How to test a Bi-Directional Triode Thyristor

[Salvador]

ok right, stupid me , thanks for pointing that out.I was thinking all the time of a transformer with no bridge rectifier and filter caps at the secondary but those added they do change the picture , hence everything having capacitors also talks about power factor.

according to this graph then we could just use a simply TRIAC for every mains transformer operated psu in it's primary even without the input signal control over it , simply cut off the very starts of rise and fall on each cycle since they would rarely ever go to the cap bank , unless it's being driven so hard the caps are empty almost all the time but that would be beyond hard clipping and distortion anyway.
and so get away with smaller transformers for the same power output ,
why don't others do that ? especially considering the tiny price a thyristor has compared to copper and iron.

 

[jim hardy]

See ? You knew already... Much of learning is discovering what we know.

 

[Planobilly]

Hi guys,

First, I am not put off in the least about what anyone said. A lively bunch of guys are always going to express different thoughts about any subject.

I also did not understand the Bob Carver statements about a transformer resisting the sine wave 4/5 th's of the sine wave. I see Jim is explaining that as I write.

The Carver amp design may in fact be very good, well until one has to repair it...lol Heavy iron may cost lots of money and well, be heavy but it is a hell of a lot less trouble to understand and work on.

One thing is for sure, this project and reading everything everyone has said has forced me to learn a bunch of new stuff!

Cheers,

Billy

 

[Salvador]

now that I'm coming to terms with this approach i would say it's not as complicated as an smps and not as easy as a simple iron with wire outputs , somewhere inbetween.I guess I was a bit out of my manners when I replied to " me big guy" because when I did that I wasn't fully aware of how this thing actually works until Jim pointed out the fact I had forgotten that once you put a rectifier with a cap bank at the load of any transformer it then charges the top of the AC cycle because the rest is already there in the capacitor and this creates uneven load for the AC sine wave.
hence the PFC in the smps units and other stuff.

So Planobilly this is what Carver meant when he was writing about the 4/5ths of a cycle getting through , although I would say the white paper could have been more directly about physics than comparisons and half of it simply about manufacturers that " cheat" and the ones who don't but I guess it's business you have to say some nice words about yourself first.

 

[Tom.G]

There is a link to the triac spec sheet in the OP.

Yup, I missed that. So the patent

This is a link to the patent for the power supply. ...https://www.google.com/patents/US4218660

with its high frequency switching is not applicable here.

 

[Planobilly]

Hi Salvador,

I sort of read around the sales pitch part of the white paper. All this seems straight forward now that Jim posted the graph. Thanks Jim !

Bob Carver seem to be saying "I can build you a amp for $1500 that is just as good as one costing $10,000 and in a blind taste test you can not tell the difference"

I will be glad when the parts get here so I can finish this project.

Cheers,

Billy

 

[Planobilly]

Hi Tom,

I sort of assumed the patent was not directly related to this amp. I did not read all of it. It was more general background information for me into what direction Bob Carver was going toward.

Repairing this amp has taken on a life of it's own and has caused me to consider a good many new things. I guess if one is going to take a dip in the deep end of the pool this amp is as good as any..lol I am sure to many of you this amp design is easy to understand but I have a ways to go to be able to really understand it.

Now if I can just get Jim to show me how to draw on a schematic and get it in a post !

Cheers,

Billy

 

[jim hardy]

Now if I can just get Jim to show me how to draw on a schematic and get it in a post !

click "copy image"
open MSPAINT
click PASTE
click the pencil icon, select a color, draw

click "save as" and give it a name ,
i always save in jpg format because it makes a smaller file than png

Back at pf click UPLOAD IMAGE and start guessing where Windows hid your drawing. I made a folder named "for pf" on desktop where i put mine.

Find Microsoft's "snip" feature, it let's you select and copy part of any image.

 

[meBigGuy]

Just want to share an insight (from the white paper) about why this is a "magnetic" power supply, and can have a significantly smaller transformer and output caps compared to conventional supplies.
(also, it is a class G amplifier, which vastly reduces output stage dissipation)

When the triac switches off, it opens the primary (like an ignition coil) and causes most of the transformers magnetic field energy to dump into the output capacitors.

 

[meBigGuy]

@Planobilly
I really wish you could have made the simple measurements I suggested. That would have helped locate the issue. Who knows how much has been damaged now.

BTW, starting with low line voltage is a good way to destroy things.

Eventually you will need to make the measurements I suggested unless you get real lucky.

