Charging supercapacitors without circuit

Thread Starter

WondrousBread

Joined Oct 26, 2022
4
Hi all. New to this forum, hopefully I've found the right section.

I'm working on an electronics project and I'm having trouble finding an answer to one particular question. Is there any reason you can't charge a supercapacitor without a charging circuit? I see lots of circuits online that may work, but I'm not sure that I actually need one for my application.

My project is a controller board that takes a PWM input and controls a stepper motor with it. It's designed to be installed in a vehicle, connected to the IGN switched 12V.

The project is functional, but I want to add a function that moves the stepper motor to a certain position when the PWM signal goes to zero (ie. when the vehicle is turned off). This means the Arduino and driver need to be powered momentarily after turning the key from the IGN position. Obviously I don't want to wire the it to constant 12V as it would cause a parasitic draw on the battery.

I was thinking the best thing to do would be to add a 12V supercap across the input power. This way it would charge up whenever the vehicle is running, then on shutting the vehicle off provide a few seconds of power for the stepper to move to it's set position.

Is it as simple as what I'm suggesting, or do I need a circuit to handle this? Is there any difference if I use two 6V supercaps in series with one another (still parallel with the input power), since 6V supercaps seem to be more cost-effective?

Thanks, Chris
 

Alec_t

Joined Sep 17, 2013
13,226
Welcome to AAC!
Having supercaps in series without "a circuit" would cause a problem because no two are identical. The one with the lowest capacitance would charge/discharge sooner than the others and could easily get charged above its rated voltage, causing damage.
 

Thread Starter

WondrousBread

Joined Oct 26, 2022
4
Hi Alec;

I understand. So the two 6V in series definitely requires a circuit then. Would the one 12V supercap in parallel with the 12V supply also require a circuit?

I apologize if my terminology isn't accurate here. I'm half decent with electronics but I'm all self-taught.

Thanks :)
 

Alec_t

Joined Sep 17, 2013
13,226
Would the one 12V supercap in parallel with the 12V supply also require a circuit?
Is there really such a thing as a 12V supercap, or is it actually two or more caps in series? The highest voltage rating listed on Farnell's site seems to be 5.5V.
Bear in mind, too, that the voltage on a nominally12V vehicle battery is actually a good bit higher when the alternator is charging the battery.
 

Thread Starter

WondrousBread

Joined Oct 26, 2022
4
I have seen some 12V supercaps available, although they are a lot more expensive. One on Mouser is $34 / unit.

I was hoping to keep the process automatic, but I think another solution would be to just wire the controller to accessory 12V. Then when I turn the key from IGN to ACC the PWM signal is cut, leave it in ACC for a moment to let the controller move the stepper, then turn the key all the way off.

Based on the data I'm gathering from the controller, the key would have to be in ACC for only about 1s to achieve this. I'm just thinking a fully automated solution would be better so one doesn't have to think about it at all and can just take out the key as normal.
 

shortbus

Joined Sep 30, 2009
9,755
The project is functional, but I want to add a function that moves the stepper motor to a certain position when the PWM signal goes to zero (ie. when the vehicle is turned off). This means the Arduino and driver need to be powered momentarily after turning the key from the IGN position
How about an off delay relay? They power up for a time you select, when the main circuit(ignition switch) is turned off.


Quote -

OFF DELAY
Delay on Release
Delay on Break
Delay on De-Energization



Upon application of input voltage, the time delay relay is ready to accept a trigger. When the trigger is applied, the output is energized. Upon removal of the trigger, the time delay (t) begins. At the end of the time delay (t), the output is de-energized.

From this site - https://www.macromatic.com/support/knowledge-base-articles/understanding-time-delay-relay-functions
 

WBahn

Joined Mar 31, 2012
27,862
Hi all. New to this forum, hopefully I've found the right section.

I'm working on an electronics project and I'm having trouble finding an answer to one particular question. Is there any reason you can't charge a supercapacitor without a charging circuit? I see lots of circuits online that may work, but I'm not sure that I actually need one for my application.

My project is a controller board that takes a PWM input and controls a stepper motor with it. It's designed to be installed in a vehicle, connected to the IGN switched 12V.

The project is functional, but I want to add a function that moves the stepper motor to a certain position when the PWM signal goes to zero (ie. when the vehicle is turned off). This means the Arduino and driver need to be powered momentarily after turning the key from the IGN position. Obviously I don't want to wire the it to constant 12V as it would cause a parasitic draw on the battery.

I was thinking the best thing to do would be to add a 12V supercap across the input power. This way it would charge up whenever the vehicle is running, then on shutting the vehicle off provide a few seconds of power for the stepper to move to it's set position.

Is it as simple as what I'm suggesting, or do I need a circuit to handle this? Is there any difference if I use two 6V supercaps in series with one another (still parallel with the input power), since 6V supercaps seem to be more cost-effective?

Thanks, Chris
Why not just have it wired directly to the 12 V system via a relay. The relay is engaged when the IGN is turned on and, when it is turned off, there is a time delay (via some means -- a number of options there) that is long enough to any shutdown housekeeping to be finished before disengaging the relay. The simplest might be to simply key the disengagement of the relay to the stepper motor reaching the desired shutdown position.
 

MrChips

Joined Oct 2, 2009
27,670
If you are using a microcontroller you can program it to go into sleep, power down, or stop mode. To power the MCU when the 12V power is removed just use a lithium coin cell.
 

DickCappels

Joined Aug 21, 2008
9,508
The following applies to the newer supercaps, originally referred to as ultracaps.

Another readin a charging circuit may be desirable is that without some kind of current limiting some (all?) supercaps can draw enough current to bring a power supply into current limit - which. may be ok I’m cases in which it is ok to short the power supply,

in the case of charging from a battery
- is a momentary short ok?
 

Thread Starter

WondrousBread

Joined Oct 26, 2022
4
I just wanted to follow up; I found a different solution.

I mentioned above that if the controller is wired to ACC, then when IGN is cut the PWM signal stops. I implemented the park function in software so that when the controller is powered but has no input (ie. Key to ACC), it parks at the maximum valve opening.

Then by optimizing the code I was able to reduce the time required for the valve to park to around ~100ms. So turning the key to off at a normal speed should be sufficient, but waiting even 1/10 of a second before turning the key all the way to OFF parks the valve reliably.

I want to thank everyone for the help, I really appreciate it :)

Chris
 
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