Hi.
I'm still a beginner in electronics.
I'm making a 12V to 5V/3V circuit to run on a motorcycle battery. Maybe you have a better suggestion than this.
In some conditions, the power supply on a motorcycle may be unstable, ranging from 11V (min) >= 16V (max) when the vehicle is running. I'm not sure if the schematic I've designed is safe for this.
I appreciate all your answers.
Thank you.
I think your J6, barrel Jack is wired backwards. The shell part usually is frame ground, so any plug always hits ground first.
I suspect it will fry when you least expect it. You have no transient or reverse battery protection to start with. Here is some reading that will help/ Most important is the AQ100 series of specifications.
Resources for Automotive Electronics:
STMicroelectronics Application Note AN2689:
This application note provides guidelines on protecting automotive electronics from electrical hazards, focusing on design and component selection. Reading this will greatly enhance your understanding of automotive circuit protection.
Read AN2689
Analog Devices: Automotive Electronics Design:
This article distills key insights into designing automotive electronics, offering practical advice for engineers.
Read the article
Diodes Incorporated: Transient Voltage Suppression in Automotive:
Learn about techniques to protect automotive circuits from transient voltage, which is critical for ensuring reliable operation in harsh conditions.
Read the article
AEC-100 Standards Webinar:
This webinar from Monolithic Power Systems provides a detailed overview of AEC standards, essential for understanding automotive electronics requirements.
Watch the webinar
Understanding Automotive Electronics, An Engineering Perspective by William B. Ribbens:
This comprehensive book offers an in-depth look into automotive electronics from an engineering perspective, making it an invaluable resource.
Access the book
Check this for OBD ISO9141 Code Meaning & How To Fix OBD negotation initialization procedure https://cdn.standards.iteh.ai/samples/16738/20ff360c25c6462b811ebc0c9256eee0/ISO-9141-2-1994.pdf Then there is GitHub - muki01/OBD2_K-line_Reader: OBD2 K-line Reader (ISO9141 and ISO14230) with microcontrollers like ESP32, Arduino PID codes: OBD-II PIDs - Wikipedia
Load Dump and Cranking Protection for Automotiv Backlight LED Power Suppl https://www.ti.com/lit/an/snva681a/snva681a.pd
Quality requirements:
These resources should provide a strong foundation for anyone involved in automotive electronics design. If you need further help or more resources, feel free to ask!
Hopefully it helps and explains what a lot of the parts are in the design form Note:A motorcycle 12 V electrical system is much harsher than a car’s because the wiring is smaller, the battery is smaller, and the regulator/rectifier is usually simpler.
You should assume very dirty power unless you fully isolate your electronics.
Below is an engineering view of realistic transients seen on motorcycle 12 V buses.
Load Dump (Worst Case): +40 V to +80 V
When the battery disconnects while the alternator is charging (loose terminals, corroded connectors), the regulator cannot absorb the spike fast enough.
Typical motorcycle load dump: +40 to +60 V for 100 ms to 400 ms
Extreme cases: up to +80 V for short durations
This will destroy unprotected electronics.
When the starter engages:
Bus voltage can collapse to 6–8 V normally
In cold weather or with a weak battery: as low as 4 V
Your device must survive undervoltage and not reset or corrupt memory.
Motorcycle ignition coils produce large inductive kicks that couple into the wiring harness:
Positive spikes easily +60 to +120 V
Negative spikes -100 to -200 V on poorly grounded systems
Very fast rise times: nanoseconds to microseconds
These can enter through the DC wiring or through electro-magnetic coupling.
Compared to cars, motorcycle regulator/rectifiers often produce more AC ripple on the 12 V bus:
1–3 V ripple is common
Bad rectifier: 5–8 V p-p (dangerous for electronics)
Switching regulators on bikes are often "shunt" designs, meaning excess alternator current is simply dumped as heat, adding noise.
Immediately after the starter motor releases:
The regulator may overshoot
Brief spike to 15–16.5 V is normal
With aging components: 17–18 V possible
Jump-start mistakes or bad grounds can create:
-12 V to -14 V for tens of milliseconds
Momentary negative spikes on shared grounds
Devices must survive accidental reverse connections.
Small inductive loads (cooling fans, fuel pumps, relays) create:
±40–60 V spikes
Duration: microseconds to milliseconds
Motorcycle frames have poor shielding. Expect:
Wideband EMI from ignition
Alternator whine in the audio range (100–3000 Hz)
High-frequency spikes from cheap USB chargers
QUICK SUMMARY: WHAT YOU MUST DESIGN FOR
At minimum:
Transient suppression: TVS diode (SMBJ or SMCJ series) at 600 W or more
Reverse polarity protection: MOSFET or Schottky
DC filtering: LC filter or buck converter with good input filtering
Load dump survival: 50+ V rated input components
Undervoltage tolerance: Must run down to 6 V, survive to 4 V
EMI protection: Grounding, ferrite beads, shielded wires
The hardest thing to design for in a motorcycle is waterproofing. MY former company used to build the electric control unit for a motorcycle builder. All connectors were automotive grade feed-thru type connectors. Had to buy a box of 1500 to get just one. Then use RTV to seal the edges of the connector, then fill the box with epoxy potting compound and more RTV to seal the cover to the box. The box went under the seat which was the place with the least water spray.
Note the current restrictions of your ancient analogue 7805.
It will cut out at about 100mA without heatsink.
Just buy a cigarette lighter to USB plug in the supermarket.
Use it with a matching socket.
Or remove it's innards and put it in your own case.
It contains a switching buck converter that is way more capble than an 78M05.
It's OK to downconvert to 3V3 with the 1117.
Leo..
That would be my advice as well. Maybe a reputable brand, rather than “supermarket.”
