I have been fixing up an
ancient LEM amplifier for a friend of a friend.
It has been a learning experience: it has two 550VA toroidal transformers and originally one of the 4x 15000µF 80v bulk capacitors was shorted above 50v.
That is to say it tested fine on my ESR meters but when you hooked it up it blew the fuse.
Through that I also learned the hard way how much energy such a transformer requires with no load!
So I felt quite confident that with new capacitors it will work fine but it does not. (Probably because their ESR is much lower than the originals).
I bought
these on Mouser rated for 100v instead since the unit runs nominally at around 80v.
Now with these to get the amplifier to start is a matter of luck:
With the toroidals on their own you could at least get lucky and time it with the AC mains cycle at its peak and then it worked.
But with the new capacitors it pretty much trips the 10A house circuit breaker 9.5/10 times...
I first researched to use inrush limiting NTC thermistors but soon found that they are not available to handle such large capacitance values.
So a soft start seems mandatory now, and I thought I had figured out the most elegant solution:
Just a 100VDC relay with its coil connected to the bulk capacitors.
They would charge through a high wattage resistor and as they reach 70v the relay should pull in (at 70% of its rated voltage).
But then I read that this is bad because the pull-in will be "soft" and create more arcing and wear on the contacts than normal.
So I made the circuit a bit more complex by using a MOSFET with built in zener protection diode:
I have tested this on a breadboard with a 24VDC relay and have made the following conclusion:
With the resistor values shown in the attached schematic I reach 70V before it pulls in, so that is very good.
However when it does pull in it does so at only 12VDC, i.e. half of the rated coil voltage, so the pull-in is very soft.
My question is is there any way around this, i.e. to give the relay a hard turn-on with 24VDC?
(I should clarify that the coil does see 24V+ when I reach 80v).
Or would I then have to admit defeat and go for one of the more complex designs like these:
Project39 or
SST-01 that use a dedicated power rail?
I have tried to understand if for example a SCR would have a less linear region than a MOSFET does.
Or if some relay driver could help but I'm not sure really when ramping up the voltage like this?