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Old 17th April 2011, 10:40 AM   #611
Baztien is offline Baztien  France
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Quote:
Originally Posted by AndrewT View Post
Hi,
A motor and/or transformer should have the fuse rated at three times the nominal maximum current.
Try T800mA or standard 1A
thanks andrew
what about on the secondary side? How do you calculate the fuse rating? what kind of fuse should I use?

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Old 17th April 2011, 12:23 PM   #612
AndrewT is offline AndrewT  Scotland
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I have found that maximum output current into the nominal load is a very good guide to the rail fuses that can supply maximum power and transient currents that could exceed the nominal maximum current.

I suggest using an F fuse rating of half the peak output current.
Quasi suggests an F fuse rating of half the rms output current.

If you have a 60W into 8ohm amplifier, the maximum output currents are 2.7Arms and 3.9Apk.
You could use F1.6A or F2A and find the amplifier will never blow the F fuses when connected to valid speaker load and pass a valid audio signal.
You will find that peak transient currents up to nearly 12Apk (for 8ohm speaker) will not blow the fuses.
You may find the amp will work with slightly lower rated F fuses, but I would not fit any higher rating.

These supply rail fuses must be fitted after the main smoothing capacitors.
Fitting fuses before the smoothing capacitance requires such high rated fuses, that they offer virtually no protection in event of mis-use.

Look at the preceding table for T rated fuses to see what is happening.
Take the T2A fuse. At 2A it lasts forever.
@ 4A is blows at ~ 1.5Seconds. No music transient ever last for 1.5Seconds. A DC fault could easily last that long and blow the fuse, saving the speaker. Obviously an F2A will blow more quickly.
The following is all guesswork based on scientific principles. Actual testing could easily prove what pulses a fuse can pass without failure.
Now look at how long the fuse takes to rupture at 12Apk. ~100ms. Again a transient output current of 12A cannot last as long as 100ms. By definition a transient is a fast and quickly passed current condition. It might last 10us or 100us or 1000us. The fuse might pass 1000 pulses of 1000us before fatigue makes the fuse blow.
But it might take 10,000 pulses of 100us for the fuse to fatigue fail and maybe a million 10us pulses for the fuse to fatigue fail.
The F2A fuse could last forever when passing valid signals to a valid load. That is why you may find that a smaller fuse might suit your system assembly.
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Last edited by AndrewT; 17th April 2011 at 12:41 PM.
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Old 18th April 2011, 05:49 PM   #613
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Originally Posted by AndrewT View Post
These supply rail fuses must be fitted after the main smoothing capacitors.
Fitting fuses before the smoothing capacitance requires such high rated fuses, that they offer virtually no protection in event of mis-use.
If you put the fuses behind the smoothing caps, you might as well skip them. They will not be able to protect the transformer from faults in wiring, rectifiers and smoothing caps that way. Their function will be reduced to protection of amp and speakers. Chip amps usually have their own overcurrent protection built-in, so the fuses are dispensable for that. Speaker protection with fuses is unreliable. If you want to protect the speakers, install a speaker protection circuit.

Fuses belong before the rectifier as close to the transformer as possible. Use slow-blow fuses with a rating equal to the transformers nominal secondary current or the next lower rating. If you use lower ratings than that, you have wasted money on too big a transformer.
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Old 18th April 2011, 06:31 PM   #614
Baztien is offline Baztien  France
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Hi I don't think I've got the choice of the location as I don't use rectifier. My unit (K4003) uses AC current directly. It also claims it's short circuit protected. I'm using a 72VA 2x12V 3A transformer but a 50VA 2A was recommended. Should I use T2A fuses? on each rail between the transfo and the amp? Besides when considering a 2x30W amp do we use 60W in calculations using power? I'm very confused with that. cheers
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Old 19th April 2011, 06:04 PM   #615
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Originally Posted by Baztien View Post
I'm using a 72VA 2x12V 3A transformer but a 50VA 2A was recommended. Should I use T2A fuses? on each rail between the transfo and the amp?
You can try T2A fuses on each rail. T3A won't hurt however.

