Schematic Guidance

Thread Starter

james211

Joined May 29, 2012
277
I need to build a custom power distribution board for an enclosure. I've started putting the schematic together and I'm curious if anyone would be willing to give me some guidance / advice? One of the things I'm unsure of is fuses - proper calculation, one for each output or one large fuse?

I'm attaching a list of the items that will live in the enclosure along with the schematic as it exists now.

Thank you in advance. I have some knowledge, but not enough to be dangerous, but I understand the importance of delivering good power.

Happy Holidays!

Screen Shot 2020-12-18 at 1.59.17 PM.pngScreen Shot 2020-12-18 at 2.03.49 PM.png
 

Thread Starter

james211

Joined May 29, 2012
277
Draw your schematic so we don't need to turn our heads to read it.
What is the purpose of the fuse(s)?
;)Sorry about that...I'm having a bit of a hard time with the schematic layout in Fusion 360, I liked Eagle better. I just assumed fuses would be necessary, are they not?
 

jpanhalt

Joined Jan 18, 2008
11,088
Since you have a dual supply, I would add reverse polarity protection for the battery. As for fusing, you definitely need at least one for the main supply. Should you fuse the wall supply and battery separately? Worth considering, as the battery might be able to supply far more current than the 12V supply, assuming the latter is a wall wart.

Rather than fuse each output, you could add in-line a mA/ampere meter for the total drain. A step beyond that would be to allow that meter via a switch (rotary) to monitor the drain of individual outputs.

As for the schematic, it displayed properly on my monitor.
 

dl324

Joined Mar 30, 2015
12,929
I just assumed fuses would be necessary, are they not?
It depends on what you want the fuses to do.

If you want to protect things connected to the voltages, you'd put a fuse inline with that device. If you wanted to protect the battery from an overload (so it wouldn't do something like start a fire), you'd fuse for what it can source safely.

If you wanted the fuses to be convenient for the end user or someone servicing the equipment, you'd use a PTC fuse which resets after an overload is removed.
 

Thread Starter

james211

Joined May 29, 2012
277
It depends on what you want the fuses to do.

If you want to protect things connected to the voltages, you'd put a fuse inline with that device. If you wanted to protect the battery from an overload (so it wouldn't do something like start a fire), you'd fuse for what it can source safely.

If you wanted the fuses to be convenient for the end user or someone servicing the equipment, you'd use a PTC fuse which resets after an overload is removed.
Okay that's great to know as they would be in a place that will not be easily accessible. The batteries are high quality batteries and I do believe they have overload protection built in to them, but I will double check.

Additionally, do I need to add any additional components to the circuit, or will it be fine if the original source is providing good clean power?
 

Thread Starter

james211

Joined May 29, 2012
277
Since you have a dual supply, I would add reverse polarity protection for the battery. As for fusing, you definitely need at least one for the main supply.
I actually have it in my notes to add a schottkey diode on both, and I completely forgot to put those in, thank you for the reminder.

Rather than fuse each output, you could add in-line a mA/ampere meter for the total drain. A step beyond that would be to allow that meter via a switch (rotary) to monitor the drain of individual outputs.
Do you have a specific hardware suggestion for this meter/monitor? I like the idea of that.
 

jpanhalt

Joined Jan 18, 2008
11,088
I actually have it in my notes to add a schottkey diode on both, and I completely forgot to put those in, thank you for the reminder.


Do you have a specific hardware suggestion for this meter/monitor? I like the idea of that.
It depends on what you want to spend. I used a cheap HF multimeter for mine until I got a power supply with it built in. The milliammeter does not have to be particularly accurate, and it doesn't need to be on every output. For example, the fan doesn't need one. But for those outputs that I use for my circuits, e.g., the 5V ones, I find it helpful to watch when first powering up. A short or other misconnection will be obvious, and you may be able to react soon enough to prevent damage. Another help is a 100Ω resistor in series. You could wire that to an output with a toggle switch and label the positions,"Safe" and "Full." Or, anything to the effect.

