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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
. I think 'around 470' nF (i.e. 0.47 μF) is a much more likely value - in fact, I'm not sure the circuit could be made to work at all with a big fat 470 μF 200V aluminium electrolytic stuck fast on it like a boiled pig's trotter stuck to a plate of pork jelly.
You are correct, as usual, PM. Values were taken for Dr. Kilovolt's schematic in this thread:
A TVS and opto-isolator in addition to the series capacitors and resistors would probably provide complete protection if my if my understanding is correct. possibly with an additional resistor across the output of the sig. gen.
I assume that the diode and capacitor across the 2N3055 should prevent the ground rail from going positive, so maybe the capacitor and resistor on the return to the sig. gen are unnessesary, and maybe I only need a zener instead of a TVS, but the way I see it, the more protection the better.
Even if I was to use a 555 or something instead of the sig. gen. I'd still want some protection.
The basic concept here is that by using a transistor with a gain of 5000 I can block excess current from getting to the sig. gen, (through the series capacitors and resistors) in the event of a failure when running this off a car battery, and the diode and capacitor across the 2N3055 should take care of voltage spikes. If this is the case then my proposed circuit above should provide protection. (probably just one capacitor and resistor is needed, between the output of the sig. gen. and the 5000 Hfe transistor?)
A Darlington transistor with a gain of 5000 might have stability problems - just a smidgin of feedback from poor layout etc, and oscillation will take off like a rocket. And I can't see why or how such a high gain device is necessary.
Could you say a few words about the signal generator? What is its ouput impedance? What is the ouput voltage, and can it be varied? What frequency range are you planning to use?
What is the transformer in the collector circuit? An LOPT, or what? The worst case that I can imagine is that a murderous-looking transient in the output might decide to couple itself into your signal generator during destructive testing, when unforeseen circuit re-arrangement could occur.
A common weakness in 2N3055 HV circuits seen around the web is inadequate base drive current, for which this veteran transistor has a great thirst. For example, you sometimes see a 2N3055 driven directly by a 555 timer, which can only supply 200 mA, when the 3055 may want as much as 7A on a hot day! If it was my problem, I'd drive the base with a transformer, as we see in TV HOT circuits, but this configuration is seldom seen in hobbyist designs for some reason.
Addendum: yes, TVS diode plenty fast! Choose a value just higher than the maximum output voltage of the sig gen.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
. A Darlington transistor with a gain of 5000 might have stability problems - just a smidgin of feedback from poor layout etc, and oscillation will take off like a rocket. And I can't see why or how such a high gain device is necessary.
The basic Idea is that, due to the very low current required to drive the Darlington, I can place a series capacitor and resistor between it and the sig. gen., which will also block excess current getting to the sig. gen. in the event of re-arrangement of the circuit due to failure. (the sig. gen charges the capacitor, which then supplies current to the Darlington via the resistor)
Proud Mary wrote ...
Could you say a few words about the signal generator? What is its ouput impedance? What is the ouput voltage, and can it be varied? What frequency range are you planning to use?
I'm planning on using 20-30 kHz. I think I mentioned this earlier. Output voltage can be adjusted.
I've attached a pdf of the Farnell LFM2 operating manual.
]lfm2.pdf[/file]
Proud Mary wrote ...
What is the transformer in the collector circuit? An LOPT, or what? The worst case that I can imagine is that a murderous-looking transient in the output might decide to couple itself into your signal generator during destructive testing, when unforeseen circuit re-arrangement could occur.
Yep, LOPT. This is basically a modified single transistor flyback circuit, but driven by a sig. gen. rather than feedback winding, so that frequency can be controlled.
Proud Mary wrote ...
A common weakness in 2N3055 HV circuits seen around the web is inadequate base drive current, for which this veteran transistor has a great thirst. For example, you sometimes see a 2N3055 driven directly by a 555 timer, which can only supply 200 mA, when the 3055 may want as much as 7A on a hot day! If it was my problem, I'd drive the base with a transformer, as we see in TV HOT circuits, but this configuration is seldom seen in hobbyist designs for some reason.
It's funny you should mention that, I discovered this myself by accident a couple of days ago, I found what, according to the schematic, was a GDT driving a 2SD2553 (datasheet here: )
in a Toshiba TV I was stripping. It was the first time I've come across a GDT driving a BJT.
The 5000 Hfe transistor (also from the same TV) can take 6 or 7 amps C-E, this is one reason I thought it might be suitable.
Proud Mary wrote ...
Addendum: yes, TVS diode plenty fast! Choose a value just higher than the maximum output voltage of the sig gen.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, this is what I'll probably do.
The sig. gen. ouputs 5V peak to peak square wave.
The capacitors C1 and C2 protect the TVS from overcurrent (In the event of it shorting out a 12V car battery), and the TVS clamps the voltage across the sig. gen. to 6V.
I'll probably order some 6V TVS's and maybe some bigger freewheeling diodes (What would be a good value. Is 5A enough?)
In the meantime I might try it on reduced power (current limited) to see if it works.
Should I also put a resistor in parallel with the TVS? Do I need R1 and R2? Do I need C2? Would a zener do instead of a TVS?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
What is the output impedance of the signal generator? Depending on that you might want to put some extra series resistance to make sure you don't blow the TVS diodes entirely if you accidentally crank up the amplitude on the signal generator.
I wouldn't worry about DC blocking both of the outputs of the signal generator, if you want DC isolation doing it in just the "signal" path should be enough. Is the low-side output isolated or grounded to chassis/earth?
