High Voltage DC Power Supply 4. - The phase switching module

!!!DANGER!!!

The circuit described bellow is dangerous. Using it improperly will kill you. On the other side I may also made mistakes during the design and built. I'll take absolutely no responsibility for it. If you want to build it, please don't ask for circuit board or help. I won't help you.

In my previous posts about the high voltage supply I already wrote about the small electronics able to keep the GND of the DC side on earth potential without isolation.

No this is not voodoo. It just a simple circuit.

The theory:

The protective earth connection in the mains connector is connected to the mains null somewhere in your system. Probably somewhere around the utility meter. So if we measure the voltage between the Line and Earth, it will be the full mains voltage. If we measure the voltage between the Null and the Earth, it will be zero. To be able to work with, I draw the following circuit:

This is just two simple zener regulator, what provide the necessary sense signals for the further processing.

I designed a complete circuit around this, with an MCU, a 5V power supply, MOSFETs for driving relays and two DPDT relay.

The board design:

Based on this design I ordered the boards from allpcb. Actually I made a mistake around the design. I bought my relays after I sent the board for manufacturing. The result is that I connected the switched circuit on wrong direction. This means if I build the circuit the relays create a short circuit to the mains.
Throw away the boards (without even soldering a single component), ordered a new batch.
It also arrived within a few days:

The finished board:

I wrote a small arduino code to run the board. It has an ATTINY84 in it. Actually the code would fit in an ATTINY24 easily.
Assembled the whole thing into it's final place:

I connected the final thing to the mains. Switched on the power switch. Everything looked fine, until I pressed the output button.
The main circuit breaker in my house went off. Shitt.
Few more tries. The result get even worst. It killed the mains even switching on the power switch and not the output button.
Removed the phase switching board. Measured. It looked like both of the relays killed. There was a short circuit between the line and the neutral on the board.
Here is when Murphy comes into the picture. How many components buy I usually for my circuits? Way more than needed. Except this case. I had only those two relays on the board. Weekend. I should wait until Monday, to replace the relays.
In the meantime I removed the relays from the board - I hate the THT desoldering.
Removed the cap from one of the relays to be able to make a picture of the burnt contacts:

The surprise come here. No burned contact. Even I made a picture with my microscope. The result is the same:

Nothing, the contacts are intact. Measured the second desoldered relay, what still has it's cap. the result is the same, no sign of the failure anymore.
Just my guess that the heat and mechanical torture of removing the relays of the board resolved the temporal latch-up of the contacts.
Now back to the basics. As I can't find out the source of the problem (didn't wanted to play this game with new relays). I just guessed that the inrush current of the capacitors causing the problem (as I wrote earlier, I had problem with it already). So I hacked a solution. Together with the new relays I bought some inrush protector NTCs. It isn't an ideal solution, but hopefully will be sufficient. Hacked a PDU board to accommodate the NTCs. I've a few spare anyways:

The relays replaced. The whole thing works as expected.
I'm still thinking about the proper solution. The device is a tool and not a target for me. So it may not worth the effort to build a second version.

High Voltage DC Power Supply 2.

!!!DANGER!!!

The circuit described bellow is dangerous. Using it improperly will kill you. On the other side I may also made mistakes during the design and built. I'll take absolutely no responsibility for it. If you want to build it, please don't ask for circuit board or help. I won't help you.

As the other PCBs are arrived for the supply, I continued the build.

Finished the separated isolated supplies for the panel meters (these are nothing else just a simple dual 7809 based linear supplies):

Also added the power distributor and some wiring for the backpanel:

Connected the panel meters and, milled parts of the front panel (manually as my CNC mill is still broken)
How the whole thing looks like today:

And its working, partially:

As you see the decimal dot is on the bad place (it is more likely 330V than 33V)
In addition I took a picture of the board on thermal camera. You can clearly see that the two capacitor draining resistors getting warm:

So I can tell, it is progressing, but quite few things still ahead of me:

• Mill a hole for the mains socket and the circuit breaker to the backpanel
• Mill the holes for the banana jacks to the front panel
• Finish the wiring
• Set the correct decimal dots
• build a dummy load to be able to test the current measurement
• Calibrate
• Paint the front panel
• Build the phase switching electronics (not mandatory, maybe after finish)
• Create the console for the caps

continue...

High Voltage DC Power Supply 1.

!!!DANGER!!!

The circuit described bellow is dangerous. Using it improperly will kill you. On the other side I may also made mistakes during the design and built. I'll take absolutely no responsibility for it. If you want to build it, please don't ask for circuit board or help. I won't help you.

I've many ideas in my had, what to build, what to experiment with.
A few of them involve using mains voltage directly. Some of this need to rectify and clean the input before using.
My lab power supplies are able to produce 60V DC maximum, what is clearly not enough for those experiments. A cost of a proper high voltage variable DC supply is enormous, and I don't need most of the features of it right now:

• Not need to be variable. My built circuits will have rectifications on its own. The input comes from mains anyways so this is the only voltage I need.
• Not need to be stabilized. Same concept as above.
• Not need to be isolated. I know, at this point the real professionals start to scream: IDIOT!!!! Let me explain why I'm not:
  1. I've built a proper isolation transformer in the past. I can use it, when it is necessary.
  2. My negligence: two way rectified mains after filtering in my country is around 650V DC. It is lethal anyways. If you make a mistake, it will kill you. It can't kill you twice because of the missing isolation.

So the plan:
Build a simple two way rectified supply with large can electrolytic capacitors (I bought 3300uF/500V ones for a good price. These puppies are huge).
Add necessary circuits for small panel meters to be able to measure, what is coming out from the circuit.
Add necessary circuit to discharge the capacitors. If they left unattended, they can still kill you after long time.
Add a special circuit I designed for it. It has two functions:

1. Test if the equipment grounded to the protective earth (it is not a proper grounding test, but if your lab is properly grounded, it will tell you if you have cabling problems)
2. Test the polarity of the incoming supply and change it if necessary. So the 0V output line of the equipment is always connected to the Null and never to the Line.

This circuit is still under development, so I don't know if it will work or not, but it isn't absolutely necessary for my power supply design. It adds some protection to the circuit, but nothing substitute the extreme care and the proper isolation.

I started to design this something like two weeks ago. Ordered the circuit board on the last Saturday. Actually it is already in my hands since Thursday, thanks to the extremely cheap, fast and high quality PCB manufacturer I found recently: ALLPCB (10 pcs 100x100 two sided boards for $5.49 delivered in 5-6 days - insane):

Yesterday I had some time to populate the board:

Those cans are huge, I told you.

Let see some measurement:

I've an isolation transformer, as I told before.
Ouch! My Fluke 117 unable to measure the output voltage.  According to the specification it is able to measure until 600V. In fact it was working until 650V, but the supply is above this. Lets change to a 1000V rated tool:

So, the PSU works now. My first impression, that the 100k/5W resistors I connected in parallel with the capacitors are improper for draining them. It take quite a long time (several minutes) to do their job, and generating some (not to much) heat during the operation (the 3W consumption on the display of the isolation transformer). It is sufficient  now, but I'm thinking to replace them to some active solution. (an AC Relay with some lower value/higher power resistors maybe).
This is it for now. Next is building into the enclosure, adding and calibrating the panel meters.