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Power Distribution Board

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therabi
(@therabi)
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Joined: 3 years ago
Posts: 10
Topic starter  

@jonnyr Thank you.  Those look like the ones used in the DB1 robot.

 


   
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therabi
(@therabi)
Active Member
Joined: 3 years ago
Posts: 10
Topic starter  

@jonnyr Thanks.  I would agree.


   
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AlexTheProgrammer
(@alextheprogrammer)
Eminent Member
Joined: 1 month ago
Posts: 19
 

Does anyone have any schematics for this board? I got a bit confused on the motor in vs motor out and how they're wired and the video doesn't have any schematics for this board. I think I got it and managed to reconstruct the schematics here (ignore the values for the components, I only wanted to depict the connections). 

power distr

 

And a few more questions about it: 

1. Is there a reason why Bill kept using the bullet connections for the motor instead of cutting them? The motors only came with female bullet connectors and the male ones had to be bought separately. Instead, I have a ton of JST-XH connectors for 2, 3, 4, 5, and 6 pins and I'm using them for everything in my project. They're really cheap. Is there any risk of using those connectors with 12v? Just to increase the pitch, for 12v and GRN I use a 3 pin connector where I heat up and remove the middle pin with soldering iron. Also, do you think the connectors won't overheat on the 12V line given the amps below? 

2. What are the values for the fuses here? My motors have stall current of 9.2A and I wanna squeeze as much as I could from them but safely (I guess 25% under stall current?) as my robot is gonna be pretty heavy and be able to lift quite a bit of payload. 6.9A fuse found be exactly 25% under the stall current per each motor but the standard fuse available is 6A. Should I just go for that one? My power supply can output 54.1 A on 12V line. Not sure if that output is if all the output is just on 12V or it's ok to consume max outputs on each line at the same time. After all, I want this supply mounted on the robot to also power a motherboard with a CPU and a bunch of other motors. The motors wouldn't be operated at the same time though. And I guess the fuses for the 12V line from the power supply should be around 20A for now with a possibility to upgrade to up to 54A (max power supply output) and up to 24A for 5V line?

power supply spec

 


   
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codecage
(@codecage)
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Joined: 3 years ago
Posts: 1168
 

@alextheprogrammer 

Here is my schematic, and I've made this into a PCB as well.  Works great.

DB1PowerDistributionSchematic

And as for the bullet connectors, why cut off existing connectors when it is easy to use the male bullet connectors via a short length of red/black zip cord to the distribution board itself?  My first iteration of this board had the male bullet connectors mounted on the board as Bill has done.  And wouldn't the JST-XH connectors be a little bit to small to carry the load these motors might be drawing?  I made my traces for the 12v lines a little larger than those of the 5v traces.

SteveG


   
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AlexTheProgrammer
(@alextheprogrammer)
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Joined: 1 month ago
Posts: 19
 

@codecage Thank you a lot of the schematics! 

btw, speaking of the JST-XH - do you think I shouldn't be using them at all for the 12v? The reason why I decided to use JST-XH for everything is price - I got a bulk pack of them really really cheaply. And one male bullet connector costs like $3 and it's also hard to buy them. I'd also need to use a bunch of bullets on the way from power supply through the board to every motor. Most likely, I have to order them from servocity.com and they charge $8 for shipping and handling of every order. Or... should I be buying elsewhere given that RadioShaks are gone. Or... What about using multiple cables and multiple pins for the same line? Btw, here are what my cables from the power supply look like. Note how I doubled-down on the 12V line as technically, this 18 gage of wire is too small for 12V but I only have a bulk pack of that gage. I'm also worrying about the 12v line shorting with others or arcing with others given how close they are in the connector. 

12v line

 

 

On a separate note, did you use a service to print this as a PCB or did you just 3D print it at home?

 


   
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codecage
(@codecage)
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Joined: 3 years ago
Posts: 1168
 

@alextheprogrammer 

IMHO the JST connectors are not beefy enough to handle the max current for the motors, while the barrel connectors a more than suitable for the higher current.  And it has been awhile since I purchased the barrel connectors so  do imagine the price may have risen a good bit.

