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filtering circuit for automotive pic supply

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Thunderchild

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I'm designing a pic based circuit for use in a car. now I'm having trouble with the 78L05 regulator I'm using to supply the pic as it keeps blowing, the obvious reason being that its being caused by all the spikes in the car power system.

My plan now is to eliminate the spikes and dangerous transients etc with filter circuitry. but I'm unsure how to go about it,

something like: power in > inductor > 24 V zenner > inductor > capacitor,

somewhere in there I could ad a specific component to eliminate spikes etc (I'm looking into it), my main concern is the value of the inductors, I'm supplying up to 3 A.
 

Mike_2545

Super Moderator
You will need a regulator for automotive use like LM1085
 
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kchriste

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Forum Supporter
I'm designing a pic based circuit for use in a car. now I'm having trouble with the 78L05 regulator I'm using to supply the pic as it keeps blowing, the obvious reason being that its being caused by all the spikes in the car power system.
[SNIP!]
I'm supplying up to 3 A.
How are you supplying 3A with a 78L05?
Mike's idea is best: Sub a regulator designed for automotive use.
 
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Thunderchild

New Member
I need to protec 3 amps worth of components, a portion of which runs off 5 volts through a regulator, I also need to protect a mosfet
 

Boncuk

New Member
Hi Thunderchild,

besides using a regulator for automotive applications you might also use a transil diode for reliable spike suppression.

Take a look at this datasheet.

Boncuk
 

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Thunderchild

New Member
yes thats a good idea, I need to kill spikes to preserve the power mosfet so need to look into both avenues: filtering and a hardy reg
 

Thunderchild

New Member
You will need a regulator for automotive use like LM1085
so what makes this reg any better than the 7805 ? apaart from the 3 A versus 1 A capacity
 

MrDEB

Well-Known Member
read a data sheet

and/or google about low drop out voltage regulators.
they are what is used for automotive aps.
LM2931, lm2937, lm2940 etc the list goes on.
they are all low drop out regulators.
as for 3 amps?
should be available?
 

mneary

New Member
The key feature is they don't blow when subjected to high voltage and reverse voltage spikes.

They may all be Low Drop Out,but I would suggest that this is a coincidence.
 

Thunderchild

New Member
Yes I've been lookingminto alternatives, I think the 78L05 I was using was 40 V input but i may have misread the datasheet. I have now ordered some low drop out regs (0.4 V) and I'm adding apike protection etc before the reg so hopefully all ok now.

what beats me is this work for quite some time before blowing, I suppose its hardy but goes eventually
 

3v0

Coop Build Coordinator
Forum Supporter
so what makes this reg any better than the 7805 ? apaart from the 3 A versus 1 A capacity
They are engineered to survive in a cars electrical system. And not all LDO's are. Look for this sort of thing in the data sheet
Designed primarily to survive in the harsh automotive environment,
these devices will protect all external load circuitry from input fault
conditions caused by reverse battery connection, two battery jump
starts, and excessive line transients during load dump. This series also
includes internal current limiting, thermal shutdown, and additionally,
is able to withstand temporary power−up with mirror−image insertion.
3v0
 

MrDEB

Well-Known Member
this subject pops up from time to time and your not the first to think that a 7805 will work in an automotive environment. that's why the LDO regulators say for automotive environments.
good luck
 

Thunderchild

New Member
this subject pops up from time to time and your not the first to think that a 7805 will work in an automotive environment. that's why the LDO regulators say for automotive environments.
good luck
well I'm using an LDO now (I presume 0.4V counts), I'm now after killing the spikes from the ignition system
 

3v0

Coop Build Coordinator
Forum Supporter
Danger will robinson !

this subject pops up from time to time and your not the first to think that a 7805 will work in an automotive environment. that's why the LDO regulators say for automotive environments.
good luck
LDO means "Low Drop Out" . That simply meands the regulator will continue regulating when the (Vin -Vout) number gets closer to zero. It has NOTHING to do with if or not the regulator is designed or suited for use with a car's electrical system.

Both Mneary and myself pointed this out earlier in this thread.

3v0
 

kpatz

New Member
I'm wondering the same thing, as I'm working on some automotive gizmos with PICs and want to make sure they're robust enough.

something like: power in > inductor > 24 V zenner > inductor > capacitor
Is there a need for 2 inductors? I've thought along the lines of: power in -> fuse -> inductor -> zener -> capacitor -> regulator -> capacitor.

If the circuit has low power draw, a small resistor (say, 10 ohms 1-2 watts) on the input power as well to make an RC or RLC filter for the noise. Also, if the 0.6V drop is acceptable, a series diode to protect the circuit against reverse polarity.

So, for ultimate protection: power in -> fuse -> diode -> resistor -> inductor -> zener -> capacitor -> regulator -> capacitor.

Much of the discussion has been on the regulator, but what about the other things? If you use an automotive-rated regulator, is there a need for the inductor? The zener? Etc?
 
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MrDEB

Well-Known Member
I realize the LDO regulator has nothing to do with being designed for automotive use it just that all the data sheets I have seen for LDO regulators have stated that it is recommended for automotive use.
 

Thunderchild

New Member
LDO means "Low Drop Out" . That simply meands the regulator will continue regulating when the (Vin -Vout) number gets closer to zero. It has NOTHING to do with if or not the regulator is designed or suited for use with a car's electrical system.

Both Mneary and myself pointed this out earlier in this thread.

3v0
I think LDO regs can be ideal in cars where the powr supply could get low (in my case of a vintage car 6 V if the battery gets flat) and in my case would be handy as I have had an issue when the battery got to 7 V, but yes I understand that i will stillneed some beafy protection
 

3v0

Coop Build Coordinator
Forum Supporter
I think LDO regs can be ideal in cars where the powr supply could get low (in my case of a vintage car 6 V if the battery gets flat) and in my case would be handy as I have had an issue when the battery got to 7 V, but yes I understand that i will stillneed some beafy protection
Prior to the 90's when they started putting digital logic into cars you needed significant external circuitry to deal with the nasty electrical environment. With the modern regulators designed for use in cars you need little or no additional protection. Start reading datasheets to find on that best fits your needs, it is not enough that it an LDO regulator.

Some regulators have what I have seen referred to as voltage tracking. When Vin drops below Vout the regulator provides a Vout which is just a bit less then Vin.
It may come in handy with your 6V car.

Luhan Monat used a voltage tracking regulator when he designed the SuperProbe. That allows one to power the probe from a 5V or even 3.3V circuit. Neat trick given that the SuperProbe uses a 5V regulator. :)

3v0

3v0
 
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