cloudapplepi
Posts: 18
Joined: Wed Dec 05, 2018 6:35 pm

Fuse between motor driver board and power source?

Fri Feb 15, 2019 1:03 pm

Hi (again).

I'm working on a robot, based on the Devastator platform (Link) and I use a 7 A Dual DC motor driver board with it (this one).

In the instructions of this motor driver it says to use a 20A fuse between the board and the power source (in my case batteries; 6 x 1.5 V AA).
I'm using a fuse that is normally meant for use in cars or boats, I think. It is deployed between the + of the battery pack and the motor driver board. At the end of this post is an image of the setup.

Currently the robot is remote controlled (it works fine) and I want to make it work with ultra sonic sensors (at least).

Now I have some questions regarding that fuse, considering that it has quite a thick cable and is a bit big (I'm thinking of shortening the cable):

1) Is this fuse really required/necessary? :?:
2) Is this fuse supposed to protect the board, the power source or both? :?:
3) I'm sure that there are other, smaller fuse types existing, but I don't know under which name to look? :?:
4) Could I replace the fuse with something else to get the same effect? :?:
5) Is there anything else that I should consider. :?:

I was wondering about this earlier, but didn't ask back then. I appreciate any help or hint here very much.

Thanks.

Almost forgot the picture:
Image

User avatar
ptimlin
Posts: 87
Joined: Tue Jan 08, 2019 7:44 pm

Re: Fuse between motor driver board and power source?

Fri Feb 15, 2019 2:40 pm

A 20A fuse for your project will be useless. Both your motors on your project as well as AA batteries will self limit current well before 20A. So if you have a short or a stall the motor will draw and the AA battery will deliver whatever current they can until something else breaks or overheats. The fuse will be happy to allow that current through since it will be well under 20A.

Realistically a AA battery (or any number of them in series like you are going to do) can probably deliver a bit under 1A continuously without overheating the battery. Looks like the drives in the tank project can pull about 2.5A when locked up per motor. I would suggest something closer to a 5A, high enough not to nuisance blow a lot but low enough that it will actually blow when something gets wired wrong or shorted.

The purpose of a fuse it to cut current when something goes wrong and try to protect everything (supply, motors, boards, etc) and also to protect humans from getting injured and fires from starting. But sometimes even with a blown fuse you might have already damaged something in your project depending on the types of components and the voltages and currents involved.

In your case the fuse is there to protect your boards and Pi from mistakes like connected positive battery terminal directly to negative terminal when you turn on your motor drive because it is hooked up wrong. Or if you apply power backwards and it is just conducting freely through a diode and that sort of thing.

User avatar
OutoftheBOTS
Posts: 711
Joined: Tue Aug 01, 2017 10:06 am

Re: Fuse between motor driver board and power source?

Fri Feb 15, 2019 9:36 pm

A fuse is basically a weak point. When something shorts out then current can flow very high and what ever can handle the lowest current will burnout first. So to protect the rest of your components you will add a fuse(weak point) so if there is a short and current goes high the fuse burnouts rather than other components.

Now the size of the fuse is the size of how weak it is (what current it will burn out at) there is no point to have a fuse so big it isn't the weakest point in your circuit other wise it won't burnout first. If the fuse is too weak then it will burnout when at too low a current when nothing is wrong.

So when choosing a fuse you have to look at all components on your circuit. AA batteries have very high internal resistance and are just not capable of supplying high current (1amp if your lucky). So atm your power supply can't even supply the needed current that your motor want to draw. Most hobby Lipo battery can supply crazy high current and would better suit your project.

The H-bridge your using can handle continuous 7amp current so it will hand much more than your motors need. All H-bridges have a forward voltage (the amount of voltage used up by the H-bridge). As a general rule of thumb the higher the rated current of the H-Bridge the higher the forward voltage. Your using 6 x x1.5v battery so your getting 9v into the H-bridge, if you measure the output to the motors I am willing to bet ti is much lower, on a 7a H-Bridge it could easily be 4v drop. You will need to know the voltage drop across the H-bridge so you know how much extra voltage to put in to get the correct voltage for motors out.

