Power Supply Soft Start Board (V3)

[UKToecutter]

OK,
So your mains Live / Hot goes to Live/SW1.
One end of your mains switch also goes to the same terminal.
The return from the switch (switched Live) goes to SW2.
The neutral is the neutral straight from the power inlet

Cheers

 

[u1022186]

Thanks for your help UKToecutter. Much Appreciated. I just wanted to check one more thing before I hook it up to test it. C! appears to be a cap across the switch contacts for turn on spikes. The switch in my amp currently has a .005 uF cap across the switch contacts. Should I remove that one from the switch or can it stay in place. Thanks
Best Wishes,
Bob

 

[BigE]

Is there a manifest at digikey or mouser that we can use to order the parts? That would be

 

[6L6]

Not that I know of... but here's the BOM http://www.diyaudio.com/forums/atta...ft-start-board-v3-soft-start-board-bom-v3.pdf

 

[BigE]

Thanks. Pity that the diystore projects don't have "projects" at each of the major suppliers. Makes building easier. I've often thought that everyone that offers a PCB should do that. At least at one place.

 

[6L6]

I'm sure that if you make a project BOM at Mouser it will be gladly posted in the top of the thread!

 

[JojoD818]

There are only a handful of parts in these projects, I'm sure you'll have no trouble finding those parts in Mouser and you'll be up and running in no time.

 

[jnzz001]

I just purchased the PCB but I can't seem to find where the small standby transformer goes and low signal cable to power switch on the front panel.
Just don't tell me they forgot about it and I have to pull my 110V all the wayyyy.
I can still cancel the order.

Thanks

 

[SGK]

Hi. Can I use one of these boards on the primary side of two transformers? If so, does anything on the BoM need to change? TIA

(Hmm... $25 per board, only need the soft start board, speaker turn-on delay will be discarded, $14 for postage to the UK...)

 

[AndrewT]

A two pole relay. One pole to one transformer.

dpst (double pole single throw).

 

[SGK]

Ok but can that be implemented with this board or does one have to create a new board with (from the circuit in post 46) K1.2 replaced with a dpst and R3-R6 and the connectors replicated for the other transformer? Perhaps I am being slow.

 

[AndrewT]

or add a second spst relay for the second transformer.

 

[SGK]

in other words, don't use this board, produce a new one (albeit one can use the circuit with the modifications we've been discussing)? What are the issues attached to simply connecting two toroids to the output of this board?

 

[u1022186]

Hi. It shouldn't be any problem hooking the soft start to 2 transformers. I have it hooked up to 2. I built a dual mono supply for my amp that has 2 - 500VA 64 volt transformers and 160k capacitance (80K per channel) using just one of these and it works perfectly. The only change I made was to double the amount of 5 watt resistors by mounting them back to back on both sides of the board and adjusting the ending resistance based on the formula given in the instructions. It works great.

 

[AndrewT]

Two transformers coupled in parallel would need roughly half the resistance of one transformer.

But you still need to fuse each transformer.
If one fuse blows you have one transformer with the halved start up resistance.
That may cause the second fuse to blow on next start up, even though there is no fault in that transformer and it's load. Looking inside you cannot tell which channel blew the fuse because of a fault.

 

[u1022186]

My amp is only on when listening to music. If one fuse were to blow, I would know right away by which channel is playing music and which one is silent.

 

[AndrewT]

What's the first and second operations you carry out when a channel fails?

 

[SGK]

Two transformers coupled in parallel would need roughly half the resistance of one transformer.

But you still need to fuse each transformer.
If one fuse blows you have one transformer with the halved start up resistance.
That may cause the second fuse to blow on next start up, even though there is no fault in that transformer and it's load. Looking inside you cannot tell which channel blew the fuse because of a fault.

Maybe I have this backwards but the in-rush is for energising two transformers rather than one and two or more capacitor banks rather than one. So the in-rush through the switch to primaries is substantially more than with one transformer and less capacitance. Why half the resistance on the soft-start board rather than more?

Also, why two fuses and not one at IEC entry? If there's a problem the entire power supply goes down.

 

[AndrewT]

The chance of both transformers going faulty at the same time is low.
If one fails and needs fuse shutdown, then a double rated fuse for powering two transformers may never rupture, or may take many minutes to rupture.

Fuse each transformer separately.

If you do not use close rated fusing and rate the fuse value @ 3times VA / Vac and then double that for two transformers you will end up with enormous fuse rating that could power a 8kW to 10kW instant shower.

The soft start limits the current flowing through the Primary.
If one transformer has a current limit of 10Apk from 115Vac (162Vpk) then you would use ~10r to 15r

If you use two transformers in parallel then the current limit needs to be doubled to 20A.
That requires two resistors each of 10r to 15r, or one resistor of 5r1 to 7r5.

It's the same when considering what resistance to use for a single small transformer compared to a single large transformer.
A large transformer needs a lower value resistance to allow a high peak start up current.

I have used 140r (7off 20r in series) for a 80VA transformer to allow a T500mA fuse to be used and never blow at start up. Whereas I use 40r for a 1kVA transformer on a T4A fuse.

 

[Liliya]

I have built the Soft Start according to the schematic shown in post 46 on veroboard. Instead of 1N 4007 I use a diodebridge W 06 F. The relay is a 12 V so I use a 12 V zener as D13. C9= 220 nF. The rest is identical.The voltage out form W06F is to low to feed the relay to act(I have tried with a 9V battery on the veroboard and know that it is ok). I have also measured the AC voltage after R14//C9 and it is steady on 9v. What is going wrong?

Test was done without any transformer connected.

Eivind Stillingen