It's my first time trying to build a electronic product and I am not sure on how to pick the right fuse for my application.
The nominal current consumed by load is 1.2A and peak inrush current is 21A for 185 micro seconds. There is also a power supply that draws 0.15A of AC current with a inrush current of 40A, but I am not sure about the time until which it lasts for. I have been trying to select the right fuse to use. Finally, a microcontroller, 7 segment display and a relay, I am planning to go with a 1A fast acting fuse for this, would this be fine?

I read Little Fuse's guide . Going by the step, I will need to calculate the value of \( i^2t \) to properly select the fuse. The guide also give different waveshapes for inrush current and forumlas, I went with the formula that would give me the highest value since I do not have an idea about my inrush current waveshape. Here is the calculation :
\[ i^2t = \frac12*21^2*0.000185 = 0.0407 \]
For 100,000 pulse cycles:
\[ 0.0407/0.22 = 0.1854 \]

So I should select a fuse with a current rating of 2A and a \( i^2t \) value greater than or equal to 0.185, is that correct?

How do I calculate the fuse for power supply where I do not know the inrush current timing? And will a 1A fast acting fuse be a good choice for the micro controller block?

How do I choose the main fuse(the fuse as soon as the power from mains enter)? Do I just add up the values of individual fuses to get the fuse rating and keep the i2t the maximum value from the individual fuses?

Thanks

 

A inrush current of 40A sounds a lot? Have considered using an inrush limiter?

 

It's my first time trying to build a electronic product and I am not sure on how to pick the right fuse for my application.
The nominal current consumed by load is 1.2A and peak inrush current is 21A for 185 micro seconds. There is also a power supply that draws 0.15A of AC current with a inrush current of 40A, but I am not sure about the time until which it lasts for. I have been trying to select the right fuse to use. Finally, a microcontroller, 7 segment display and a relay, I am planning to go with a 1A fast acting fuse for this, would this be fine?

I read Little Fuse's guide . Going by the step, I will need to calculate the value of \( i^2t \) to properly select the fuse. The guide also give different waveshapes for inrush current and forumlas, I went with the formula that would give me the highest value since I do not have an idea about my inrush current waveshape. Here is the calculation :
\[ i^2t = \frac12*21^2*0.000185 = 0.0407 \]
For 100,000 pulse cycles:
\[ 0.0407/0.22 = 0.1854 \]

So I should select a fuse with a current rating of 2A and a \( i^2t \) value greater than or equal to 0.185, is that correct?

How do I calculate the fuse for power supply where I do not know the inrush current timing? And will a 1A fast acting fuse be a good choice for the micro controller block?

How do I choose the main fuse(the fuse as soon as the power from mains enter)? Do I just add up the values of individual fuses to get the fuse rating and keep the i2t the maximum value from the individual fuses?

Thanks

I look at it the other way around,
what are you trying to protect with a fuse ?

Typical its a fault condition,
So if your unit was designed to normaly take say 2 amps, and you put a 3 Amp fuse in,
then provided all the wiring can take 3 amps, all is happy.

Startup surge is typiclay less than half a cycle of the mains, on this PSU Im looking at its 120 uS, of which only 20 uS is at 40 amps, and it rises / falls quickly back to 2 amps.

 

A inrush current of 40A sounds a lot? Have considered using an inrush limiter?

Thanks, I will check it out.

I look at it the other way around,
what are you trying to protect with a fuse ?

Typical its a fault condition,
So if your unit was designed to normaly take say 2 amps, and you put a 3 Amp fuse in,
then provided all the wiring can take 3 amps, all is happy.

Startup surge is typiclay less than half a cycle of the mains, on this PSU Im looking at its 120 uS, of which only 20 uS is at 40 amps, and it rises / falls quickly back to 2 amps.

So you mean I can just plug any fuse until it satisfies the current ratings?

 

hi Redgear,
It not only the fuse rating thats important, but also where you place them in the circuit.

Do you have a block diagram of your project you could post.?
E

 

Thanks, I will check it out.


So you mean I can just plug any fuse until it satisfies the current ratings?

Its about what do you want the fuse to do , and to ensure the fuse is the weakest link in the system.



Example.. For many years , people put fuses in the output circuit of amplifiers, Till the realised, a) the conic sound effect but mainly b) the fact normaly on a short, the output transistors would blow first, making the current then go high, which blew the fuse.

