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schand10

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HI all,

i work with a car company. We used a gps tracker system for some of our cars which were used for rental purposes. The problem which occcurred was tht the gps system used to restart everytime the ignition was turned on. To solve this i actually used a diode in series with +ve and then some caps in parallel.
The caps i used were 470pF, 100uF, 4700uF and 22000uF. Without the 22000uF the circuit used to restart. I thought of using an inductor and tried some out instead of the diode as an Inductor ideally has zero resistance. But this dint work for any of the values i tested for like 1uH, 10uH, 100uH and 1000uH.
I wanted to use this actually to reduce the big cap(22000uF) i used initially. Could someone please suggest any other useful techniques or something im ignoring.

Thanks
Srid
 
Do you want to reduce the physical size? Or the capacitance? Because you can get 500mF to 1F supercaps (2.5-5V ratings only) that are "small". THey aren't cheap though at $1-$5 a piece. You coudl also try using a capacitor with a lower voltage rating since it would be smaller than one of a higher voltage rating.
 
hey thnhx for d reply..............i just wanted to understand the concept better as to why it requires such a big cap and is not working with a 4700uF.....im workin on 12V circuits....so ratings of arnd 16V is reqd.
 
Let me see if I understand the problem correctly.

You have this GPS device and with out external intervention resets itself when the ignition is off? And you wish to prevent this?

At this time you're using a capacitor to do so? Please let me know if I understand the problem correctly before I attempt to go further.
 
All he's doing is use the capacitor to keep the power supply up when the ignition is switched on.

What are the power requirements of the tracker?

You could run it from a switched mode power supply, this will continiue to boost the 10V (or whatever voltage the battery drops to when the ignition is operated) to 12V but it will be more complicated.
 
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also y the inductor is not working instead of a diode.....if someone cud help me undstnd these facts it wud b awsome....
thanx
 
Why don't you wire the GPS system directly to the battery? If that would drain the battery too much, then use a hidden switch or wire it to the accessory circuit. I know some vehicles disconnect some of the circuits (Usually accy and some ign) during starting to minimize the load on the battery. Is this what your capacitor is supposed to solve?
 
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@3I
the GPS resets wen i turn on the ignition ......during ignition all d supply is taken for starting which i thnk cuts off d supply to GPS and so it resets......then its normal....
@HERO
its d same for the tracker 12V

thnx
 
I don't believe that the power is totally cut off when the ignition is started.

What normally happens is, the battery voltage drops as the huge current is drawn. This causes many devices in the car to stop working properly. A switching regulator might be able to help by boosting the voltage to the GPS which will keep it working.
 
I don't believe that the power is totally cut off when the ignition is started.
When the key is turned to the "start" position, many "high current" circuits are disconnected. Such as wipers, lights, rear defrosters etc. to avoid robbing the starter from the all important start up. Easy to see by activating wipers and starting the car. The wipes will stop mid flight until the key is released to the run position

If the GPS is reseting, then it is most likely connected through one of these cut offed circuits. The OP needs to find a circuit that is "hot at all times". If the direct to batt is used then an inline fuse must be incorporated. On board modules and memories like radios etc are kept alive by using "hot at all times" circuits. The necessary currents are typically in the low tens of milliamp area, however, I have seen TSB's for some high end cars have been known to bleed down a battery if stored over long periods of time.

Reserve capacities of car batts today is very limited, headlights for maybe 10 minutes as apposed to the old days of 45 minutes +. To save a buck, they only design in what they need.

What normally happens is, the battery voltage drops as the huge current is drawn.
This is true...
However, a good battery should not drop below 10 volts, if it does then it is discharged or toast.
On a cold day, a starter could drop it down closer to 9 volts. Around 9 volts, the starter will just groan. Why block heaters save/extend a batterys life. Remember that the battery is only used to start the car. A working alternator keeps it going.

A lot of automotive "add on" devices account for this by regulating the 12 volt batt to an internal working voltage that accommodates the potential drops in voltage during these peak surges.

cheers
 
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thnx a lot guys......
but could someone explain abt the ckt im using rt now to solve the prob. that is.....y large cap is needed and y an inductor is not working......

Thanks
Srid
 
What you need is a small battery. A capacitor behaves like a small, temporary, but very fast on-demand battery. An inductor does not behave like this. Comparing a capacitor to an inductor like comparing a very very very rough approximation of a voltage source to a very very very rough approximation of a current source. A capacitor behaves more like a voltage source than an inductor does while an inductor behaves more like a current source than a capacitor does (but this is only relative to each other, they are nothing like actual current or voltage sources).

That is why. And please don't use chat speak and use whole sentences.
 
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thnx dknguyen
but, im using an inductor instead of a diode....not a cap....i undstnd abt the cap as battery...but y such a big cap?
 
You need a big cap because the the cap's voltage drops as you draw current from it. If you draw too much current from it, the voltage drops too much shutting off the GPS. It's the same as if you used a battery that is too small for the power you are drawing from it- it doesn't run long enough. In your case, the capacitor is unable to provide enough power to keep the GPS from resetting during the ignition period (it is unable to provide enough current to the GPS without the voltage dropping enough to cause the GPS to reboot).

PLEASE DON'T USE CHAT SPEAK!
 
An inductor won't work because it stores electricity as a magnetic field and tries to keep the current the same.

An inductor effectively stores current and a capacitor stores voltage. If your load required a constant current then an inductor would do but it doesn't, it requires a constant voltage so an inductor is no good.
 
If the tracking device is taking 350 mA, then a 22000 :mu:F capacitor will drop at 0.35 / 0.022 = 15.9 Volts per second.

That is probably worst case as 350 mA is probably the maximum and 22000 :mu:F capacitors are often larger than that.

I assume that the problem is caused by the battery voltage falling as the engine is started. The battery probably falls to about 8 V during starting.

That size of capacitor will only hold the tracker above 10V for about 1/8th of a second.

I guess that the tracker usually only takes about 10 mA, so the capacitor is enough to hold the voltage above 10 V for about 4 seconds.

Anyhow, that is why you need a capacitor that large. Even large capacitors cannot supply more than a few mA for a few seconds.

One problem that I have seen on some vehicles is current limiting devices instead of fuses. When those feed ordinary bulbs, the cold fillaments are like a short circuit and the voltage can dip a long way down as a lamp turns on.

I have seen voltages dip to 4V. That was on a Ford Transit, on the feed to the hazard warning lights, each time they flash.
 
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