 

[jim hardy]

meBigGuy is thinking straight.

I too would like to see 'scope traces at top of R403-C405 to see of the cap is getting charged
and gate of triac to see if any of the charge on C405 makes it on over to gate

 

[Salvador]

I wanted to say that actually I'm quite happy that I jumped into this thread, at first I was much like big guy already said kind of stupid towards the whole concept , went on with what I think it does instead of looking at what it really does.
I must admit I'm taken by surprise.
But happily I now know better.I must say of all the HI FI bla bla bla stuff which is there like 80% of the time and except the sales things of the Carver , this power supply idea is rather simple but very cool , he really manages to squeeze a mains transformer to it's max with not that many extra parts.Quite interesting.
Basically Jim's post about the tops of the rectifier cycle and another place at the internet gave me some clues as one other folk had mentioned about the ignition coil thing besides big guy at one other place.

 

[jim hardy]

as one other folk had mentioned about the ignition coil thing besides big guy at one other place.

i'm still chewing on that one



Thyristor stops conducting when current through it is zero.
At the instant of zero current there should be zero energy in the transformer, energy being ½LI² with I=0 .
So which is true - intuition or math ?
Actually the the thyristor ceases conducting when current through it drops to its "holding current" value , near zero but not quite.
from its datasheet at http://www.datasheet.hk/view_online.php?id=1927120&file=0465\sm16gz41_6629879.pdf

So i am relieved - the "ignition coil" effect is small but very real and there. Math and intuition agree.

meBigGuy is right the filter caps enable graceful turnoff by capturing the stored energy.
R310-C404 help too.

Hang in there Salvador - we all continually add to our understanding one tiny piece at a time. I had missed the significance of C405.

my two cents.

old jim.

 

[Salvador]

well correct me if I'm wrong but I guess I'm starting to get there myself , the whole " secret" is in how and when you turn on or off the sine wave.
if I'm correct the magnetic field strength is larger when the rise or fall of a waveform is steeper or closer to vertical and also when the point at either turn on or off is at a higher potential , in other words if you switch the sine wave to the transformer primary while it's at about few volts above zero the field begins weak and then builds up with the sine wave but that happens evenly and quite slowly the same with turn off , but if you turn on the TRIAC when the sine wave is already at say near maximum , it has high potential at that point so the primary current starts quite quickly and the rise is fast so the field in the transformer starts much stronger than when it would begin together with the sine wave.

this is why they call it an " ignition coil" and ignition coil wo5rks much like an smps only the switching is the other way around , but in both cases you rapidly and suddenly apply a full steady DC potential to where no current and voltage was before and that gives you a rapid change in flux and if the energy source is strong enough a lot of power with it.
the sine wave starts much slower and ends the same way so it doesn't create the fast change in flux and at lower than peak voltages were the caps are already full it simply goes in and back away without much use except for winding loss and stray fields.

this is how I have came to see this if there is something wrong I would appreciate if someone corrected me , thanks.

 

[Planobilly]

As soon as the parts arrive and get installed I will make the measurements you guy ask for. I have replaced or tested everything from the mains to the transformer. The only exception are the two diodes D402 and D403 which were 1n4002 which got replaced with 1n4004's. 1n4002's are on order. The fuse in the transformer is working because voltage got to the three main bridge rectifiers when I jumped the triac.

Edit: also I do not understand the data sheet regarding the triac. What is the max voltage that can be applied at the gate?

 

[jim hardy]

Edit: also I do not understand the data sheet regarding the triac. What is the max voltage that can be applied at the gate?

It's sort of like a zener diode , at some unspecified forward voltage it'll start accepting gate current
They're telling you that unspecified forward voltage will be not more than 3 volts
http://www.datasheet.hk/view_online.php?id=1927120&file=0465\sm16gz41_6629879.pdf

and triac will "fire", ie commence conduction, at gate current ≤ 50 milliamps.
A line or two later they tell you the unspecified gate voltage will be not less than 0.2 volts, so we should see gate pulses between 0.2 and 3 volt .
Up in Maximum Ratings they tell you that if you want to use the triac ever again, don't force enough current into the gate to exceed ten volts between gate and T1.

 

[jim hardy]

if I'm correct the magnetic field strength is larger when the rise or fall of a waveform is steeper or closer to vertical and also when the point at either turn on or off is at a higher potential , in other words if you switch the sine wave to the transformer primary while it's at about few volts above zero the field begins weak and then builds up with the sine wave but that happens evenly and quite slowly the same with turn off , but if you turn on the TRIAC when the sine wave is already at say near maximum , it has high potential at that point so the primary current starts quite quickly and the rise is fast so the field in the transformer starts much stronger than when it would begin together with the sine wave.

