ESL Power supply question?

[dochungwell]

How critical is the 18m ohm resistor in the power supply, can it be subsituted with either a 15m/1kv or 20m/5kv ohm resistor feeding the panels?

Also i am having difficulty locating 5kpf capacitors for the ladder circut, i can however find .01/3kv will this suffice?


BTW, Thanks Charlie/Jazzman for updating your website, Your build truley is an inspiration to many of us under taking this project!

Jerry

[CharlieM]

Originally Posted by dochungwell

How critical is the 18m ohm resistor in the power supply, can it be subsituted with either a 15m/1kv or 20m/5kv ohm resistor feeding the panels?

Also i am having difficulty locating 5kpf capacitors for the ladder circut, i can however find .01/3kv will this suffice?


BTW, Thanks Charlie/Jazzman for updating your website, Your build truley is an inspiration to many of us under taking this project!

Jerry

Thanks Jerry,
Hey I'm still a newbie too... I didn't know a thing about ESL's when I started that project and I still struggle with simple circuits. My bias supply is basically the one shown in Sanders' Cookbook with a few tweaks. My logic for those small caps is to limit the potential current available to any arcing that might occur [and available to shock me if I happen to touch it]. The larger caps you referenced will work fine (will give less ripple and even a bit more voltage-- half-wave has tons of ripple but you won't hear it unless you have a leak path draining excessive charge off the diaphragm).

My bias supply worked well with the 11M ohm charging resistor I was using previously so it's can't be that critical. I think the 15M or 20M you referenced will work fine -- anything above 10 Meg should be OK (then you won't get bitten too bad if you grab the wire).

I show a high-voltage type resistor in my new parts list but I actually haven't ordered any yet -- and in the interim I've been using a standard 18 Meg resistor rated for just 400 volts with no problem. I'm thinking that, in operation, the charging resistor and caps only have to handle the difference in voltage on either side of them in the circuit (not the full circuit voltage relative to ground)-- that's my theory anyway -- someone please jump in if I've got that wrong!

[dochungwell]

Thanks Charlie, appreciate the honesty and advice as always. I was set to order everything on Friday. I had sourced all my components on Mouser….. and then my laptop crashed, I just recovered it so I am ordering everything today.

Charlie, isn’t the Licron and aerosol, cant it just be sprayed evenly with out the need to be spread with a brush?

[CharlieM]

Originally Posted by dochungwell

Thanks Charlie, appreciate the honesty and advice as always. I was set to order everything on Friday. I had sourced all my components on Mouser….. and then my laptop crashed, I just recovered it so I am ordering everything today.

Charlie, isn’t the Licron and aerosol, cant it just be sprayed evenly with out the need to be spread with a brush?

Yes, the Licron Crystal comes in an 8-ounce aerosol can and it's sprayed on. One coat "just wet" is all you need. The one coat looks quite thick when you spray it on but it's 97% solvents so it dries very thin, clear, sounds great, won't come off and it's unaffected by humidity.

[bolserst]

Originally Posted by CharlieM

I show a high-voltage type resistor in my new parts list but I actually haven't ordered any yet -- and in the interim I've been using a standard 18 Meg resistor rated for just 400 volts with no problem. I'm thinking that, in operation, the charging resistor and caps only have to handle the difference in voltage on either side of them in the circuit (not the full circuit voltage relative to ground)-- that's my theory anyway -- someone please jump in if I've got that wrong!

That's correct. The air gap and bias resistor form a voltage divider with essentially all of the voltage showing up across the air gap since it has a much larger resistance than the bias resistor.

However, when/if an arc occurs the air gap resistance is low and all the bias voltage shows up across the bias resistor. I only mention this because twice in the past I have had small amount of arcing on some perforated sheet metal stators. I would patch them up with corona dope at the arcing point. Then, a few days later that speaker would stop playing. In both cases, the problem was traced down to an open bias resistor. I started using 3 resistors in series to spread "the pain" of any potential arcs.

Just something to keep in mind if you are still perfecting your stator coatings and one of your speakers go silent. It may simply be the bias resistor.

