Cheap AndEasy 6.3 Volts

upsid...@downunder.com wrote:

------------------------------
 
> 
> >
> >>"** I would try a 6.3V AC "wall wart" rated at say 0.5amps. "
> >
> >Do you really think it will take that 3,500 volts ? 
>
> 
> Usually tjose devices are rated at 2500 V and only tested for a short
> time (1 min ?).
> 


** Class II appliances require a 3,500VAC proof test  -  but in reality the rules concerning permitted insulation materials, minimum creepage and clearance distances that are at the heart of the matter.

FYI:

The dielectric strength of common plastics like PVC, Polypropylene and Polyethylene is around 20kV per mm.  

The split bobbin places a couple of millimetres of insulation grade plastic between the primary and secondary windings. 

http://www.cettechnology.com/wp-content/uploads/products/pst-pdst-split-bobbin-transformer-series/PST-PDST-crop-2-web.jpg 

Being a simple AC to AC type, there are no components like ceramic EMI suppression caps to cause trouble -  plus the iron core remains floating. 

Bet it works a treat. 



....  Phil 

>"Usually tjose devices are rated at 2500 V and only tested for a short time (1 min ?). "

I'll let you know what happens. 

>'Use an (auto)transformer to convert  5 V to 6.3 V. "

Don't have one and have no idea where to get one. 

People may be figuring out I am trying to do the maximum with the minimum here. I fixed a couple of equalizers lately. I need them because my hearing is going. It was nice to crank up that 16 KHz control and hear timbre again. And now I got the TV equalized so I can understand that. 

I like good sound, there is a discussion right now at a high end audiophile (audiophool ? almost, mainly a love for sound and too much money on their hands) forum about hearing aids that are made to reproduce music. Learned a few things about them lately as well, they make them but they cost. Of course some of these people have six figures into their stereo system.

>"The dielectric strength of common plastics like PVC, Polypropylene and Polyethylene is around 20kV per mm."

Wow, I would've never thought it that high. What is that about 20 times that of free air ? (dry free air that is)

Found a 9 volt and a 12 volt. I'll have ot see if I have or can make the right resistor for  it. I was sure I had a 5 volt one but it disappeared with many other things, including my sanity LOL.

<jurb6006@gmail.com> wrote in message 
news:0cbc2ac7-f911-493e-900f-bc01c9dcb4af@googlegroups.com...
>I am playing around with this old scope in which the power transformer 
>winding for the CRT filaments is arcing internally. In fact years ago I did 
>put in a hi-pot 6.3 VAC transformer and wired it to switched AC internally 
>and it worked for a long time.
>
> But now the replacement is arcing.
>
> The filament is elevated to about 3,500 volts negative DC. What is the 
> possibility I could find a cheapo switched mode for this purpose. The 
> insulation would be much easier for the higher frequency. In fact it could 
> possibly be AC coupled.
>
> Is there something off the shelf for this ?

What is the make and model of the scope?

On 02.02.2018 07:49, jurb6006@gmail.com wrote:
> I am playing around with this old scope in which the power
> transformer winding for the CRT filaments is arcing internally. In
> fact years ago I did put in a hi-pot 6.3 VAC transformer and wired
> it to switched AC internally and it worked for a long time.
>
> But now the replacement is arcing.
>
> The filament is elevated to about 3,500 volts negative DC. What is
> the possibility I could find a cheapo switched mode for this
> purpose. The insulation would be much easier for the higher
> frequency. In fact it could possibly be AC coupled.
>
> Is there something off the shelf for this ?

Try to avoid anything switch-mode for this application.

Switching power supplies can't stand very much in terms of high voltage
when the high voltage stress is being applied permanently. The hi-pot
test is only a minute (type test) or even less (production test) and the
components across the insulation can only withstand it for a short time.

Switching power supplies have several components across the insulation,
including "Y" capacitors and opto-couplers for the feedback path of the
control circuit. They are designed to withstand mains voltage and the
occasional surge (as is typical in public power networks) but not the
continuous high voltage stress of a circuit that "sits" on a 3.5 kV
potential permanently.

Also, switching supplies are strongly capacitive (between primary and
secondary) due to the "Y" capacitors and they can generate a whole lot
of noise. Both of these "qualities" are no good when the intention is
to operate a CRT filament (while maintaining the CRT in a good shape as
well as keeping the trace clean).

Your best bet would be a transformer constructed with the primary and
secondary windings on separate bobbin halves.

If you could somehow get some "TEX-E" type (triple insulated) wire, you
could rewind the secondary of a somewhat larger (compared to the small
wattage you need) mains transformer for 6.3 V with the TEX-E wire.
The main problem is that this particular wire type is hard to come by,
and (to a lesser extent) that the insulation is relatively soft and
requires taking care to avoid sharp edges, denting, or scratching.

>"What is the make and model of the scope? "

It is a Tektronix 561A. 

It is also the only scope I have with dual timebase. Not much bandwidth but I could pick a video signal apart and a few other things. I could borrow a scope with a dual timebase but it is better to have one lurking in the corner. 

Dimitrij Klingbeil wrote:

---------------------------
>

> >
> > Is there something off the shelf for this ?
> 
> Try to avoid anything switch-mode for this application.
> 

 ** Correct.


> 
> Your best bet would be a transformer constructed with the primary and
> secondary windings on separate bobbin halves.
> 

 ** Correct again.


> If you could somehow get some "TEX-E" type (triple insulated) wire, you
> could rewind the secondary of a somewhat larger (compared to the small
> wattage you need) mains transformer for 6.3 V with the TEX-E wire.


** What for? 

The secondary winding only has 6 volts between its ends no matter what. 

The 6.3V transformer the OP fitted to fix the old scope likely had paper insulation and the core grounded. 


....  Phil

>"Try to avoid anything switch-mode for this application. '

I would have to agree. It doesn't have much bandwidth but like 100 KHz could cause some aliasing. It would have to operate above several times the 10 MHz (mc ?) bandwidth of the scope. 

>"Your best bet would be a transformer constructed with the primary and secondary windings on separate bobbin halves. "

As Phil described a wall wart with an AC output. 

I couldn't find the 5 volt one, and I know I don't have a 6.3 volt one. I did find a 9 volt and a 12 volt. If the CRT filament is 300 mA that means 21 ohms. I doubt it is less than 300 mA so that is where I'll start. I'll find or make a 21 ohm resistor and use the 12 volt one. We'll see how that goes.

>"The 6.3V transformer the OP fitted to fix the old scope likely had paper insulation and the core grounded. "

OH DEFINIELY !

The one in there is a typical I and E job and screws to the chassis. therefore when it is powered by a non-grounded circuit you get a shock off all the metal parts of the scope, and they are just about all metal parts. 

Maybe it would make a good alarm clock. With cardiac arrest you'll never be late again.

What's more, right when I discovered the fault it blew the chip out of my PC to TV convertor. I used to like watching videos on my bigscreen, but that thing is history now. Anyone need a Sony SD-186B that is very strong ? And it pisses me off that I had two of them but gave one away. But I guess I am lucky it didn't fry my old PC. Thing runs like lightning.