Guesstimate A Transformer

It would help if you can find out the primary inductance or get an approximate read on that figure. I like the energy-perspective for transformer/inductor design; the core geometry you need is proportional to how much instantaneous magnetic energy the core has to hold without saturating (the peak AC component plus any DC component of the magnetic field if the amp is single-ended output.)

The following set of slides outline a procedure to derive a ballpark core geometrical constant/"effective electrical size" from design requirements like the max saturation flux density of the core material, peak primary current, required primary inductance, etc. You know what the required primary inductance will need to be because it forms a high pass filter with the tube(s) output impedance and determines bass response. You know what the required peak primary current will be for your desired power output into the load on the secondary. You can ballpark the rest of the required parameters to get a geometrical constant that will let you infer the required size of the core.

Conversely if you already have a core you can run the equation on page

11 "backwards" to infer what kind of energy handling ability it might be able to provide.

a certain feature I need for this. This is a mobile audio amplifier. The ba sic configuration is shown at the end of this post in Spice. The coils, L1 and L2 are actually a center tapped winding.

n't have much to go on. The core is 3.757" X 3.158" X 1.931". The DCR acros s the whole thing is about 0.5 ohms.

about 30 watts into 8 ohms. Theoretical about 50 watts but with losses 30 i s more like it. I have used much smaller transformers for this at 30 watts. Now I intend to multiply the voltage. I could double it or triple it. Doub ling it should get me 4 times the power, theoretically but we know that is not going to ha[pen. Tripling it should get me 9 times the power but we KNO W that's not going to happen.

oss is going to be in the transformer. That means heat. Well now just how m uch heat ? I am sure there is a way to figure it out but I don't know it, a nd am not an expert on transformers. I don't know if I even have the right parameters. I guess I got total surface area, and I have DCR which is where much of the loss will go, what else is a significant factor ?

uilt, all I have to do is populate it with transistors and drive it but I a m pretty sure it will be too much. I might be able to regulate it down, a b it of inductance and maybe synchronous rectifiers or something. I'll burn t hat bridge when I come to it. If I do that I might go with a tracking power supply, hey, it beats commutators.

er in the area as it is pretty insulated, so just on the sides I got 26.7"^

2 right ? Should I stick some fins on it ? I could probably find enough scr ap heatsinks around here to do it on all sides, but one might be inaccessib le.

tion of the windings ? I am sure even going to 24 volts Vcc is going to be more than this thing was ever made for.

sed 24 CT power transformers and they worked, though performance was not st ellar to say the least. I know I was inducing a bunch of voltage back into the primary, and that will happen with these as well. I will have to clamp it somehow because if it ever gets run without a load I don't want it blowi ng smoke rings. that's also why I would like to enhance the insulation.

what it can take ? If anyone like smoke I think I can make regulated about

50 volts at about 8 amps. If necessary I can get drastic but it wouldn't be regulated. (what would be the point ? lol)

If you look up mains transformers with similar dimensions you'll get a roug h idea on power.

NT

This is for 50/60 Hz:

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It depends on the method of cooling ( free air, confined, forced ) and acceptance criteria for hot-spot temp rise in the windings e.g. 50'C.

I would expect for conventional cold-rolled Grain Oriented Steel (CRGOS) ( not Amorphous type)

- core losses about 1W/kg +/-30% better with thinner Laminate.

- core capacity < 100 VA/kg depending winding efficiency, winding design, copper losses, cooling design

- for resistive load only

- if bulk unregulated load with 10% ripple V then I peak:RMS= 10:1 - then derate max VA by at least 35%

the required core size is a strong function of the frequency. As an audio output transformer for HiFi you may ant to go down to 30 Hz. If its only PA system voice then maybe 100Hz.

m

Thanks folks. It'll take some time to sift through all that. At this point I think I would be alright feeding it 24 volts but not 36.

It seems a couple do not realize that I am not using the primary.I am using the secondary as a split primary, which is why I need the 4 and 16 ohm tap s. It must be balanced.

On that Spice print, that IS the secondary. I am also going to incorporate Shottky diodes at the collectors of the outputs to emulate tube sound best as I can.

I am really thinking about a tracking regulator now, that way I can cut bac k the peak power when the temperature rises. I also will incorporate a sabo tage proof protection system, like another one, if you hook the speaker wir es to say, the AC line it just burns the wires off and still works after be ing turned off for a few seconds and turned back on. Any amp I build that c osts more than a couple of cases of beer will have that. Simply indestructi ble, in fact I jst might make a challenge. You can connect anything to anyt hing but you can't go inside the case. Reward money in escrow.

I just might not...

Does that circuit actually work? I guess it should, barring any DC imbalance issues.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I just wonder why on earth the transformer. In tube designs, it was a mandatory part, to get a ballpark match to the low impedance of the speaker(s), but a complementary transistor amplifier can be made to work directly to the speaker.

I've made a bunch of tube amplifiers (about 5 decades ago), and the output transformer was a PITA, honestly.

--

-TV

That transformer allows the outputs to be emitter followers, and to get more voltage into the speaker. True, there are simpler ways to drive speakers... like buy a class D full-bridge chip.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

imbalance issues. "

I built about a half dozen amps with similar topology. They worked. In fact people would bring them back and say they went bad but in all cases they h ad blown the speakers.

Those were common collector outputs, Darlington in fact and were driven by a 1:9 transformer from a normal speaker output of an amp. I am changing tha t because I do not want the input transformer and I want a normal feedback loop that I can control. I also want line level inputs. Tone controls. Thin gs like that.

So is a classic complementary-symmetric audio output stage. What's the difference here?

The transformer adds a degree of difficulty to get the overall feedback from the output to stay stable.

--

-TV

and a nest of other issues. Output transformers were always the curse of valve amps. So try some PD500s & electrostatic speakers. 25kV 1.6mA 30W.

NT

** All of which any transformer designer worth his salt knows how to deal with.

** You do love to exaggerate. ** Now doing that IS going " out of the frying pan and into the fire ".

... Phil

You could have a free x-ray at the same time

NT