Also if you circuit, could take 1 amp , and you put a 3 amp fuse in , but only put 1 amp wire to the circuit, that would be a fail,

 

hi Redgear,
It not only the fuse rating thats important, but also where you place them in the circuit.

Do you have a block diagram of your project you could post.?
E

Hi Eric,
Thank you.
Here is the schematic of my circuit.


The AC_live and AC_Neutral are connected to the load. The load like I said consumes, 1.2A nominal and a 21A inrush for 185 micro seconds. The PSU consumes at max 2A, with inrush of 50A for 20 micro seconds as told by @andrewmm . The PSU ouputs power a microcontroller, a relay(max 100mA) and a 7 seg display that display time.
I am also thinking to have a fuse as soon as the line supply enter the board(i read that's a good practice), will a 5A fuse with the i2t value of the F1 be a good choice?

Its about what do you want the fuse to do , and to ensure the fuse is the weakest link in the system.



Example.. For many years , people put fuses in the output circuit of amplifiers, Till the realised, a) the conic sound effect but mainly b) the fact normaly on a short, the output transistors would blow first, making the current then go high, which blew the fuse.

Also if you circuit, could take 1 amp , and you put a 3 amp fuse in , but only put 1 amp wire to the circuit, that would be a fail,

Thank You.
I think I have fused the weakest link, can you please take a look at the schematic above? Except for the load, the other parts will be on the PCB. A wire should be selected based on the nominal current ratings, am I correct?

 

What is F3 going to do ?
fuses blow so slow that its only going to blow after the board has gone up in flames, and that wont happen as probably the PSU will go into current limit first .

F2 is needed for safety reasons, an type approvals, I'd probably put a 3A slow blow in there, your only protecting against catastrophe, such as short circuit on the PSU input , it wont protect against anything else,

The "front panel switch" would normally be on the AC side, to isolate the AC from the PSU.
Currently the PSU is permanently powered, which I don't think will pass type approvals.

This involves wiring the power to the front panel..

One other thing,
if this is for a commercial product, that PSU does not meet EN55032 , so will probably require further filtering on the input.

 

In your diagram, you show what Meanwell power supply you are using and the voltage regulator that you are using, but don't show the load. If you are referencing the Meanwell power supply as able to handle this huge current inrush, you need to look at their specification. It may handle this amount of inrush already without a needed fuse. Otherwise, the load is somewhat going to determine the actual current draw. The primary fusing should be based upon the load draw and not the 40 amps you wrote. Since you are using a 3-volt regulator output the current draw of this load is not going to be large at all and will never draw 40 amps without a short circuit occurring on the load side. I can't imagine you are using a 40 amp circuit from your breaker panel to start with. Remember that when fusing is done, that it is a descending. For example 15 breaker to a 3amp fuse is ok. A 15 amp breaker to a higher rated fuse would be very dangerous as the wire between the breaker and the fuse would really become the fuse. Depending on the load (like a motor lock load rating) this is how the fusing should be determined.

 

What is F3 going to do ?
fuses blow so slow that its only going to blow after the board has gone up in flames, and that wont happen as probably the PSU will go into current limit first .

F2 is needed for safety reasons, an type approvals, I'd probably put a 3A slow blow in there, your only protecting against catastrophe, such as short circuit on the PSU input , it wont protect against anything else,

The "front panel switch" would normally be on the AC side, to isolate the AC from the PSU.
Currently the PSU is permanently powered, which I don't think will pass type approvals.

This involves wiring the power to the front panel..

One other thing,
if this is for a commercial product, that PSU does not meet EN55032 , so will probably require further filtering on the input.

Thank You. F3 is to protect the micro + 7-segment. I will move the front panel switch to the input side of the power supply. The datasheet states that the supply meets EN55032 without any external components. I will any how add a 0.1uF and 10uF cap on the output.

In your diagram, you show what Meanwell power supply you are using and the voltage regulator that you are using, but don't show the load. If you are referencing the Meanwell power supply as able to handle this huge current inrush, you need to look at their specification. It may handle this amount of inrush already without a needed fuse. Otherwise, the load is somewhat going to determine the actual current draw. The primary fusing should be based upon the load draw and not the 40 amps you wrote. Since you are using a 3-volt regulator output the current draw of this load is not going to be large at all and will never draw 40 amps without a short circuit occurring on the load side. I can't imagine you are using a 40 amp circuit from your breaker panel to start with. Remember that when fusing is done, that it is a descending. For example 15 breaker to a 3amp fuse is ok. A 15 amp breaker to a higher rated fuse would be very dangerous as the wire between the breaker and the fuse would really become the fuse. Depending on the load (like a motor lock load rating) this is how the fusing should be determined.

