I've just completed building and testing a soft start DC blocker ('SSDCB') board.
You can read about it here:- https://hifisonix.com/projects/soft-start-dc-blocker-for-mains/
Here's a presentation you can download https://hifisonix.com/wp-content/uploads/2022/01/DC-Blocker-Soft-Start.pdf
Here's a video about DC on the mains by Paul Gowan of PS Audio DC on the Mains
The circuit uses a 32mm Ametherm thermistor to provide 10A current limiting for 3~5 seconds after mains power is applied with a fast (<0.5sec) reset when power is removed. A separate function provides DC blocking to remove the annoying audible transformer 'growling' that arises from DC on the mains.
You can read about it here:- https://hifisonix.com/projects/soft-start-dc-blocker-for-mains/
Here's a presentation you can download https://hifisonix.com/wp-content/uploads/2022/01/DC-Blocker-Soft-Start.pdf
Here's a video about DC on the mains by Paul Gowan of PS Audio DC on the Mains
The circuit uses a 32mm Ametherm thermistor to provide 10A current limiting for 3~5 seconds after mains power is applied with a fast (<0.5sec) reset when power is removed. A separate function provides DC blocking to remove the annoying audible transformer 'growling' that arises from DC on the mains.
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Hi Andrew
The link to download the BOM doesn't appear to be working on your site (it doesn't download the excel file)
The link to download the BOM doesn't appear to be working on your site (it doesn't download the excel file)
Hi - I've just checked and it is working ok. Can you PM me and I'll send it to you separately if you still cannot down load it (I'm using Office 365 to create my BOM's btw - might be that's an issue ?)
Looks like it was an issue with Chrome; I switched to IE and downloaded it. It's very strange as I downloaded the BOM for the speaker protection, via Chrome, yesterday, but can't do the soft start with Chrome.
Here's a very stupid/novice question. I note that it says the soft start is ok up to a full AC load current drawer of up to 2.2 Amp. How would I work out if my requirements don't exceed that (I want this to go in front of a 300VA, 25V+25V transformer, which will power a 77 watt Amp with 35V rails.)? Oh, and they'll be hooked up to 8 ohm speakers. I know, assuming 90% efficiency, that my 8 ohm speakers would draw 2.4 A at the full 77 watts, but not sure if that has any bearing on the AC current load.
Here's a very stupid/novice question. I note that it says the soft start is ok up to a full AC load current drawer of up to 2.2 Amp. How would I work out if my requirements don't exceed that (I want this to go in front of a 300VA, 25V+25V transformer, which will power a 77 watt Amp with 35V rails.)? Oh, and they'll be hooked up to 8 ohm speakers. I know, assuming 90% efficiency, that my 8 ohm speakers would draw 2.4 A at the full 77 watts, but not sure if that has any bearing on the AC current load.
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Why not arrange the board to also switch the AC mains on and off? It appears that this design requires an on/off switch rated for 230V and 10 amperes peak (?).
Agree - but that's just for the in-rush Mark. For steady state load its c. 2.2A (the fully de-rated DC block cap current). Might be a function to add at a future date though.
The load current is the AC mains current going into the transformer. Since your transformer is 300VA, you are well below the max AC load current of the SSDCB which is 500VA fully loaded.
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By the way, there's a typo in the spreadsheet for item 1. You've put 100000 uF, but it should be 10000 uF 🙂
I could not find R14 on the BOM. BOM item 10 remarks says 10 Ohms and 20 Ohms; should be 10K and 20K.
On page 4 of the PDF it says "The inrush delay is c. 3 4 seconds with a very rapid (<0.5seconds) reset when the AC power is removed"
The thermal time constant of the 10 Ohm thermistor is 64 seconds. Electrically the circuit may reset in 0.5 seconds, but the thermistor remains hot for a time. Repeated rapid AC mains cycling will cause high inrush current.
On page 4 of the PDF it says "The inrush delay is c. 3 4 seconds with a very rapid (<0.5seconds) reset when the AC power is removed"
The thermal time constant of the 10 Ohm thermistor is 64 seconds. Electrically the circuit may reset in 0.5 seconds, but the thermistor remains hot for a time. Repeated rapid AC mains cycling will cause high inrush current.
I happen to like illuminated pushbutton "anti vandal switches" like the ones in this picture. Regrettably, more than 75% of the ones I can find are only rated for low current, low voltage, DC use. Such as vending machines. And about half of them are momentary pushbuttons (not push-on, push-off). So my mains-AC-switch-plus-soft-start board includes a digital latch { CD4013 IC } and a PCB jumper option, to accept both types of switch action, momentary and/or push-on, push-off.
https://www.diyaudio.com/community/attachments/av_switches_all-jpg.846806/
https://www.diyaudio.com/community/attachments/av_switches_all-jpg.846806/
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I believe the comment about 10/20 ohm against item 10 is about the voltage measurement across the relay terminals when using 110/229 thermistor (10/20 ohms are the figures mentioned in the testing notes).
@Bonsai:
With regards to R4, R8, R9, you have 1K in the BOM, but the presentation says we need to work the values out depending on the supply voltage the amp is using. For me, using 35V rails, I get a resistor value of 687ohm ((35-34/0.16) and a power dissipation of 0.176 watt (256E-6 * 687). 687 ohm isn't a value I can find for a resistor so I'm not sure what to do.
The reset time I quote is indeed the electrical reset and not that of the thermistor. However, on the tests I've done (admittedly not with a large reservoir capacitive load but a c. 2kVA transformer), the thermistor remains cold - its only conducting high currents for 3~4 seconds. If you are using just a thermistor and relying on it getting hot due to the load current as an inrush current limiting technique, then the thermal time constant would cause a problem if you shut power off and then back on again within a few seconds. I am using the Thermistor here because its compact (32mm x c. 8mm disc) and has high energy handing capability.
BOM updated - thanks for catching the R14 thing.
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Those are nice looking switches. the 12A/250 VAC rated versions I've seen are about $25.
In the Excel BOM the note "10 Ohms for 110/120VAC and 20 Ohms for 220/240VAC" is in the row for item 10. The note should be moved to the row for item 11, the R3 thermistor.
Panasonic makes newer capacitors than the FC series that may be advantageous in this application. Series FS has a 10,000uF 6.3V part that has a 120 Hz rated current of 3056mA and a 10,000 hour life. The Mouser part number is 667-EEU-FS0J103.
Panasonic also recommends not blocking the pressure relief valve at the top of the capacitors. Also they do not guarantee the sleeving on the capacitor to be insulation quality. See the attached file sections 1.3(8) and 1.4(5) to 1.4(7).
Panasonic makes newer capacitors than the FC series that may be advantageous in this application. Series FS has a 10,000uF 6.3V part that has a 120 Hz rated current of 3056mA and a 10,000 hour life. The Mouser part number is 667-EEU-FS0J103.
Panasonic also recommends not blocking the pressure relief valve at the top of the capacitors. Also they do not guarantee the sleeving on the capacitor to be insulation quality. See the attached file sections 1.3(8) and 1.4(5) to 1.4(7).
Bonsai, I want to double check my maths for the resistor value in R4 -R9. Having revisited my earlier calculations, I now reckon I need a 660 ohm resistor of a quarter watt value. Assuming a current value of 0.0166667 (24/1440), and with my amp supply being 35V, then 35-24/0.0166667 = 659.99 ohms, or 660 ohms. Is my logic sound?