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I need a simple, stable and low consumption dc/dc converter

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Dec 21, 2001
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dc dc converter consumption

I have build a motorycle alarm and i used LM317 to reduce voltage from 12V to 5V DC. This alarm is using a microcontroller (AVR AT90s1200A) and i am having problem when the motorcycle is moving. To be more specific, when motorcycle is moving, the microcontroller is acting strangely, doing resets, running code that souldn't be running e.t.c.

I have come to the conclussion that there is to much noise to the power supply (to motorcycle's battery) when the motorcycle is moving or when i use the ignition.

What can i do? I do not want to use a dc/dc pulse converter because they are not 100% efficient and in this project the power consumption is very critical. I want a power supply circuit that has very low self power consumption

Can anyone help me? I am depserate

First of all, I am sure that the bike power supply is very noisy, and also has high overvoltage transients. Those could be bigh enough even to kill the regulator chip.

I would consider two approaches:

1) To add a LC filter and a Zener clamp to the existing power supply, or
2) To redesign the supply - maybe even using a switchmode converter!

I think your worries about power supply idle current are unnnecessary, if the design is done right - and the ficiency will be better when loaded, because your linear regulator is definitively of low efficiency. Let us see what you are doing: A conversion from 12V to 5V with a linear regulator, based on current being about equal on both sides. (Outcurrent=Incurrent+regulatorconsumption). I assume for simplicity that the regulator uses 0 mA itself. Then the best case efficiency would be at 12 V nominal voltage: 5/12 = 41.7%. With a switchmode regulator you can easily get around 80% efficiency.

But back to the main problem: Noise. You might use an iron-core inductor of some Henry from the battery, and capable to handle the DC current maximum, with a electrolytics capacitor and a protective zener diode in the regulator side. That kind of filter has been used in the past in cars. To wind an inductor is not a big deal, you have to get the materials (core, copper wire, and bobbin). You must remember that an air gap is needed in most cases due to DC load. Programs for calculating it are widely available (sorry, have none for this moment at hand, may see little later around).

For the noise problem, another important thing is the grounding and filtering of external wires. The electronics should have it's own ground system, with only one point connection to the bike ground. All inputs should have filters (RC filters should do for slow signals), and the driver for alarm horn must be considered because the high currents. A relay might be the easiest isolating component, allowing the horn current (and noise) to be kept outside the logic.

This is some general about the design - and in spite that I disagree considering the unsuitability of a switch mode converter, I would as a first approach try without redesigning the power supply. You see, you have the same noise from the motorbike's power system affecting anyway, and something must be done with it even when using another regulator.

Good luck,


P.s. I came to a thought where you might get a suitable inductor: Consider loudspeaker bass/diskant division filter coils. Depending on the current you might find a neat ready-made coil from some electronics hobby shop. There are other surplus inductors around as well.

hi,if you only supply a cpu ,it will consum small is is good mothed of using a LM317.but the LM7805 will be better,you can save two resistences.maybe this is the cheapest way to slove this problem.
what you need is to get ride of noise from can :
1:use a LC filter from input,the cap must be a low esr ,big capacity one,such as a NICHICON 25v 1000u or more,and parallel a low esr MKT ,such as a 1u or more MKT cap.
2:do the same at ouput;
3:shield the whole pcb with metal;
4:get a good ground connector,you can find a good grong point after several tests.
5:put the PCB far away from motor;
i think this can slove your problem.
good luck.

For switch mode buck converter, if the output current is less than 1A, you can try LM2575 chip, you only need to add several passive components for implementation.


I will part my recommendation in two parts:
First the electronic design.
Second some finger rules of the real world(include some nice joke).

I agree with Ted an want to add some info:

1. In the input of the power supply you must use lc filter,
and the protection zener(that called transzorb) it must be little big
then the max voltage continus of your alternator (12v baterry need ~15
carge voltage so recommended transorb of 16v otherwise it will burn).

2. dc-dc converter is most recommended for power save.

3. Some time the motorycle have some power fail like
when use starter,etc.. so you will get a power fail some time of sec or more
so you need inside the system on the 5v a good capacitor to backup
you from that power-fail . You can use a great cap that
called SUPERCAP is a small special cap of 5v and 0.5-1F !!!

other options is to use a baterry + additinal power fail chip that will
backup your system in cause of power fail.

4. I hope that you known your motorycle and the system is 12v!!!

some kind of motorycle have small baterry of 6-8.6v !!! and not 12v
It will nice if you give us some info of the motorycle like vedor,cc,baterry(volt/amp).