There's a long long thread "how power transfers across an ideal transformer"...

Go back to your basics
B=μNI/L_{ength of magnetic path}
The only term that's a function of time is current I .
So magnetic flux is in proportion to current.
The derivative relation between flux and voltage makes it counter-intuitive until you have accustomed your brain to work that differential .

So break up your run-on sentence into bite sized thoughts,
one step at a time,
one thought per line
and your thinking will get better.

AC circuit theory is usually taught without emphasizing that sinewaves are a mathematical oddity -
they and their derivatives look just alike.

 

[jim hardy]

The fuse in the transformer is working because voltage got to the three main bridge rectifiers when I jumped the triac.

That fuse is for gate drive current, not power

with it unplugged - got no ohms between p5 and p6 ?

on another track.......

The PCB track where D402's anode lands - any components nearby where a previous troubleshooter might have accidentally shorted something?

Seems to me this area would require caution when poking around.. check for smoke residue

 

[Planobilly]

Hi Jim,

I built this circuit and why I was asking about the max gate voltage. The lamp was 72 watts, as I did not have a 100 watt bulb. Close SW1 and the lamp lights as would be expected.

I also replaced the 100 ohm resistor with a 1000 ohm resistor. The voltage was 115 VAC 60hz. I did not have a heat sink attached to the triac so I did not leave it on for too long.

So, with this circuit, based on a 72 watt lamp ( lamp resistance of 15 ohms apx. Z=?) and a 1000 ohm resistor and 115 VAC 60hz what is the gate voltage?
I am still stuck on stupid trying to understand the triac...lol

To answer your question about D402 and the area near it...no sign of any shorts. I replaced all the resistors, the two diodes, and the four transistors connected to D402. I have tested the bridge rectifier and the optocoupler which I think are OK.

Cheers,

Billy

Update: after replacing the filter cap I smoked and disconnecting the 100V secondary the triac is working and sending current to the transformer. That is the good news.

Now I need to find the fault in the 100V part of the amp and when I do I think everything will go back to normal. This amp is a PITB to work on...lol...Don't try this at home...lol

 

[jim hardy]

what is the gate voltage?
I am still stuck on stupid trying to understand the triac...lol

The gate voltage in your TEST circuit will be a squarish-looking wave at line frequency
with flat tops between 0.2 and 3 volts
and maybe a spike at front edge that reaches 3 volts.
So a DMM set to AC should read between ~0.15 and ~2 volts.

The gate voltage in your amplifier will be a short pulse once per line half-cycle, again between 0.2 and 3 volts.
It's narrow because its source is C405.
So it has not enough area to cause a DMM to show much of anything. You'll have to measure it with a 'scope.

That's my thinking.Think of the Triac as a snap action switch , initially open,
that snaps closed when gate current reaches a few milliamps
at which point all 3 terminals become very nearly shorted to one another
and remain so until current falls to zero with next sinewave zero crossing , at which time it snaps back open.

That's your simple mental model.

Your "new" SCR manual describes in detail how the gate strikes conduction in a small region and it spreads out from there as current rises , That's why they need di/dt protection. And why it has a finite turn-off time.

A useful device to have in your "bag of tricks" - can be a power control device, also a memory device.

old jim

 

[Planobilly]

OK...Thanks Jim, I think I understand now. Looking forward to reading the SCR manual. Books are generally slow to arrive so it may be a bit before it gets here.

As a side bar I was able to sell two of the Fender Hot Rod DeVille amps so I got more money to feed my amp addiction...lol All these electronic toys seem to be multiplying like rabbits. I need a smaller house and a bigger shop but don't let my wife know I said that...lol

It's back to the farm early in the morning. I have work to do on the back hoe and the potting machine. It helps if one is not put off by the smell of cow manure and diesel fuel !...lol

Cheers,

Billy

 

[jim hardy]

I have work to do on the back hoe and the potting machine.

agreed on shop ...

I have to put torque converter back in neighbor's backhoe. Never a shortage of things to fix, is there ?
Ah well this is after all the age of technology and it should be enjoyed..

IMHO high school should teach more physics and appliance repair .

