AJT & AndrewT:
Given the late stage of this project and my very limited understanding of the way the circuit works causes me to want to resolve this problem as simply as possible. If I can get the diyaudio softstart board to work in this application then I'd like to do so, and worry about a better softstart implementation the next time.
Until the resistors blew, both amps seemed to be working just fine; both F5T V2 amps were cycled on and off several times before I started letting them warm up to set the bias and, even then, they worked for a few hours before the resistors gave up their smoke. I'm still unclear what caused this problem to occur in both amps.
The BOM recommended X-class caps in the 0.33 to 1 uF range and posts indicated that the smaller the cap the longer the delay. Because the toroids are 1,000 VA (with 24 VAC secondaries in one amp and 32 VAC in the other) and the capacitor banks are 160,000uF, I figured a longer delay made sense. Clearly, that wasn't the case. I could try adding .56uF X-class caps in parallel or replace them altogether with something larger. I could also change the power resistors (presumably to something higher).
My inclination is to keep things simple, leave the power resistors at 75R apiece and get 1uF X-class caps. Does this sound reasonable to you?
Regards,
Scott
Given the late stage of this project and my very limited understanding of the way the circuit works causes me to want to resolve this problem as simply as possible. If I can get the diyaudio softstart board to work in this application then I'd like to do so, and worry about a better softstart implementation the next time.
Until the resistors blew, both amps seemed to be working just fine; both F5T V2 amps were cycled on and off several times before I started letting them warm up to set the bias and, even then, they worked for a few hours before the resistors gave up their smoke. I'm still unclear what caused this problem to occur in both amps.
The BOM recommended X-class caps in the 0.33 to 1 uF range and posts indicated that the smaller the cap the longer the delay. Because the toroids are 1,000 VA (with 24 VAC secondaries in one amp and 32 VAC in the other) and the capacitor banks are 160,000uF, I figured a longer delay made sense. Clearly, that wasn't the case. I could try adding .56uF X-class caps in parallel or replace them altogether with something larger. I could also change the power resistors (presumably to something higher).
My inclination is to keep things simple, leave the power resistors at 75R apiece and get 1uF X-class caps. Does this sound reasonable to you?
Regards,
Scott
Folks:
Just an update at this stage: the replacement power resistors and 1 uF x-class capacitors have arrived and I'm home this weekend, which means it's time attempt a repair. I removed the blown power resistors and x-class 0.33 uF cap and, as you can see from the first photo, this SoftStart pcb was scorched by the toasted power resistors (the gap of about 8mm between the resistors and the pcb was clearly inadequate). The damage does not appear to have affected the board traces, which were checked with a meter, and everything else looks okay.
The second photo shows the damage the 0.33 uF x-class cap endured.
I'll post a follow-up regarding this, the first of my four F5T amp builds.
Regards,
Scott
Just an update at this stage: the replacement power resistors and 1 uF x-class capacitors have arrived and I'm home this weekend, which means it's time attempt a repair. I removed the blown power resistors and x-class 0.33 uF cap and, as you can see from the first photo, this SoftStart pcb was scorched by the toasted power resistors (the gap of about 8mm between the resistors and the pcb was clearly inadequate). The damage does not appear to have affected the board traces, which were checked with a meter, and everything else looks okay.
The second photo shows the damage the 0.33 uF x-class cap endured.
I'll post a follow-up regarding this, the first of my four F5T amp builds.
Regards,
Scott
Attachments
...and the results appear to be in. The power resistors and x-class caps were replaced in the two F5T V2 amps and all is well. I'm still using 75R/5W power resistors (per AndrewT's advice) and swapped in 1uF caps for the 0.33uF caps (per AJT's advice). Both amps have been running beautifully for about 10 hours each.
I suggest that the diyaudio SoftStart BOM be updated to provide a more nuanced recommendation for the x-class caps based on mains voltage.
Thanks to 6L6 (for the diagnostic advice), AndrewT and AJT!