One other addition, I installed switches on every output, including my -5V output. That allows eacy on/off and help prevent mistakes plugging pins in.

Just noticed, you don't have any negative voltage. I would add a -5V at a minimum.
 

Thread Starter

james211

Joined May 29, 2012
277
One other addition, I installed switches on every output, including my -5V output. That allows eacy on/off and help prevent mistakes plugging pins in.
I will be adding switches, I just haven't found a compact, illuminated DC switch that I like yet - any suggestions?

Just noticed, you don't have any negative voltage. I would add a -5V at a minimum.
This is COMPLETELY new to me, can you explain further? Additionally how do I add negative voltage?
 

jpanhalt

Joined Jan 18, 2008
11,088
I will be adding switches, I just haven't found a compact, illuminated DC switch that I like yet - any suggestions?


This is COMPLETELY new to me, can you explain further? Additionally how do I add negative voltage?
You need to generate it or use a supply that has it. I was fortunate early on and had a Motorola wall wart with negative voltage. How about getting the general design set with either -5V or -12V or both shown*, then we can discuss how to get it. Minus voltage are usually very low current, and there are several IC's that can do it for you, such as the ICL7660 (https://www.renesas.com/eu/en/www/doc/datasheet/icl7660s-a.pdf ).

*-12V can be reduced easily to -5V with an appropriate voltage regulator. Going the other direction is more difficult. So, if I had to pick just one, it would be -12V.
 

Thread Starter

james211

Joined May 29, 2012
277
okay, but my question is, what is the purpose of it for my use case? I'm not questioning you, I just don't understand the purpose of negative voltage.
 

dl324

Joined Mar 30, 2015
12,929
okay, but my question is, what is the purpose of it for my use case? I'm not questioning you, I just don't understand the purpose of negative voltage.
A classic example is an opamp such as LM308. To have ground in the common mode input voltage range, you'd need to have a negative supply of at least -1V:
clipimage.jpg

If you were using an inverting opamp with a positive input voltage, you'd need an appropriate negative supply.
 

jpanhalt

Joined Jan 18, 2008
11,088
If you stick to the NE555 and arduino, you don't need it. If you expand your horizons in the analog world (e.g., op-amps) , it is nice to have. For example, I had a project using op-amps for filters, and a negative voltage was necessary.

EDIT: A negative supply is also essential for proper LCD operation:
Most displays have it built in, but not all.
 
Last edited:

bassbindevil

Joined Jan 23, 2014
168
Looking at your list of outputs, I don't see a need for negative voltage.
If you're using a 3-terminal linear regulator, those are inefficient at high currents; I'd use a car USB adapter to get 5V from 12V, since those contain efficient switching regulators.
A fuse on the battery and wall-wart inputs would be reasonable.
On ebay (or your favourite overseas electronics bazaar), you can find a range of LED panel mount digital voltmeters and ammeters, including comination units. Being LED, they'll draw enough current to drain a battery if left on all the time, so I'd include a switch to disconnect the power. They come in various forms, from modules without a case, to water-resistant types designed for mounting in a vehicle dashboard. Linearity may not be perfect since they're based on cheap microcontrollers, but some have trimpots so you could calibrate them for the voltage you care about the most.
 

Thread Starter

james211

Joined May 29, 2012
277
For Schottky diodes, reverse voltage protection, I've always just used a 1N4001, will that work or do I need something more specific?
 

jpanhalt

Joined Jan 18, 2008
11,088
A 1N4001 may work, but the Schottky usually has a faster speed and lower voltage drop in the forward direction. That may be needed in situations such as a single Li-ion cell poweringt a 3V3'ish IC/MCU.
 

Thread Starter

james211

Joined May 29, 2012
277
A 1N4001 may work, but the Schottky usually has a faster speed and lower voltage drop in the forward direction. That may be needed in situations such as a single Li-ion cell poweringt a 3V3'ish IC/MCU.
Can you assist me in choosing the right diode?
 
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