If you’re set on building it yourself, there are some regulators similar to the 7805 that are specifically designed for the automotive environment, like the LM2940 that address at least some of the issues.
Look for automotive grade components.
Even in favourable conditions, it's often worth going up a grade.
As mentioned by @westfw , the LM2940 is an example.
Just a mention, but component grading from bottom to top goes roughly Consumer, Commercial, Industrial, then the Medical/Aerospace/Nuclear/Military grouping.
@beginner_699
I've done a CDI design for an old Kawasaki, it had just one TL750L08 regulator with just one 100uF cap on the input and one 10uF cap on the output. Never had any problems.
So I see no problem with your design other than you have the wrong values for some of the capacitors
For automotive design i do something like this
Do observe the current requirements for the 7805, and add heatsink if required.
Right.
We don't know the current requirements of OP's project.
It should be clear that a design with lineair regulators quickly runs into heat dissipation problems. 12 to 5volt is 0.7Watt for every 100mA. A 50 degrees C hotter part if not cooled.
Leo..
Sometimes people get lucky. What does your design do if it gets with a 80V transient, maybe 60V on newer vehicles. Look at the data sheet and see what the surge voltage is. It is also important to use the correct capacitors for high frequency bypass.
An (18volt standoff) TVS diode with a fuse in front of it is good input protection.
Better than a forest of series and parallel diodes.
Linear regulators are slow. Fast noise spikes blow right through it.
Only capacitors (and inductors) can stop them.
Leo..
From his earlier thread i have the suspicion of a rough idea. Somehow i doubt it is going to be a lot.
But OP doesn't know exactly yet either i think.
A 7805 does have an overheat protection. And a TO-220 package has decent heat-dissipation out of the box. Easy enough to add a bit of Alu if needed.
Well the main thing to counter out is the reverse voltage. I do agree that a 100v input Capacitor.
I don't know i have been using this sort of thing for years in my Renault 16 & Volvo 66 for the Attiny WW interval relays, without issue. And in my Megane for the window controllers. Nice and close to the electromotors.
Yes that is what the capacitors are for. Mainly the capacitor needs to be big enough, and probably quick enough.
You might want to look at what the electrical system offers. My background is on the OEM and after market devices. Here is a simple spike:
Check this link: https://www.ti.com.cn/cn/lit/pdf/sszt243
One thing that is not given is the amount of current that can be driving these transients. It is depended on on the alternator inductance and other inductors and capacitors on the system.
OP has the 78M05 in the diagram (not TO-220).
There are three variants I know
7805-1A
78M05 0.5A
78L05 0.1A
The higher current ratings can only be achieved with heatsinks.
See the thermal specs of the packages in the datasheet.
The 7805 won't survive 40V. Why should the cap.
I think it's interesting that you don't hear more about people's expensive phones and devices getting fried while plugged into "car chargers", since a lot of them ARE built very cheap. Either the switchmode topology is particularly immune to the usual problem (it does have that "big" inductor), the chips are hardier than you'd expect, the oft-stated "problems" with the automotive environment are less common than we think, or the devices' input protection is surprisingly good. (or some combination of those.)
One thing that hasn't been mentioned yet: with a 12V nominal input (11V-16V range) stepping down to 5V, a linear regulator like the 78M05 wastes about 60% of the power as heat. At 500mA, that's over 3W dissipation - significant for a small package without heatsink.
For automotive applications with varying input voltage, a buck converter is much more efficient (typically 85-95%). Many automotive-grade buck converter modules handle the wide input range and already include input protection.
If you want to design your own buck converter circuit, you can calculate the required inductor and capacitor values here: Buck Converter Calculator | Schemalyzer Tools
Just enter your input voltage range (11-16V), output voltage (5V), and load current to get the component values.
Will the voltage drop across D2 (around 0.6V) not lift the output to 5.6V?
No because all GND for the MCU and non-opto-isolated parts connect to the 7805 pin 2, so they all have that offset.
I suspect the location is also relevant. If the wires to the alternator and ignition are short, these spikes may be bigger. (the starter motor is another beast, but that is never close) All of the metal surface will also act as a capacitor up to some extend.
I think that may have something to do with those devices as well. Most LiPo charger chips take a fair range of input voltage and most of those devices are 3.3v devices, stepping down a bit further from the 5v input voltage. LM1117 or AMS 1117 regulator have an input voltage ranging up to 12v themselves, so if the step down to 5v is not completely successful, no harm done i guess. But location plays a big part in it as well i think.
Ah and also i suspect that most car manufacturers put a filter on the 12v socket & radio power supply.
I recommend the TO-220 package (the 78M05 is not the one usually found in your local electronics shop and i don't have it in my box, i have the TO-92 & TO-220 package, never even seen the 'M' )
AVR's are quite rugged i'd say. And for what i used them for, which is to do simple button or similar sensing and drive electromotors, the reverse voltage coming from the motor is the bigger issue.
For the electric window replacement i detect button presses and since most of those new button actually press both the up & down button and direction is determined by which one is pressed first, a small MCU was the easiest solution. 3 of the windows have switches in more than 1 location, so i decided to stick with the existing wiring and tap into that, but opto-isolate the input. The first prototype i built had didn't have that and causes issues when the window closed, causing a recoil, cause by the motor itself, not from any other spike.
The windscreenwiper interval relays never had any of those issues, just the sensing of the dead-zone was a tad tricky but not so much because of voltage spikes, but because i am measuring something that is directly connected to the electro-motor and drive an opto-couple with that. The opto-coupler needs a current limiting resistor, where as the motor it self has very little resistance relative to that.
There is that. Good IP67 enclosures are available and ip67 wartels as well. Once the design is complete, a bit of varnish on the PCB can help a lot.
