fuses

[kristopher612]

what kind of fuse do you guys use with your 3886 amps?  this is something i didn't even think about until i was going over all the stuff i've bought.  oops. im just glad to have realized it before i tried to fire it up!  my transformer ratings are
load capacity 160VA
primaries 115V+115V
secondaries 22V+22V

im not looking to save anything more than my house/life with the fuse so anything that will work for that is great.  thanks in advance
kris

[teemuk]

An ordinary F-type fuse will carry 135% of its rated current for about an hour but will blow within one second at 200 %. The maximum load current is about Urail / Rload plus some when the amplifier clips so the fuse should be maybe 50% – 60 % of that value. That for rail fuses, secondary fuses before the rectifier-filter circuit and mains fuse should be higher in value because they draw current in transient peaks. I have never tried fusing before the rectifier but I think you could use the same value as rail fuses but instead of type F use type T fuses (delay, slow blow whatever people wish to call them).

I don't use speaker fuses and I would rather replace that type of protection with a proper relay-based DC/thermal overload detector-protection that disconnects the load.

For mains side my only rule of thumb is to start with a fuse value derived from the secondary current draw that is converted to current draw at primary side using the turns-ratio of the transformer as guide. Use T-type fuse since the current is drawn in surges. Likely the initial fuse value does not last over the powering up due to high inrush current so advance increasing the fuse size until you find the one that tolerates the initial current surge. If the final fuse has some insane value then there's something wrong in the circuit (like insane amount of rail capacitance, high-power transformer etc.). In that case you need to redesign the circuit or add a "soft start" that limits the inrush current. Anyway, this behaviour shouldn't be an issue with a VA rating of 160 and if the rail capacitance is typical (about 1000uF – 6000uF per rail). Sometimes the transformer's datasheet suggests some fuse values.

Anyway, those are just my guidelines. I'm glad you asked about this topic: Fusing is not a thing that would be typically discussed in literature concerning amplifier electronics. Most just mention that one should put fuses here and there but do not really care to discuss about their ratings and characteristics. Hopefully we have more discussion about this issue.

[kristopher612]

We most definitely do need more discussion about this end of amplifier building.  To not discuss the things that can be the difference between life/death is kind of silly.  although after talking with RG Keen over at diystompboxes, and reading other posts by him choosing the correct fuse is not an easy task. 
so my math (mains side fuse) should be 160VA/22V+22V with a secondary current draw of 3.64A
do i need to multiply the 3.64A by 2 to gget what i should be using?  if im reading the datasheets correctly i need to add the secondaries together to get the correct voltage into the PS to run the amp at 50W output per channel.  this is why i ask about multiplying the secondary current draw. 
my final math should be 160VA/44V=3.64(rounded up)
so 3.64*2=7.28
7.28*3.64=26.5A 
unless i am wrong here a 30A fuse should work???  also being a newbie to fuses in general is a t-type fuse the same as a slo blo fuse?  thanks abunch

[joecool85]

We most definitely do need more discussion about this end of amplifier building.  To not discuss the things that can be the difference between life/death is kind of silly.  although after talking with RG Keen over at diystompboxes, and reading other posts by him choosing the correct fuse is not an easy task. 
so my math (mains side fuse) should be 160VA/22V+22V with a secondary current draw of 3.64A
do i need to multiply the 3.64A by 2 to gget what i should be using?  if im reading the datasheets correctly i need to add the secondaries together to get the correct voltage into the PS to run the amp at 50W output per channel.  this is why i ask about multiplying the secondary current draw. 
my final math should be 160VA/44V=3.64(rounded up)
so 3.64*2=7.28
7.28*3.64=26.5A 
unless i am wrong here a 30A fuse should work???  also being a newbie to fuses in general is a t-type fuse the same as a slo blo fuse?  thanks abunch

3amp slow blow fuse is what Brian at chipamp.com recommends for a LM3886 amp.  That's what I used for mine and it's been fine.

Here is the assembly manual for brian's LM3886 kit: http://www.chipamp.com/docs/lm3886-manual.pdf

[teemuk]

Kristopher612, Your equations are terribly wrong. You derive the 3.64A correctly but you must remember that the amplifier draws current from both the transformer and the filter caps. Therefore, the momentary peak current draw can be substantially higher than what the transformer allows.

You still do not state the load impedance that is an important part of the equation. Anyway, for dual supply amp you need two rail fuses. The peak current draw (per rail) is approximately 22V / Zload and a little bit more. A proper fuse value is about 50 – 60 percent of this.

If the amplifier is single rail one you would calculate the peak current draw from 44V / Zload.

...

At primary side the voltage is notably higher and therefore current much lower. You can determine how much lower based on turns ratio. In essence N2 / N1 = U2 / U1 = I1 / I2, where N means turns and the numbers mean primary (1) and secondary (2). The current draw of the secondary is about the 3.64A you derived.

Anyway, like I said before this will only give you the theoretical minimum mains fuse value – should be in the range of milliamperes. However, it has no use in real-life, as the inrush current can easily be few amperes, therefore the need for slow-blow fuses and higher rating.