After we talk of the power supply let go to other parts of the design
(I'm not sure that the only problem is the power supply)

Tell us some of the i/o interfaces and also whit type of rallay you use?

1. All your io must be protected from spike
recommended rc filter and also add zener after the resistor for protect
the system from over-voltage.

You can't solve all the noise in the io only the rc filter!
recomended to add digital filter in program for example
if you wan't arm the alarm(block the motorbyke) sample the signal for
20sec(exapmle) for stability (that protect you from long spikes)
before lock it.

2. Recommended to use a power-fail reset chip and not rc on the reset of
the uc in case of big-power fail or slower power up it can go to :?: :x

3. Is not recommended to use not a normal relay
use special pulse relay after you move the connetions he continue
to stay in that position without any current
(in case of the system crash on ride it not disconnected the motorbike)

3.1 is recommend also to store in ee mem the status of drive
(after a power-fail when the uc wake up to have know is he is in ride mode
to not disconnect your relay in functions of the digitals program filters).

4. Recommended to use extend temp comp like ind temp
(commercial components work from 0-50c not good to non-dor system)
Water resistant.
Chemicals resistant.
Vibration resistant.(comp on dip socket will get out so solder them).

I think that i talk about all (sorry if i forgot something).

:!: :!: :!: :!: :!:
Now the most fun part the second part
safety :? finger rules 8O and joke :wink:

A day ago a try write all thats(it take to me ~1hour) but when i send the mail it lost :cry: :x :x :x so today i write it again and bakup it!!!! :D

A motorcyclist to motorcyclist some little rules:

1. If you are not in hospital start with the nurses :p or
you drink a cup of coffe with god :? or
you are with your new boss :twisted: :x
So that good because you read that usefull info 8O :!: :wink:

2. I hope that you are a good designer so your life is in your hand. :?

3. You have more courage than mind. 8O

4. Some little info 8O of the real world :
The insurance comp don't like to pay :x for most accidents 8O !
So in case of accident is not important if that your mistake or not
if they will find a non apruvved aditional part to your motorbike
you will the quilty :!: the insurance comp will say to you :x ,
and you find 8O yourself pay for the mistake of other. :?

5. if you see :twisted: in the side of the road wait for tremps
don't stop ,continue to drive but don't look to himself a lot of time if
you don't want to enter on a tree. :wink:

6. Not good idea to be a pilot on air . :?

Aproval test for system (how you know is not a indoor system)
that appruve system have:

1. Vibration test: take a shirt put the system inside and put
inside the dryer machine now for road motobike , now run the
macine with hot air for
half hour for road motobike :?
for others&harlly-davisons use 1.5 hour :wink:
now open the system see in all the parts are fixxed. 8O

2. Water& chemicals test: put the system in the wash macine
full prog + soap. :wink:
Now see if the color not down.

2.1 Water test: put the system in aquarium power it
if the fish jump out so you have now two problem. :?

3. Temp test:
put in oven in 75c for 2 hours the system when work
see 8O if somting is cook :?
put in frizzer for night in the morning powered the system on
see if it not shorted for the low temp 8O

4. fuel&fire test: close the door and windows open the gas wait half
hour power the system . You never know if a problem was. 8O
(child don't do it at home!!!)

5. Last test road test: find a "fried" that you don't want see him
let him take a tour on the motobike.
if you will not sem him so you solve one problem 8O
and you have two probems no system&motobike :?
if you see him you have only one problem him! 8O

In the last row i add you info of my two ridder friendsfrom loony-toons
that was on the road
one of them have a lot of experience on design additional parts to road
machine and the other still drive this will get you all the picture

Please post us what going?
Best regards Gabby.

(to admin/mod sorry but life is not joke but it ussefull tool to open
his eys before it going sleep between the lines)


Thanks a lot guys!!! I am bound to you all. I will try all these useful info and i will let you know about the result.

The truth is that i tried the LC filter and i had a little problem. I do not use an external siren enabled by a relay. I have microcontroller producing the sounds and i use a darlington transistor, an up transformer and a piezohorn to make sounds louder. So, when i used this LC filter, i got lower sounds. This was done because my serin needed about 0.8A to work and couldn't take it because of the inductance. Should i use a big inductance? How big? I think i used 150uH. Is it enough?

