No I mean C9 on the PSU schematic for the F5 recommended by Mr.Pass or C1 on the PSU schematic for the F5T recommended by Mr.Pass.
I see that some members propose not dealing with a soft start when transformer VA are under 400VA or so. It is true that it may not be needed.
But, the way I see it, the benefits are (correct me on that):
(1) You can use smaller fuses, so higher protection.
(2) The rectification components will suffer less, so long term reliability is increased.
Maybe (2) is not a real issue, but I don't think a soft start could do more (if any) harm than good.
I am planning to build a power supply using a 100VA toroid, with 10 13.5V secondaries and 2 19.5V secondaries, each one of them using a CRC filter of 1200uF - 5.4R - 1200uF. So I thought I might use AndrewT's proposal, that is 4 390R resistors. Do you think this is good? Necessary? Let alone my opinion on necessity.
PS: Has anyone ever used a zero-crossing switching relay to power up? Wouldn't it take away the need for a soft start? Or would the initial current to magnetize the core still be big enough to need a soft start?
But, the way I see it, the benefits are (correct me on that):
(1) You can use smaller fuses, so higher protection.
(2) The rectification components will suffer less, so long term reliability is increased.
Maybe (2) is not a real issue, but I don't think a soft start could do more (if any) harm than good.
I am planning to build a power supply using a 100VA toroid, with 10 13.5V secondaries and 2 19.5V secondaries, each one of them using a CRC filter of 1200uF - 5.4R - 1200uF. So I thought I might use AndrewT's proposal, that is 4 390R resistors. Do you think this is good? Necessary? Let alone my opinion on necessity.
PS: Has anyone ever used a zero-crossing switching relay to power up? Wouldn't it take away the need for a soft start? Or would the initial current to magnetize the core still be big enough to need a soft start?
I don't agree. Even with smaller transformers the fuse cannot be close rated. A 400VA should be able to run on a T2A fuse forever. It will probably run forever on domestic duty with a T1.6A fuse fitted, Without a Soft Start it will almost certainly blow the T2A on first start up. Apply similar logic to 300VA and 200VA and 160VA. In my view they all benefit from using a Soft Start in that they don't then blow the close rated fuse.
This does not sound like Andrew T's idea.
I suggested series resistors of small value because I believe they are more robust and tolerate the start up condition better.
Last edited:
You are correct.
I posted a suggestion for discussion on how to adjust this particular Soft Start PCB to suit different transformers.
But that unfortunately does not condone my lapse of memory.
I still believe that it is better to series connect the resistor or Power Thermistors rather than to parallel them.
But you are stuck with this PCB.
I have made comments on this Soft Start schematic before and as a result I seem to have lost any friends I may have had on the Moderation Team.
I posted a suggestion for discussion on how to adjust this particular Soft Start PCB to suit different transformers.
But that unfortunately does not condone my lapse of memory.
I still believe that it is better to series connect the resistor or Power Thermistors rather than to parallel them.
But you are stuck with this PCB.
I have made comments on this Soft Start schematic before and as a result I seem to have lost any friends I may have had on the Moderation Team.
So for a 100VA toroid, based on your initial suggestion combined with your comment, I could use 4 25R 5W resistors in series. Is that correct?
AJT, thanks for the response. I really don't worry about the switch because as you say, I can buy one rated at 8A or what.
But a soft start should stress the reservoirs less, plus the rectifiers. And it would provide better protection, since smaller fuses would be used. On the other hand, I have no evidence that indicates the fact that capacitors live longer if soft-started for years, to support my claim.
Anyway, point well taken, I understand that you say that it is not necessary, but can't harm. I translate your comment into "don't get yourself into unnecessary trouble, buying parts that are not actually needed". Which is appreciated. I may finally choose to use a soft start based on my comments above, and to actually build one and gain experience. Next project should involve around 300VA for a guitar amp, so yeah, I will eventually use a soft start as projects progress, so why not become familiar with it earlier?
