Spice simulation

Doubly Inscrutable.

takes tables of data... in examples\Educational\FRA folder
inscrutable directions - but I got it to work
could be an approach to answer David's interest in Nichols plotting?

It is certainly obscure, as is your own response, since it's in a different thread in a different forum;) You do like to hide your lamp under a bushel.
But thanks for the idea, I will see if it works.

Best wishes
David
 
in any plot window you can see what data it can plot by right clicking, selecting Add Trace or select from tool bar View>Add Trace

there is a ReadMe.txt in the FRA folder too - doesn't add much but you may not see it if you navigate there inside LTspice with .asc default

Thanks for the reminder. I don't update LTSpice very often and I had only read the old txt. RTFM and it works.

Best wishes
David
 
ISC and NC?

I have asked about this in Solid-State without response, so perhaps there is an expert here?
Any comments on the ISC and respective NC parameters?
It is the Base/Collector version of ISE and NE which control low current Beta fall off.
As far as I can see it should be mostly irrelevant in typical audio circuits.
But some of the "reverse" parameters have an impact on normal operation because of the way the Ebers-Moll and Spice models handle all quadrants with one set of equations.
So, any tricks or recommendations?

David
 
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I've been searching all afternoon to try and work out what is a reasonable series resistance to put in a voltage source for an AC voltage in a PS simulation. (I use a sine wave to simulate the AC voltage).

All I have been able to find is "Voltage sources are ideal you should add some realistic parasitics for resistance and capacitance" The problem is I have no idea what is realistic!!

I was seeing RMS current in the first caps in my PS of around 5.5A Which is a lot. I figured this was unlikely as the transformer in reality would not be a perfect voltage source. I added in 0.1 ohms of series resistance and the current in the caps dropped quite significantly (along with the rectified voltage).

The thing is I don't know of 0.1 ohms is a reasonable number to put in. It may be too high or too low. The main reason I was simming was to try and get a feel for whether the caps I was looking at would be up to the task.

Anyone able to make some suggestions?

Tony.
 
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I've been searching all afternoon to try and work out what is a reasonable series resistance to put in a voltage source for an AC voltage in a PS simulation. (I use a sine wave to simulate the AC voltage).

All I have been able to find is "Voltage sources are ideal you should add some realistic parasitics for resistance and capacitance" The problem is I have no idea what is realistic!!

I was seeing RMS current in the first caps in my PS of around 5.5A Which is a lot. I figured this was unlikely as the transformer in reality would not be a perfect voltage source. I added in 0.1 ohms of series resistance and the current in the caps dropped quite significantly (along with the rectified voltage).

The thing is I don't know of 0.1 ohms is a reasonable number to put in. It may be too high or too low. The main reason I was simming was to try and get a feel for whether the caps I was looking at would be up to the task.

Anyone able to make some suggestions?

Tony.

Connect a 100-watt light bulb to your mains and measure how much the mains voltage falls. Then measure the current being drawn by the light bulb (you can put a 0.1 ohm resistor in series with it and measure the drop to see what the bulb current actually is). Then simply divide the observed mains voltage drop by the measured current of the light bulb. If the mains drops 0.2V and the bulb draws 0.8A, then the mains resistance is on the order of 0.25 ohms. Try to do this at a time when the mains voltage is not moving all over the place.

Cheers,
Bob
 
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ah thanks Bob. I probably should have specified I'm not using a transformer in this model (being a bit lazy) I was just setting a voltage source up with a 50Hz sine wave and 1.414 X the expected rms transformer secondary voltage. But what you have just told me makes me think it would be best to actually make a transformer in spice :) Time to go read those spice chapters in your book me thinks :)

Tony.
 
You can get a good Transformer resistance from its regulation spec.

eg 500VA 50V 10A transformer with 10% regulation means the output voltage will drop 10% at full load of 10A. ie 5V at 10A which give you 0R5 series resistance. All calculations with AC

For da nitpickers, there is a difference between da rebel colonies (US of A) and da old world over whether the voltage spec is at full load, no load or something in between.

But if you want to sim charging currents in a conventional PSU, you need the leakage inductance of the transformer too. I think I saw a page with typical inductances of big toroids somewhere.

I think this is enough to simulate a transformer. No need to do a full model.
 
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Thanks kgrlee and Andrew! I'd be looking at a 5% regulation 300VA toroid so that works out at about 0.2 ohms (for the voltage I'm looking at). That also fits with what Rod Elliot has said in one of his PS projects that to get 35V you need about 28V (which is higher than expected) secondaries. Came out smack on the money when simmed.

Andrew I did find gootee's transformer and downloaded but I don't actually have any transformers yet.. I was partially doing this to try and work out what voltage I needed to get a 30V rail when pushed hard (BA3 with 30V rails and complementary output stage one transformer per channel). Also was making sure that I had caps with high enough ripple current rating.

with 0.2 ohms series resistance and the standard BA PS with 10,000uF caps I'm getting 30V rails with a 25-0-25 transformer and load of 75W per rail. However each voltage source is showing 5A RMS which is more than half of the 300VA rating of the transformer (around 250VA) I guess because of the higher rail voltage) I might need to consider 400VA torroids.

Tony.
 
with 0.2 ohms series resistance and the standard BA PS with 10,000uF caps I'm getting 30V rails with a 25-0-25 transformer and load of 75W per rail. However each voltage source is showing 5A RMS which is more than half of the 300VA rating of the transformer (around 250VA) I guess because of the higher rail voltage) I might need to consider 400VA torroids.
You'll find using a higher VA rated transformer (with the same regulation) gives less series R which has a much greater effect on the rectified voltage under load. Much more effective than another zillion uF of caps.