Serin is producing me another problem. Because it is internal, when it sounds i can disarm the alarm with the remote control only is i am standing close to the alarm. Apparently there is too much noise. :cry:

I used 7805 in the past and i had self power consumption problem. You see, 7805 needs about 5mA for its own and my motorcycle can't affort it :D

Yes, my motorcycle works with 12V DC and i think it is about 5.5AH. I know it is small, that is why the 9mA that my alarm needs to work totally discharges my battery in about 3 days. :cry:

Yes, i do use latch (pulse) relay to prevent engine from starting when alarm is armed. I am telling you, i have done everything i could to turn down the power consumption.

I am thinking of re-designing the whole system using a switchmode regulator. Can you advise me any component?

That' all

Thank u all
I am looking forward for your advices

Some comments and details:

You say: "The truth is that i tried the LC filter and i had a little problem....This was done because my serin needed about 0.8A to work and couldn't take it because of the inductance. Should i use a big inductance? How big? I think i used 150uH. Is it enough?"

I would use bigger inductance, much bigger. The problem arising might be, except for DC resistance, also that when the sirene is switching on, your power consumption increses and the inductor needs time to "charge". (In opposite case, when sirene is switched off, the zener would dissipate the excess energy). But the inductor has to have low DC resistance, and capability of that 0.8A you need. Also, like shanren says, you need a good and big low-impedance electrolytics capacitor after it.

Gabby proposes to have a tranzorb or big zener, which I fully agree, but would have slightly higher voltage - maybe about 20V so that it will not conduct too often -- only when needed for protection. And those parts have tolerances, too, so a 16V part might get in worst case a bit close to the maximum charging voltage.

The noise issue to your remote: The noise is very high frequency one if it enters the RF range your remote uses. That means fast edges! I would suggest: Good bypass of the microcontroller power supply (short wires, or a chip capacitor as close as possible). Also, the digital port(s) driving your sirene transistor should have either a small resistor or a ferrite bead close to the IO port. The transformer should have also a RC snubber over the primary. (Btw, I hope you do not use regulated voltage for the sirene circuitry, but the 12V supply)

In my opinion your alarm processor uses too much current in the first place. Modern low-power chips, using relatively low clock frequency, and "sleeping" much of the time, should be able to use much less.

You should consider a "telecom" type switchmode regulator chip, they are of low power, and can take high input voltages, actually much more than you need. The other extreme for low power is for portable phones etc. They tend to be optimized <10V inputs from battery.
Vishay-Siliconix parts (for example Si9105 or Si9108) might do the trick, and they have very low self consumption. See **broken link removed** I have used that family sevral years ago for an ISDN phone product power supply where power economy is very critical.
There may be more integrated designs needing only a simple inductor (not a transformer like these), but it is hard to beat the <1mA self consumption of these! You might look at Maxim and Linear tech, too, just to mention a couple of companies with many regulator designs.


Hi abouras.
In the time i write that rows Ted send also a mail.

Again i must agre with Ted and add some info(sorry a part was write
before he send the mail or in the same time so you find some equal info)

First 0.8A (what freq you use?and from what voltage you drive it 5v or12v,i belive from 12v) is a lot a current as you know
the inductor don't let you big current changes .

Now how much big the inductor will be it will filter more best your system from
power changes but when you make inside big changes it not let you,
one option to use a more big cap but that space.
My simple better solution is only the part of
the output of the darlington + transform drive it directly form the 12v
without the inductor(but protect is with the zener),
this part of the
system in drive mode is of so the noise don't take effect on it.

Now for power reducion some question?
I don't know your schematic but see first if you can cange the desing
from 5v to 3.3v your avr avaliable also in low voltage but i don't know
if the other parts can work from 3.3v ?
If you can cange the voltage to 3.3 you save 50% of the current.

Other option is to reduce the clock of uc it will save you current but
you los form the power calculation and that not nice BUT!!!
take a look on the idel/standby mode of the uc maybe you can
make a change in the sofware that in the most time it will be in idel
I don't kwon your uc well i take a look on it maybe is an option to
put it in idel and the counter run in that mode it will wake up
only once to some time (in funtion of the timmer) if you can do that
the consuption of the uc will go down unter the 0.1ma-1ma
that will be nice? :roll:

Now back to the power supply you are going to use dc-dc converter!!!
give us only how much current you need from him :?:

The most popular companny that make dc-dc are: :?

1. maxxim
2. linear
3. nationalsemiconductors
4. micrell

give me little time and the current you need and i find you some dc-dc
that are nice . :p
(now is 2:26AM ups 3:00AM and i going zzz....)

Because you come back i will finish the story :cry: that i start in the first mail :wink:
(i have few row more) in the next mail tommorow == today afternon

are you like the fingers rules?

I going to idel mode . :wink:
Best regards Gabby. 8) ZZZ.....

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