For the PCB, I used KiCad to design and the sent the gerbers to PCBWay.  I have used PCLPCB for some of my boards as well.

SteveG


   
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DaveE
(@davee)
Prominent Member
Joined: 2 years ago
Posts: 714
 

Hi @alextheprogrammer,

  At the risk of annoying you, by pointing out what may already be obvious, I hope this will reinforce, and perhaps explain a little of the "why?" behind @codecage's good advice.

---------

If you imagine flow of electricity along a wire (and through the connectors), is like the flow of water through a pipe, then the voltage would be the pressure of the water and the current would be the quantity of water.

Thus a pipe with a large diameter is needed to carry a large volume of water, whilst a physically strong pipe is needed to contain a high pressure, but the pipe can be narrow if the volume to water to be transported is small.

Note that plastics like polythene and PVC are very good insulators, so even a very thin layer will effectively insulate voltages up to (say) 50V.

Hence, a thin cable with very thin (fractions of a millimetre) insulation can easily carry voltages up to (say) 50V, providing the current flow is low. However, to carry a higher current, it is necessary to increase the thickness of the conducting (copper) part of the wire. This principle includes the connectors, switches, etc. The thickness of insulation does not need to increase for a higher current, only for a higher voltage.

------------------

Having said that the insulation may be very thin, it is also necessary to think of the practical implications. Very thin insulation is likely to be mechanically weak, so that if the wire is subject to handling, pressure, etc. it is quite likely that it will be punctured and rendered useless at some point. Hence, wires in an enclosed, protected environment may be fine with thin insulation, but in other environments a more generous allowance is advisable. Mains wires are often required to have two layers of insulation for this very reason.

-----

As for connectors carrying (say) 12V. It is probable that the connectors you mention will be fine with 12V at low current, maybe less than 1 Amp. Ideally, you should try to find a data sheet if you are in doubt of their capability.

However, if the supply to a connector or wire is capable of supplying much higher currents these conductors can handle, then some form of protection, like a fuse before the connector or wire, is needed to limit the current that can pass through in the event of a fault.

------

Note that with connectors, and also other places like switch contacts and relay contacts, the current carrying capability depends on both the surface area of the touching contacts and the pressure between the two surfaces. Hence, although the general principle of bigger contacts for higher current applies, it can be difficult to estimate the current carrying capability. Try to find a data sheet!

Also, many 'economy' connectors are only 'really' designed to be clipped together once, reflecting the throwaway society approach. Whilst you can usually connect and disconnect several times, each cycle tends to bend the contacts, reducing the pressure they can exert. If it is a connector that will be connected and disconnected many times, look for a robust design with an ample current margin!

-------

In addition, with systems using more than 1 voltage, say 12V and 5V from a PC supply, consider the risk of the 12V supply shorting to the 5V line. This is unlikely to happen if the connectors, etc. are good mechanical condition, in a physically protected environment, but if they more exposed it is easy for them to get pulled, or maybe a meter probe slips, etc. In a computer, a momentary connection of 12V to the 5V line will typically subject many chips to a massive overvoltage surge, which they are unlikely to survive.

So it is wise to assume that anything bad that can happen will happen. Try to imagine the ways something can break or bend, be accidentally shorted out, etc. and design to minimise the consequences.

----------

Hope you find something above useful, and it doesn't sound too boring or patronising.

Best wishes, Dave


   
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AlexTheProgrammer
(@alextheprogrammer)
Eminent Member
Joined: 1 month ago
Posts: 19
 

Oh, yeah! That's definitely useful! will need to redo the board with bullet connectors or screw-terminals for anything high voltage. 

