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When to use fuses in circuit

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M4krD4d
(@m4krd4d)
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I am working on a bench power supply. I posted the topic here: Link to My PSU project

Many designs used to adapt repurposed psu's I find on the web show added fuses. 

How do I determine when a fuse is needed and where to add it to the circuit?

 

Thanks!


   
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triform
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@m4krd4d

I usually add a fuse for just over my total current needs. Say I have a PSU that will deliver 20amps. I use only 4amps in total, so I will fuse the output for just over 4 or as close as I can.  Now, you may have a PSU that can only deliver 5amps and your circuit only nominally needs 3amps, BUT could pull more than 5amps in rare cases (i.e. a stalled motor)  I would put a 5amp fuse in to protect the PSU.  Now, for motors and motor controllers - You have a motor controller that can deliver 2amps max, but the stall of the motors is 3amps. You would want to fuse between the motor controller and the power source for just over 2amps.

I hope this makes sense.

Scott


   
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M4krD4d
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@triform

Hi Scott - 

Thanks for the advice. Sorry it took so long to respond. I've been sideswiped by covid. I've been using my PS without additional fuses for now. I will add those fuses you recommend in the circuit. 

Appreciate the help.

 

Merci!

 

Norm

 


   
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Apeshaft
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I'm slowly working on building my own bench power supply and have a n00b question. Can I use a thermal fuse instead of the glass fuse you most often find in (or that I see) in different type of electronics?  I have no problem with using glass fuses. I'm more curious if I could use a thermal fuse in my bench power supply and if the answer is "NO!". Why not?


   
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Ron
 Ron
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@apeshaft It's ok as long as it protects the same, it's the cost that drives the decision and the physical size might be relevant as well.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
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Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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Inst-Tech
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Posted by: @apeshaft

I'm slowly working on building my own bench power supply and have a n00b question. Can I use a thermal fuse instead of the glass fuse you most often find in (or that I see) in different type of electronics?  I have no problem with using glass fuses. I'm more curious if I could use a thermal fuse in my bench power supply and if the answer is "NO!". Why not?

Hi @apeshaft,  Indeed, some good advice from previous post on the subject..However, there is a bit of misconception of what fuses are for... generally thought to protect the devices they are connected to, but in reality, they are their to protect the wiring as they are the current limiters, Wire size for current capabilities goes hand in hand with fuse or circuit breakers, and therefore should be considered when deciding what fuse size you will need. while it is customary to fuse power supply and the like to max current you intend to use, you need to make sure that the wiring is capable of carrying  that current load. Most large motors have thermo protection for overloads, and many device are protected by fuses, mainly for high voltage spikes which will increase current significantly for a very short time..hence slo-blow fuses. Here a link to more specifically answer your question:

https://www.hunker.com/13414303/how-to-identify-a-glass-fuse

about glass vs thermo ( cermamic types) fuses.

Hope this helps .

Regards,

LouisR

 

 

LouisR


   
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Ron
 Ron
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@inst-tech FYI @apeshaft, I was taught a little differently in that the fuse was to prevent the wiring insulation from catching fire. I think that is a slightly more conservative approach. In practical terms, it's more art than science but there is science if you want to go that deep.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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Apeshaft
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Thank you for your answers, they really helped. One more question. Sometimes when I connect a PSU I suspect is faulty, I plug it into a 230 Volt outlet and hope for the best. I make sure I disconnect everything else from that phase first. I'm thinking that if I leave something like my TV or a computer plugged into the same phase and the PSU I'm testing blows the fuse I may damage things like my TV or PC before the fuse goes off. 

I'm off course making sure that I'm not holding on to anything that grounds me and might kill me. Is there something I can use as some sort of firewall between a 230 Volt outlet that I use for things like testing a PSU that will increase the safety for me, my house and any other electronics I have connected throughout my house?

I would prefer something that I can plug straight into the 230 Volt outlet myself since most other thing is illegal if you're not a licensed electrician. And I'm not.

If I not mistaken I might have seen a video where Mr. Dronerobotworkshop himself talks about this, but I can't find since youtube now days and for no appernt reason only recommends videos in hindu or 10 year old CNN broadcasts etc. 🙂


   
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Inst-Tech
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@zander, "@inst-tech FYI @apeshaft, I was taught a little differently in that the fuse was to prevent the wiring insulation from catching fire. I think that is a slightly more conservative approach. In practical terms, it's more art than science but there is science if you want to go that deep."

Hi Ron, Yes you are correct, the type of insulation and wire size are the main considerations to prevent excessive heat build up in the wiring. Ohms law applies to this as the wire is rated at "ohms per foot, or ohms per meter" and the insulation at deg. C max.

@apeshaft, There should be no concern with other devices sharing the same mains phase as you put it, as the mains circuit breaker is designed to take care of high current spikes. The only thing you need to be aware of is not exceeding the PSU's power output rating, and that is what the fuse is for in this case..making sure that the output current can't exceed the max rating of the unit. You can always fuse the out put for a lower rating than what the PSU's max current rating is, but not higher!

