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A tribute to the UD2.7x

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Finn Hammer

Sun Oct 02 2016, 12:59PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

I bought a bunch of UD2.7C boards, now as long as stock lasted, and have just finished populating the boards as well as testing them and built them into enclosures. An investment into the future, which will allow me to "just pick a driver" from the shelf, whenever I want to build a nice Tesla Coil.

Since I have added a few personal touches to the build, I thought i would share, so here goes:
My focus was on the OCD circuit, in particular the adjustment of it during setup of the coil.
It never appealed to my attitude, to have to open up the box, find the reference sheet telling what voltage corresponds to which current trip point, and then handling the diddle stick with one hand, trying to hit both ground and test point at the same time, with 2 DVM probes in the other hand.
Instead, I bought cheap DVM modules from the far east traders, and incorporated them into the enclosure, together with a nice 10 turn wirewound pot at the front panel:

To keep in line with the original design, where all peripherals are connected via headers, I soldered a 4pin header into the vacant spot of the HP fiber receiver, to bring out V+, Ground, Pot Wiper and Pot top connection. I just had to cut one trace going from Receiver to HC14, pin1

The corresponding jumpers on the back of the board can be seen here:

The extra circuit can be seen here:

The parts are arranged like this, where the VDM Cal. Pot. is glued to the back of the OCD Pot with 5sec. glue.

My test setup looks like this, and contains a mock primary coil made from some fancy wire out of an elevator, it is a multistranded flat cable with several 1.5mm^2 wires. The resonant cap is just a single 1.5uF 940C12W1P5K-F, the motor is an old one from the worlds first musical DRSSTC's, with Fairchild 40N60A4D switches, justifiably legendary switches which still reliably conduct 600APeak, the interrupter is Steve Wards old triple triple 5 circuit.
Voltage across switches is monitored with a Metrix MX 9003 Diff. Probe, and current is Monitored with a Pearson 101 current Monitor, both signals perused with a Rigol DS1054,

As you noticed, the current burden resistors are 2x10ohm, for a total of 5 ohms, and this gives nice numbers, which I appreciate.
With a 400 turns (20 x 20 cascade) current transformer I get 1 amp @ 400 ampsPri, and 5V across the sense resistors. With a 7.5Volts headroom, the 400:1 transformer is good up to 600A.
The calibration procedure goes as follows:
I set the trip voltage to 5 volts and turn up the supply voltage untill the OCD kicks in, then I verify that this trip value, (400A) is correctly returned with the Pearson.
Then I adjust the VDM Calpot till the VDM shows 400mV, which then corresponds to a 400A reading.
I turn the OCD pot to 600, and check that it kicks in as expected, which it does, then I test it at various other settings, to verify accuracy, and it is within a few amps, so more than sufficient accuracy.
A coil that uses bigger switches will need another current transformer ratio, for example the big CM600 type coils, which should be allowed up to 3000A would need a 2000:1 (50 x 40 cascade fx.), and the VDM will need recalibration to show 2V " 5V acrosss sense resistors.
If you build a UD2.x board, I suggest you replace the R8 resistor with a 3.3k type, particularly if you are going to do a bit of debugging, otherwise the blue on board ON-Led will tire your eyes prematurely, at least that is how I felt it.
Happy building, and a warm thanks to Steve Ward, Eric Goodchild and Gao Guangyan for evolving this wonderfull driver board.

Cheers, Finn Hammer

nzoomed

Tue Oct 04 2016, 08:35AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

WOW!

Those are awesome mods.
Yeah, I too find it a bit of a pain when setting the OCD too. This mod makes it easy!

BSVi

Tue Oct 04 2016, 05:25PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

Can you please show what gate waveform took with that differential probe looks like? Is it any cleaner than with conventionl probe?

Finn Hammer

Tue Oct 04 2016, 07:47PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

BSVi wrote ...

Can you please show what gate waveform took with that differential probe looks like? Is it any cleaner than with conventionl probe?

Bulavinov Sergey,

I am not scoping the gate waveform, but the voltage across the switch.
The Differential probe shows the same waveform as I would have got with an ordinary probe, but the diff. probe is isolated from ground, so I can measure top leg of bridge without floating the scope.
The same effect can be produced by using 2 probes, both connected to the same points as the diff. probe, and subtracting them in the scope.

Cheers, Finn Hammer

Mads Barnkob

Wed Oct 12 2016, 04:02PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Woah, great stack of drivers!

Having 9 ready at hand is certainly staying prepared, even more than staying prepared, you are staying coiled.

Fine idea with the DVM module, usually I just mark out 3-4 positions on a little plate around potentiometer and thats it, but then that only goes for that coil.

I am still using my one-sided UD2.1 that is in Rev. C by now, one day ill order a bunch and others like me that love through-hole can join in :)

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