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Scopeing a Full or half bridge

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LAN_403

Goodchild

Fri Sept 04 2009, 04:00PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Hello I am new to this form and new to tesla coiling in general and I searched this forum to try and find some help.

I recently built an SSTC it works very well, but like most coilers I was pulled in by the tempting taste of DRSSTCs.

My question is this, when scoping a bridge on a tesla coil whether it be half or full and on a SSTC or DRSSTC I herd that
you have to use an isolation transformer between mains power and your coil so that you can GND one side of the
bridge. Is this true? If not what is the best way of going about scoping a bridge on a SSTC and DRSSTC?

Thanks

Eric Goodchild

Sulaiman

Fri Sept 04 2009, 09:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

The 'best' way is to use an isolated differential probe = expensive
The 'worst' way is to remove the earth connection in the 'scope
(Please DO NOT put your life at risk like this)

You can use the tips of two 'scope probes with the 'earthy' side not used,
set your 'scope to Y1 + Y2 with Y2 inverted. (Y1 - Y2)
(most 'scopes can do this as it's a common requirement.)
Use x10 setting on the probes and set gain to an appropriate range
(e.g. 5V/div. x10 = 50V/div.)
Connect both tips to ac mains and adjust the variable gain of Y1 or Y2 until NO ac waveform is visible.
Now the 'scope will display the difference in voltage between Y1 and Y2.

Goodchild

Sat Sept 05 2009, 12:49AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Ok I thing I got it, but just to be safe ill just clarify before I go hooking up my scope to ac mains

Ok so i will set my scope to these settings: I will set it to "ADD", "CH 2 INVERT", "Both probes set to 10x",
but as for the gain I don't think my scope can be set for 50V/div. (sorry i'm a bit of a nooby when it comes to scopes as well)

here is a pic of my scope, I believe that at 10x it can only read 20v/div.

291683092 O

Thanks

[Edit: God almighty that was a huge picture]

Sulaiman

Sat Sept 05 2009, 02:50AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I meant the 'scope is set to (for example) 5V/div and with the probes at x10 that's 50V/div effective.
Other than that everything should be ok.

You could/should practice on lower voltages first to get the feel for the technique.
And be SURE that the earthy side of the probes CANNOT touch any of the live circuitry.
If the probes have clip-on earth leads remove them for safety,

P.S. for high frequency signals you may need to adjust the trimmer capacitor in the probe on x10
to really cancel the common-mode signal.

Arcstarter

Sat Sept 05 2009, 07:02PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I have the exact same scope :D. Nice simple old scopes.

Anyway, i see that your level knob is i guess broken off? And that is why you've set it to TV field to make the waveform actually show up?

You should try to fix that knob. The knob hooks straight to the potentiometer. That should be easy to fix with some superglue or something. The scope is very easy to take apart, too. I had to tighten up channel one's BNC connector, as well as put in a new resistor, the old one broke off :/.

Goodchild

Sat Sept 05 2009, 08:17PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

That is not a pic of my my scope just a pic I found online my scope has all of it's knobs and is in good working condition.

also Sulainman thanks for all your help i will start off with a 24VAC transformer.

Thanks much

Eric Goodchild

Goodchild

Sat Sept 05 2009, 08:47PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

I can't seem to get the waveform to disappear I have my trigger set to normal and the level set so that I can
see the waveform, also both channels volts/DIV are set to 5v so at 10x = 50v/DIV. I can't set my gain any higher on ether
channel 5v/div is as high as i can set it.

thanks for halpin this noob

Sulaiman

Sat Sept 05 2009, 11:08PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

ok, start at the beginning;

1) Both probes set to x10
2) Both channels set to 1 V/div DC, timebase to 2 ms/div,
MODE = Both, Norm, Chop
3) Both earth clips to one side of the 24 Vac transformer, both tips to the other.
You should see two identical waveforms, if not I forgot something !
4) When you have two identical waveforms change MODE from Norm to CH2 invert,
now one waveform will be the inverse of the other.
5) MODE from Chop to ADD
Now there should be a very small signal visible,
adjust CH1 OR CH2 Variable gain to minimise the waveform.

Now you are ready for differential mode;
6) Remove the earth connections and put one tip on each side of the transformer
you should see the 24 V ac signal.

If this doesn't work indicate at which step things are different and I'll try sort it out.

Goodchild

Sun Sept 06 2009, 02:16AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Thanks much that worked well. Now all I should have to do is repeat the same processes with AC mains and then hook it up to
my bridge.

Thanks for the help

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