I would like to model power supply effects in my Rotel 850 simulation (specifically what high power operation of the output transistors does when the PSU isn't an LTSpice voltage source).
This tutorial on modelling a transformer-rectifier-caps PSU seems easy enough:
ACDC Linear Power Supply Simulation in LTSpice Step by Step Guide | ElectronicsBeliever
EXCEPT
I have no idea what the primary inductance of a reasonably realistic Rotel 850 transformer would be.
The user manual says the power output is max 50watts per channel, and the power consumption is max 260watts.
Can anyone help me? Even approximately?
BugBear
This tutorial on modelling a transformer-rectifier-caps PSU seems easy enough:
ACDC Linear Power Supply Simulation in LTSpice Step by Step Guide | ElectronicsBeliever
EXCEPT
I have no idea what the primary inductance of a reasonably realistic Rotel 850 transformer would be.
The user manual says the power output is max 50watts per channel, and the power consumption is max 260watts.
Can anyone help me? Even approximately?
BugBear
Mains transformers have primary inductances in the realms of Henries. For a 250VA power transformer you could start at 10H. The secondary inductance is then defined by the primary inductance divided by the turns ratio squared.
Apoligies - what I actually want to do is see the effects of output transistor current draw on the power rails (and the consequent effect on the "front end" of the amplifier circuit).
Further reading led to an article suggesting that for my purpose it is sufficient to model the secondary of the transformer ON ITS OWN as a simple sine wave voltage source with a series resistor, so that drawing current drops some supply voltage across that resistor.
Questions:
BugBear
Further reading led to an article suggesting that for my purpose it is sufficient to model the secondary of the transformer ON ITS OWN as a simple sine wave voltage source with a series resistor, so that drawing current drops some supply voltage across that resistor.
Questions:
- is this proposal reasonable?
- what is the (rough) effective net resistance present in the secondary of the 850's transformers?
BugBear
I don't HAVE a Rotel 850, I'm just using its schematic as a test bed for designing.
I found the numbers I needed by working backwards from the regulation numbers for some commercial transformers; the spec include the unloaded voltage, the loaded voltage, and the loading current. Ohm's law does the rest.
BugBear
I found the numbers I needed by working backwards from the regulation numbers for some commercial transformers; the spec include the unloaded voltage, the loaded voltage, and the loading current. Ohm's law does the rest.
BugBear
Perhaps if you are quite keen to do a sim and use it for investigative purposes then a much better way to approach that is to have the amp yourself (any cheap amp for that matter) as that provides not only real parts to do static measurements on, but also dynamic measurements of operation, and to compare waveforms. It also helps to confirm the simulation is correctly modelling specific parameter changes as you can add a known series resistance, or double a known filter capacitance, or other change that only varies one key parameter so that any result is more objective than subjective. Also the net PT resistance includes the primary and secondary winding resistances and voltage ratios. Although it may seem useful to just do a sim by itself, the old adage of 'shite in, shite out' can apply.
Linear power source has output voltage at rated transformer VA.
Depending on regulation, voltage goes up unloaded, voltage sags under load.
Assuming regulation of transformer is around 6 to 8 %
just make life easy and assume unloaded voltage rise, and voltage sag under load is around 3 to 5 volts.
if your model is using say +/_ 25 volts rails
then test at +/_ 28 for unloaded
then test at +/_ 22 for loaded
you'll be able to see usual behavior since current sources/ DC offset will change.
you can also vary transistor temperature of input stages
since some current sources will get hotter than others
otherwise if the voltage source in model has internal resistance
of 0 its closer to perfect voltage source.
Change internal resistance to .8 to 1 ohm to observe benefit of decoupling capacitance
or if not possible add series resistance.
Depending on regulation, voltage goes up unloaded, voltage sags under load.
Assuming regulation of transformer is around 6 to 8 %
just make life easy and assume unloaded voltage rise, and voltage sag under load is around 3 to 5 volts.
if your model is using say +/_ 25 volts rails
then test at +/_ 28 for unloaded
then test at +/_ 22 for loaded
you'll be able to see usual behavior since current sources/ DC offset will change.
you can also vary transistor temperature of input stages
since some current sources will get hotter than others
otherwise if the voltage source in model has internal resistance
of 0 its closer to perfect voltage source.
Change internal resistance to .8 to 1 ohm to observe benefit of decoupling capacitance
or if not possible add series resistance.
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