Advice for a 12VDC circuit

Thread Starter

Grez

Joined Nov 18, 2021
6
Hi this is the first time I made a circuit and I would like to have your opinion on it, is it wired properly, what can I improve, are my fuses correctly selected ... I want to build a dune buggy and this would be my entire project's 12V circuit

also I would like to know if you got any other circuit simulator. I found the CircuitJS sim but it's pretty basic and I would like to download a better one9EA5B5A4-57FB-45B6-A94A-EB93976948AB.jpeg
 

dl324

Joined Mar 30, 2015
14,307
Welcome to AAC!

  1. You can't use potentiometers to control heaters and motors; PWM is the preferred method. You'll burn them up.
  2. Some of the fuse values seem unreasonably high to me (e.g. 35A for a 300W load). What gauge wire is that branch going to use? That's an important factor for fuse selection.
  3. The diodes on your light switches aren't required.
  4. The voltage divider won't work if you have an alternator to charge the battery and it's probably not stiff enough. The current in the divider should be 10 times the load current.

Regarding your schematic. Black and white is easier to read and ground should always point down.
bw.jpg

You did a pretty good job of not having too many unnecessary wire bends and using wires with 90 degree angles. Those are typical problems with people just starting out; unless they were already accustomed to reading well drawn schematics.
 
Last edited:
A dune buggy with windows and windshield wipers? How are you supposed to roll it?
The circuit looks OK but a few observations - your main 12V bus is huge, it could see 200A with accessories on. I would break it up. Most fuseholder block's total current is far below that.
You would use a 5V buck-converter for the Bluetooth or USB charger etc. instead of a resistor divider because they are wasting the excess voltage as heat and really inefficient.
Same for the heater and fan motor, you can't use resistors at such high power levels to drop down the 12V unless you want to contribute to global warming, instead use PWM controller.
 

Jon Chandler

Joined Jun 12, 2008
156
Voltage dividers provide a reference level; they won't provide power to a load. Consider the one you want to support 5 volts to your Bluetooth devices.

Screenshot_20211118-115459_Edge.jpg

At no load, this divider will provide ~ 5 volts.

Using Ohm's Law, consider the current through the resistors:

I = V/(R1 +R2) = 12/(11,000 + 8,000) = 0.63mA

and the voltage drop across R1:

V = IR = 0.00063 × 11,000 = 6.9 volts.

What happens if we try to draw some current from this 5 volt source? Let's say a tiny current of 1mA.

V = IR = 0.00163 × 11,000 = 17.9 volts.

Even a tiny 1mA current will drop your 12v supply to zero (because the full 12 volts and more would be required to get a 1.6mA current across an 11,000 resistor).
 

Thread Starter

Grez

Joined Nov 18, 2021
6
Welcome to AAC!

  1. You can't use potentiometers to control heaters and motors; PWM is the preferred method. You'll burn them up.
  2. Some of the fuse values seem unreasonably high to me (e.g. 35A for a 300W load). What gauge wire is that branch going to use? That's an important factor for fuse selection.
  3. The diodes on your light switches aren't required.
  4. The voltage divider won't work if you have an alternator to charge the battery and it's probably not stiff enough. The current in the divider should be 10 times the load current.

Regarding your schematic. Black and white is easier to read and ground should always point down.
View attachment 252984

You did a pretty good job of not having too many unnecessary wire bends and using wires with 90 degree angles. Those are typical problems with people just starting out; unless they were already accustomed to reading well drawn schematics.
Thank you

-Ok I didn't know but I thought that I could use a potentiometer because the heater and fan only had 2 wire so I thought that I could not use it.

-The reason I chose most of the fuses is by calculating the amperage from W=VA so 300W/12V=25A then I found on a forum that I need this formula to calculate the fuse required A/(0.89x0.75)=25A/0.6675=37.45A fuse so I took a 35A
As for the gauge wire I don't know what to use on this circuit so I would also need your advice on this

-For the diodes when I started the simulation both lights would light up even if I selected the left or right flasher and same thing for the white lights

-For the alternator I won't be using one instead I wanted to use EV Motors for this build so I would be using a DC-DC converter to get the 12V required to power all those components
-Ok thank you but I wanted to use a LM7805 chip as shown in this diagram but the simulator I worked with didn't had any voltage regulator option so I took the resistor way to get the 3.6-6V required

