Freewheel diode

[throbscottle]

I built a variant of the circuit halfway down this page: http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/ and it works ok, but I'm mystified by the position of the freewheel diode, D2. Why has he put it across the driver transistor, and not the transformer primary?

TIA

 

[AnalogKid]

History of the Flyback Damper

www.earlytelevision.org

Extra like for tech geezer porn.

AND, the museum is about 6 miles away.

ak

 

[Nigel Goodwin]

Extra like for tech geezer porn.

AND, the museum is about 6 miles away.

ak

Cool

Where my daughter lives in the Netherlands she's walking distance from a small radio museum - which rather bizarrely is in a bungalow on a housing estate?.

Radio Museum, Hengelo

Museum Electro en Radio Nostalgie

The owner was very excited to have visitors from England, and insisted I signed his visitors book.

If you ever visit, be aware the owner doesn't speak English - in fact hardly anyone round there does, as it's close to the German border presumably German tends to be the second language?. Very difficult in shops or restaurants, unless you have a daughter or son-in-law with you My daughter, along with a friend from Eastern Europe somewhere (I forget where) are currently taking their Dutch Language tests, working towards Dutch citizenship - my daughter has managed to avoid one of the tests, as it's about job interviews and getting a job, and she already has a permanent position at the University.

 

[Dick Cappels]

It should be noted that in Horizontal deflection circuits the deflection yoke is AC coupled to the switching transistor meaning that SOMETHING has to conduct for the later approx 40% of the forward scan time, and that something was usually a diode.

In the boost converter, if cleanly done, there should be little or no reverse current for the switch to deal with. The voltage across the switch should be above ground throughout the cycle. I thank Danadak for his idea in post #12 of why the diode should conduct at all.

 

[throbscottle]

You might like a read here:

History of the Flyback Damper

www.earlytelevision.org

Very interesting. I need to read it twice, though. Back when I used to strip down TV's for parts, I tried drawing out a line output circuit from the board. Did not make sense to my 16yo brain! Now I see why...

But I take inspiration from your mention of TDK cans - maybe self oscillating is the better route.

 

[Nigel Goodwin]

Very interesting. I need to read it twice, though. Back when I used to strip down TV's for parts, I tried drawing out a line output circuit from the board. Did not make sense to my 16yo brain! Now I see why...

But I take inspiration from your mention of TDK cans - maybe self oscillating is the better route.

They were used in a lot of Sony products that used florescent displays, VCR's, DVD's, Audio Surround Systems etc.

I presume they were fairly universal across most makes that used florescent displays, but we were a Sony dealer

 

[throbscottle]

These ones? https://xdevs.com/doc/HP_Agilent_Keysight/3458A/pdf/TDK-CD1867N.pdf

 

[Nigel Goodwin]

Yes, those are the sort of things - although often they were in large IF style cans. TDK also made bias osc/erase cans for cassette recorders.

 

[MrAl]

I built a variant of the circuit halfway down this page: http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/ and it works ok, but I'm mystified by the position of the freewheel diode, D2. Why has he put it across the driver transistor, and not the transformer primary?

TIA

Hello,

You can not put a diode across a transformer primary unless maybe it was driven with unipolar signals and there was absolutely no chance of reverse voltage across the bipolar CE junction.

The diode across the transistor protects the transistor from reverse voltages from the transformer which result because of the inductance in the transformer. It's a standard practice and usually necessary. When using MOSFETs (like N MOsFETs) you dont always see diodes like that because the MOSFETs have diodes built it, but even then you may see external diodes when the maximum efficiency is a goal or the transistor dissipation is getting near max.

In a bridge circuit for example you always see diodes unless again the transistors are MOSFETs. Bipolars dont have built in diodes so they need added diodes.