High Voltage Power Supply for CRT

[Noor_99]

I want to achieve 7KV and 25KV for the CRT without using a flyback transformer.
Is it possible to achieve it using doubler circuits with a 700V output from a chopper circuit?

 

[Colin]

You can use a tripler from an old black and white TV.

 

[Noor_99]

You can use a tripler from an old black and white TV.

assuming that i have to design multiplier circuit on my own, i assume i would be needing high voltage diodes?
How much high voltage can be practically achieved using multipliers?

 

[Colin]

You can get up to 10 times but the current gets less and less.
Colour TV's had triplers too. This is the cheapest and best way to get an output as it has all the insulation and the anode connector.

 

[Nigel Goodwin]

assuming that i have to design multiplier circuit on my own, i assume i would be needing high voltage diodes?
How much high voltage can be practically achieved using multipliers?

As much as you want, but starting from a lower voltage will obviously require a lot more components - you obviously will need high voltage (fast recovery) rectifiers and capacitors.

Can't you put an extra winding on the chopper transformer?.

 

[Noor_99]

As much as you want, but starting from a lower voltage will obviously require a lot more components - you obviously will need high voltage (fast recovery) rectifiers and capacitors.

Can't you put an extra winding on the chopper transformer?.

My input is 28V DC with a very low current. I am concerned whether by using doublers current will drop so low that it wont be enough for CRT.

 

[ronsimpson]

My input is 28V DC with a very low current.

"Very low current"
So your 25,000 volts will have an output current of:
("Very low current"/1,ooo) * Efficiency=

Efficiency is maybe 25%.

To output 100uA might take 28V at 500mA.
How big a CRT are you using?

 

[Nigel Goodwin]

My input is 28V DC with a very low current. I am concerned whether by using doublers current will drop so low that it wont be enough for CRT.

I would imagine if your input is only 'very low current' you haven't got a prayer

You mention 25,000V, so that suggests a colour CRT - EHT on those is normally limited to 1mA maximum, and 1mA at 25,000V is 25W - so probably well over an amp from your 'low current' 28V supply.

What are you trying to do?, usually with such 'vague' questions it's something 'silly', and either impossible or the complete wrong way of thinking about it.

 

[Noor_99]

I would imagine if your input is only 'very low current' you haven't got a prayer

You mention 25,000V, so that suggests a colour CRT - EHT on those is normally limited to 1mA maximum, and 1mA at 25,000V is 25W - so probably well over an amp from your 'low current' 28V supply.

What are you trying to do?, usually with such 'vague' questions it's something 'silly', and either impossible or the complete wrong way of thinking about it.

Alright.. Silly it may be. Just trying to experiment with a high voltage power supply, by not using a flyback. Its a very old radar CRT and the input source is 28V but how much current the load is consumig? I do not know. I am just assuming the current req by any other CRT based displays. Had I known all the answers, I would not have asked.
So If i try to achieve high voltages through voltage multipliers, would that do? (And I'd be converting it back to ac at the end??)

 

[Nigel Goodwin]

So If i try to achieve high voltages through voltage multipliers, would that do? (And I'd be converting it back to ac at the end??)

The output of a Walton-Cockcroft multiplier is DC - assuming you have 700V AC, at high frequency, and sufficient power, then it should be possible.

 

[ronsimpson]

How much power a CRT pulls is related to how much white there is on the CRT.
If you are building a radar system there will be mostly black so low current.

 

[Noor_99]

Thanks. That greatly improved my concepts. Though I am still not clear about the AC or DC requirements of a CRT. The heater and grids, do they consume AC or DC voltages. 25kV from a flyback goes straight to CRT so its AC. But as Colin suggested triplers above, its output will be DC. As Nigel pointed out that if I achieve 700 V AC and then use multipliers, it should work. Can you please explain it a little.

 

[ronsimpson]

Do you have a schematic showing how the CRT was used?
OR
Do you have a data sheet for the CRT?

Most of the voltages are DC. The only AC is the video signal.

 

[Noor_99]

Do you have a schematic showing how the CRT was used?
OR
Do you have a data sheet for the CRT?

Most of the voltages are DC. The only AC is the video signal.

I do not have any datasheet. I have this Schematic though, for CRT. However, no schematic for the Power Supply which provides 700V, 7KV and 25 kV. I have deigned a circuit for 700VDC. I even tried taking out 7kV and 25 kV from a flyback. But so far 25kV is an issue. Plus, flyback available in market are bigger than my required size (I am using a TLFA01365). Can i use a custom made flyback in smaller size?

 

[Nigel Goodwin]

Though I am still not clear about the AC or DC requirements of a CRT. The heater and grids, do they consume AC or DC voltages. 25kV from a flyback goes straight to CRT so its AC.

No, it's DC - if there's no external rectifier then the rectifier is inside the transformer. In fact a common technique at one time was 'diode-split transformers', these had multiple windings inside, along with the rectifiers for a tripler, and used the capacitance between the windings for the multiplier capacitors.

So everything is DC, although the heater can be either (and is usually AC).

 

[ronsimpson]

Do you have the yoke? (deflection coils) (The coils on the neck of the CRT)

 

[Nigel Goodwin]

Do you have the yoke? (deflection coils) (The coils on the neck of the CRT)

He said it was a very old radar tube - which means it 'could' be electro-statically deflected - which was why radar units were often converted to oscilloscopes.

 

[debe]

The one in his picture is a Rasta scan type with fixed deflection coils, probably late 80 to 90s radar. earlier ones used a rotating deflection yoke on the tube neck.

 

[ronsimpson]

'could' be electro-statically deflected

fixed deflection coils,

Too many questions.

 

[debe]

This is a Tripler from early Philips colour TVs (threw my stock out but kept this one for old time sake). These are still available & you feed 8KV ac in & earth the blue & white wires Theres 8KV DC available from Uf & the long lead gives 24KV DC. Be a help if we knew what Brand & model radar we are dealing with here. Most Radars ive worked on use a separate EHT supply in a metal box & its full of a silicone like compound, for obvious reasons in a Marine environment.