Wiseguy New Inverter Build Nano R6


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wiseguy

Guru

Joined: 21/06/2018
Location: Australia
Posts: 1102
Posted: 12:27pm 14 May 2024      

Copied from KeepIS's thread where I accidentally posted this reply.

Klaus I apologise in advance for the verbose reply.....war and peace here we go

With regard to your question about what the Variac is, I will assume you have seen and used a Variac before and I would be most surprised if you had not.  For newbies who are maybe not familiar, a Variac is special type of adjustable transformer or a VARIable AC device or VARIAC.

The normal units are essentially a special type of toroidal tranformer that has a single layer winding and for descriptive puproses has the varnish missing from the bottom side. There is a shaft through the centre with an arm and a wiper contact that can touch individual wires as the shaft is rotated. Lets assume it has 100T and we apply 100V across the winding ends. If we meter from one end of the winding as we touch the wiper to the first winding  - 1 Turn - we will measure 1V, and if we rotate the wiper to touch the 20th turn we will measure 20V etc.

Sometimes it is handy to have a variable AC source that we can slowly increase the  voltage to something we wish to test in a safe manner and we can monitor input current against the increasing voltage and stop if things are not looking good.

In the Nano inverter there is a fixed reference voltage within the Nano and it compares the output AC level to the fixed reference and drives the PWM higher until the sample of the output voltage matches the reference level, thereby regulating a constant fixed AC output.

Poida created some code that had no feedback from the output, instead it has the usual feedback pin tied to a variable resistor (5V on the pots top end, ground on the other) and the code creates a PWM that is proportional to the wiper setting. So in theory a 2.5V midpoint pot setting will create PWM of 50% duty cycle modulation.

The only issue with that is the dynamic performance of the Toroid and output stage  will cause the output voltage to sag when a heavier load is applied.  So I created some analog control that compares the output level to the pot setting and if the output starts to sag it increases the control voltage to the Nano slightly to compensate for it.

Next unless you have paid a lot for a Variac it has no volt or current metering and a rough and ready 0-100% or 0-260V scale that is very inaccurate and dependent on the actual mains voltage at any given moment.  The Nano variac has volts & current metering and a settable current limit, which causes the output volts to drop to whatever level is required to maintain the current limit setting.

If you ever want to calibrate something to a fixed AC level, be it current or voltage the Nano Variac is a very stable AC source and stays exactly where it is set so you are not trying to chase the varying mains voltage or current as it wobbles around ad infinitum.

Lastly it operates from a DC source from 16 - 60V but the transformer primary has to be wound to suit the applied voltage.  No doubt 99% of users will either use their 24V or 48V storage supply and suitable Toroid. Of course if there is no mains power and the primary inverter has also died the Variac can be set for 240V and with the small 8 FET Power board should easily supply 2.5kW to bring your fridge to life and turn on a lamp and power your soldering Iron for the primary inverter repair.
Edited 2024-05-14 22:22 by wiseguy