CRT - heater connected to cathode - 6.3V AC or DC

[Max DN]

Hello,

Reposting with a new title for good order, as I didn't get a reply before, probably because this post was comingoled in another post.

I have built a Scope Clock based on David Forbes design. I hand wound the transformer.

All secondaries, including for the heater are on the same core, so the voltage for the heather output depends on duty ratio, frequency and load on the other secondaries.

I noticed that even if I’m careful with the winding ratio, it’s difficult to get an exact 6.3VAC RMS under load and the output voltage would also depend on the overall load on the primary, being the regulated secondary winding on the same core (SMPS regulated to +250VDC). 

So now I’m thinking to use a +6.3VDC power supply that I had designed on the same board. I have rectified one of the secondary voltage to about +12VDC, then I stepped it down to +6.3VDC regulated. The step-down power supply works well this way. 

However when I connect the ‘IsolatedGround’ from the 6.3VDC power supply to the Cathode PIN of the CRT (internally connected on this tube), the power consumption increases and the focus pot gets hot very quickly, taking the a full 600mA load. 

So, clearly something isn’t right in this last step, given the huge voltage differential between CATHODE and IsolatedGround.

My schematic attached.

>> Any suggestions on how to connect the +6.3VDC isolated power supply to the Cathode pin of the CRT welcome. I’m sure I’m missing something obvious.

>> Also, any tips on what RMS voltage I should read (with and without load) if I power the heater straight from the secondary. This works well when connected to the Cathode pin of the CRT, I can even read 6.2VRMS but a test light bulb of 6.2V 500mA got quickly very dark when receiving 5.7V under 450mA load, not good.

So, not testing it on the CRT again until I get this right. I manage to get a green dot on the CRT, which is a good sign. But I need to get the heater voltage right before I plug the CRT in again.

Thank you

[gregebert]

I see a +5 v supply going to the filament supply, but there is no mention of that being isolated like the 12V_isolated supply. I suspect there is missing isolation somewhere. The filament is biased near the cathode voltage, which is several hundred volts negative.

[Max DN]

That was a mistake, I have corrected the schematic, attached. Grahame pointed it out too. Apologies.

What I don't understand is: is it even possible to connect IsolatedGnd to GridGround? There is 1.2k Volt differential. I see Sgiteach does it but Grahame is using two different transformers for the highvoltage supply vs heater supply and then he connects IsolatedGnd to GridGround, I'm trying to do that from the same core, maybe isn't possible? What I don't understand is that if I connect the heater secondary (AC voltage) to GridGround, it works all fine.

If I connect a resistor between IsolatedGnd and GridGround it gets hot very quickly, rightly so...

I must be missing something very simple here...

[gregebert]

Where is the power to the regulator for the +5v isolated supply coming from ? That source needs to be isolated as well.

[David Forbes]

The Ccope Clock has a rather unusual power supply. It makes a 100kHz square wave for the tube heater. You can't measure it as 6.3V RMS because it's not a sine wave.

When I designed and tested the transformer, I went by the color of the orange heater glow to get the voltage correct. It seems to work well with the original SC100 clocks, as their CRTs still glow after 20 years of continuous operation. 

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[gregebert]

20 years ? Amazing. How is the phosphor holding up after 20 years ? Do you dim the CRT (ie, with a PIR sensor or manually turning-down the brightness) , or just let that run as well ?

I have an 8SJ31J CRT clock kit I bought from an overseas seller (it wouldn't surprise me if they plagiarized your design, David....), and I'm reluctant to keep it running a lot out of concerns for the phosphor lifetime.

[Max DN]

Found my error! I had retrofitted a 5V linear regulator to get Isolated+5V on my schematic. But I hadn't realised that pin 8 of the optocoupler was still connected to +5V non-isolated on the PCB! All good now. Thanks everyone.