Delay Capacitor

Discussion in 'General Electronics Chat' started by szotyi, Jan 14, 2016.

1. szotyi Thread Starter New Member

Jan 14, 2016
19
3
Hi!

I have a car electronics directly connected to the battery. I switch off and on with a switch button.
My problem is that if the electronic is already on, and I start the car, during self-starting the voltage drops and my electronics restarts. So I was thinking to use capacitor/diode combo to prevent this situation.
My question is how can I calculate how big capacitor I need? The consumption of the electronics is 2A. And the time is max 5 sec.

Thanks,
Szotyi

2. RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89
Yes, it happen may be due to Short circuit while Automobile Starter it consume lot of Current.

3. szotyi Thread Starter New Member

Jan 14, 2016
19
3
And how can I prevent this situation? Thanks

4. kubeek AAC Fanatic!

Sep 20, 2005
4,688
806
Again stating the obvious? OP seems to know pretty well why this is happening.

A capactior that would drop its voltage for example by 2V while you pulled 2A for 5s would need to be 2s*5A/2V= 5 farad.
That is a serously large supercap and will cost a lot. You might be better off getting a larger main battery or using a small secondary battery. Or just live with it.

5. kubeek AAC Fanatic!

Sep 20, 2005
4,688
806
Exactly, no need to repeat the obvious.
By simple math derived from capacitor equations. https://en.wikipedia.org/wiki/Capacitor#Current.E2.80.93voltage_relation
I= C*dV/dT which for constant current means simplifies to I=C*V/t, rearrange it to get C=I*t/V

6. kubeek AAC Fanatic!

Sep 20, 2005
4,688
806
2V is how much you allow the voltage on the capacitor to drop while you are discharging it.
No, I am an electrical engineer, just like you claim you are.

7. szotyi Thread Starter New Member

Jan 14, 2016
19
3
Ok, guys if you can not agree with each other, how could I do? You are the experts, I do not know about it a lot..
My situation is more complex, I just wrote it simple to not bother you with a lot of info. Actually it is a tractor and not a car. It has an RTK GPS on it (1-2 cm accurate) with autosteering. In the real life, I start the tractor, then the electronics. I work with it. I am middle of row, something brakes, or I have to go somewhere else for whatever reason - thats the real life. I would stop the tractor, but I can not, because if I start the tractor again the rtk looses the coordinates, the heading etc. I need to go forward and backwards to get back the heading and the right coordinates... But it is impossible to go back right there I left, if every few cm is important. So I can not live with it sorry. The main battery is already 160Ah...
I am asking help, because I do not know it.

Last edited: Jan 14, 2016
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8. RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
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ok, i say connect another battery with it.

9. szotyi Thread Starter New Member

Jan 14, 2016
19
3
What do you mean?

So if we have a second battery how I connect to the main battery? To charge the second battery, and to prevent the selfstaring to sink from the second battery?
The second battery cost also quite a money, would be a 5 farad capacitor good? Or costs so much?

10. RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89

<snip>

Last edited by a moderator: Jan 14, 2016
11. szotyi Thread Starter New Member

Jan 14, 2016
19
3
My hobby is programming. I wrote also myself an autosteering software for tracked tractors. I have made the electronic circuits for it myself from reading posts, and tutorials from the internet...
Input voltage is 10-16 V.

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12. kubeek AAC Fanatic!

Sep 20, 2005
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2 amps for a GPS seems like a lot, would it be possible to divide the system into parts that don´t really need the backup and into parts that do?
I would suggest you simply instead of a capacitor use a small 12v battery, conected though a diode to your main battery. You will not be using a lot of energy from it, so it can be fairly small and keep recharging through that diode. Just mind you would need to turn everything of in order not to drain the battery when the motor is off.

13. szotyi Thread Starter New Member

Jan 14, 2016
19
3
the GPS consumption is ca. 750ma, the monitor is 1 A, thats why I have wrote 2Amps.
So I connect like the attached circuits?

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14. hp1729 Well-Known Member

Nov 23, 2015
2,094
232
Almost. A diode connected to each battery. 1N5822 is suggested. A low loss Schottky Rectifier, 40 Volt, 3 Amp.

Re: the capacitor:
The suggested capacitor runs \$100 to \$200. Your suggested circuit would be correct for using the capacitor as the second battery.

Re: 125 acres
What do you raise on 125 acres that makes good money? Avocados? In America a "small farm" is 500 to 600 acres, with most farms being in the thousands of acres before they are profitable. Are you subsidized by the state? No insult intended. A 125 acre avocado farm is worth about \$2,000,000 in Ventura California.

• Design 567 12 v backup.pdf
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Last edited: Jan 14, 2016
15. szotyi Thread Starter New Member

Jan 14, 2016
19
3
thank you very much!

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16. szotyi Thread Starter New Member

Jan 14, 2016
19
3
Both, thanks again for your help guys!

17. kubeek AAC Fanatic!

Sep 20, 2005
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The problem with your circuit is that the small battery will never get recharged, that is why I suggested a single diode. It will leave the smaller battery a bit below full charge, but it will allways recharge to that level once the motor is running.

However if you put a resistor across the diode from the smaller battery to limit the charging current, then it will be probably the best solution.

Last edited: Jan 14, 2016
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18. szotyi Thread Starter New Member

Jan 14, 2016
19
3
ok, so my circuit is good? thanks

Mod edit: removed off topic material.

Last edited by a moderator: Jan 14, 2016
19. #12 Expert

Nov 30, 2010
16,682
7,322
Ignore everything posted by RRITESH KAKKAR and all the side-tracked responses to him. I have reported this so the Moderators will clean up this irrelevant conversation which hinders your progress.

I say, use a small 12V battery isolated by a diode from the tractor's main voltage supply. A battery is much more reliable and stable than a capacitor in this use. Your tractor will recharge the small battery to almost the same level as the starter battery. This small weakness is a good thing to avoid overheating your small instrument battery. Check it often until you see how it is going to act in the face of being charged by a much larger system.

20. #12 Expert

Nov 30, 2010
16,682
7,322
Apologies. I answered something on the first page without realizing there was a second page. Kubeek seems to have worked this out. Ride with him.