Most Power Supplies have two fuses.
One in the mains circuit which is to protect the transformer against catching fire.
One in the secondary circuit to protect the electronics that is being powered.
In your case you have a 30VA transformer that requires a 500mA fuse in the mains (230V) side.
AND
A 5VA Load which requires a 1A fuse after the reservoir cap(s)
You dont need to try to protect the 5VA load from the mains.
The 500mA fuse is simply there to protect the transformer against failure.
It is the 1A fuse after the capacitor that is protecting your delicate electronics.
One in the mains circuit which is to protect the transformer against catching fire.
One in the secondary circuit to protect the electronics that is being powered.
In your case you have a 30VA transformer that requires a 500mA fuse in the mains (230V) side.
AND
A 5VA Load which requires a 1A fuse after the reservoir cap(s)
You dont need to try to protect the 5VA load from the mains.
The 500mA fuse is simply there to protect the transformer against failure.
It is the 1A fuse after the capacitor that is protecting your delicate electronics.
Last edited:
Your calculations to protect the power transformer 30VA is a 500mA fuse in the mains side (before the transformer), could you elaborate the difference between your calculations & Andrewt's calculations?
He's calculated the fuse required to try and protect your 5VA load at the mains side.
Vis-a-Vis his calculations would be those required to protect a 5VA mains transformer.
You have a 30VA transformer running at 230/240V
The mains current that the transformer can use at maximum output is therefore
30/230 = 130mA
To allow for surge currents this is uprated by 3 - 3.5X
3 x 130mA = 390mA
The next higher value is 400mA (or 500mA)
That is the transformer protected against fire in the event of a failure.
The secondary fuse is simply that that is quoted ie 1A.
This assumes that the secondary fuse is after the rectifier and smoothing capactitor. If you wish to protect a few diodes and the capacitor then consideration needs to be taken into the charging current of the capacitor. Most designers dont bother and simply fit the fuse after the cap.
The mains current that the transformer can use at maximum output is therefore
30/230 = 130mA
To allow for surge currents this is uprated by 3 - 3.5X
3 x 130mA = 390mA
The next higher value is 400mA (or 500mA)
That is the transformer protected against fire in the event of a failure.
The secondary fuse is simply that that is quoted ie 1A.
This assumes that the secondary fuse is after the rectifier and smoothing capactitor. If you wish to protect a few diodes and the capacitor then consideration needs to be taken into the charging current of the capacitor. Most designers dont bother and simply fit the fuse after the cap.
Last edited:
So you use the same formula/equation like AndrewT but referenced to the power transformer not to the load, right?
About the secondary fuse is 1A because it's load needs or how did you calculated?
About the secondary fuse is 1A because it's load needs or how did you calculated?
Yes the mains fuse is for the transformer.
The secondary fuse is to suit your load.
If the load states 1A then use a 1A fuse.
The secondary fuse is to suit your load.
If the load states 1A then use a 1A fuse.
I can't leave this unchallenged. It is misleading and confusion arises in the mind of those trying to learn.
The mains fuse is to prevent the house cables setting fire to the house. All wiring downstream of a mains fuse or circuit breaker must be rated at greater than the fuse/breaker rating.
There is little in a transformer to support a fire. The fuel will be limited to the organics in the insulation and little else. The metal in the transformer could become so hot as to set fire to the remainder of the equipment and it's surroundings. That's what the mains fusing is there for, to limit the time and power/energy available to so seriously overheat the damaged equipment from setting fire to the house.
The secondary fusing is not there to protect the equipment from damage. If the secondary fuse blows then the abuse has already occurred and the equipment has already been damaged. The fuse is there to limit the damage upstream, i.e. the transformer, which could set alight to the surroundings if the continuous overload is allowed to continue.
The secondary fuse is not there to prevent or limit damage to the downstream components.
Last edited:
OK FIRE is a bit dramatic.
The mains fuse is to protect the mains.
The secondary fuse is to protect the supplied equipment.
I agree - If the secondary fuse fails then the damage has already been done.
In all cases we are trying to prevent overheating of failed components.
The mains fuse is to protect the mains.
The secondary fuse is to protect the supplied equipment.
I agree - If the secondary fuse fails then the damage has already been done.
In all cases we are trying to prevent overheating of failed components.
Are you reading what we are writing?
At 230V to protect the transformer 400/500mA.
You can use a smaller fuse but it may well rupture due to the inrush current of the transformer.
Once everything is stable a 100mA fuse will be OK.
A 500mA fuse will protect the mains side against failure.
your use of FIRE is OK. The fusing is there to help prevent operators burning down their house and maybe their neighbours as well.
Sort of. To prevent the mains cables setting fire to the house. The distribution board first stage fuse/s is/are there to prevent you damaging the electricity suppliers equipment.
No. The secondary fuse it there to prevent the damaged equipment from putting an excessive continuous load on the upstream equipment and there by exposing the house to greater risk of catching fire.
Merlin.
you posted a pic in another thread showing a 5Vdc 1000mA PSU.
Is that the PSU you are referring to here?
It tells you on the PSU that it is 20VA. (not 5VA nor 30VA)
We should not have to guess at transformer efficiency nor at what rated voltage the secondary windings are nor how many there are.
you posted a pic in another thread showing a 5Vdc 1000mA PSU.
Is that the PSU you are referring to here?
It tells you on the PSU that it is 20VA. (not 5VA nor 30VA)
We should not have to guess at transformer efficiency nor at what rated voltage the secondary windings are nor how many there are.
I'm doing a shunt regulated BiB 5VDC 1A to use instead the wall wart psu in the pic, that's the reason I asked fuse value?
- Status
- Not open for further replies.