User avatar
rpdom
Posts: 14424
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 7:29 am

OutoftheBOTS wrote:
Fri Feb 15, 2019 9:36 pm
AA batteries have very high internal resistance and are just not capable of supplying high current (1amp if your lucky).
That only goes for standard AA batteries. NiCd or NiMh batteries can supply an obscene amount of current due to their low internal resistance.

However, the OP does say 1.5V, which does imply standard non-rechargeable cells, so your comment is good advice.

cloudapplepi
Posts: 18
Joined: Wed Dec 05, 2018 6:35 pm

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 10:48 am

Thanks for the replies.

Yes, I use non-rechargable batteries indeed.

Why is it that rechargable batteries provide more current or in other words why do they have a lower internal resistance?

Then this would also mean that I should rather use a 20A fuse when I use rechargable batteries and the 5A fuse is good for non-rechargable batteries?

Another question regarding the fuse or rather the fuse-holder. Are there any smaller fuses or fuse holders that could be used here?

User avatar
OutoftheBOTS
Posts: 711
Joined: Tue Aug 01, 2017 10:06 am

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 12:01 pm

cloudapplepi wrote:
Sat Feb 16, 2019 10:48 am
Thanks for the replies.

Yes, I use non-rechargable batteries indeed.

Why is it that rechargable batteries provide more current or in other words why do they have a lower internal resistance?

Then this would also mean that I should rather use a 20A fuse when I use rechargable batteries and the 5A fuse is good for non-rechargable batteries?

Another question regarding the fuse or rather the fuse-holder. Are there any smaller fuses or fuse holders that could be used here?
Please read my earlier post explaining what a fuse does and how it works then you can work out what size is needed. With the current batteries you will never blow the fuse because the batteries just can't put out that much current even if you shorted a wire from 1 side of battery to other so a fuse is useless.

Setting the size of fuse will depend upon what protection over the rest of the components you want. If you used batteries that can supply loads of current that it could damage other components then you need to decide what is the max current that I would like to have flow though the rest of the components before I want the fuse to blow. Your H-bridges r rated for continuous current of 7amp with peaked of 10amp (crazy high). The link to your motors quote "No-load current: 0.13 A" and "Locked-rotor current: 2.3 A". This means if the motors are not under any strain at 7.5v they only draw 0.12amps but if you totally lock the motors so they can't turn at 7.5v then they will draw 2.3amps (and will be starting to burn out the motors)

If you wanted to be super cautious then you could use a 2amp fuse as it would blow if the motors locked/stalled and this would stop them burning out. But most people wouldn't do this as anytime you locked/stalled the motor the fuse would blow which is a bit of a pain, so many people will give a little head room like 2.5amp or maybe 3amp as this wouldn't blow easy but also wouldn't overly protect the motors either.

User avatar
Burngate
Posts: 5884
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 12:28 pm

cloudapplepi wrote:
Sat Feb 16, 2019 10:48 am
Why is it that rechargable batteries provide more current or in other words why do they have a lower internal resistance?
You want a lesson in chemistry?

Primary cells are designed to hold their charge for long periods so they don't have to be replaced too often, and the circuits they feed are generally designed to be frugal for the same reason.
Carbon-zinc cells were even worse than the alkaline cells you're using.

Secondary - rechargeable - cells don't have that constraint, and indeed aren't very good at holding their charge - their self-discharge rate would be considered abysmal for a primary cell.
But it doesn't really matter, because recharging is relatively cost-free.
Your car battery doesn't have to do much other than hold enough energy to start the engine on a cold day. After that the engine does all the work.
But when you want its stored energy, you want it now, not during the next half-hour.
Then this would also mean that I should rather use a 20A fuse when I use rechargable batteries and the 5A fuse is good for non-rechargable batteries?
A 20A fuse is designed to carry 20A without blowing.
It'll probably blow if you try to take 100% more than that, but don't rely on it to do so.

If your load shouldn't take more than 5A, then only a fault condition will cause it to blow, whatever your power source.
Putting in a larger fuse just because your power source could provide a larger fault current is putting carts before horses.
The design route should be:
* what current does the load take?
* what power source can provide that current?
* what fuse will protect everything?
Another question regarding the fuse or rather the fuse-holder. Are there any smaller fuses or fuse holders that could be used here?
The fuse holder has to be able to withstand the heat when the fuse blows
It also needs to be possible to change the fuse easily.

