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Patrick

Thu Mar 31 2016, 09:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I like schematics and nice repeatable results, but many of us get stuck mucking around and guessing about the internal potted wierdo elements. So when I was informed of the HR Diemen miracle site, I was elated.

So the question is . . . I see in many flybacks have this one particular layout.

This with a transistor, resistors, a center tap marked "Vpp", and then a "Vcc"

why is there a center tap that is so high in voltage ?

heres mine the HR 6329

Notice the upper left primary side is exactly the same. but theres a 850 Vpp ! Whoa.

I always prefer using the internal primary winding. How much input voltage does it take to drive a primary that has a dam 850 V tap.

and . . .

I figured all of these out but for the "TYPE" with the 110 value.

Erlend^SE

Thu Mar 31 2016, 10:56PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Simple enough: add a diode + capacitor and you get high voltage dc (1000v ish) that is useful for dynamic focus control etc.

A separate winding would take 2 pins, while this only use one pin and requires no extra windings, just a tap.

Patrick

Fri Apr 01 2016, 02:18AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

so the Vcc input must be higher than that tap, 85 or 850 in these two cases ?

Erlend^SE

Fri Apr 01 2016, 07:53AM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

No, actually lower.

When the transistor turns off there is generated a voltage spike, and the winding give a gradient between the spike voltage and vcc over the winding length.

Proud Mary

Fri Apr 01 2016, 08:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

HR Diemen manufacture a dedicated LOPT test instrument, and some of the figures in the box on the datasheet relate to settings and findings on that instrument. MAT is the EHT o/p in kV.

An autotransformer tap on the LOPT primary, and assorted secondaries with and without taps, are used to supply power to receiver subsystems where voltages in the high tens and hundreds are required.

A salvaged LOPT suddenly looks hopeful when its HR Diemen equivalent is discovered!

Patrick

Sat Apr 02 2016, 05:19AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I just wish I knew how to operate the original primary. like which voltage it expects. I see the transistor, but I use the alternate primaries.

Proud Mary

Sat Apr 02 2016, 09:28AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

I just wish I knew how to operate the original primary. like which voltage it expects. I see the transistor, but I use the alternate primaries.

All about LOPTs:

A few helpful points: HR Diemen LOPT type 110 means that the HT+ (B+ to you) supply across the series HOT and primary is 110V DC. Type 90 means 90V, and so on. In practice, the physically larger LOPTs for larger CRTs tend to be of the 110V type.

As the HOT switches off, potentially destructive voltage spikes in the low kV regime would appear across the primary, so the values of the timing components are organised to reduce them.

The CRT field windings are part of the oscillatory circuit formed by the primary and the Booster Diode, also known as the Efficiency Diode, or Damper Diode. If one was to copy the very reliable LOPT circuitry used in TVs, an inductor of suitable value would have to stand in for the CRT field windings.

TV LOPT circuits can and often do provide tens of thousands of hours of faultless operation, so there is clearly something for HV enthusiasts to learn from them.

Erlend^SE

Sat Apr 02 2016, 01:09PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

You are basically dealing with a variant of the flyback converter (I have a board from a monitor that use a actual UC3842, a common flyback converter controller).

Unless you go the full original quasi-resonant circuit, you NEED voltage feedback (one of the other windings, or the HV resistors inside) or spark-gap to stop it self-destroying.

Patrick

Sat Apr 02 2016, 05:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

heres some pics

does the above look like the below?

HV scope pics...

Current pulse, zoomed in. 6.6V means 6.6V / 0.5Ohms = 13.2 amps.

I just cant repeat that result.

Proud Mary

Sat Apr 02 2016, 06:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Given that the waveform in the diagram has been idealised for clarity, I'd say they were substantially the same.

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