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Voltage Protection from car battery

AbarthGT

Oct 23, 2012
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Hi guys this is my first thread on the forum, hope to get the help needed.

Basically my car battery produces 12v when the car ignition is off then 14v when driving. I want to plug a 12v device and want to make sure I only get 12v and not more even while driving.

I need a protection that reduces any voltage higher than 12v to 12v .

Tried voltage divider but was reducing all the current which my device won't power on. I'm thinking of a zener diode can anyone help?
 

Raven Luni

Oct 15, 2011
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It depends what youre plugging into it. If its a device designed to be plugged in to the lighter socket for example then you should have nothing to worry about. These things should have their own power regulation.

As for the voltage divider, that is a bad idea especially with a car battery. Unless you know the exact impedence (or DC resistance in this case) of your load, you will never be able to calculate your resistor values properly since the load acts as a parallel resistor to whichever one its connected across. Plus, the more current you need, the lower the resistor values have to be and the more power is dissipated in them. If you exceed the watt rating of the resistor, you will soon have smoke and fire especially with the large currents a car battery can put out.
 

davenn

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hi abarthGT
welcome to the fourms :)

firstly... what is the device ?
am just wondering how sensitive it may be to voltage fluctuations .... chances are that it has internal regulators anyway maybe

is the device only in use when the car engine is running ?

Dave
 

AbarthGT

Oct 23, 2012
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Thanks for the replies, its a touch screen lcd screen. Its meant to be plugged to a 230v wall socket but I took the stepdown apart and desoldered the wire that goes straight the the screen power module. It was not designed for cars etc..

I read the stepdown it says output 4.3 amps and 12v. Dont want to blow my screen with a 14v.
 

Raven Luni

Oct 15, 2011
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Ah. I've been working on something along those lines (one of my many projects on the back burner due to getting distracted by something else). Mine is more with a vew to current protection rather than voltage though - car equivalent of an adjustable mains adapter.

Anyway, a zener diode wont work - you need at least 2V more than the required voltage and your 14V isnt likely to stay at 14.

4.3 amps is quite a big load. You will need some power transistors and heat sinks for starters. You could maybe use an op amp to stablise the voltage at a certain level but youd have to be careful and buffer the output properly to get your 4.3 amps. Anyway, you're looking at a non-trivial circuit design. Maybe someone else can help you more or has a better idea.
 

AbarthGT

Oct 23, 2012
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Ah. I've been working on something along those lines (one of my many projects on the back burner due to getting distracted by something else). Mine is more with a vew to current protection rather than voltage though - car equivalent of an adjustable mains adapter.

Anyway, a zener diode wont work - you need at least 2V more than the required voltage and your 14V isnt likely to stay at 14.

4.3 amps is quite a big load. You will need some power transistors and heat sinks for starters. You could maybe use an op amp to stablise the voltage at a certain level but youd have to be careful and buffer the output properly to get your 4.3 amps. Anyway, you're looking at a non-trivial circuit design. Maybe someone else can help you more or has a better idea.

I agree with you, but I'm thinking to use the cigarette lighter plug to wire it to. I think the current is around 4amp anyway, but will use a fuse just in case. Then maybe the 14v won't do much harm as long as the current doesn't go over, is this right?

Found something on ebay, would this work? Quite cheap swell, would save me some time.

http://www.ebay.co.uk/itm/300688084419?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
 

CDRIVE

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AbarthGT

Oct 23, 2012
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The concept is OK and not unorthodox but this one is under rated for your needs. You'll want a model that's not being pushed to its maximum limits. Before I'd spend any money I'd investigate whether your monitor would be hurt with a 14V supply.

Chris

How could I find out if it will handle 14v? Other than trying and see if it blows :p
 

KrisBlueNZ

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You will need input overvoltage protection to suppress voltage surges during cranking and load dump in the automotive electrical system. These can exceed 80V. Varistor(s) and/or TVS diode(s). Do some googling.