Google is your friend

cloudapplepi
Posts: 18
Joined: Wed Dec 05, 2018 6:35 pm

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 1:23 pm

OutoftheBOTS wrote:
Sat Feb 16, 2019 12:01 pm
cloudapplepi wrote:
Sat Feb 16, 2019 10:48 am
Thanks for the replies.

Yes, I use non-rechargable batteries indeed.

Why is it that rechargable batteries provide more current or in other words why do they have a lower internal resistance?

Then this would also mean that I should rather use a 20A fuse when I use rechargable batteries and the 5A fuse is good for non-rechargable batteries?

Another question regarding the fuse or rather the fuse-holder. Are there any smaller fuses or fuse holders that could be used here?
Please read my earlier post explaining what a fuse does and how it works then you can work out what size is needed. With the current batteries you will never blow the fuse because the batteries just can't put out that much current even if you shorted a wire from 1 side of battery to other so a fuse is useless.

Setting the size of fuse will depend upon what protection over the rest of the components you want. If you used batteries that can supply loads of current that it could damage other components then you need to decide what is the max current that I would like to have flow though the rest of the components before I want the fuse to blow. Your H-bridges r rated for continuous current of 7amp with peaked of 10amp (crazy high). The link to your motors quote "No-load current: 0.13 A" and "Locked-rotor current: 2.3 A". This means if the motors are not under any strain at 7.5v they only draw 0.12amps but if you totally lock the motors so they can't turn at 7.5v then they will draw 2.3amps (and will be starting to burn out the motors)

If you wanted to be super cautious then you could use a 2amp fuse as it would blow if the motors locked/stalled and this would stop them burning out. But most people wouldn't do this as anytime you locked/stalled the motor the fuse would blow which is a bit of a pain, so many people will give a little head room like 2.5amp or maybe 3amp as this wouldn't blow easy but also wouldn't overly protect the motors either.
Thanks. I was refering to the physical size of the fuse/fuse holder. It carries these midi size fuses from cars. I was wondering what other types of fuses or fuse holder could be used. I saw some fuses made of glass, seems to be something like this: https://cdn.instructables.com/FXC/WCJO/ ... .LARGE.jpg

I couldn't really find anything when searching.

cloudapplepi
Posts: 18
Joined: Wed Dec 05, 2018 6:35 pm

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 1:28 pm

Burngate wrote:
Sat Feb 16, 2019 12:28 pm
cloudapplepi wrote:
Sat Feb 16, 2019 10:48 am
Why is it that rechargable batteries provide more current or in other words why do they have a lower internal resistance?
You want a lesson in chemistry?

Primary cells are designed to hold their charge for long periods so they don't have to be replaced too often, and the circuits they feed are generally designed to be frugal for the same reason.
Carbon-zinc cells were even worse than the alkaline cells you're using.

Secondary - rechargeable - cells don't have that constraint, and indeed aren't very good at holding their charge - their self-discharge rate would be considered abysmal for a primary cell.
But it doesn't really matter, because recharging is relatively cost-free.
Your car battery doesn't have to do much other than hold enough energy to start the engine on a cold day. After that the engine does all the work.
But when you want its stored energy, you want it now, not during the next half-hour.
Then this would also mean that I should rather use a 20A fuse when I use rechargable batteries and the 5A fuse is good for non-rechargable batteries?
A 20A fuse is designed to carry 20A without blowing.
It'll probably blow if you try to take 100% more than that, but don't rely on it to do so.

If your load shouldn't take more than 5A, then only a fault condition will cause it to blow, whatever your power source.
Putting in a larger fuse just because your power source could provide a larger fault current is putting carts before horses.
The design route should be:
* what current does the load take?
* what power source can provide that current?
* what fuse will protect everything?
Another question regarding the fuse or rather the fuse-holder. Are there any smaller fuses or fuse holders that could be used here?
The fuse holder has to be able to withstand the heat when the fuse blows
It also needs to be possible to change the fuse easily.