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Originally Posted by Baztien View Post
Besides when considering a 2x30W amp do we use 60W in calculations using power?
Yes, the two channels add up. 2x30 W appears quite optimistic for the TDA2616, though. When the datasheet specifies 2x15 W at 10 % THD, then 2x30 W could be DC or square wave power.
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Old 20th April 2011, 08:44 AM   #616
Baztien is offline Baztien  France
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Originally Posted by pacificblue View Post
You can try T2A fuses on each rail. T3A won't hurt however.


Yes, the two channels add up. 2x30 W appears quite optimistic for the TDA2616, though. When the datasheet specifies 2x15 W at 10 % THD, then 2x30 W could be DC or square wave power.
You're right I've actually got an old IC version the TDA1521 for which Velleman gives 2x30W but the data sheet gives 2x15W as you said.

I'm not sure I understand why they add up. In the example given above by AndrewT it looks like 60W comes from 2x30W (value I first gave) but it's for a calculation of fuse rating per rail (I understand).

Or maybe I am wrongly assuming that one rail corresponds to one channel.

Thanks in advance for helping me clarifying the matter
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Old 20th April 2011, 08:54 AM   #617
AndrewT is offline AndrewT  Scotland
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Hi,
the amplifier rating is used to determine a compatible transformer size.

Once you have selected your transformer you use transformer to determine the mains fuse rating.

If you want secondary fuses to the amplifiers, (I prefer each amp to have it's own two supply rail fuses) then use speaker impedance to determine peak current and from there select a secondary fuse.

Note: each fuse uses different criteria for selection of fuse value. Each fuse must be sized for the duty it is expected to perform (pass operating current) and when to rupture (break fault current).

BTW,
normal operating current in the secondary feed before the main smoothing capacitance is enormous. Follow Pacific's advice and determine "operating" current and select a fuse to suit the duty. Then see what faults can rupture that fuse. Good luck.
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Last edited by AndrewT; 20th April 2011 at 08:58 AM.
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Old 20th April 2011, 02:26 PM   #618
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Quote:
Originally Posted by AndrewT View Post
Hi,
the amplifier rating is used to determine a compatible transformer size.

Once you have selected your transformer you use transformer to determine the mains fuse rating.

If you want secondary fuses to the amplifiers, (I prefer each amp to have it's own two supply rail fuses) then use speaker impedance to determine peak current and from there select a secondary fuse.

Note: each fuse uses different criteria for selection of fuse value. Each fuse must be sized for the duty it is expected to perform (pass operating current) and when to rupture (break fault current).

BTW,
normal operating current in the secondary feed before the main smoothing capacitance is enormous. Follow Pacific's advice and determine "operating" current and select a fuse to suit the duty. Then see what faults can rupture that fuse. Good luck.
Yeap, it all starts with a well thought out power supply ...
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Old 20th April 2011, 03:08 PM   #619
Baztien is offline Baztien  France
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Quote:
Originally Posted by FastEddy View Post
Yeap, it all starts with a well thought out power supply ...
I selected a transformer as recommended by Velleman's kit assembly manual

Quote:
Originally Posted by AndrewT View Post
Hi,
the amplifier rating is used to determine a compatible transformer size.

Once you have selected your transformer you use transformer to determine the mains fuse rating.

If you want secondary fuses to the amplifiers, (I prefer each amp to have it's own two supply rail fuses) then use speaker impedance to determine peak current and from there select a secondary fuse.

Note: each fuse uses different criteria for selection of fuse value. Each fuse must be sized for the duty it is expected to perform (pass operating current) and when to rupture (break fault current).

BTW,
normal operating current in the secondary feed before the main smoothing capacitance is enormous. Follow Pacific's advice and determine "operating" current and select a fuse to suit the duty. Then see what faults can rupture that fuse. Good luck.
Is the operating current given by the manufacturer? I read 3A on my trafo
what do you mean by "see"? look up on a graph like the one you described? or test what faults can rupture that fuse?

Sorry but there's just a lot of electronic terminology I'm not familiar with.
Pacific's solution leaves me less stumped than the ones with all this terminology.

thanks for your patience
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Old 20th April 2011, 04:40 PM   #620
AndrewT is offline AndrewT  Scotland
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Post 610 has the graph from a manufacturer for their T rated (slow blow) fuses. Go and look at it.
Most manufacturers will be similar but not identical. F rated fuses and standard rated fuses have their own sets of graphs. Some manufacturers have a range of ultrafast fuses.
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