In your diagram, you show what Meanwell power supply you are using and the voltage regulator that you are using, but don't show the load. If you are referencing the Meanwell power supply as able to handle this huge current inrush, you need to look at their specification. It may handle this amount of inrush already without a needed fuse. Otherwise, the load is somewhat going to determine the actual current draw. The primary fusing should be based upon the load draw and not the 40 amps you wrote. Since you are using a 3-volt regulator output the current draw of this load is not going to be large at all and will never draw 40 amps without a short circuit occurring on the load side. I can't imagine you are using a 40 amp circuit from your breaker panel to start with. Remember that when fusing is done, that it is a descending. For example 15 breaker to a 3amp fuse is ok. A 15 amp breaker to a higher rated fuse would be very dangerous as the wire between the breaker and the fuse would really become the fuse. Depending on the load (like a motor lock load rating) this is how the fusing should be determined.

Thanks for the reply. I think you have misunderstood my posts.

The meanwell supply will power only the microcontroller and the 7-segment LED. The PSU's datasheet states that there will be inrush current of 40A. I'm considering this in my fuse calculation so the fuse does not blow asa the power supply starts. My fuse will be rated only for 2A but I need the inrush current to calculate the i2t value to choose the correct fuse(atleast that's how Little Fuse's guide recommends to do it).

The load has a separate fuse of 2A. The load draws a current of 1.5A and inrush current peak up to 21A for 185 micro seconds.

I am also considering to add a 4A fuse as soon as the mains enter the component.

 

Your post and response are confusing. If you plug in your phone charger for a smart phone into a wall outlet you don't get a 40 amp in rush. This would mean you would be buying a new phone every week. The current draw is based upon the device's resistance and is designed for fusing with this factor. Maybe if you show the complete diagram someone can help. If you are using an Arduino Uno for example the board draws about 1 amp at 12 volts. The LM7805 on the PCB (voltage regulator) drops the voltage to 5 volts. The board will not draw more than one amp from the 12-volt power supply even with a 20 amp output and if you have used this microcontroller board you will notice that there isn't a fuse onboard. Again, look at the breaker supplying this circuit. Is it rated at 40 amps or higher? Most likely not. If there was more then a 40 amp draw the breaker would trip first. If the breaker was 20 or 15 amps which are more likely, a forty amp inrush would trip the breaker, and the traces on the PCB would fry. The Meanwell power supply will have a bridge rectifier that will take the ac input and convert this to it's output dc voltage and will limit the amperage too. Maybe if you show your complete circuit and not just part of it we can help you.

 

It's my first time trying to build a electronic product and I am not sure on how to pick the right fuse for my application.
The nominal current consumed by load is 1.2A and peak inrush current is 21A for 185 micro seconds. There is also a power supply that draws 0.15A of AC current with a inrush current of 40A, but I am not sure about the time until which it lasts for. I have been trying to select the right fuse to use. Finally, a microcontroller, 7 segment display and a relay, I am planning to go with a 1A fast acting fuse for this, would this be fine?

I read Little Fuse's guide . Going by the step, I will need to calculate the value of \( i^2t \) to properly select the fuse. The guide also give different waveshapes for inrush current and forumlas, I went with the formula that would give me the highest value since I do not have an idea about my inrush current waveshape. Here is the calculation :
\[ i^2t = \frac12*21^2*0.000185 = 0.0407 \]
For 100,000 pulse cycles:
\[ 0.0407/0.22 = 0.1854 \]

So I should select a fuse with a current rating of 2A and a \( i^2t \) value greater than or equal to 0.185, is that correct?

How do I calculate the fuse for power supply where I do not know the inrush current timing? And will a 1A fast acting fuse be a good choice for the micro controller block?

How do I choose the main fuse(the fuse as soon as the power from mains enter)? Do I just add up the values of individual fuses to get the fuse rating and keep the i2t the maximum value from the individual fuses?

Thanks

So as to choose the correct amperage of the breaker, you first need to realize the full-load consistent state current of the circuit at a surrounding temperature of 25º C (68º F). When the current worth is resolved, at that point a wire rating ought to be chosen as to be 135% of this worth (taken to the following standard worth)