Btw, where you do you regularly buy the components? In local brick-and-mortor electronics shops? Or on expensive sites like Sparkfun or ServoCity? Or on AliExpress that's kinda sketchy but the components there look really really cheap. https://www.aliexpress.us/item/2251832468316751.html?spm=a2g0o.productlist.0.0.64753384dbrB6s&algo_pvid=4f665ac2-1bbd-4914-9661-4743e2cec738&algo_exp_id=4f665ac2-1bbd-4914-9661-4743e2cec738-4&pdp_ext_f=%7B%22sku_id%22%3A%2212000021110254424%22%7D&pdp_npi=2%40dis%21USD%211.9%210.01%21%21%21%21%21%40210318cb16670964377913565e6e7c%2112000021110254424%21sea&curPageLogUid=KJOR3sCbc2dF

 

I'm also thinking to physically separate this power distribution board into 3 boards: 
- one with just two fuses in between the power supply and everything else. 

- another with distribution of 12v on screw terminals and bullet connectors.  There'll be just a few connectors, they'll be spaced apart pretty generously from other wires and connectors to reduce the chance of shorts or breakages. When I tried to wire the entire board like this into a tiny PCB board... I didn't even finish testing it with my multimeter - half of the connections didn't work and shorts were everywhere - had to take it apart right away! 

old power distr front
old power distr back

 

- and finally for 5v distribution on cheap JST - just line them up and wire together. The voltage is low, the amperage is nearly zero, there're a lot of them so the risk of using the cheap connectors is nearly zero too and the cost of using high end ones is way too high.

 


   
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DaveE
(@davee)
Prominent Member
Joined: 2 years ago
Posts: 714
 

Hi @alextheprogrammer,

 I am sure there are others on this forum who can offer more advice than I can, but I'll offer a couple of personal comments.

There is no universal answer to questions like "What are the best connectors?" or even "Where to purchase them from?", because there is no universal project or aim.

e.g.The good side of the general bullet connector style you have identified is typically cheap, able to pass a fairly high current, good insulation, can be fitted with cheap tools, robust against vibration and so on, which makes them a good choice when you only have as small number of connections to make to a mechanically large component like a motor or a battery.

But they also have down sides, such as:

  • if you have a considerable number of wires to connect, then the total bundle can become unwieldy
  • they are designed to be 'connected for life' ... obviously they can be pulled apart if absolutely essential, but they tend to be stiff, they distort, each wire must be individually parted, etc.
  • Most projects have a mix of (relatively) high current and low current parts .. these connectors are generally clumsy for the low current stuff

---------------

As an example of a contrast, look at the IDC (Insulation Displacement Connectors) connectors which come in a range of sizes and scales

e.g. https://uk.farnell.com/w/c/connectors/idc-connectors

(This style of connector has been ubiquitous for at least 4 decades, probably more .. so they are available 'everywhere' - the reference is just one of many.)

The most common with connectors consisting of two rows of pins at 0.1 inch spacing.

Whilst the 'pin' half of the connector is usually soldered to PCB or similar, the 'socket' part can simply be 'pressed' onto the wire with a vice.

Although repeatedly connecting and disconnecting these connections will eventually cause a failure, they generally last a long time, especially if you add the 'flip ears' to lever the connector out.

Clearly these are not suitable for high currents, and common sense says "don't put the 5V line adjacent to the 12V line", because a mechanical 'yank' on the cable might short them, but they are far more suitable for an 8-bit bus than individual bullets!

----------------

So I recommend you look around, and gradually build your own small "library" of different connectors for different tasks. That "library" should not just be a few boxes of different types, but also the personal knowledge and experience of how to choose the best one for the next task. And include some 'reading matter' like data sheets of some of the common parts so you can compare there current, voltage, etc. characteristics. 

-----------

As for where to purchase, that partly depends upon usage and view about risk.

Sources like Aliexpress are basically just a marketplace fronting up a vast number of concerns. Some of the products seem to be good quality; many are clones of established products, some virtually identical, others not so close; and some are rubbish. You may find most will reasonably well, but a few are useless.

Of course companies like Adafruit, etc. put a lot of effort into supporting their products, often designing them, providing software, etc. Typically, there will no support from the cheaper sources.

So, if the item is much cheaper than alternative, and failure of the product will not be dangerous, then you might consider it is a worthwhile risk ... but only you can make that choice and be responsible for the consequences.

---------

Best wishes, Dave


   
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