 

[Salvador]

hopefully we will one day get to the age were technology repairs itself much like robots build cars in an automated conveyor line these days. And then we will be finally able to proceed to the next step which would either be finding out aliens and fighting with them (Star Wars) or the rebellion of the machine and civilization would be taken over by robots (I like Stanley Kubrick's Space Odyssey)

as for the thread topic , I didn't understand from your last response to my post Jim , so was I right in the understanding of this psu design or not or half way there ?
Now it seems logical that opening a primary loop while the sine wave through it is still near its peak causes a larger B field to develop in the core than if the sine wave goes down to zero the usual way it does.Although I guess one would have to calculate the field strength in the switch opening case VS the one you would get if the current runs the way it normally does.
Although Carver talks about getting as much out of the mains through a given size xformer as he can so since the secondary has cap bank to feed this sudden current on/off switching may do the trick.

 

[jim hardy]

so was I right in the understanding of this psu design or not or half way there ?

I apologize Salvador... Iwas having so much trouble parsing out that long long long sentence into individual thought steps that i gave up.

if I'm correct the magnetic field strength is larger when the rise or fall of a waveform is steeper or closer to vertical and also when the point at either turn on or off is at a higher potential , in other words if you switch the sine wave to the transformer primary while it's at about few volts above zero the field begins weak and then builds up with the sine wave but that happens evenly and quite slowly the same with turn off , but if you turn on the TRIAC when the sine wave is already at say near maximum , it has high potential at that point so the primary current starts quite quickly and the rise is fast so the field in the transformer starts much stronger than when it would begin together with the sine wave.

that doesn't sound right to me.
One step at a time. Transformers first.

Here's steady state flux and voltage.

if you energize a transformer by closing the switch(or Triac) at the voltage peak , you start with flux at zero and voltage at peak, which is normal and it's a gentle start for the transformer. That's why they sell solid state relays that switch at peak instead of zero crossing for use with inductive loads.

If you energize a transformer at the voltage zero crossing, you stare art with flux and voltage both at zero which is not normal and some period of adjustment will be required for the flux-voltage relation to establish normalcy.

from http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/practical-considerations-transformers/

since the transformer core will probably saturate there'll be a current inrush

That inrush can persist for several cycles
from a decent article here http://www.globalspec.com/reference/74817/203279/10-3-transformer-protection

you need to think derivative. For that it's helpful to use non-sine wave.

so closing the triac near peak is easier on the transformer's iron.
But it gives a current surge into the filter capacitors.

You might peruse that ideal transformer thread.

 

[Salvador]

Hmm I will have to read up some more on this but it sounds weird the fact that at zero crossing or starting from zero the flux is higher than when the primary is connected at near peak of half wave.
I was thinking like this , when you are near or at zero the AC value is basically zero for a very short moment of time there is no voltage , that I think we can agree upon.
This is what I can't understand , how can it be when you basically connect almost no voltage to an inductive load there develops a bigger than usual flux and inrush current respectively ?
I've always though it's about how fast the change happens from zero to max that determines many things including core saturation inrush current and flux.
in an smps for example each time a semiconductor connects the primary to the DC load the primary starts off from usually no voltage to suddenly up to max supply voltage and this creates a large flux in the core.
if each of the cycles would be made longer than necessary the core would probably saturate and the current inrush would get even bigger but I can't see how can one saturate a mains iron core with a since wave starting at zero.

Ok I will go read some on my own too otherwise I am turning this into a theory discussion

 

[dlgoff]

I need a smaller house and a bigger shop ...

My whole house is becoming a shop.

 

[meBigGuy]

At any point in time the triac can turn on and the transformer field (current) builds up. When the triac opens, the field collapses causing a voltage surge in the secondary which is captured by the filter capacitors.

 

[jim hardy]

..

When the triac opens, the field collapses causing a voltage surge in the secondary which is captured by the filter capacitors.

That's how a flyback converter works

but to pick nits, the triac won't open until current through it falls to zero (well actually to less than its holding current, 50ma for this one) .

caution - that's the current zero crossing which is not exactly at the voltage zero crossing.

 

[Planobilly]

If all this, let's call it "Carver Power Amp idea" is really such a good idea, could this type of power supply be used in guitar amps? If so, why is it not being done?
I have never met a guitar player who is happy about how heavy his amp is. This is really true with 100 watt plus guitar amps. Ever load a 100 Watt Fender Twin Reverb into the club? Is there anything special about tube amps that would make this system not workable?

Cheers,

Billy

 

[NascentOxygen]

When the triac switches off, it opens the primary (like an ignition coil) and causes most of the transformers magnetic field energy to dump into the output capacitors.

I don't think so!