Here are two 5-pin 5A low dropout regulators that should be suitable.
Micrel MIC29502: http://www.digikey.com/product-detail/en/MIC29502WT/576-1145-ND/771614
Micrel MIC29503: http://www.digikey.com/product-detail/en/MIC29503WT/576-2242-ND/1029199

Just because the device is powered from a supply specified as 12V and 4.3A maximum, you can't assume that the device draws up to 4.3A, and you can't assume that it needs exactly 12V and would be damaged by 14V. (I would assume that it would be damaged by an 80V surge though!)
 

AbarthGT

Oct 23, 2012
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You will need input overvoltage protection to suppress voltage surges during cranking and load dump in the automotive electrical system. These can exceed 80V. Varistor(s) and/or TVS diode(s). Do some googling.

Here are two 5-pin 5A low dropout regulators that should be suitable.
Micrel MIC29502: http://www.digikey.com/product-detail/en/MIC29502WT/576-1145-ND/771614
Micrel MIC29503: http://www.digikey.com/product-detail/en/MIC29503WT/576-2242-ND/1029199

Just because the device is powered from a supply specified as 12V and 4.3A maximum, you can't assume that the device draws up to 4.3A, and you can't assume that it needs exactly 12V and would be damaged by 14V. (I would assume that it would be damaged by an 80V surge though!)

I see what you mean, you might be right, I'm still on my second year of engineering and not sure how to use this voltage regulator. 4pins 2 input 2output?
 

Raven Luni

Oct 15, 2011
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You will need input overvoltage protection to suppress voltage surges during cranking and load dump in the automotive electrical system. These can exceed 80V. Varistor(s) and/or TVS diode(s). Do some googling.

I did not know this. That could have been some pain for me further down the line. Thanks :D
 

KrisBlueNZ

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I'm still on my second year of engineering and not sure how to use this voltage regulator. 4pins 2 input 2output?
That and many other questions are answered by the data sheet for the Micrel MIC29xxx devices, which is just a click away from those pages I linked to.

They are LDO (low-dropout) linear regulators. You would connect the regulator between the automotive supply rail and the device you're powering. At 5A load current, the typical dropout voltage is about 0.4V. This means that if your automotive supply voltage is 12.4V or more, the regulator output will be regulated to 12.0V. If the automotive supply voltage is less than 12.4V, the regulator output will be about 0.4V lower than the automotive supply voltage.
 

CDRIVE

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Kris, those are impressive regulators.

Chris
 

MrEE

Apr 13, 2012
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you can use a shunt regulator that will allow a device to stay at the desired 12 volts. Theoretically a hefty zener diode in parallel with a load or device will do (with a small valued power resistor in line with V+). In practice you can build a simple amplified zener diode that will handle the job. I can post a schematic in few moments.
 

KrisBlueNZ

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you can use a shunt regulator that will allow a device to stay at the desired 12 volts. Theoretically a hefty zener diode in parallel with a load or device will do (with a small valued power resistor in line with V+). In practice you can build a simple amplified zener diode that will handle the job. I can post a schematic in few moments.
That's not really workable with a 4.3A load current and a maximum input-output differential of 2V.
 

MrEE

Apr 13, 2012
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there are a couple of points that need to be considered:
1- measure the actual current drawn by the device. is it 4.3A or less.
2- monitor the current for the minimum and maximum values based on operating conditions.

A shunt regulator can be optimized based on these values. If supply is 12V, the shunt regulator is practically out of the circuit, but at 14V, the current drawn (diverted by the regulator) can be quite large in order to "get rid" of the 2V differential.

One idea is to use PNP switch in parallel with a line resistor. This transistor is on at or around 12V and off above a certain threshold. (say about 12.5V)
 

CDRIVE

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There's a reason that shunt regulators were never popular for anything other than low current requirements. They're wasteful by their very nature.

Chris
 
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