SLA Battery (Filament Supply) DC Reference limits?

[Ludwig Haus]

Hi,

I'd like to ask if anyone has information regarding how safe it is to reference a SLA battery (filament supply) to a 250V supply?. I've ran them before referenced to 100V or so, but this new design is more than double that and I cant find any info out there.

Any replies much appreciated.

L.H/S

 

[Mooly]

The safety risk is simply that the battery is at whatever voltage you are floating it on. In practice that means the battery should be in an insulated area of the unit and inaccessible to prying fingers. There is also the real possibility of slight electrolyte seepage from the terminals and possibly vents (it can happen even on SLA's) and that could make the whole battery case conductive.

So I would say for total safety that the battery must live on a plastic tray and that the no part of the battery case must contact anything conductive. There must also be no possibility of anyone being able to access the battery unaware that it is all at high potential.

 

[JonSnell Electronic]

Sounds like a lethal idea! Pre 1945 dry batteries of 90Volts were used in radios, using one 2Volt lead acid cell for the heaters. There has never been over 200Volts using series batteries for obvious reasons, gassing is not safe either.

 

[Mooly]

I think he mean hanging a single battery off the 250 volt line rather than generating 250 volts from series SLA's.

 

[trobbins]

L.H/S what are you actually proposing?

Is the 250V supply a secondary side DC supply referenced to safety ground?

How are you going to maintain the battery at nominal full SOC and moderate temperature, and avoid a deep discharge?

I assume you are aiming at using a small 2-7Ah 6V monobloc. They present a risk from electrolyte leakage and explosion if not managed properly.

Ciao, Tim

 

[Ludwig Haus]

L.H/S what are you actually proposing?

Is the 250V supply a secondary side DC supply referenced to safety ground?

How are you going to maintain the battery at nominal full SOC and moderate temperature, and avoid a deep discharge?

I assume you are aiming at using a small 2-7Ah 6V monobloc. They present a risk from electrolyte leakage and explosion if not managed properly.

Ciao, Tim

Thanks for the replies.

250V supply is B supply of the amplifier and is referenced to safety ground, yes. 12V 6Ah SLA to power 250mA 5V filament with series rheostat and choke between the -225V node in the schematic and the filament return of the tube.

I had something similar but at 100V for a couple years and it worked ok, but not wanting to push things without knowing the risks (and I dont know).

Otherwise, I can always look to a CCS based regulator which can be safely referenced to the B supply..

Thanks,

L.H/S

 

[trobbins]

Why a 12V batt for a 5V filament ?? Why a choke?

 

[Ludwig Haus]

Simply because its what I have..

 

[trobbins]

Sorry, I must be missing something. That scheme shows a choke and a rheo, are they the parts you say are powering the filament from a 12V battery?

 

[Ludwig Haus]

ah, no.. the filament supply schematic is not shown.. just to say that it connects between -225 and the other end of the tubes filament..

 

[trobbins]

Cheap eBay step down converter will give much longer battery time, and may have low voltage shutdown to save the battery.

 

[Ludwig Haus]

ok, but is it safe to connect the battery to the circuit as shown in the way described - thats all I really need to know. in other words, is it safe?

 

[trobbins]

If you don't touch the rheostat. If all the parts are insulated, isolated and non-contactable. If you charge the battery out of circuit.

Imho it's not safe until you have shown that you have made it all safe.

 

[Ludwig Haus]

Charge out of circuit, theres a good point.. the charging cct is switched with 240v coil and 30V contacts, I wonder what the insulation rating is between the coil and the contacts.. I'm not liking it so much...

Thanks Tim & Mooly.