Hello, i was wondering if i got things right,
assume we have 2 transformers in this configuration:
1x230V Primary
6x8V 5A Secondary
1x230V Primary
2x15V 5A Secondary
what would be the safest route to protect the whole device? i read some people just use a fuse on the primary but that doesnt make much sense... the 8V 5A secondary draws 40W max, so to protect the secondary effectivly i would need a (40/230=0.17) 0,17A primary fuse (?!) please enlighten me here
assume we have 2 transformers in this configuration:
1x230V Primary
6x8V 5A Secondary
1x230V Primary
2x15V 5A Secondary
what would be the safest route to protect the whole device? i read some people just use a fuse on the primary but that doesnt make much sense... the 8V 5A secondary draws 40W max, so to protect the secondary effectivly i would need a (40/230=0.17) 0,17A primary fuse (?!) please enlighten me here
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im currently planning my build, i wanted to get the 6x8V (or possibly even 6x8V and 2x15V in one transformer) transformer because i wanna save some space... but i guess this doesnt help with choosing fuses..
i also wanted to play around with supercaps, but i guess its no problem because the regulators cap out
thats what i try to figure out, can i get away with just a primary fuse? it seems kinda safer to protect each secondary and use some arbitrary primary fuse that just protects from short circuit... but can still let the transformer run under full load, which wouldnt be possibly with calculated main fuse in my example above...
i could also up the 8V rails to 9A .... so they can get away with 70W like the 15V 5A rails... this would allow for a 0,3A main fuse, i guess because of the transformer thing it will be around 0,5A, could this work?
its a dac, i guess this is not too critical... better a pop than the house burning..
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hmm i guess its really better to fuse each secondary seperately, if we talk about "fuse sound" we should try to dimensionate the fuses as big as possible, right?
so i probably go with this scheme:
Primary: 230V 400VA, fused with (400/230=1.74A, lets take 150% which is 2,56A, prob the next fitting value is 2A..)
Secondarys:
6x8V 5A, protected with 4A each before the rectifier
2x15V 5A, protected with 4A each before the rectifier
i probably could live with a 0,17A main fuse but for a transformer with more windings than two this seems kinda strange todo
i got things right, right?
so i probably go with this scheme:
Primary: 230V 400VA, fused with (400/230=1.74A, lets take 150% which is 2,56A, prob the next fitting value is 2A..)
Secondarys:
6x8V 5A, protected with 4A each before the rectifier
2x15V 5A, protected with 4A each before the rectifier
i probably could live with a 0,17A main fuse but for a transformer with more windings than two this seems kinda strange todo
i got things right, right?
PT?
the example above was for one transformer with 8 secondarys, the primary fused with 2A like in the quote
yea im currently planning on fusing both sides because i think that way you can take a way bigger fuse on the primary, else it seems like a compromise, tho i guess this only a primary fuse works pretty fine for transformers with only one or two secondarys
Choosing a fuse in the primary & secondary is not so simple. The first question is whether it is a toroidal transformer or an EI. I don't use higher power EI transformers, so I'll tell you about the toroidal ones (they draw a higher starting current). A 4AT or 5AT slow-type fuse in the primary is required for a 400VA torus. For that transformer (and larger) it is recommended to have a soft start (inrush current limiter), when the fuse can be reduced to 2.5AT. Fuses on the secondaries are not mandatory, but they can be installed.
The starting currents also depend on what is on the secondary, if there is a lot of capacity, then there is a bigger problem with the fuse. It is not so easy to calculate. The nominal current value of the secondary cannot be taken, but at each secondary slow fuse (T) must be taken at least 50% stronger than the nominal current. If it burns on power-up, take the next higher value.
You can try some simulations with PSUD2 and you will see what happens when you turn on the secondary. That will also show you whether you have chosen the diodes in the bridge correctly. The capacitors are charged by a series of strong current pulses in periods when the voltage of the secondary is higher than the voltage on the capacitors. There is no sinusoidal current for which the transformer is dimensioned. That's why it's not a bad idea to oversize the transformer when the secondary is loaded with diode bridge and a lot of capacitors.
The starting currents also depend on what is on the secondary, if there is a lot of capacity, then there is a bigger problem with the fuse. It is not so easy to calculate. The nominal current value of the secondary cannot be taken, but at each secondary slow fuse (T) must be taken at least 50% stronger than the nominal current. If it burns on power-up, take the next higher value.
You can try some simulations with PSUD2 and you will see what happens when you turn on the secondary. That will also show you whether you have chosen the diodes in the bridge correctly. The capacitors are charged by a series of strong current pulses in periods when the voltage of the secondary is higher than the voltage on the capacitors. There is no sinusoidal current for which the transformer is dimensioned. That's why it's not a bad idea to oversize the transformer when the secondary is loaded with diode bridge and a lot of capacitors.