Regards,
Scott
I suggest that the diyaudio SoftStart BOM be updated to provide a more nuanced recommendation for the x-class caps based on mains voltage.
Thanks to 6L6 (for the diagnostic advice), AndrewT and AJT!
Regards,
Scott
One way to confirm or refute this suspicion, might be to run a circuit simulation (for those who are really interested).
It might turn out to be very important, to get the relay's coil resistance correctly modeled in simulation. It might also be important to simulate at the max/min possible values of mains voltage, for that unlucky person farthest away from the generating station, trying to play their class A amplifier on a day when generator output voltage is exceptionally low.
maybe we can start a database starting with yours...
mains voltage, cap value, relay coil dc resistance...
120vac, 0.86ufd, (please fill in your relay dc resistance)
data will surely be useful to others....
thanks pthep, this will assist others to determine suitable capacitor size...
please post your data SRMcGee....and others who have successfully built this soft start board...
so now we have values that work.....Hi everybody here.
I am building a Regulated PSU for a modular synth, it is a big one.
I am using a 300VA toroid trafo, 230V primay, 15-0 & 15-0 secondaries.
My filters are two 10000uF caps, using two 25amp bridges and two LM338 regulators.
The idea is getting +15 and -15 volts 5 Amps.
Should I need a soft start? I believe that I should, but after reading the whole 16 pages of the forum still I am not sure if this soft start is suitable for Regulated PSUs.
Any advise will be apreciated.
Thanks a lot in advance.
I am building a Regulated PSU for a modular synth, it is a big one.
I am using a 300VA toroid trafo, 230V primay, 15-0 & 15-0 secondaries.
My filters are two 10000uF caps, using two 25amp bridges and two LM338 regulators.
The idea is getting +15 and -15 volts 5 Amps.
Should I need a soft start? I believe that I should, but after reading the whole 16 pages of the forum still I am not sure if this soft start is suitable for Regulated PSUs.
Any advise will be apreciated.
Thanks a lot in advance.
Thanks a lot AndrewT for your really fast answer.
I was reading carefully all the posts and as far as I can understand I should use C9 1uf/400v R15 to R18 1K5 5W. Right?
R= 230x230/5/30 = 352.67 comercial value: 360R
360R x 4:1440 ohms commercial value 1K5
Is that right or I am doing something wrong here?
Thanks a lot!
Guillermo
I was reading carefully all the posts and as far as I can understand I should use C9 1uf/400v R15 to R18 1K5 5W. Right?
R= 230x230/5/30 = 352.67 comercial value: 360R
360R x 4:1440 ohms commercial value 1K5
Is that right or I am doing something wrong here?
Thanks a lot!
Guillermo
get a mains switch good for at least 10 amps, i do this all the time...no need for soft start...
Hi AJT, thanks a lot for your answer.
I have another PSU (51X Group DIY Lunchbox) and I did that (big switch), the Toroidal Transformer there is 250VA and I had to place a 3.1Amp fuse because the fuse was blowing everytime I turned it on.
I think for this new one which has a 300VA toroid I should use a 4 Amp fuse t avoid it blowing, then reading the all the posts I think a 4 Amp fuse will be almost useless. Am I wrong? That's why I ask for a suitable soft start.
Unfortunately I haven't got enough knowledge to be really sure about it and the consequence of not fiting one.
Cheers.
I have another PSU (51X Group DIY Lunchbox) and I did that (big switch), the Toroidal Transformer there is 250VA and I had to place a 3.1Amp fuse because the fuse was blowing everytime I turned it on.
I think for this new one which has a 300VA toroid I should use a 4 Amp fuse t avoid it blowing, then reading the all the posts I think a 4 Amp fuse will be almost useless. Am I wrong? That's why I ask for a suitable soft start.
Unfortunately I haven't got enough knowledge to be really sure about it and the consequence of not fiting one.
Cheers.
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