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Topic: super-cap charger circuit (Read 2121 times) previous topic - next topic

Smajdalf

The wattage on R3 does not have to be equal to the peak watts. The peak current is only for a short time.
I was considering it. The dissipation will decrease to about 25% of peak value while charging the cap - but not very quickly. With large capacitance the charging time may be quite long - many seconds. I have no experience with power resistors but I don't think you can safely reduce wattage of R3 considerably - maybe to 50%? Not so great saving. OTOH with properly sized R3 the circuit will survive short on the output - I think it is a valuable property of a "simple circuit for noobs".

@ReverseEMF: You can use "normal" buck topology I think. Your circuit is a voltage inverter - there will be negative voltage on A0 of Arduino. While switching circuit is much much more effective than a linear one it is not as simple and a noob may easily run into problems.

6v6gt

What, incidentally, is the application ?  If it is a simple, low current application and you can tolerate longish charging times (say a backup power source for an RTC) then the circuit can be very simple.

MorganS

#17
Jun 18, 2019, 09:31 pm Last Edit: Jun 18, 2019, 09:31 pm by MorganS
Here's a Switch Mode strawman [untested]:
The advantage of a dedicated switcher chip like the LTC3824 is it applies current limiting to the individual PWM pulses. It switches on until the input current reaches the programmed threshold and then switches off. Then it switches modes halfway through the charge cycle to increase efficiency.  It's a standard buck converter topology with no special features. You don't need to invert the voltage.

Your scheme of "run for a bit then stop to check" is not as precise.

Speaking of the inverter, this isn't going to work as already pointed out - the voltage on A0 is negative. It also removes your control over Q3 as it will now need a negative voltage to switch off.

The simple Wikipedia diagram of a buck converter is:


The inductor "protects" the input circuit by limiting the rate of rise of current going into the supercap. In the supercap application, the initial charge at low voltage actually sources most current through the diode. It may be getting 1A from the input and 4A from the diode. Despite sourcing current, the diode is not providing power. Just one of those odd things about current and voltage and power. Power comes from the input of course.

Here's a complete circuit using the LTC3824. This uses almost all of the advanced functions of the dedicated switcher chip. (I have left out the ideal diode circuit here.) Obviously PCB layout is critical - you can't just place all these components on a board and expect it to work.





"The problem is in the code you didn't post."

ReverseEMF

#18
Jun 19, 2019, 12:06 am Last Edit: Jun 19, 2019, 06:30 pm by ReverseEMF
Speaking of the inverter, this isn't going to work as already pointed out - the voltage on A0 is negative. It also removes your control over Q3 as it will now need a negative voltage to switch off.
Holy <expletive,> you're right -- removing!

How about something like this, then:




Your scheme of "run for a bit then stop to check" is not as precise.
True, but, consider that it's a fixed load.  This thing has one job: Charge that capacitor.  And as long as it's the same capacitor, every time, and as long as conditions don't change much, you might just be able to get away with it.   Besides, what I'm really trying to do here is provide a stepping-off point for thought in this direction.  And, I actually did throw some components together, on a solderless breadboard, and ran it with a function generator, and Whoo-hoo, it caused the coil to kick charge into a SuperCap!  I tried three different typologies. and they all worked.

So, my reasoning is:  because this is a fixed application, it might be possible to "tune" this thing by choosing an inductance and setting the PWM frequency to it's electrical characteristics, and then "run it a bit", like I said [and the timing of that "bit" is part of the "tuning"].  And, yes, not as efficient as can be -- but perhaps more accessible to the Arduino Hobbyist most likely to visit these pages [for instance, all the parts I suggested, are "through-hole"].
"It's a big galaxy, Mr. Scott"

Please DON'T PM me regarding what should be part of the Public Conversation -- Let it ALL hang out!!
Unless, of course, it's to notify me of a mistake.

MorganS

I like the buck converter topology. What else did you try?

We still have no idea what the supercap is for. What is it powering? But we can have some fun speculation while we wait for the OP to tell us why these suggestions won't work.
"The problem is in the code you didn't post."

ReverseEMF

#20
Jun 19, 2019, 06:32 pm Last Edit: Jun 19, 2019, 06:32 pm by ReverseEMF
We still have no idea what the supercap is for. What is it powering? But we can have some fun speculation while we wait for the OP to tell us why these suggestions won't work.
Too true!
"It's a big galaxy, Mr. Scott"

Please DON'T PM me regarding what should be part of the Public Conversation -- Let it ALL hang out!!
Unless, of course, it's to notify me of a mistake.

tjones9163

Hi All,
JCA is correct: Q2 must be connected correctly so that it can turn ON!
Also, it would be better to interchange the values of R6 & R7. Now,
look at the gate of Q1: R5 & R9 form a voltage divider so the gate
rests at 6V while U2:A is off. The Vgs(on) will keep the super cap
from taking a full charge. (How can messed-up schematics like
this be published?)
Herb
Thanks for the response, and yah you are right, i am trying to find better sites to study and eventually build easy-medium circuits. Any suggestions?
right now i use Circuit Digest, ElectronicsHub, and Makerpro.
Thanks again for the rsponse

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