Freewheel diode

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throbscottle said:
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?
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Not sure what it does, but it's not a freewheel diode.
The circuit appears to be a flyback type, and if you put it across the transformer primary, you would kill the output voltage.
 
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D2 clamps the collector voltage to a diode drop below ground when the collector swings that low. In a flyback CRT display or TV set that would be called the damper diode.
 
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Dick Cappels said:
D2 clamps the collector voltage to a diode drop below ground when the collector swings that low. In a flyback CRT display or TV set that would be called the damper diode.
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Over here they were called efficiency diodes, it's a bit like the rectifier in a modern buck PSU - the transistor puts the energy in the coil, and the diode allows the energy to flow back round the circuit.

As here:

 
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crutschow said:
Don't see why that's needed(?).
If the voltage drops below ground, then the transistor will operate in the reverse direction to clamp the voltage.
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Why would you expect a transistor to do that?, and if it did it would likely be destroyed. It's basically a line output stage from a TV, and the transformer 'rings' with the applied pulse.
 
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Nigel Goodwin said:
Why would you expect a transistor to do that?
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Because a reverse connected transistor still acts as transistor (albeit with lower gain).
Nigel Goodwin said:
if it did it would likely be destroyed.
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Why?
The voltage would be low and the current, less than the normal forward current.
Nigel Goodwin said:
the transistor puts the energy in the coil, and the diode allows the energy to flow back round the circuit
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A flyback puts energy into the coil's inductance and that's transferred into the primary when the transistor shuts off, generating a voltage pulse on the primary that's transferred to the secondary as determined by the turns ratio.
Any current carried by the diode would be due to ringing after the main output pulse has transferred the energy to the secondary.
 
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crutschow said:
Why?
The voltage would be low and the current, less than the normal forward current.
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The voltage certainly won't be low - a TV line output stage, fed from 100V or so, produces a 1000V pulse on the collector of the transistor. In later years some LOPT transistors had the efficiency diode built-in - and if you replaced it with one that didn't have the diode, you had to add an external diode, or the transistor instantly failed.

If I recall correctly?, one (TO3) example was the BU208, and the BU208D - the D version having the diode.

But by all means, remove the diode and see what happens - monitor the input current and output voltage before and after, just in case the transistor does survive, to see if efficiency drops.

The circuit isn't tuned, so won't ring properly like a LOPT stage, so it would be interesting to see what happens.
 
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Does not seem like diode does much at all -



Of course I do not have all the correct P/N's, xfmr specs.

Timestep step at 1 nS, diode spec is good for 4 nS.


Regards, Dana.
 
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When I get my (now indoors!) workshop set up again I'll have to play with it some more.
As I understand it, when you stop applying voltage to an inductor, the back emf tries to keep current flowing in the same direction as it was whilst the voltage was applied.
But with this circuit, that can't happen - the current would have to reverse for the diode to do anything. What am I missing here?
 
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Dana, the transformer is hand wound on a small toroid. it only has to supply some tens of mA at about 3v or less. I tried it with an assortment of cores pulled from CFL bases but they weren't big enough. Settled on a very small E core eventually (can't remember if it's gapped but I don't think so). Don't know if this helps with your model!
 
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I used same number turns, in schematic, and 10 uH in primary, 1.4 uH in secondary's.

I looked up 13 digit filament current in VFDs, saw ~ 150 - 300 mA.


Regards, Dana.
 
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Q1 is a low-side driver... when it turns off the transistor collector->inductor connection will try to go +++ in the positive direction, so what would that diode do? I don't see it ever conducting the way it is now, and nothing is protecting the transistor. In the original, it's a BD137 which has a Vce rating of 60V, so since it's only switching 5V that probably explains why it never blew up.

If it was a high-side switch THEN it would go negative.
 
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crutschow said:
I was referring to the reverse "ringing voltage" not the normal flyback voltage pulse which reverse biases the diode.

Are you sure the diode isn't a Zener?
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It's not shown as one, and no reason for it to be one - it looks obvious it's 'supposed' to be an efficiency diode, as used in CRT TV sets.

However, it's quite likely that the circuit is just poorly designed, with the designer copying the idea from TV LOPT.

If it works?, don't worry about it - it's pretty crude and nasty anyway

Such modules were available from companies like TDK, and were often built-in a slightly larger IF can, but used a single transistor oscillator, with a feedback winding on the transformer, giving a fairly close sine wave output - and often included a high voltage winding to power the display as well. They did occasionally fail, and often their internal schematic was shown on the service manuals, inside a dotted 'can'. I've repaired a few over the years - usually just the transistor, and occasionally a failed rectifier for the high voltage supply.
 
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It's basically the same as the rectifier in a buck converter:

Buck converter - Wikipedia

en.wikipedia.org

The efficiency diode name was given to the component long, long ago - 1950's?, earlier? - I don't recall ever repairing any TV's that didn't have one. Quick google - supposedly the first (UK) production TV with one was the Pye B18T in 1948 - so earlier than the 50's.

You might like a read here:

History of the Flyback Damper

www.earlytelevision.org
 
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