Transformer tap switching between series and parallel

[nemail2]

Hi

yasq - yet another (probably very) stupid question:
In the attached screenshot you'll see a pulldown for the "SW-WINDINGS2" net. are there any objections? I don't want that pin to float during any failure mode cause that might leave T8 in linear operation which might fry it.

I kinda don't like R27 and R26 being the same value, don't know why (really not sure about this). It certainly is not a voltage divider but is there any other issue I might run into with this?

thanks!

[Zero999]

Hi

yasq - yet another (probably very) stupid question:
In the attached screenshot you'll see a pulldown for the "SW-WINDINGS2" net. are there any objections? I don't want that pin to float during any failure mode cause that might leave T8 in linear operation which might fry it.

I kinda don't like R27 and R26 being the same value, don't know why (really not sure about this). It certainly is not a voltage divider but is there any other issue I might run into with this?

thanks!

Adding a pull-down is a good idea. I advise moving R27 to the base, which will reduce the current taken from the circuit driving it, as well as provide a better pull-down. There is no issue with the resistors being the same values.

I've learned a bit from this thread. I didn't know that the idea diode bridge controller doesn't work with a tapped transformer.

[nemail2]

Adding a pull-down is a good idea. I advise moving R27 to the base, which will reduce the current taken from the circuit driving it, as well as provide a better pull-down. There is no issue with the resistors being the same values.

ok thanks! unfortunately it was too late for this spin of the PCB but it fortunately my version also "works".

I've learned a bit from this thread. I didn't know that the idea diode bridge controller doesn't work with a tapped transformer.

happy to see that my thread managed to teach something even to a pro obviously I didn't know either that this ideal diode bridge rectifier doesn't work with a tapped transformer. I went with schottkys now and have just finished building the PSU up, all files and photos attached.
the rectifier diodes now don't get too hot (around 60 to maybe 70°C abs. max), even at 5A current.

Features of this linear Lab PSU (no switching prereg, completely linear):
- 0 - 24V in 1mV steps
- 0 - 5A in 1mA steps
- accuracy: pretty accurate :-D no idea how to characterize and measure everything properly
- 100W max
- 2x 12V 100VA toroidal transformer
- Teensy 4.0 - virtually unlimited resources for going wild on software features
- crystal clear, bright and crisp 128x64 2.7" OLED display
- output relay (disconnects V+ and GND)
- tap switching from this thread using a P-FET, no relay clickedy-action
- dual BD139/TIP3055 darlington pair
- active cooling for output stage with temperature monitored (PID) and PWM controlled 80mm fan
- 2x TI DAC70501 14 bit with 5ppm internal reference for Voltage and Current setting
- 2x ADS1115 16 bit ADC for voltage and current measurement
- OPA4197 and OPA2197 precision opamps for control loop/gain stage
- precision/low drift resistors for high initial and temperature-stable accurracy
- 10 mOhm 15ppm 0.1% current shunt with LT6102 Precision Zero Drift Current Sense Amplifier for low heat generation and low voltage drop
- Schottky diodes for voltage rectification to minimize voltage drop and heat
- display as well as front panel diodes brightness is software-controllable
- piezo buzzer
- there is no ripple up until about 10.8V at full load (5A) with one winding and at up to 23V at 5A with two windings (IIRC, maybe a bit less).

now imagine the possibilities if one would have time to go wild on the firmware... I have only implemented the very basic features for now.

This is the n-th incarnation of my lab psu design which I started by copying and changing Dave's old µSupply project. Years and many forum threads here and on mikrocontroller.net later and thanks to many people helping me there, I have come to the point where I'm quite happy with the result. Me on my own would have never been able to accomplish this and I have learned a metric crapton on the way.
Sure, this thing has its flaws and it is nowhere near a professional Lab PSU but it is the very best I'm able to create and I'm happy with it as it is

[tautech]

- there is no ripple up until about 10.8V at full load (5A) with one winding and at up to 23V at 5A with two windings (IIRC, maybe a bit less).

Looking good but pray do tell us what qualifies as no ripple ? 

[nemail2]

- there is no ripple up until about 10.8V at full load (5A) with one winding and at up to 23V at 5A with two windings (IIRC, maybe a bit less).

Looking good but pray do tell us what qualifies as no ripple ? 

Thanks! Maybe I did not express myself correctly. When I wrote no ripple, I meant that the scope shows a completely flat line in AC coupled mode, set to 20mV/. Except for noise and crap it is picking up, of course. When tuning the voltage up a bit over the values I mentioned, you start to get dips in 100Hz frequency so that's where the input caps are too small or the voltage headroom of the transformer is to little...
At least that's how I understood the concept

[tautech]

Yes zero ripple at full load is not easy to achieve but in reality it's not like that will be absolutely required when instead very low ripple at say 2A would be expected and is a more realistic target.

A DIY linear PSU I did years ago used a LM339 for 5A and it would do that however the transformers weren't a stiff enough supply so voltage did sag some near full load. It used 2x 12V transformers manually switched for parallel and series connection to provide 15 and 30V modes although no current limiting other than what a LM339 does.

Maybe by probing yours close to the PSU is influenced by EMI where a scopes 20 MHz BW limit can help get accurate readings. Use 1x probes and some averaging too.

[Barneyview]

I think this (attached diagram) relay / switch connection removes any danger of a short during switching