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12v 5 second backup via capacitors

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troy310589

New Member
Hi everyone, i am trying to come up with a circuit for my car,
i need a setup that will deliver a constant 12v for around 5 seconds once the unit stops recieving power,

it is for electronic gauges to go in my car which calibrate when they receive power. if connected to the ACC position (the one the stereo usually runs off) power is cut white starting the engine to give the starter motor as much power as possible, then the gagues re-calibrate when the engine is running, not returning to a true 0 position, and if i connect it to the ON position, it retains constant power but the gagues dim really bad as the starter motor draws a lot of current and this has the possibility of damaging my gauges,

thank you guys
 

MikeMl

Well-Known Member
Most Helpful Member
You need to measure how much current it takes to operate the gauges (on 12V). I dont think the voltage sag during cranking will hurt the gauges.
 
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Diver300

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Most Helpful Member
Fit two diodes so that the guages are run from either the ACC or the ignition supply. That way it will keep going while the engine is cranking.

There is still the question of whether the guages will work with the lower voltage. Do you know the voltage / current requirements of these guages? It seems to me that gauges that self calibrate when turned on will suffer from the same problem on any car, so the manufacturers have probably got a solution.
 

troy310589

New Member
i will try that idea with the diodes, the gagues do still work when the engine is cranking, but being a digital readout it dimms severly when doing so, mostly it looks bad and it just bugs me, and being a digital gauge im not sure if the drop/change in voltage could in the long term be damaging.

i am unable to get a current draw reading at the moment as the car is getting some work done to it until saturday. but when i get the car back how do i measure the current draw? do u use a volt meter set to 12v and just use it in-line with one of the power wires for the gague?
 

MikeMl

Well-Known Member
Most Helpful Member
No, you use an ammeter in series - try the 10A range for a start (in case it takes a lot), then work down to the lower ranges.
Your multimeter should have ranges for AC Volts, DC Volts, AC Amps, DC AMPS, OHMs, .... If you still have the instruction manual or can download it, read the section on how to measure DC AMPS (usually means that the red probe must be moved to a different jack on the multimeter).

After configuring the meter to measure DC AMPS, remove the fuse that powers the gauge(s), and use the meter probes to short across between where the two ends of the fuse connect (no probe ever touches ground during a current measurement).
 
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troy310589

New Member
ok, got my car back now i have tested the current, i chose the 200ma setting, and these are the readings i got,


200ma (resolution 100uA, accuracy +-1.6%of+-2digits)
ign ON (engine off) 62.5
Cranking 27
ign ON (engine running) 66.4.
 

Hero999

Banned
It depends on the minimum voltage requirement too, you'll never get a constant 12V using a capacitor, the voltage will drop.

Assuming it'll work down to 10V, you'll need a >0.166F capacitor, the nearest standard value is 0.22F 16V which will be huge.
 

troy310589

New Member
ok cool, so how would i connect this capacitor, accross the earth and the positive? and does this mean that when i turn the car off the gague stays on for a little while then eventually dimms down untill it turns off? (would look kinda cool:p)
 

Hero999

Banned
Yes, connect it across the earth and positive.

You'll need a Schottky diode in series to stop it from discharging back into whatever is feeding it.

Yes, it'll probably dim before it turns off.

No, I don't think this is the best way of doing it though, taking a feed which stays permanently live to power the gauge and using a timer triggered from the ignition switch is a better idea. The capacitor will be very large and expensive, a timer will be lightweight and cheap.
 

troy310589

New Member
ok, i was thinking a small circuit similar to one used to keep the interior light on longer for an extra 5 seconds. but on that is activated when the positive wire is disconnectedm and begins to discharce a capacitor(s) at an almost constant voltage and then swithces off (possibly via an IC of some sort)this way it will stay on while cranking the engine, and will stay on for a few seconds after turning the car off.
 

MikeMl

Well-Known Member
Most Helpful Member
You need a HUGE capacitor. Look at the attached, which shows the voltage dip for a 5 sec cranking time vs different size capacitors. You need the resistor to limit the inrush current when the switch recloses.

I dont think it is practical to do it this way!

Alternatives: Use a boost-switching power supply to feed the gauges from the battery. The switcher doesn't care that the battery voltage is pulled down during cranking.

Use a small 12V sealed lead-acid battery. You need a diode for isolation. After the diode's forward voltrage drop, the SLA should float charge while the alternator is on-line.

Install a timer which doesn't apply any voltage to the gauges until after the engine is running?

Call the gauge maker and ask them. They will tell you that this whole idea is totally unnecessary because the gauges are designed to cope with it, which I told you 10 posts ago :D
 

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Hero999

Banned
You'll need a permanent live from the battery.

Here are a couple of circuit ideas.
 

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MikeMl

Well-Known Member
Most Helpful Member
Hero,
He is not trying to generate a delay; he is trying to store enough energy to keep the voltage to his gauges near 12V while the starter motor drags the car battery down to ~8V during cranking. How does your circuit do that?
 

Hero999

Banned
That's not what he said:

i will try that idea with the diodes, the gagues do still work when the engine is cranking, but being a digital readout it dimms severly when doing so, mostly it looks bad and it just bugs me, and being a digital gauge im not sure if the drop/change in voltage could in the long term be damaging.
does this mean that when i turn the car off the gague stays on for a little while then eventually dimms down untill it turns off? (would look kinda cool:p)
 
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