Protecting a Signal Generator while Driving Transistors. / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Protecting a Signal Generator while Driving Transistors.

first previous 2 3 4 5 next

Move Thread

LAN_403

Proud Mary

Mon Aug 01 2011, 11:13AM

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

Ash Small wrote ...

Well, the manual for the sig. gen. is a bit vague (I posted a link earlier in the thread), but according to my 'scope the output does go negative.

The circuit diagram shows the dual rail 10V Â± supplies needed for the output to swing around 0V.

Ash Small

Tue Aug 09 2011, 04:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've now mounted the circuit on a breadboard and added a 2N3055 and four resistors as per the schematic below.

My 40 year old 2 channel 10MHz 'scope is showing some voltage spikes of around 28V, but, surprisingly, it is showing the highest spike when the 2N3055 switches 'on'. It is also showing a lot of ringing, both from the spike when it turns 'on, and when it turns 'off'. I think the frequency of the ringing is indicative of 'stray capacitance', maybe from the probe itself, maybe from the 5000uF smoothing caps I'm using on the supply, or maybe just from the longish wires attached to the 2N3055.

The coil only has ~1 amp average current going through it at the moment (2A @ 50% duty cycle).

The first photo shows the peaks and ringing at 15mm arc length, and the second shows the peaks and ringing at 5mm arc length. The second also shows the sig. gen output, which is deformed because my 'cheap probe' (Â£4.95) is only good up to 20kHz (I'm running at 27kHz at the moment, as this seems to be the 'natural' frequency for the coil, and is X1 (~3V Pk-Pk), My 'expensive' probe (Â£19.95) is good for 100kHz and is set at X10. The supply I'm using is 12V and the spikes are over twice this voltage.

Next step is to see if running from a 12V car battery with current limiting (eliminating the smoothing caps) and shortening the leads to the transistor has any effect on the ringing, but it does seem that my 'scope is sensitive enough to display the transient spikes, albeit not very clearly.

Sulaiman

Wed Aug 10 2011, 12:25AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I find it best to drive the main transistor via a resistor from some +V and use the 'driver' transistor to short base-emitter as it's the turn-off that usually needs most attention.
A 'pre-driver' transistor that shorts the driver base-emitter (resistor to +V from driver base) gives a non-inverting overall response which keeps the main transistor off until the pre-driver is turned on, with the benefit of low power drive requirement for the pre-driver.

Ash Small

Wed Aug 10 2011, 03:07PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

I find it best to drive the main transistor via a resistor from some +V and use the 'driver' transistor to short base-emitter as it's the turn-off that usually needs most attention.
A 'pre-driver' transistor that shorts the driver base-emitter (resistor to +V from driver base) gives a non-inverting overall response which keeps the main transistor off until the pre-driver is turned on, with the benefit of low power drive requirement for the pre-driver.

I don't fully understand your second sentence.

Do you mean like this?:

Sulaiman

Wed Aug 10 2011, 10:39PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

More like this

e.g.
R1 = 22R 10W
R2 = 220R 1W

With no input signal R2 turns on Q2 which keeps Q1 turned off.

Ash Small

Thu Aug 11 2011, 03:54PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Won't that allow the signal generator to 'see' the voltage spikes, or, at least, the ringing, across the coil?

Having the signal inverted isn't a problem, as far as I can see, as the duty cycle is 50%.

Ton of the Darlington is 0.8uS, and Toff is 1.1uS. Tstg is 6.2uS

EDIT: Also, won't the ringing be 'seen' by the base of the Darlington, and 'interfere' with it? (Hfe is 6500 to 20000)

Sulaiman

Thu Aug 11 2011, 11:59PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Do not use a darlington transistor to turn off the main transistor
as the collector-emitter on voltage is too high,
use a single npn.

Any 'spikes' will not harm your sig-gen as C1 & R3 will protect it.

Ash Small

Fri Aug 12 2011, 08:11AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

Do not use a darlington transistor to turn off the main transistor
as the collector-emitter on voltage is too high,

Because there will still be enough voltage across the B-E of the 2N3055 to keep it on?

I'm currently switching the Darlington with ~0.5mA, so that R1 can be large, and C1 can be small, to provide maximum protection for the sig. gen.

A single NPN capable of driving the 2N3055 to destruction will need orders of magnitude greater base current.

Below is the saturation curve of the Darlington, and the 'On' voltage curve of the 2N3055. This 'seems' to indicate that, even at 0.2V B-E there will be 2.5A C-E, and the saturation voltage of an NPN will still be around 0.3V, which 'seems' to indicate that the 2N3055 will still be 'On' even with an NPN across the base and emitter.

Ash Small

Wed Aug 17 2011, 08:44PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've now established that something else is limiting the current flowing through the LOPT primary, maybe the impedance of the primary, maybe the Darlington isn't fully turning on.

I'll try removing the 100Î© resistors and see what's happening between the collector and emitter . The waveform looks a bit suspect at the moment.

If that has no effect I'll try removing some turns from the primary.

BTW, it's now running best at 70kHz instead of 27kHz. All I've changed is running from a battery charger with 5000Î¼F added smoothing capacitance to running from a car battery, with and without current limiting. It also still seems to run better with the smoothing caps fitted, bizzare.

Ash Small

Thu Aug 18 2011, 10:10PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'ts now running best at just under 80kHz, and I've also had it running at around 130kHz. (The LOPT is from a large Toshiba TV, about ten years old)

I'm coming to the conclusion that C1 is too small. I'll try and find one around 5 or 10uF. I've already removed the two 100Î© resistors.

first previous 2 3 4 5 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob