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Old 19th September 2013, 10:39 AM   #1171
AndrewT is offline AndrewT  Scotland
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Gootee developed a SPICE sim of a typical transformer.
One simply measured a few standard parameters and inserted them into the sim.
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Old 20th September 2013, 03:55 AM   #1172
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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.
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Old 20th September 2013, 04:03 AM   #1173
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Quote:
Originally Posted by wintermute View Post
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

Anyone remember the thread with typical leakage inductance for big power transformers?
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Old 20th September 2013, 08:46 AM   #1174
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Quote:
Originally Posted by kgrlee View Post
...
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
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Old 20th September 2013, 09:38 AM   #1175
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Quote:
Anyone remember the thread with typical leakage inductance for big power transformers?
Quote:
Originally Posted by Dave Zan View Post
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.
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Old 20th September 2013, 11:37 AM   #1176
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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 +-V I would not worry. The Mains voltage variation is far bigger @ ~ +-1.8V.
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Old 20th September 2013, 12:20 PM   #1177
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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!!

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Old 20th September 2013, 02:13 PM   #1178
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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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Old 20th September 2013, 02:15 PM   #1179
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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

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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Old 20th September 2013, 03:42 PM   #1180
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Quote:
Originally Posted by wintermute View Post
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
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