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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'm planning on blowing up some 2N3055's while driving them using a signal generator and I thought I ought to take some measures to protect the sig gen from shorted transistors,etc.
Will this work?
The circuit on the left is what I have been using, my proposed modifications are on the right.
I was going to establish the values of the resistors and capacitors by trial and error, increasing the resistance and decreasing the capacitance until performance is affected, but might try some maths first, to try to get an idea of values.
the additional transistor (2SD2493) has a gain of 5000 and max. frequency of 60 mHz, so should be up to the job. Data sheet here:
I'll be driving the circuit at about 20-30 kHz.
EDIT: there is a MISPRINT in the schematics. The capacitance of the capacitor across the 2N3055 is 0.56 uF, I think (not 560 F). (The diode across it was 4 A I think, but I might increase the value when I increase the current)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Even 560 μF looks large to me, Ash.
It's around 500 something. I've not measured it tonight, but it's of the order that I was advised on here (when I was asking how to protect the 2N3055 from voltage spikes), and that is in previous threads on 4HV. I'll do some research in the morning, but whatever the units are, it's what I was advised to use.
I know I should have checked, but, once I found that extra transistor (2SD2493) in my 'parts bin' ( from a Toshiba TV I stripped recently) I wanted to know if the basic principle would work. (In my haste I mistakenly posted 560 F, which is obviously wrong).
Whatever the units are, I think I was advised to use 'around 470'. The closest I had was 560)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
Even 560 μF looks large to me, Ash.
It's around 500 something. I've not measured it tonight, but it's of the order that I was advised on here (when I was asking how to protect the 2N3055 from voltage spikes), and that is in previous threads on 4HV. I'll do some research in the morning, but whatever the units are, it's what I was advised to use.
I know I should have checked, but, once I found that extra transistor (2SD2493) in my 'parts bin' ( from a Toshiba TV I stripped recently) I wanted to know if the basic principle would work. (In my haste I mistakenly posted 560 F, which is obviously wrong).
Whatever the units are, I think I was advised to use 'around 470'. The closest I had was 560)
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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 uF 200V aluminium electrolytic stuck fast on it like a boiled pig's trotter stuck to a plate.
It's not an electrolytic, I think it's a foil cap of some sort. I'll post more details in the morning.
I was limiting current to 4 amps at the time, to protect the signal generator, because I knew the transistor wouldn't blow at that current. I'm planning to blow some transistors, to establish how much power they will take, so I want to add some protection for the sig. gen.
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 uF 200V aluminium electrolytic stuck fast on it like a boiled pig's trotter stuck to a plate of cold pork jelly.
It's not an electrolytic, I think it's a foil cap of some sort. I'll post more details in the morning.
I was limiting current to 4 amps at the time, to protect the signal generator, because I knew the transistor wouldn't blow at that current. I'm planning to blow some transistors, to establish how much power they will take, so I want to add some protection for the sig. gen.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Inducktion wrote ...
Why not use an optoisolator?
The question must be "Why over complicate things?"
Why spend time figuring out a protection circuit for your precious signal generator that you want to use to drive some 2n3055 transistors into oblivion.
Use a simple 555 or TL494 oscilator and get blowing up stuff right away.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
. I think 'around 470' nF (i.e. 0.47 μF) is a much more likely value .
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 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 TVS across the sig.gen. should provide additional protection in the event of failure of the freewheeling diode. Is my understanding correct?
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Protecting a Signal Generator while Driving Transistors.
