Power Supply Soft Start Board (V2)

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JojoD818

JojoD818

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Joined 2003
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RCruz said:
I need to build this softstart circuit for my new amplifier psu that uses two 250VA TX in parallel.

I will buy the boards but want to keep other expenses to a minimum so I would like to avoid buying the 24v relay as I allready have some 12V ones in my stock. Can I replace the 24v zener (D13) by a 12v one and use my existing 12v relay ?
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Hi,

Unfortunately, the circuit calls for the use of a 24V coil relay.
 
A

AndrewT

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Joined 2004
Originally Posted by AndrewT View Post
Two transformers equals two relays to bypass the two sets of soft start resistors/thermistors.

Another unhelpful post?
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and two 12V relays will operate from the same 24V circuit supplied by DIYaudio

Tony sees my posts as unhelpful.
indeed, why complicate matters when simply paralleling two transformer primaries will work?
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Why?
because each transformer sees it's own current limiter. and each is unaffected by what the other transformer is doing/loaded.
 
TonyTecson

TonyTecson

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Joined 2003
AndrewT said:
and two 12V relays will operate from the same 24V circuit supplied by DIYaudio

Tony sees my posts as unhelpful.Why?
because each transformer sees it's own current limiter. and each is unaffected by what the other transformer is doing/loaded.
Click to expand...

if you are doing mono blocks, i agree, but if the amps are going to be built on a single chassis, i do not see any need for separate soft starters.....

remember that the series current limiters will be shorted out after a few secs delay so that Andrew's concerns are unfounded.....Have a very merry Christmas AndrewT.....


@ RCruz
if you want to use 12 volt relays and change the zeners to 12 volts, i'd say go ahead, after you have purchased the boards, it becomes your property and you are free to modify it to suit your needs....
 
RCruz

RCruz

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Tony said:
@ RCruz
if you want to use 12 volt relays and change the zeners to 12 volts, i'd say go ahead, after you have purchased the boards, it becomes your property and you are free to modify it to suit your needs....
Click to expand...

Judging by your comments, I would say it is feasible.....

I used spice to sim the circuit and in reality the zener seems to clamp the voltage at 12V.

The wattless dropper is used to set the time delay and I believe we can "play" with the cap value without fear of "overdriving" the relay.

I will build it p2p just to be sure....

Now regarding safety... what if the relay stops woking.... During power up, with a large amp TX with some 40000uF smoothing, there will be around 5.5amps on the 180r resistors, but after the 40000uf powerbank is charged, the current should go down, so even if the relay does not start, maybe the current on the 180r resistors is not so high ?!
 
RCruz

RCruz

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Tony said:
just one ntc is good enough...this ntc once heated up will be like a sub-ohm resistor and so will be invisible to the transformer....
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Thank you Tony

That is good to know, in fact Papa uses these in his A75 power amp....

I will experiment with a single ntc and if I feel it does someting nasty to the sound, I will build the relay thing with the timer :)
 
S

steelwil

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Joined 2008
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Why does R14 a 1M resistor need to be 3W?

I = V / R = 250V/1000000 = 0,25mA
P = I * V = 0,00025A * 250V = 0,0625W

So even if it is directly applied to 250VAC a 0.25W resistor should still be fine.

I would imagine this has something to do with safety or the 250VAC.

I am just having some difficulty getting a 3W 1M resistor.

Thanks
 
N

nrg2009

Member
Joined 2009
AndrewT said:
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.
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Andrew,

I read your interesting posts on thermistor use.
I am just getting started and want to learn more. You mention that thermistors are very effective at startup of an amp, even better than the softstart board offered by te diyaudio store.
However you also mention that they limit the amp from reaching its full current potential.
Also, you mention placing the thermistor on the secondary, whereas i see most people placing the thermistor on the primary side of the trafo, for us that is 110volt. In the guidelines from GE who manufactures the thermistor also shows it on the secondary winding.
Any suggestions on how i should use thermistors?

Thanks.
 
TonyTecson

TonyTecson

Member
Joined 2003
steelwil said:
Why does R14 a 1M resistor need to be 3W?

I = V / R = 250V/1000000 = 0,25mA
P = I * V = 0,00025A * 250V = 0,0625W

So even if it is directly applied to 250VAC a 0.25W resistor should still be fine.

I would imagine this has something to do with safety or the 250VAC.

I am just having some difficulty getting a 3W 1M resistor.

Thanks
Click to expand...

your call, you can do it if you wish.....when using 240volt mains the 1ufd capacitor must be rated 630volts dc or better....
 
TonyTecson

TonyTecson

Member
Joined 2003
nrg2009 said:
However you also mention that they limit the amp from reaching its full current potential.
Also, you mention placing the thermistor on the secondary, whereas i see most people placing the thermistor on the primary side of the trafo, for us that is 110volt. In the guidelines from GE who manufactures the thermistor also shows it on the secondary winding.
Any suggestions on how i should use thermistors?

Thanks.
Click to expand...

not Andrew, but i have had extensive experience with these NTC thermistors repairing hundreds of ATX psu's....

these thermistors have cold resistance values from say 4 ohms to about 20 ohms, but as soon as it gets warm, the resistance drops to less than an ohm so that it does not interfere with the working of the power traffo, the dc resistance of the primary of the power traffo will have more resistance....

you can verify what i say by connecting a thermistors to an ohmmeter and training a heat source to it like a hair dryer and see for yourself how the resistance drops....

use it in circuits where you think adding a series resistance to limit surges will help, doesn't matter primary or secondary....
 
T

TANDBERGEREN

Member
Joined 2010
Just pointing at a tiny little case here Tony.

It does matter wether the thermistor is to be mounted on the primary or the secondary.
One should always mount them where the nominal current is the lowest.
This to secure the best (longest) service time for the cirquit.

But otherwise I totally agree with You. Although I always bypass them with a relay after a suitable inrush-time. Again to secure extended service-time.
 
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