fires and discharges the cap.
The charging process restarts when the the blinker 'switches off' again, at a voltage which typically isn't specified in the DS.
The ignition voltage of the flashlight is stated in its specs (94V is a common and practical value. Just ensure that the rating of the cap is higher).
And You know the capacitance of the cap.
Now the problem is that the charging current is also dependant on the charging resistor and the leakage.
There's a rule of thumb for the charging time: tau~5x RxC
As the associated tolerances of all three 'participants' are quite high the results would be just rough estimates instead of precise figures.
The blinker is a nice qualitative measuring device but a bad quantitative measuring device.
Use it as such and don't worry about 'precision'

jauu
Calvin

Last edited by Calvin; 21st June 2016 at 05:28 AM.
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Old 21st June 2016, 12:57 PM   #7
tyu is offline tyu  United States
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Ok...right now I remember ..at night I would play my 57s in the dark...an the flashing light would drive me nuts...I put tape over the red bulb...
All ESL panels leak right...so I see the neon bulb .. dose nothing...but flash...
May turn some off....they cant fix the ESL panel or make it flash less...if some one tell them ..it flashing to much...
thanks for your info an time...have fun
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Old 21st June 2016, 02:23 PM   #8
bengel is offline bengel  United States
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Just curious on how this works.....

So you put the neon light in series with a 10 MOhm resistor, in series with the output of the step up ladder (which will be a few thousand volts) and then put a .1 uF cap (or whatever) across it. So if you have no leakage (ideal) the light would just stay on constantly? If you do have leakage, the slower it blinks the more leakage you have?

Think I am missing something here :-)
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Old 21st June 2016, 03:55 PM   #9
Calvin is offline Calvin  Germany
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Hi,

yes, You missed it
At the beginning both, the output of the bias supply and the membrane are at 0V.
When the supply is switched on its output voltage rises.
As the cap blocks dc and the flashlight also doesn't conduct, the membrane remains at 0V.
Now the cap is charged up and the voltage across its pins rises too.
When the voltage reaches the ignition treshold of the flashlight it fires, thereby acting like a switch that is closed, it conducts.
The cap discharges through the flash into the membrane.
Its voltage sinks and the memrane voltage rises (membrane is charged up).
When the cap voltage has reached a lower limit the flashlight stops firing (like a switch that is opened) and the charging process begins again.
The only difference is that the membrane is now at a voktage >0V.
As while the start up the voltage of the supply rises quickly the firing rate of the flashlight is high.
The rate of flashes sinks with rising membrane voltage/charge.
When the membrane is charged up fully (exactly: to a voltage lower than the flashlights treshold voltage below the bias supply voltage) the blinking stops completely.
In praxis the blinking will never stop completely as there is always leakage (even if not connected to the membrane, but just hanging in free air).
Which means, that the membrane voltage sinks slowly and the voltage over the cap rises.
When the treshold is reached the flashlight fires thereby recharging the membrane ... and so on and so on.
A slow blinking rate of say 2/min indicates low leakage/fully charged membrane while 2/sec indicate either start up or heavy leakage.
The blinker acts like a switch, that decouples the membrane from the bias supply when its off and connects to the supply while it blinks.
As the cap and flashlight are bi-directional the blinking will also happen when the bias supply is switched off and its voltage sinks blow the membrane voltage.

jauu
Calvin

Last edited by Calvin; 21st June 2016 at 04:04 PM.
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Old 21st June 2016, 04:51 PM   #10
bengel is offline bengel  United States
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Ah, missed the DC blocking aspect of the cap...... the cap is also in series with the output of the step up ladder......


Thanks for the explanation. I think I will add this into my bias supply build.
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