Need a soft start?

[pooge]

not enough info. doesn't specify the current it can handle. 5-8 seconds seems like a VERY long time to be in circuit, especially for a power amp. could burn out resistor if not adequate, unless it is a high value resistor to really drop the current, and therefore require a really long time constant.

[Leolabs]

Toroidal needs softstart but not EI.

[maxpou]

Hi,
this is my soft start and it's very simple. i bypass 2 resistances 10W 39ohms in parallel after 1s. What do you think of this circuit? Thank You! Maxpou

[AndrewT]

Hi Max,
what is your circuit connected to?

if the connections on the left come from the soft started transformer then they are going to sit low and delay the time constant.

Why is c1=10mF, this should work with less than 100uF.
Why is the RC of C2*R1=22seconds? how will this give 1 second delay? 200mS to 300mS is OK for a soft start delay.

When the voltage does come up then a higher voltage is going to appear on C2 & R4. will the high voltage be OK?

are the diodes on the right connected to a pair of relays?
Why two diodes?
What are the two relays doing? One for soft start presumably, and the other?

[maxpou]

Hi AndrewT,
my circuit is connected to another transfo 9-0-9Vac and the 10mF cap is for dc filter.

The delay is 1 second because the cap must be charge at 0.6V for the transistor Q1 turn on, not 12V.

Yes, you are right I could have put one diode. I have 2 relay one by side because i have 2 transfo. Thank You! Maxpou

[smithy666]

I have just finished a PCB for a soft start and switch on circuit. The soft start uses 4 x 10R/5W resistors in series to limit the inrush current to about 5.5A, which are then taken out of the circuit by a relay which operates after about a half second or so. The other part of the circuit is a remote switch on, allowing any switch to be used as the power switch (I am using the anodised aluminum push button - TH Contact). There are also 2 LED connections - standby and power on, with trimpots to allow adjusting the LED intensity. Board size is about Board size is 168x97mm. Just tested and works beautifully.
I've been using the soft start circuit alone in my power amps for about 8 years now with no problems to report.
Pic of completed PCB attached. I can make the boards available (bare), with components or completed if anyone is interested. Sorry, but PCBs have been designed for 220-240V use only.

[maxpou]

Hi,
i would like to know how calculated the fuse of primary for my power amp? Thank You! Maxpou

[AndrewT]

Hi Max,
transformer VA divided by mains voltage.
eg. 400VA and 120Vac gives T3A.
If this blows at start-up then fit more soft start resistance.

[Jan Volkering]

John,

Maybe you can check if Hypex is working on SMPS for their UcD's.
What is the kick of an efficient amp if the powersupply is bulky?
In that case I asume the PFC, hence inrush limiting will be included.

Alternatively, Have a look at Per Anders' website.
This is by the looks of it a relay based limiter.
Pro: its simple
con: high probability it switches at the peak of the mains sinewave, causing a massive bang in the torroid.
You can hear it buzzing for several seconds if that happens.

Last option: be patient, I am working on a microcontroller controlled TRIAC version that can switch up to 3 transformers, switches on at the zerocrossing with a delay between each transformer.
Sounds like overkill but if you spend so much on your torroids, high power rectifiers and high voltage caps, 10 bucks for a protection isn't gonna bankrupt you.

[peranders]

Originally posted by Jan Volkering
Alternatively, Have a look at Per Anders' website.
This is by the looks of it a relay based limiter.
Pro: its simple
con: high probability it switches at the peak of the mains sinewave, causing a massive bang in the torroid.
You can hear it buzzing for several seconds if that happens.

Not quite... max current is set by the resistor which in my case will give me 3 A peak... not a massive bang

Rember also that the main purpose of a softstater is to save the fuses, not make a silent start.