LM7805.png

A dune buggy with windows and windshield wipers? How are you supposed to roll it?
The circuit looks OK but a few observations - your main 12V bus is huge, it could see 200A with accessories on. I would break it up. Most fuseholder block's total current is far below that.
You would use a 5V buck-converter for the Bluetooth or USB charger etc. instead of a resistor divider because they are wasting the excess voltage as heat and really inefficient.
Same for the heater and fan motor, you can't use resistors at such high power levels to drop down the 12V unless you want to contribute to global warming, instead use PWM controller.
-I just hooked everything together so it would be easier for me to see everything

-That is because I want to do both road and dirt trails and windshield and windows arre required to be road legal
 

dl324

Joined Mar 30, 2015
14,307
Ok I didn't know but I thought that I could use a potentiometer because the heater and fan only had 2 wire so I thought that I could not use it.
The power rating for a potentiometer is for the entire winding and a high power pot might only be 2W. If you tried to amps through a fraction of the winding, it won't last long. PWM is the preferred method for varying a high power load.
The reason I chose most of the fuses is by calculating the amperage from W=VA so 300W/12V=25A then I found on a forum that I need this formula to calculate the fuse required A/(0.89x0.75)=25A/0.6675=37.45A fuse so I took a 35A
As for the gauge wire I don't know what to use on this circuit so I would also need your advice on this
While you need to size the fuse to avoid nuisance openings, you also need to size them to prevent the wires used from overheating and causing fires.
For the diodes when I started the simulation both lights would light up even if I selected the left or right flasher and same thing for the white lights
Is this what you're referring to?
1637280540320.png
When "Light Switch" is closed, both relays will be energized; so the diodes are useless.

Did the simulator tell you that this was working the way you wanted?
For the alternator I won't be using one instead I wanted to use EV Motors for this build so I would be using a DC-DC converter to get the 12V required to power all those components
What is the capacity of your battery? You could be drawing a lot of amps from it and without an alternator to charge and supplement it, runtime could be an issue.
 

Thread Starter

Grez

Joined Nov 18, 2021
6
While you need to size the fuse to avoid nuisance openings, you also need to size them to prevent the wires used from overheating and causing fires.
Ok what would you recommend on those high fuse ratings for exemple the 35A what other amperage would you recommend? and I would vary the gauge for each sub circuits a big for the main 12v then smaller depending on the amperage

Is this what you're referring to?
View attachment 253015
When "Light Switch" is closed, both relays will be energized; so the diodes are useless.

Did the simulator tell you that this was working the way you wanted?
What is the capacity of your battery? You could be drawing a lot of amps from it and without an alternator to charge and supplement it, runtime could be an issue.
On the Light Switch it is a 3 throw switch ON/ON/OFF which is
bottom : off
Middle : on 2 60W light + park lights
Top: on middle switch + 300w LED bar
I needed diodes to prevent the current to pass to the 300w when on middle and to prevent the current from cycling until depletion of the current when I turn off

And same thing on the Hazard + Flasher Switch the Flasher is also a ON/OFF/ON but the hazard switch is a ON/ON switch used to power the flasher switch or bypass the switch and power the 2 flasher relay

What is the capacity of your battery? You could be drawing a lot of amps from it and without an alternator to charge and supplement it, runtime could be an issue.
The battery would be 14 Tesla Model S 22.8V 232Ah totaling ~75kWh and ~320V that the DC-DC converter would send to the Motor Controller in 320VDC and to the rest of the circuit in 12VDC which could supply me enough power to last 350KM (220 miles)
 

Tonyr1084

Joined Sep 24, 2015
6,866
this is the first time I made a circuit
WOW! That's quite a circuit for a first time build. Looks like you've gotten some good and important input. But I'm not sure this is the kind of "First Time" circuit a newbie would be attempting. I remember my early circuits - and the many many errors committed, along with the grossly understated understanding of how things work. I thought a pot was a pot. I didn't realize it couldn't be used to control a fan motor at 12 volts and survive. I recall the magic smoke released from that circuit. I also remember the first small gauge wire I tried to use to power the fan directly and how that wire turned to instant beads inside the insulation, no longer able to conduct electricity.

If this is your first circuit - good luck. We'll help all we can, but be careful about how much money you throw at learning some hard lessons.
 
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