Help with winding a toroid pulse transformer to drive back-to-back SCRs

My biggest worry would be the breakdown voltage of the magnet wire, especially after it has been dragged through the hole in the toroid many times. What is the peak voltage that will appear between any pair of the three wires? You may need better insulated wire, or an insulated core and three separated windings to achieve reliable resistance to breakdown.

What inductance do you need?

Is the core black ceramic or painted? It might be a powdered iron core, that includes distributed gaps, that make it useful for designs that include DC in the windings or emphasize energy storage in the inductor, rather than simple transformer action of pure AC.

about 170 volts

I guess I assumed pulse transformers needed to be wound that way (trifilar). Separate windings are actually easier to wind from my experience.

Actually, I haven't got a clue. I hoped someone more familiar with triggering SCRs would fill me in on that. I've done searches but all I've found is on commercial sites that want to sell pulse transformers.

Actually it is painted blue on one side and the other side and circumference is painted very light green. The core is from a Pulse Engineering transformer but I couldn't find any information on it anywhere on the web.

I'm guessing it is ferrite from what I assume was it's intended use. I have two cores so I can bust one open if that would give helpful information. I would think the low inductance I measured would mean it is a low mu core intended for high frequency use, but I stand to be corrected and educated on that assumption.

Thanks for reply.

These transformers are usually called "Trigger Transformers". Mouser sells the Vishay-Dale line, but looking at their stock, they only have one that looks like it would meet your needs

. Newark looks the same, and Digikey doesn't stock any. Pitiful.

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

"In theory, there isn't any difference between theory and practice.  In 
practice, there is."  - Yogi Berra

Yes, with a mod to the circuit. The transformer is driven by

6 volts, and just needs to turn the SCRs on. As drawn, with a home wound unknown transformer, you risk burning out the PNP's, so put a 100 ohm resistor in series with the winding that is connected to the PNP collectors. If you use sensitive gate SCR's, you can do that (turn them on) with very little current. Build the circuit, and try it out with a 6 volt supply to a 6V load on the SCR's rather than the heater, just to make sure it turns the SCR's on when it should.

Once you're satisfied with the circuit, you can connect it to the heater.

Ed

A much simpler solution, and likely cheaper, more robust, and higher longevity would be to use solid state relays to energize the AC fed heater elements, and use a simple Low Voltage DC circuit to perform the sensing, and relay activation signal.

Probably more efficient as well, as the DC sense/control circuit could be built to us very little energy.

*solid*%20*state*%20*relay*&Ntx=mode+matchall&Ns=P_SField&Mkw=solid%20state%20relay&N=1323038%20254134&Ntk=Mouser_Wildcards

ITS PROBABLY -52 MATERIAL IRON-POWDER FROM MICROMETALS.

oops, sorry about shouting.

what you need to do is a "splat test"

charge up a known, large cap to some voltage V, then "splat" it across onw winding (other open-cct) and measure current (eg 0.1R resistor off to scope). you will the current slope (dI/dt) gives you V/L, and if the cap is large (c.f. energy in choke) then V is pretty much constant so you can work out L. you can also see the core saturate (slopes upward, L decreases), which will tell you how many Volt-Seconds the core can handle (with that many turns), which then tells you what pulse width it can cope with - you know the volts the driver cct will apply.

trifilar wound produces lowest leakeage, so best pulse shapes. if leakage is too high, primary saturates before 2ndary goes very far.

Cheers Terry

I wouldn't trust ordinary single layer varnish to stand that. I would probably use kynar wire wrap wire or Teflon insulated wire if I was going to do a trifilar.

It improves the energy transfer at the edges to do trifilar, but the compromise with separate windings may be acceptable.

A first cut would be to calculate the inductive current over the pulse duration and the transformed load current. As long as the inductive current is less than the transformed current, you are getting close.

I think it is a powdered iron core, not ferrite, so it is probably non conductive enough to not worry about insulating the core. But the colors probably rule it out as a Micrometals core:

But, regardless, based on your test of 2 uH with 8 turns, you can predict pretty well the inductance of other windings based on the inductance being proportional to the square of the number of turns.

The inductance is probably too low for the core to be ferrite. But powdered iron is fine for a pulse transformer, since you don't have to worry about forcing exactly zero DC bias in the drive circuit. The distributed gaps in the powdered iron material make these cores much more resistant to saturation than ungapped ferrite is.

Double strength mag would/should be fine. Even single strength has over 350 Volts breakdown resistance, on a bad day. Also, if the turns count is low enough, he could pipe the mag wire through a Teflon sleeve.

A good thing to do is wrap the toroid with transformer tape the way they used to wrap tires a few decades back. It usually has about 1000V per mil strength, if not more.

One could also dip the toroid in q-dope paint.

Do I assume correctly that the resistor goes across one of the secondaries in the "splat test"?

I will use some kynar ww wire for stronger insulation as suggested. I will also put a resistor in series with the primary - good suggestion.

BTW - I tried searching both digikey and mouser for a SS relay that would handle the amps - no dice. I know I've seen them surplus from time to time but iirc they were pretty pricey compared to a couple of SCRs (3 bucks).

I'm trying to use stuff I've got laying around. I've got all the parts except the pulse transformer and SCRs.

yes. unless you happen to have a current probe (most people dont).

Thanks Terry.

MOC3xxx will trigger parallel SCR's .

trigger pulse may need to be extended

because of inductive loads on the SCR's ,

so i dont use transformers , they of course

would saturate , stop triggering the SCR's

just before they could accept the trigger

pulse !

---------

Measure inductances with a scope .

known voltage dumped into the inducter ,

the slope is so consistant at start , it

has no change , so put a strate edge on scope

and use lots more scope boxes , to get a mo

accurate "division" of the hor and vert boxes .

volts/henries = dir(amps)/dir(time in seconds). -------- or ---------- henries = volts/ ( dir-amps/ dir-seconds ).

Put a low value R in the ground leg of your

ckt to send signal to scope .

------

figure the caps inductance to be far lower than

induct under test , figure the inductors R to be far

higher than all other R's ...

I got SolidState Relays from

BG Micro that use SCR's , rather than TRIACs .

They also have ALTERNISTORS , about $0.40 .

BTW I live in Guadalajara , San Juan de Dios , by the Tube ... I saw the nut and bolt store south of me on Revolution street

, tossing the old 8 foot flour tubes and replacing with 8 twist C.F. tubes ! Dark as a cave ! Fools .

Compact Flour' lites are BOGUS , they have starters and heaters that limit life .

I had a 220 vac , 8 foot tube , i got used , it lasted the 23 years , til i sold the house in 2000 ! No heaters , no starter .

You do not seem to have mentioned doing the dip before the windings are wound. Though it was implied.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

If you can't get your hands on a scrap "Mr Coffee" to salvage parts from (sorry, couldn't resist) consider using a MOC3010 (or similar) optocoupler to switch a triac. (datasheet available from onsemi.com)

I used it, a thermistor, and a LM304 to make a thermostat a few years ago. no PCB - I just epoxied the ICs down and soldered the Rs and Cs point-to-point. it worked great. for the adjustment I used a multi-turn pot from an old VCR.

I mounted the whole lot in an earthed metal case.

Bye. Jasen