Capacitor to smooth-out crank voltage drop

Thread Starter

john2k

Joined Nov 14, 2019
146
I have the following 12v powered USB hub https://www.startech.com/en-gb/cards-adapters/st4200usbm and a phone charger plugged in to ACC wire from my car. All working fine. However, when I start the engine during the crank there is a slight power outage and seems like this is common across most cars. I'm told I can smooth this out with a capacitor and a diode. I've drawn the following diagram. Will the capacitor be able to cope with the load? what sort of load would the capacitor be able to handle? and what is the purpose of the diode in this diagram? Should I add a 3A inline blade fuse or something on the +12v line? Thanks

 

prairiemystic

Joined Jun 5, 2018
191
ACC is switched off during cranking, so it always cuts out. It can be for several seconds so a 4,700uF capacitor isn't going to hold up that long, it depends on the load though.

The car will have a second ACC power line that has power throughout cranking, but it can be hard to find on the car's electrical wiring diagram.
I take power off battery and added a switch/MCU timer to keep power-on to my accessories despite starting the car.

You always need a fuse for the electronics and wires. 3A to 5A is good.
1N914 is way too tiny- use a big TVS at least 1.5KE20A or 5KP20 to deal with moderate transients. I use huge automotive-rated SM8A27 6,600W TVS for protection. Why? Because phone chargers are not always tough to take vehicle transients and a phone is worth quite a few more dollars. For years now, zero troubles. The TVS clamping voltage depends mostly on the phone charger's max. input voltage. Some IC's/mosfets are 25V only or worse.
 

sagor

Joined Mar 10, 2019
490
Put the diode from your +12V on the right towards the capacitor. Remove the 1N914. That allows both devices to run from the +12V on the right, but with only a small voltage drop from the diode. On motor start, the diode blocks any instantaneous voltage drop for a short time from the car supply (prevents the drop from sucking out the capacitor). However, the capacitor may only hold the higher voltage for a fraction of a second, depending on your load.
 
The diode you have drawn is not a good choice. A 1n5402 would be better. 3A, 200 PRV
Why 200? It has to do with the alternator. You can also put an 18V rated TVS diode there too in parallel. A 1.5KE18A https://www.littelfuse.com/~/media/.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf That would protect against positive spikes. Same orientation.

The diode's position in your circuit would protect against reverse polarity and negative spikes, but the 1n914 isn't suitablr.. You would also need a 3A or smaller fuse as you suggested. Reverse polarity would blow the fuse provided the fuse is before the network.

You wanted something to help ride through the bumps. In that case you need to also put a 1n5402 diode in serie with the 12V supply before the capacitor before the network of stuff. The band would point toward the USB power port.

I was typing.
 

ronsimpson

Joined Oct 7, 2019
1,543
Assuming the power is coming from the right side of the picture. A normal diode will drop the voltage by 0.7 volts but the Schottky diode will drop only 0.35 volts.
SB5100TA 100V 5A, SB540-T 40V 5A, 1N5821-E3/54 3A,
1604962981021.png
 

Papabravo

Joined Feb 24, 2006
17,003
It might be OK except for the fact that a capacitor has an exponential voltage decay. People generally have an exceedingly poor grasp of all things that exhibit exponential behavior.
It would make more sense to diode connect a standby battery and the car's electrical system. That is the most reliable and foolproof way to do it.
 

MisterBill2

Joined Jan 23, 2018
9,493
Assuming the power is coming from the right side of the picture. A normal diode will drop the voltage by 0.7 volts but the Schottky diode will drop only 0.35 volts.
SB5100TA 100V 5A, SB540-T 40V 5A, 1N5821-E3/54 3A,
View attachment 221902
THIS version of the circuit can provide some carry-over during cranking, when the ACC is switched off.
If the phone has a battery good for a few seconds then it does not need the carry-over function. Adding another fuse would be silly because the acc circuit is already fused, and every fuse has a voltage drop across it. The diode needs to be able to handle the charging current of the capacitor, so it should be rated at least 5 amps.

And now a question of what is so important that it can not stand a few seconds of power outage while you start the engine? If the battery in the phone can not carry it for 5 seconds then replace the battery.

And if you are one of those who is constantly on the phone instead of focusing on driving, switch OFF the engine and stop driving!!
 

ronsimpson

Joined Oct 7, 2019
1,543
Many sources of 12V power in a car are switched off while the starter is cranking. (depends on what car)
In my Suburban, one outlet is powered off the battery and several are switched through the key.
I did auto design 35 years ago. The battery voltage during cranking is very undepictable. It depends on the car and how old the battery is. The 12V does not merely drop in voltage. I have scope pictures of the voltage jumping from 6V to 18V with each cylinder as it reaches top. It depends where you tap off power. With the circuit in #5, the capacitor probably does not need to hold for the entire crank time but will get some what recharged many times during the crank time.

Why? When the piston is in the compression stroke it take much power from the starter & battery. As the piston passes top it pushes back into the starter. (with or without firing) If you ever hand cranked a Model T engine you know how the handle acts.

It is hard to say anything that is true on all cars, under all conditions. Get a diode and a big cap and report back.
 

Thread Starter

john2k

Joined Nov 14, 2019
146
And now a question of what is so important that it can not stand a few seconds of power outage while you start the engine? If the battery in the phone can not carry it for 5 seconds then replace the battery.
The phone is used as a permenant mounted screen in the car and the ACC is a trigger to wake it up via the charging port. But because the ACC cranking issue the phone wakes-up annd goes off for a split second and back on.
 

Thread Starter

john2k

Joined Nov 14, 2019
146
Assuming the power is coming from the right side of the picture. A normal diode will drop the voltage by 0.7 volts but the Schottky diode will drop only 0.35 volts.
SB5100TA 100V 5A, SB540-T 40V 5A, 1N5821-E3/54 3A,
View attachment 221902
I have a 1n5822 diode on hand will that work based on this diagram you drew? But wasn't the original diode there as some sort of transiet to pass induced voltage back to the earth?
 

MisterBill2

Joined Jan 23, 2018
9,493
If the system supported is that critical then the correct arrangement would be to feed it from the battery directly, or, even better, from a separate battery charged from the main 12 volt system.
An arrangement that would be simpler is to power it from one of the "ignition on" circuits, which are not interrupted during cranking.
And if the phone goes off and then comes back on during cranking then certainly the internal battery has lost the ability to provide any support.
And what is so important about the "permanent mounted screen"???
 
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