# Automotive Power Supply Regulator Circuit

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#### Kryex

##### New Member
Hi

I am working on a tracking system for vehicles (both cars and trucks).
The system power supply is the vehicle's battery (which supplies 12V or 24, for cars and trucks respectively), which may have many fluctuations (spikes, negative voltages, noise, transient voltages, etc.).
The tracking system needs 5V and 500mA constant (it may peak at 700mA), which means a regulator circuit from the vehicle battery (12-24V) to 5V (500mA~700mA) is necessary.
Price is an important factor, it should be as cheap as possible.

I have designed the following circuit to deal with the voltage regulation:

View attachment 64935

What do you think?

Do you have a different suggestion? I am open to different circuits (specially more simple ones), which regulate the voltage to 5V and protects from the voltage irregularities in the vehicle's battery.

If you need to know anything else let me know.

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#### bountyhunter

##### Well-Known Member
7805 and a small heatsink. Total cost about 25 cents.

#### WTP Pepper

##### Active Member
7805 and a small heatsink. Total cost about 25 cents.
Depends on how reliable you want it to be. A regulator specifically for vehicle electrics is required.
There is a thing known as load dumping present on vehicles. A flat/dead vehicle battery being jump started from a healthy battery. Once the jump leads are removed there can be a huge voltage spike because the dead vehicle battery isn't taking any charge from the alternator. Result is a fried circuit.

http://www.electro-tech-online.com/custompdfs/2012/06/LM2935-DPDF.pdf is one I've used before.

#### picbits

##### Well-Known Member
I've started using the Micrel MIC2951 - great little devices but only a couple of hundred ma - they do the 1 amp regulators though for automotive use - well worth a look as they don't need any "magic" output capacitors that the LM2940 appear to need.

#### Kryex

##### New Member

Take a look at this device, the LTC4366: http://www.linear.com/product/LTC4366 or this LTC4363: http://www.linear.com/product/LT4363

There are a lot of regulators and transient regulators for the automotive environment. You should be able to accomodate both 12 and 24 V systems.
I have looked into linear's solutions (ex. LT4356), but they are too expensive for the price range I had intended for this project.

7805 and a small heatsink. Total cost about 25 cents.
This doesn't seem to be reliable enough for the automotive environment.

Depends on how reliable you want it to be. A regulator specifically for vehicle electrics is required.
There is a thing known as load dumping present on vehicles. A flat/dead vehicle battery being jump started from a healthy battery. Once the jump leads are removed there can be a huge voltage spike because the dead vehicle battery isn't taking any charge from the alternator. Result is a fried circuit.

http://www.electro-tech-online.com/custompdfs/2012/06/LM2935-DPDF.pdf is one I've used before.
LM2935 seems very nice, but neither have I found it for sale in any of my usual online stores nor have I found a replacement with similar specs.

I've started using the Micrel MIC2951 - great little devices but only a couple of hundred ma - they do the 1 amp regulators though for automotive use - well worth a look as they don't need any "magic" output capacitors that the LM2940 appear to need.
It also seems ok, but with a very low output current (150mA). It is not enough, as I need a guaranteed 500mA output.

#### bountyhunter

##### Well-Known Member
Depends on how reliable you want it to be. A regulator specifically for vehicle electrics is required.
If you want built in load dump protection, just use a linear like the LM2940 or LM2941. No problem.

#### bountyhunter

##### Well-Known Member
This doesn't seem to be reliable enough for the automotive environment.
If you are going to design electronics for automotive, you need to build in load dump protection for the module or derive your source power from a point in the vehicle where the line has load dump protection. There's nothing magic about load dump protection, it's very easy using devices like Tranzorbs or similar.

Car radios and stereos build in voltage clamps like an L-C input filter and power Zener or Tranzorb device.

#### bountyhunter

##### Well-Known Member
Another design option which makes more sense:

Have the power supply be separate from the GPS unit. Use a "plug in" device like so many small units use. That way you could build a small switcher based PS unit and the EMI would not be killing the GPS because they are not in the same module. Power dissipation would be tiny, no heatsink needed.

As I recall, there are a number of readily available plug ins that go from 12V to 5V already on the market for cheap \$. I used to use one to run my portable Sony mini disc player in the car.

#### Diver300

##### Well-Known Member
If you are running a GSM modem to get the GPS data back to the owner, you need to be able to supply the current peaks that a the GSM modems take.

GSM modems take around 2 A peak, for around 577 μs every 4.6 ms, giving an average current around 250 mA. When sending GPRS data, the current peak can be around 1150 μs long every 4.5 ms. The current will be less if the GSM modem is close to its cell tower.

If you want to use a capacitor to keep the voltage within 0.2 V during the current peak, you need around 10,000 μF, which is very large. What is usually done is to make sure that the power supply can source 2 A.