If your concerned about your safety, and you should be, then I'd recommend a GFCI that you can just plug into your 230V main outlet plug..However, I've not been able to find one rated for 230-250V for those living in EU ,or other places that use 230V mains. Since I live in North America, they are readily available for 125 Vac out lets..see linK; https://www.amazon.com/gfci-outlet-adapter/s?k=gfci+outlet+adapter

image

Safety is a good thing to be concerned about..and it's good that you recognize that fact..As a Industrial electrical/instrumentation tech for almost 50 year, now retired, I've seen my share of people hurt, and even killed by electricity because they didn't observe the safety precautions and best practices. 

Regards,

LouisR

 

 

 

LouisR


   
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Ron
 Ron
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@inst-tech In Europe they are called RCD. Have a look at this WiKi LINK

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@davee)
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Hi @apeshaft,

---------

 Please note, fuses often look very simple, but this is deceptive -  their design, construction and application is actually rather complex. This discussion tries to explain a few points, but it cannot begin to be comprehensive. It is offered in good faith, but I do not accept any responsibility for any mistakes or consequences that may result from it. If you have any doubts, please consult a local expert.

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  Re: I'm more curious if I could use a thermal fuse in my bench power supply and if the answer is "NO!". Why not?

I think you may be advised to check the terms that you are using, as you could be a little confused.

The normal use of the term 'thermal fuse' is NOT concerned with protection against too much current flowing, but rather a unit overheating ... that is a thermal fuse opens the circuit ("blows") when the environment in which it is placed exceeds a certain temperature.

An example might be a hot air heater (or hair dryer) which relies on a fan create a flow of air to remove the heat from the heating element. If the air flow stops for any reason, the heating element will rapidly become very hot which could damage the machine and may result in a fire. Thus a thermal fuse mounted close to heater could detect the temperature rise, and break the circuit supplying power to the heater.

Thermal fuses come in a wide variety of physical forms ..

e.g. https://www.farnell.com/datasheets/1702837.pdf

Note that they typically specify three principal characteristics:

  • Temperature
  • Current
  • Voltage

They will open if the device temperature exceeds the temperature rating.

The voltage and current are the maximum voltage and current the device can safely carry in normal operation and break if the temperature causes it to blow.

Exceeding the voltage or current will NOT blow the device, but exceeding these ratings may result in the fuse failing to stop the current conduction if it blows due to overtemperature.

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The common, conventional fuse which blows when too much current flows through it, consists of a ceramic or glass tube with a thin wire (or tape) connecting the metal end caps.

A Wikipedia overview for fuses is found at https://en.wikipedia.org/wiki/Fuse_(electrical)

Note, although the ceramic and glass fuses have somewhat different characteristics, their basic usage and capability is similar.

Obviously, the principal characteristic is the current rating , measured in Amps and related to the current that will cause it to blow. Note this is a very imprecise specification, and even the definition of what it means varies widely between the different standard writing bodies.

When concerned with something like mains wiring, the fuses (or circuit breakers) are essentially designed to prevent the wiring itself overheating, and risking a fire. However, fuses inside units like televisions, etc. are often used to minimise the danger of component failures, etc.

------------------ 

In addition, there are (usually) small components which will limit the current, which are sometimes referred to as resettable fuses or poly-fuses. They have a resistance which increases with temperature. Hence, if current exceeds a certain level, the current flow causes the temperature of the device to increase, and the increased resistance reduces the current low. These are often found in USB power circuits.

See https://en.wikipedia.org/wiki/Resettable_fuse for an overview

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For a power supply unit, it is reasonable to consider fuses at both the power input, and at each of the power outputs.

Mains Input

For example, Bill's (dronebot-workshop)'s linear power supply has a mains fuse in the power input socket:

See the "Power Entry Module" section of https://dronebotworkshop.com/linear-dc-power/

This should provide protection to the general house power supply from a major shorting fault within the power supply unit.

In the UK, the wall plug at the other end of the mains lead will also contain a fuse. I realise, this may not be possible in other countries.

In both of these cases the fuse should be rated as low as possible ... say typically 3A for a power supply rated at 500W or below.

Note the fuses for both the socket and UK-style plugs should always be of the ceramic type intended for this application.

Low Voltage Outputs

Assuming the power supply outputs are low voltage ... say up to 25V, and up to a few Amps current rating, then you may wish to provide each one with a suitable fuse to limit the current in the event of a short circuit.

However, if the output is regulated, the regulator MAY provide a reasonable level of protection - for example the LM317 used in the power supply just referenced includes various protection mechanisms, which may be sufficient for your needs.

However not all all power supplies, particularly SOME of the cheap buck regulator modules that available from Amazon, AliExpress, etc. have any short circuit protection. These would definitely be safer with a simple fuse in the output wiring.

------

Please remember, all of this discussion is very general. Design of power supply units for commercial use is a specialised subject which requires a lot of knowledge and experience, which a short article like this cannot begin to replicate. Anything you build or use is completely your responsibilty.

Good luck, Dave


   
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Ron
 Ron
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@apeshaft It's difficult to imagine a PS causing any damage to co-connected devices. However, if it's that big a concern to you, put an isolation transformer between the wall outlet and the PS.

Regarding safety, get a plug-in RCD to go in the outlet, then plug the PS into it. You could even piggyback both, plug an isolation transformer into the wall outlet, then add an RCD to the output side of the transformer. Now you only need to worry about jumbo jets falling out of the sky and landing on you. Both what you have now and the jet are of equal likelihood.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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