Alas, its also more expensive :mad:

Anyone remember the thread with typical leakage inductance for big power transformers?
 
...
Anyone remember the thread with typical leakage inductance for big power transformers?

I asked about this a while back, is that the thread you had in mind?
I didn't actually receive any useful answer.
Toni posted a number but sort-of disowned it when I pointed out that it was implausibly low.
An LCR tester was used and I assume the value was measured at a current too low to overcome the initial knee in the BH curve of the core.
I would still like a believable value if you find one.

Best wishes
David
 
Anyone remember the thread with typical leakage inductance for big power transformers?
I asked about this a while back, is that the thread you had in mind?
I didn't actually receive any useful answer.
Toni posted a number but sort-of disowned it when I pointed out that it was implausibly low.
You want the Leakage Inductance rather than the Primary or Secondary Inductance.

This is measured with the primary shorted. It's much smaller than Primary or Secondary Inductance.

Radio Designers Handbook 4 ed. has chapter & verse. Hope everyone knows what I'm referring too. Da old fogeys will know. :)

In da SPICE model, its the difference between the Primary & Secondary Inductances referred to the side you want.
 
300VA 22Vac feeding a capacitor input filter (rectifier and smoothing) has a maximum continuous DC current available at the output of ~3.4Adc.
For cool running I and some others adopt about 50% of this for continuous duty, i.e. 1.7Adc to a ClassA amplifier.

That 5% regulation transformer will give ~31Vdc when lightly loaded.

Expect about +-30Vdc when supplying 1.7A of bias to a ClassA amplifier.

If my "in head" model is out by +-½V I would not worry. The Mains voltage variation is far bigger @ ~ +-1.8V.
 
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Thanks Andrew. I have a 20-0-20 300VA transformer in my gain clone. I might have to second it for some real world tests :) I was actually thinking I would need a 25-0-25 for 30V based on what I was getting in spice (pretty sure the 20-0-20 gives me about 28 at light load), but that may be overkill... especially since my voltage never seems to drop below 247V and is often around 252V!!

Tony.
 
If the average RMS current is the same, copper losses will be the same regardless of peak current value. So unless the reservoir pulses cause the core to heat, I don't see why they are an issue. So my question is, does core heating cause a 200VA transformer to not be enough for a 200W amp due to poor power factor? This is theoretically the reason a trafo would be rated at VA instead of W, but it's not wise to assume anything. I've tried to show current saturation in a toroid but wasn't able to.
 
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Here is my simple spice model. 24-0-24 is looking like about the ticket for what I am trying to achieve. Though I probably need to be looking at 500VA to meet Nelson's rule of thumb of only using half the capacity of the transformer! That's 1000VA total for just 10W (class A) into 4 ohms :spin:

That is giving me around 29V rails with 2.5A of load current (per rail). factor in 5% over on the mains and I only get to about 30.5V which is still less than the absolute recommended 32V rails for a BA3.

Push that to 3A and the transformer is at full capacity and voltage drops to about 28.6V (with a 300VA toroid).

I'm starting to see why people want to use 8 ohm (or higher) impedance speakers with class A amps!!

Now I just ran the sim with close to the nominal 25V rails. and 3A bias per rail. That should be the 150W per channel that Nelson talks about. That shows about 240VA requirement on the transformer... I guess this is why Nelson says we roughly want to double it... The losses in the diodes, caps and the CRC mean that to get 150W out of the PS we need to put about 240W in... so perhaps sticking with 2 X 300VA toroidal will be fine after all.

My head is starting to hurt ;)

Tony.
 

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Thanks kgrlee and Andrew! I'd be looking at a 5% regulation 300VA toroid so that works out at about 0.2 ohms (for the voltage I'm looking at). That also fits with what Rod Elliot has said in one of his PS projects that to get 35V you need about 28V (which is higher than expected) secondaries. Came out smack on the money when simmed.

Andrew I did find gootee's transformer and downloaded but I don't actually have any transformers yet.. I was partially doing this to try and work out what voltage I needed to get a 30V rail when pushed hard (BA3 with 30V rails and complementary output stage one transformer per channel). Also was making sure that I had caps with high enough ripple current rating.

with 0.2 ohms series resistance and the standard BA PS with 10,000uF caps I'm getting 30V rails with a 25-0-25 transformer and load of 75W per rail. However each voltage source is showing 5A RMS which is more than half of the 300VA rating of the transformer (around 250VA) I guess because of the higher rail voltage) I might need to consider 400VA torroids.

Tony.

Hi Tony,

I am probably being redundant in mentioning this, but remember to take account of the fact that the rectified current pulses passing through the transformer are quite large and ultimately govern much of the regulation, more so than rms values of current. The problem is that these current pulses are of typically small duty cycle, so their peak value must be high in order to support the average current demanded by the load. In very rough terms, if these pulses have a duty cycle of only 10%, and the load needs 4A, then the peak value of these pulses could be as high as 40A in the secondary winding. DCR and leakage inductance can really get you here.

Also, the mains source impedance can be a factor that should be considered as well. Obviously this will be different in different environments (e.g., how much crappy 14 AWG wire the mains must pass through in getting to your socket), so sometimes an arbitrary value should be included.

Finally, the minimum expected value of the mains voltage (say, 110V instead of 120V) should be taken into account lest you be disappointed in the power output. Finally, you need to consider the maximum expected mains voltage (say, 130V) in assessing how your amplifier will deal with the added power dissipation and voltages under these conditions.

Cheers,
Bob