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#### Ubergeek63

##### Well-Known Member
Hi

I am working on a tracking system for vehicles (both cars and trucks).
The system power supply is the vehicle's battery (which supplies 12V or 24, for cars and trucks respectively), which may have many fluctuations (spikes, negative voltages, noise, transient voltages, etc.).
The tracking system needs 5V and 500mA constant (it may peak at 700mA), which means a regulator circuit from the vehicle battery (12-24V) to 5V (500mA~700mA) is necessary.
Price is an important factor, it should be as cheap as possible.

I have designed the following circuit to deal with the voltage regulation:

View attachment 64935

What do you think?

Do you have a different suggestion? I am open to different circuits (specially more simple ones), which regulate the voltage to 5V and protects from the voltage irregularities in the vehicle's battery.

If you need to know anything else let me know.
if you do not know what you are doing you need to get an automotive rated part. normal regulators will not withstand a 60-100V load dump (spikes from electric motors and generators in the system)

Dan

#### Kryex

##### New Member
I have read and looked into everyone's advice and I will get back to each of you tomorrow (I'm sorry, today was really busy).

After your ideas I have decided to go for a regulator designed for automotive use, and I found the MIC2941AWU (datasheet) which is said to be "ideally suited for automotive applications for a variety of reasons", such as a wide range of inputs (4V - 60V), deals with reverse battery, and load dump conditions (positive transients up to 60V), etc.

As an automotive environment may have positive transient voltages higher than 100V, I was thinking about adding a TVS diode to the suggested application design in MIC2941AWU's datasheet and that would do it.

#### bountyhunter

##### Well-Known Member
Yeah, he learned what I posted way back half way up the first page two days ago:

Depends on how reliable you want it to be. A regulator specifically for vehicle electrics is required.
If you want built in load dump protection, just use a linear like the LM2940 or LM2941. No problem.
Good to know people read my posts eventually....

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#### picbits

##### Well-Known Member
As I said before - the Micrel range are pretty good. They are much more design friendly than the older LM2940 series.

What really swung it for me was the exceptional customer service I received from Micrel - they seem an incredibly friendly company to deal with.

As a result, all my new designs for in car stuff uses their regulators if I need a lowish current linear regulator.

#### Mr RB

##### Well-Known Member
I'm wondering why a "tracking device" needs a whopping 500mA?

GPS modules only need 50-100mA and a microcontroller etc maybe another 20-30mA.

#### Kryex

##### New Member
Neat, you learned something and you did your own research. Your on the right track.
Now take a look here: http://www.electro-tech-online.com/custompdfs/2012/06/an9312.pdf
Thanks.
What a great piece of information, specially that table with the typical automotive transients.

Yeah, he learned what I posted way back half way up the first page two days ago:
Good to know people read my posts eventually....
Better late than never!

I'm wondering why a "tracking device" needs a whopping 500mA?

GPS modules only need 50-100mA and a microcontroller etc maybe another 20-30mA.
The regulated output will be used for two things: a relay and charging a battery, which will power the tracker.

As I said before - the Micrel range are pretty good. They are much more design friendly than the older LM2940 series.
What really swung it for me was the exceptional customer service I received from Micrel - they seem an incredibly friendly company to deal with.
As a result, all my new designs for in car stuff uses their regulators if I need a lowish current linear regulator.
However there is one thing worrying me, power dissipation and heat. What can you tell me about your experience dealing with them?

By using a linear voltage regulator like MIC2940A-5.0WU I believe I will be facing some problems regarding power dissipation (Pd) and heat. Considering the case of a truck, at 24V, and a constant output of 500mA, there is a Pd of (24 - 5) * 0.5 = 9.5W, which can be worse if we consider that trucks' batteries will operate at higher nominal voltages, such as 28V or 29V, leading to a Pd of 12W. That is a big Pd, which will generate lots of heat.
Can the MIC2940A-5.0WU handle it, in a hot ambient like the vehicles'? Should I use a heatsink? Which one? How should I deal with power dissipation and heat?

#### picbits

##### Well-Known Member
Yes - you should use a heatsink if you're using a linear regulator at those currents and voltages - the size is determined by the power dissipated - 9.5 watts is a reasonable amount to get rid of. If you're using a constant 500ma at 24 volts then its probably worth going for the switching regulator over a linear as it would be cheaper due to not needing as much heatsinking.

#### Kryex

##### New Member
To begin with, thanks to all of you for your wise advice and help. It has helped me develop both my circuit and skills.

I hope this is the last round of questions I have, so I can move on to the final circuit soon.
I have decided to go with a switching regulator, due to the heat cause by a linear regulator.

I am now down to the final two options:

AP1512 (the IC I began this topic with, which still seems quite reasonable)
View attachment 65056

NCV2575 (same as LM2575, but the NCV series is meant for automotive. I'm still not sure what the difference is)
View attachment 65057

What do you think about the circuits? They are both similar in design and cost. Will they be able to handle the automotive ambients?
Which one would you choose?
Do you have any suggestions regarding the external components (specially the PTC fuse)?
Heat wise will it be ok? Should I use a heat sink (suggestions)?

Thank you!

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