AJT, thanks for the response. I really don't worry about the switch because as you say, I can buy one rated at 8A or what.
But a soft start should stress the reservoirs less, plus the rectifiers. And it would provide better protection, since smaller fuses would be used. On the other hand, I have no evidence that indicates the fact that capacitors live longer if soft-started for years, to support my claim.
Anyway, point well taken, I understand that you say that it is not necessary, but can't harm. I translate your comment into "don't get yourself into unnecessary trouble, buying parts that are not actually needed". Which is appreciated. I may finally choose to use a soft start based on my comments above, and to actually build one and gain experience. Next project should involve around 300VA for a guitar amp, so yeah, I will eventually use a soft start as projects progress, so why not become familiar with it earlier?
I thought it was preferable to power a soft start using a separate low VA transformer... Although there are plenty of designs where no transformer is used, these are "less safe" and seem to be ill-advised for DIY use where someone without the proper knowledge of safety ratings for mains caps (for instance) or other components can put himself in danger of electrocution.
I need to implement a soft start for an upcoming project having a delay of 5-10 seconds. I am planning on using a separate 10VA transformer (cost is only $10 for a nice one), an inexpensive bridge rectifier and a single smoothing cap. This will be regulated to 12V by a cheap LM7812, and then a 555 timer is used to switch the resistor/thermistor bypass relay. This is very easy to design, safe, and adjusting the delay time anywhere between tens of milliseconds and tens of seconds is very simple. It's also simple to adapt the circuit to different relay coil voltages by changing the regulator, and the pull-in characteristics of the relay are unimportant.
Perhaps the added expense and complexity of a transformer and regulator are not desirable to this audience?
I need to implement a soft start for an upcoming project having a delay of 5-10 seconds. I am planning on using a separate 10VA transformer (cost is only $10 for a nice one), an inexpensive bridge rectifier and a single smoothing cap. This will be regulated to 12V by a cheap LM7812, and then a 555 timer is used to switch the resistor/thermistor bypass relay. This is very easy to design, safe, and adjusting the delay time anywhere between tens of milliseconds and tens of seconds is very simple. It's also simple to adapt the circuit to different relay coil voltages by changing the regulator, and the pull-in characteristics of the relay are unimportant.
Perhaps the added expense and complexity of a transformer and regulator are not desirable to this audience?
yes, in my case i use a small "housekeeper" traffo...
the soft start circuit was designed with simplicity in mind, regulators? not absolutely required....
the 2sc945 can be harvested out of discarded atx psu's generics will have them...
BC546 will work, just please make sure the leads are properly oriented. 2SC828 is another generic candidate.
Folks:
I thought you might find this interesting. This SoftStart board came out of an F5T V2 stereo amp. I'm not sure what the cause of the failure was but one lead on each of the four power resistors was blown out of its hole and one of the four power resistors is fried (the others appear fine). The transformer is a 1kw toroid with 24V secondaries and the power supply has 160kuF in capacitance. I was using 75R power resistors as recommended by AndrewT in this thread (as opposed to the 150 to 180R values in the BOM) and a 4A slow blow fuse.
Do you suggest I increase the resistance on R15-R18, lower the value of the fuse or try something else?
Regards,
Scott
I thought you might find this interesting. This SoftStart board came out of an F5T V2 stereo amp. I'm not sure what the cause of the failure was but one lead on each of the four power resistors was blown out of its hole and one of the four power resistors is fried (the others appear fine). The transformer is a 1kw toroid with 24V secondaries and the power supply has 160kuF in capacitance. I was using 75R power resistors as recommended by AndrewT in this thread (as opposed to the 150 to 180R values in the BOM) and a 4A slow blow fuse.
Do you suggest I increase the resistance on R15-R18, lower the value of the fuse or try something else?
Regards,
Scott
Attachments
Have you tested the board without the transformer connected? This way you can be sure that the relay works and shorts out the power resistors even without a transformer connected.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.