The value of your freewheeling diode depends entirely on how much current you put through the load. Also the diode "antiparallel" with the 2n3055 isn't doing anything right now, if it's to protect the BJT from the leakage inductance of the transformer then it needs to be parallel with the transformer coil, and still pointing upwards, because the back-emf from the transfromer primary will drive the collector node of the 2n3055 positive with respect to Vin.
And if you are driving a true flyback transformer which has rectification on the output side in the proper direction for flyback mode then you don't want a freewheeling diode at all on the primayr.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Mattski wrote ...
What is the output impedance of the signal generator? Depending on that you might want to put some extra series resistance to make sure you don't blow the TVS diodes entirely if you accidentally crank up the amplitude on the signal generator.
Output impedance is 60 Ohms. Square wave output is 12V peak to peak. I will measure it myself, though, just to be certain.
Mattski wrote ...
I wouldn't worry about DC blocking both of the outputs of the signal generator, if you want DC isolation doing it in just the "signal" path should be enough. Is the low-side output isolated or grounded to chassis/earth?
I think it is isolated. The earth connection is separate. It has four terminals, earth, two outputs and one marked 'sync.'.
Mattski wrote ...
The value of your freewheeling diode depends entirely on how much current you put through the load. Also the diode "antiparallel" with the 2n3055 isn't doing anything right now, if it's to protect the BJT from the leakage inductance of the transformer then it needs to be parallel with the transformer coil, and still pointing upwards, because the back-emf from the transfromer primary will drive the collector node of the 2n3055 positive with respect to Vin.
And if you are driving a true flyback transformer which has rectification on the output side in the proper direction for flyback mode then you don't want a freewheeling diode at all on the primayr.
Now I'm confused. Last time I asked about this (I had suggested using a freewheeling diode across the primary) I was told this wasn't required, and was referred to this circuit:
I used the term 'freewheeling diode' because that is the common name for a diode across the emitter and collector of an IGBT, and is there to protect the IGBT from reverse voltage spikes when driving inductive loads. (Are you able to clarify this point?)
I was told in an earlier thread that the capacitor is required due to the time it takes for the diode to start conducting.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ...
Now I'm confused. Last time I asked about this (I had suggested using a freewheeling diode across the primary) I was told this wasn't required, and was referred to this circuit:
I used the term 'freewheeling diode' because that is the common name for a diode across the emitter and collector of an IGBT, and is there to protect the IGBT from reverse voltage spikes when driving inductive loads. (Are you able to clarify this point?)
I was told in an earlier thread that the capacitor is required due to the time it takes for the diode to start conducting.
Diodes like this are common in a half or full bridge where the current in an inductive load will be travelling away from the collector node of or toward the emitter of another one. You can think of inductors as giving momentum to current (in a way), so when you turn the transistor off current keeps wanting to go in the same direction. If it was traveling towards the collector then the voltage at the collector will spike up, so the diode in the circuit you have will not conduct. If using a half/full bridge then there are times when an inductor will want to pull current away from the collector, which lowers the voltage at the collector and allows the diode to conduct, saving the transistor from breakdown.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
I think the transformer idea mentioned earlier is one of the best if driven by a discreet transistor driver circuit or driver ic. For two reasons.
Firstly most signal generators aren’t designed to drive low impedances or much current.
As well as isolation from the high current supply, if anything backfires through the gate drive transformer, then I suppose it could take out the driver ic or whatever, but then a resistor and a TVS to gnd between the sig gen O/P and the driver’s input would make it unlikely for anything to get past that.
A step down drive transformer gives a higher current low voltage, better suited to driving a bipolar transistor from a voltage source derived from the supply rails, which kind of has already been pointed out.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks Mattski. I think we've cleared up a misconception that seems to be widely held on here.
Electra wrote ...
I think the transformer idea mentioned earlier is one of the best if driven by a discreet transistor driver circuit or driver ic. For two reasons.
Firstly most signal generators aren’t designed to drive low impedances or much current.
I ran some preliminary tests last night and managed to drive the Darlington with ~1mA (~3.5V through a 3.5kOhm resistor). I'll be running some more tests with capacitors (for DC blocking) between the sig. gen. and the resistor, so I won't be drawing much current from the sig. gen. I not sure whether to put another, smaller resistor between the sig. gen. and capacitor, though.
I plan eventually to do away with the sig. gen., but it is useful for varying the frequency while checking for the most efficient frequency, etc.
Electra wrote ...
As well as isolation from the high current supply, if anything backfires through the gate drive transformer, then I suppose it could take out the driver ic or whatever, but then a resistor and a TVS to gnd between the sig gen O/P and the driver’s input would make it unlikely for anything to get past that.
A step down drive transformer gives a higher current low voltage, better suited to driving a bipolar transistor from a voltage source derived from the supply rails, which kind of has already been pointed out.
I do have a transistor driver transformer from the HOT circuit of a TV which I plan to do some tests with. (I suppose technically it's not a GDT as it drives the transistor's base. (BDT, maybe?.) I agree that it would make more sense.
I've posted an updated schematic below.
I'd appreciate some more comments, eg does it need a resistor in series with the TVS's?, Is the TVS, capacitor and resistor across the 2N3055 necessary? Do I need resistors R3 and R4? any other comments?
EDIT: Some here may find this interesting.
This is the waveform used to drive the standard primary winding in a flyback driven by a BJT driven by a GDT (not that a BJT has a gate, but.....)
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Protecting a Signal Generator while Driving Transistors.
