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Old 7th June 2012, 05:41 AM   #1
Ana is offline Ana
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Default Power Supply Soft Start Board (V2)

This thread is for discussions about the Power Supply Soft Start Board in the store. For more information on this product, please see the product page below.

Threads on diyAudio that relate to this product (If we have missed one, please post it in this thread and we will add it to the list):

Last edited by Jason; 27th October 2012 at 07:15 AM.
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Old 27th October 2012, 12:20 AM   #2
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Default Honey Badger


Will the PS SSB work with the Honey Badger? and if I get the 4U Deluxe Jack of all trades case, will two of these SSBs fit in there? I am also building one F5; same questions apply. Please inform, since I want to order these soon.
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Old 27th October 2012, 07:28 AM   #3
Jason is offline Jason  Australia
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Hi Bugz,

I've updated the first post in this thread with a link to the Build Guide, the main discussion thread, and the build thread for the Honey Badger. The build guide shows the use of our power supply and soft start boards as well as photos of the amp in a 5U chassis. Jojo (who wrote the build guide) thinks that the 4U will be great for a HB with single power supply, and the 5U good for dual supplies. The same should apply to the F5.

We're about to publish a build guide for the F5 which shows it all dropping straight in to a 4U. The best aesthetic will, IMHO, come from mounting some boards vertically near the front of the chassis using simple brackets but we wanted to initially show that they aren't actually a requirement. There's a lot of space in those chassis and just putting everything flat on the bottom is quite a waste of space. The perforated baseplate is brilliant for the ability to vertically mount boards wherever you want using a few brackets that can be pretty easily made up.
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Old 2nd November 2012, 11:08 AM   #4
AndrewT is offline AndrewT  Scotland
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I think there should be some "design" notes added for the current limiting resistor values.
The 4 resistors R15 to 18 are specified @ 180r.
Is this for 110/120Vac and/or 220/240Vac?
Is this for 200VA to 1000VA transformers and/or any size transformer?

I know from experience that 45r (180/4) works on 220/240Vac for medium sized transformers in the range 200VA to 500VA.

But smaller transformers would need higher value resistors and larger transformers would need lower value resistors.
I expect that 110/120Vac transformers will generally require lower value resistors to effect similar current limiting.

A simple table showing VA & Vac combinations with appropriate resistor values would be very useful to less experienced builders.
regards Andrew T.
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Old 2nd November 2012, 12:53 PM   #5
AndrewT is offline AndrewT  Scotland
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here's a suggestion for discussion

. . . . . . . . . .Vac
VA . . . 115Vac . . 230Vac
100 . . . . 270r . . . 390r
200 . . . . 180r . . . 270r
400 . . . . 120r . . . 180r
800 . . . . .82r . . . 150r
1600 . . . . 68r . . . 120r

The resistor values shown are from the E12 range. If these are difficult to find in 5W wire wound, use the nearest E6 value.

For 230Vac the values shown allow a T rated fuse that is close rated to be used for reliable start up over the long term.
eg. 500VA fused with T2A will start with 40r to 50r in the primary circuit (equivalent to 160r to 200r for the four 5W resistors)
regards Andrew T.

Last edited by AndrewT; 2nd November 2012 at 12:58 PM.
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Old 6th November 2012, 01:11 PM   #6
dazed2 is offline dazed2  Canada
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Would this Soft start be suitable for a PSU that utilizes a 1.5kVA transformer with about 700,000uF capacitance at 36V?
This is for a F5T V3 build so its going to be a Class A amp.

Would it be advisable to increase the SS time?
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Old 6th November 2012, 01:26 PM   #7
AndrewT is offline AndrewT  Scotland
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soft start is for transformer start up current surge.

If you want capacitor slow charge then consider using Thermistors in the secondary feed. This is described in most of the Thermistor literature. Use a relay or similar to take the Thermistor/s out after ten to twenty seconds.

Back to soft start. This only needs to be a few dozens of milliseconds. Use 100 to 250ms as your delay.
The circuit given here does not have a defined delay !!!! It is just an RC with ill defined starting voltage, that becomes limited by the Zener.
regards Andrew T.
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Old 6th November 2012, 02:23 PM   #8
dazed2 is offline dazed2  Canada
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Thanks for the feedback AndrewT.
What I meant by increasing the delay was based on the build guide of varying C9 to increase delay. However you're right its only in the matter of milliseconds, not seconds.

So what you are suggesting is that after the transformer, build another circuit with thermistors to slowly charge the caps and then a relay to switch them out?
How would you ensure that the thermistors have been fully heated before the relay switches them out? Or does this also occur in milliseconds?

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Old 6th November 2012, 04:55 PM   #9
AndrewT is offline AndrewT  Scotland
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Thermistors are probably the best style of resistor to use in both a soft start and in a slow charge.
But here in the UK Power Thermistors are very expensive.
Therefore I use multiple wirewounds as the current limiter.

In the mains circuit they must be switched out quickly, probably no more than half a dozen seconds. But for soft start that is not an issue the soft start limiting is redundant within 100ms and can be switched out long before the resistors get hot.

Thermistors don't have that problem, they adjust dissipation to suit the temperature they are running at. They get hot, they adjust the resistance down, they get less hot.

However in a slow charge circuit the capacitors never really fully charge. The charge voltage is asymptotic to the final voltage, They never quite get there.
This results in a current pulse when the resistance is switched out.
The sooner you switch out the resistance the bigger the current pulse.
For this reason you want a long duration on the timer. That long duration does not suit resistors, it does suit Thermistors. They don't burn out, if selected appropriately.

Initially they warm up and the resistance falls, as the charge current reduces they cool down and the resistance increases. But eventually they stabilise to the quiescent condition. But if the music starts the resistance will vary with the "beat" of the music. The supply rails jump around due to the high and variable source impedance. You must switch them out.

Read my Krell Klone post. I could see the rails change as the amp was biased into ClassA from ClassB. I also found that when biased to ClassA with the rails at a lower voltage the amp delivered more power because the Primary circuit Thermistor was still in circuit, but was hot due to the ClassA current, whereas in ClassAB the Thermistor had cooled and fed that high source impedance to the transformer and music peaks and even the 2second duration of maximum power delivery did not warm up the Thermistor enough to allow the Rails to come up to full voltage.
I wish I had tested full power in ClassA & ClassB without the limiter in place..
regards Andrew T.
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Old 7th November 2012, 04:12 PM   #10
bobfig is offline bobfig  United States
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hello i have a question about what switch i can use. i found one i like but im not sure on how much current the switch will need to handle to turn on and off the amp.

switch in question
MP0045/1E2BL012 Bulgin | Mouser
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