[CharlieM]

Originally Posted by bolserst

That's correct. The air gap and bias resistor form a voltage divider with essentially all of the voltage showing up across the air gap since it has a much larger resistance than the bias resistor.

However, when/if an arc occurs the air gap resistance is low and all the bias voltage shows up across the bias resistor. I only mention this because twice in the past I have had small amount of arcing on some perforated sheet metal stators. I would patch them up with corona dope at the arcing point. Then, a few days later that speaker would stop playing. In both cases, the problem was traced down to an open bias resistor. I started using 3 resistors in series to spread "the pain" of any potential arcs.

Just something to keep in mind if you are still perfecting your stator coatings and one of your speakers go silent. It may simply be the bias resistor.

Bolserst,

Thanks for sharing that insight. I guess my stator coatings must be pretty good, as I haven’t experienced any arcing [that I know of] since I built the stat panels in July 2008—but that was with the old 2kv bias supply. I’ve been using the new 2.7kv bias supply (shown on my web page) for only a week but so far no apparent problems with arcing—time will tell.

Bear with me please, as my understanding of electronics is not very good:
When your panels arced, how did you go about finding the location on the stator where the arc/burn-through occurred? Also, I’m not sure I follow the idea of using three resistors in series to “spread the pain”— would that be three resistors of 1/3 value in series to get the desired final resistance? Somehow it’s easier for me to imagine “spreading the pain” using three 3x resistors in parallel. I guess I just need some help understand why series resistors would be better for this.

Thanks,
Charlie

[CharlieM]

Okay, never mind... after further thought, three 1/3 value resistors in series makes sense---whereas, parallel resistors would permit three times the current flow to sustain any arcing that may occur (not a good thing). Did I get that right?

[bolserst]

Originally Posted by CharlieM

When your panels arced, how did you go about finding the location on the stator where the arc/burn-through occurred?

I was listening in a dimly lit room, so the arcing could be seen as small blue flashes that coincided with transient peaks in the music. Close examination of the panel in the area of the arcing showed small pinholes in the diaphragm. Then I looked for poorly coated edges on the holes near that area. In one case, it was easy to spot as the metal has actually started to rust. Uniform powder coating problems.

Also, I’m not sure I follow the idea of using three resistors in series to “spread the pain”— would that be three resistors of 1/3 value in series to get the desired final resistance? Somehow it’s easier for me to imagine “spreading the pain” using three 3x resistors in parallel. I guess I just need some help understand why series resistors would be better for this.

Yes, three resistors in series of 1/3 the value.
Putting resistors in series reduces the voltage across individual resistor.
Resistors in parallel reduces the current through individual resistors.
Either method distributes power dissipation between the resistors.

Most components can handle short bursts of current well over their rating as there isn't time for them to overheat.
But even short duration over-voltage can cause damage.
That was why the series connection made more since for this situation.

[AcoustatAnswerMan]

The purpose of the resistor between the bias supply and the diaphragm is to attain a constant charge on the diaphragm, rather than a constant voltage. Its value is not critical, with higher, rather than lower values being more desirable. You should try to find a high-voltage rated resistor, usually rated in the range of 3-5kV; in these types you will find the wattage rating will be in the 3-5 watt range. This way you will have a bullet-proof design that is unlikely to fail even under arcing conditions.

Acoustat, with its nominal 5-kV bias supply, used a 500-Mohm resistor.

[bolserst]

Originally Posted by AcoustatAnswerMan

The purpose of the resistor between the bias supply and the diaphragm is to attain a constant charge on the diaphragm, rather than a constant voltage. Its value is not critical, with higher, rather than lower values being more desirable. You should try to find a high-voltage rated resistor, usually rated in the range of 3-5kV; in these types you will find the wattage rating will be in the 3-5 watt range. This way you will have a bullet-proof design that is unlikely to fail even under arcing conditions.

You are right, if your coating has a low resistance it is important to use a high resistance bias resistor to ensure constant charge operation. But Licron Crystal coating has very high resistance (1e8 - 1e9 ohm/sq) so constant charge operation is inherent even without high resistance bias resistors. I could not measure any difference in distortion between 10Mohm, 100Mohm, and 1000Mohm bias resistors when Licron Crystal was used as the coating.