Google is your friend
Thanks for the explanation. :)

Regarding the fuse/fuse holder, I already googled, but couldn't find something really helpful. I'm using now a fuse holder for midi sized fuses (the same as in cars), but are there other solutions (fuses types/fuse holders) that are typically used in these type of projects (robots)?

I saw a fuse made of glass, might be something like shown here: https://cdn.instructables.com/FXC/WCJO/ ... .LARGE.jpg

Or is using these car fuses totally fine and normal? Also used elsewhere: https://www.ez-robot.com/Tutorials/Lesson/47

User avatar
Burngate
Posts: 5884
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Fuse between motor driver board and power source?

Sat Feb 16, 2019 6:18 pm

Fuses come in many different shapes, sizes, and materials, depending on where they're going to be used.

So, here in the UK (your profile doesn't indicate where you are) plug-tops have a fuse that has to be removed from the socket before you can change it.
It's got a ceramic body that won't shatter when it blows, but you can't see if it's blown just by looking.

Glass-body fuses are useful, because you can look to see if it's blown, but extracting bits of broken glass from the holder can leave droplets of blood inside - not nice for the next person needing to change it.

Car style fuses are designed for a fuse box fixed to the car body - a pair of pliers is all you need to change them. Because they're low-voltage, plastic is a reasonable material to make the body from.

I'd be quite happy with that last-linked style - you might not even need pliers if your fingers are thin enough, and being semitransparent you can maybe see if it's still intact even without removing it.

cloudapplepi
Posts: 18
Joined: Wed Dec 05, 2018 6:35 pm

Re: Fuse between motor driver board and power source?

Tue Feb 19, 2019 8:53 am

Burngate wrote:
Sat Feb 16, 2019 6:18 pm
Fuses come in many different shapes, sizes, and materials, depending on where they're going to be used.

So, here in the UK (your profile doesn't indicate where you are) plug-tops have a fuse that has to be removed from the socket before you can change it.
It's got a ceramic body that won't shatter when it blows, but you can't see if it's blown just by looking.

Glass-body fuses are useful, because you can look to see if it's blown, but extracting bits of broken glass from the holder can leave droplets of blood inside - not nice for the next person needing to change it.

Car style fuses are designed for a fuse box fixed to the car body - a pair of pliers is all you need to change them. Because they're low-voltage, plastic is a reasonable material to make the body from.

I'd be quite happy with that last-linked style - you might not even need pliers if your fingers are thin enough, and being semitransparent you can maybe see if it's still intact even without removing it.
Thanks.

Maybe I have to have a look if there is a solution that needs less space.

MarkR
Posts: 154
Joined: Fri Jan 25, 2013 1:55 pm

Re: Fuse between motor driver board and power source?

Tue Feb 19, 2019 10:36 am

I would say that it's probably strictly unnecessary, even if you would use a much more powerful battery.

Suppose you switch to a 2S Lipo pack rated at something like 30A continuous (these are easily available and are commonly used on quadcopters, etc).

The robot is never going to run:

* Unattended - i.e. without a human at least vaguely watching
* In a safety critical or flammable environment

The fuse is mostly to prevent fires in the case of a short circuit. A small fire can become a large dangerous fire if nobody extinguishes it, or if there are dangerous materials around e.g. petrol.

If you don't put in a fuse, and there is a short, something else will pop. Maybe a motor, maybe a wire, maybe your speed controller. It might make smoke, it might be moderately expensive or inconvenient to replace, but it's unlikely to put anyone's safety at risk.

User avatar
Burngate
Posts: 5884
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Fuse between motor driver board and power source?

Tue Feb 19, 2019 10:56 am

MarkR wrote:
Tue Feb 19, 2019 10:36 am
I would say that it's probably strictly unnecessary, ...
If you don't put in a fuse, and there is a short, something else will pop. Maybe a motor, maybe a wire, maybe your speed controller. It might make smoke, it might be moderately expensive or inconvenient to replace, but it's unlikely to put anyone's safety at risk.
I would put in a fuse.

Replacing any other component can get expensive; it can take time to find which component to change; damage to a component may not be directly terminal but lead to failure later on.

Replacing a fuse is cheap and easy.
You know it popped, so you can find out why it did so, and correct the error.

Return to “Automation, sensing and robotics”