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you say secondary fuse is not mandatory but how is one 230V 2A primary fuse going to protect one secondary going over 40W (8V 5A in my example) ?
it will be a toroidal, a custom toroidy audio grade one
i designed a dc rectifier/smoothing board with 2x 10000 uF, for each of the secondarys, tho i probably have to redesign it since i left a fuse out...
i probably also play around with supercaps, ... when these are charging current is quite high, but i probably will limit current with regulators, if things settled the current flowing will be quite low since its a dac
i will take a look at inrush current limiters
I have a 650VA torus wound on a core for 800VA in the amplifier. Sometimes it blew the 10AT fuse in the primary, so I added an inrush current limiter, a timer with a 4sec delay and then it activates a bypass relay switch (3x10A). Current is limited with 50ohm 50W resistor. Fuse is 5AT.
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no of course not but i just try to understand how just a main fuse calculated for the whole transformer can still protect a single secondary effectively
Just put at least 50% bigger than nominal current. It is sometimes dangerous for symmetrical power supplies to blow a fuse on only one secondary. And transistors burn faster than even the fastest fuses.
well the question is how to i protect the dac effectively in my case, neither the primary or the secondary should be able to burn down.... and also not because a symmetrical power supply fails on one side
You worry a lot. In 40+ years of this hobby I have had one 300VA transformer burn out and that was the primary winding. Wire has not completely burned, but the thermal fuse in the primary .This happened due to some manufacturing error, probably, or poor quality varnish on the wire. Your secondary will not burn that easily, it will burn all the electronics before that. The transformer is quite a durable beast.
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i just wanna be on the safe side, i will probably add secondary fuses
ah yea this seems like a good solution, i just found the these thermistors but they dont work if the unit is already warm... the relay and resistor is basicly the same principle but works at all times, and i guess the relay offers lower resistance, how do you realize the delay?
Ah... but how to power it?
i also found this post: https://www.diyaudio.com/community/threads/selecting-ntc-inrush-current-limiters.313248/post-5528035
so i guess i could inrush current limited the transformer and use 5V from a secondary to power on the relay, no aditional circuit required as the post states but i could also use a microcontroller or the NE555 then
10 ohm 50W resistor seems about right for a 230V 2A circuit
i also found this post: https://www.diyaudio.com/community/threads/selecting-ntc-inrush-current-limiters.313248/post-5528035
so i guess i could inrush current limited the transformer and use 5V from a secondary to power on the relay, no aditional circuit required as the post states but i could also use a microcontroller or the NE555 then
10 ohm 50W resistor seems about right for a 230V 2A circuit
PT?
PT means Power Transformer.
Most used abbreviation in this Forum.
By the way, OT means Output Transformer.
It is not and nobody said so.
Individual Secondaries are protected by their own individual fuses.
In general "regulators" and "supercars are not used together.
Think Inrush limiting instead.
No fuse will "protect your DAC", funny idea.
Fuses protect the supply feeding a device, not the device.
You work at hobby level, too small a sample to be statistically significant.
As a repair guy, I have replaced over 100 PTs, in any level of damage, from partially molten plastic bobbins to toasted paper tape to burnt wire insulation to copper molten to a blob.
Other Techs here can testify to similar horror stories.
Fuses generally are to prevent fire. Usually wiring fire (but also windings of inductors/transformers as here). Fuses are too slow to protect semiconductors in most situations (because bond-wires are much faster to blow than regular fuses). Fuses can't even protect other fuses, its not unusual to blow several at once with a hard short.
in house electric a fuse is only to protect the wires from not getting too hot/warm nothing really more ... its basicly the current you wanna max. allow before bad things happen... but it can only protect the wires you calculated it for... a device plugged in with smaller cable diameter could easly burn down if it doesnt has a fuse that is calculated for the specific circuit/device
same principle that made me question one main fuse for transformers but no secondary fuses... but like i said before this might work just fine if you got an transformer with only 1 or 2 secondarys...
same principle that made me question one main fuse for transformers but no secondary fuses... but like i said before this might work just fine if you got an transformer with only 1 or 2 secondarys...
i still have todo some research on that... the caps i wanna use have 500F ... they need a bit charging time, thats why i thought the current limiting factor of a regulator could play well together...
i read LDOs can actually be used as inrush current limiters
but calculating and limiting the current with a resistor might be a possible too, this would actually make it quite easy too, tho just "missusing" the regulator a bit seems easier
the TPS7A4700 regulator i made a pcb for reads kinda like it can current limit but this is an fault case and probably shouldnt be "abused"
i guess something like this would work:
https://www.lcsc.com/datasheet/lcsc_datasheet_2304140030_Diodes-Incorporated-AP2552W6-7_C441824.pdf
i guess something like this would work:
https://www.lcsc.com/datasheet/lcsc_datasheet_2304140030_Diodes-Incorporated-AP2552W6-7_C441824.pdf