Help with a simple charging circuit Please

[jezza103]

Hi everyone. I am just a beginner with electronics and have built a few nice little projects but i am stuck with this problem.
I have got some small super capacitors rated at 25 farad 2.7 volts and I want to build a circuit to charge them safely. I have built a little voltage regulator circuit that is based on a LM317T voltage regulator and can happily get any voltage that I want but what i want to do is have the circuit shut off when the capacitor reaches about 2.6 volts is there a simple way of doing this please. Any help would be greatly appreciated. Thank you for taking the time to read my message.
Jeremy

 

[duffy]

I don't understand... this sounds like you built a wheel and are asking how to build a wheel... ?? If you already have a regulator you can adjust to 2.6V, it will charge the caps to 2.6V.

 

[jezza103]

Hi Duffy I know it sounds strange but I want the circuit to turn itself off when the cap gets to 2.6V. I thought I could conserve some power so if I was to leave the room while i left the cap charging iThe circuit would automatically turn off. Or maybe get a buzzer to sound or led to light to remind me or something like that. I am sorry if i did not make myself clearer on that
Jeremy

 

[MrAl]

Hi,

If it is set to regulate at 2.6 then once the cap charges it should not use much power even if still running.

 

[duffy]

Nothing very simple, you could add a voltage comparator to the output and have that switch off a relay or something. Are you concerned about the tiny current the LM317 will use after it has charged the cap? It shouldn't be more than a couple of milliamps.

A better approach might be to increase the current and reduce the charge time. An LM317 is only rated for 1.5A. So to charge a 25F cap to 2.6V @ 1.5A would take about 43 seconds.

But if you gave it 6A, it would take less than 11 seconds - you could charge it just standing there holding down a pushbutton on the main power switch.

 

[jezza103]

Hi and thanks for your advice. The thing is I just wanted to build a little circuit that did what i wanted it to do. I understand that the power consumption is minimal, but as this is just for a bit of fun. This is my little hobby and I enjoy it very much, but I thought I would try to make it work the way I wanted it to thats all. Maybe I can find something on the web for a auto shut off battery charging circuit and adapt that a little bit. I know you all must think I am stupid but I just want to enjoy what I do and just wanted a little help that's all
Jeremy

 

[Roff]

What these guys have been trying to tell you is that an LM317 set to 2.6V, connected to a discharged capacitor, will charge the cap with a current determined by the 317's internal current limiting. When the cap voltage gets to 2.6V, the LM317 will turn off, except for the current through the resistive divider that sets the output voltage. It seems to me like the LM317 does what you want. If you want something different, you have to define what that is a little better.
And we don't think you are stupid.

 

[duffy]

Is it a 25F, 2.7V cap like this one?
https://www.electro-tech-online.com/custompdfs/2012/03/ESHSR-280003000500100025005029C0-002R7pdf.pdf

That one can be charged in 3 seconds. You wouldn't even have time to leave the room. Your disconnect could be the same momentary pushbutton you press to charge it.

 

[MrAl]

Hi,

Ok, it sounds like from Jer's last post that we have to switch into Hobby mode. Part of the reason for doing this is just to see how it would be done.

One way is to use a PNP transistor on the front end. The PNP is energized during start up, then after the cap is charged a comparator senses that the current has dropped to some small value, then turns off the PNP transistor. This is similar to other chargers that do actually have to turn off. There is a still a very very small amount of quiescent current but it now is down in the microamperes.

Another way is to use a relay. The relay is energized during start up, and after a sense circuit senses some low level of current the relay is de-energized and that turns the circuit off completely. It is possible to have zero quiescent current when the circuit is off this way.

 

[jezza103]

Hi guys. Sorry about my last post and thankyou for your advice. MrAl has hit the nose on the head in his reply. I would like to see how it would be done. I found this circuit and I think this is the sort of thing I mean ...
https://www.scribd.com/doc/11382730/Auto-Turn-Off-Battery-Charger
Could you tell me if I could get this circuit to do what i want? I have all the components except the correct transistors It states 3x BC558 and the oly PNP transistors I have are BC327 Would they work? If not I will get some and have a go unless of course any of you could point me in the right direction.
Thanks again for your time
Jeremy

 

[duffy]

Yes, the BC327 will substitute for the BC558. It has better current and voltage handling ability, and more than enough gain for this application.

 

[Roff]

Hi guys. Sorry about my last post and thankyou for your advice. MrAl has hit the nose on the head in his reply. I would like to see how it would be done. I found this circuit and I think this is the sort of thing I mean ...
https://www.scribd.com/doc/11382730/Auto-Turn-Off-Battery-Charger
Could you tell me if I could get this circuit to do what i want? I have all the components except the correct transistors It states 3x BC558 and the oly PNP transistors I have are BC327 Would they work? If not I will get some and have a go unless of course any of you could point me in the right direction.
Thanks again for your time
Jeremy

One problem with this circuit is that it implies that the charging current is 70mA. It actually starts out at around 450mA, dropping slowly to around around 300mA before it drops rapidly to cutoff.

 

[MrAl]

Hi again,


It will probably work with that transistor or any general purpose transistor like 2N4403.

Now we see the practical side to this too. If you want your battery charger to turn off COMPLETELY (including the input transformer) then it's a good idea to use a relay and current detection circuit to turn it off when the charge is complete. Normally we dont worry about this as much because we use a wall wart.

That circuit looks decent, but keep in mind that there will be some small temperature deviation in the output voltage because of the added diode D6, which is there to prevent the batteries from discharging back into the bias circuit for the LM317 when the charger does turn off. This probably wont make too much difference though near room temperature, but something to keep in mind.
I think we could find a small modification that would improve this circuit in that respect.

 

[Roff]

As I mentioned in a previous post, the charger circuit that was posted charged the AA cells at several hundred milliamps. Current was limited by the 10Ω resistor.
I have modified the design to charge them at 70mA. This was done by placing a current source (Q1, Q2, R3, R4, D2) in series with the LM317. The current can be changed by changing the value of R3.
Ichg≈0.65/R3
The schematic posted is not exactly the one I simulated. The simulation models the functionality of the relay, but not the relay itself. The transformer and diode bridge were replaced by a 12V DC source to simplify the simulation.
Open the .asc file in LTspice if you want to see how the simulation results were obtained.

EDIT: Charge time in the simulation is much reduced from real time by using a 0.1 Farad cap to model the battery.
EDIT:The schematic is temporarily (I hope) missing. I tried to edit it, and I kept getting an error message when I reposted it. I'm waiting for a reply from the admin.

 

[jezza103]

Hi and thank you for your help especially roff for the circuit design. I really appreciate all you have done for me I will make up the circuit and give it a try
Thanks to you all
Jeremy

 

[Roff]

Hi and thank you for your help especially roff for the circuit design. I really appreciate all you have done for me I will make up the circuit and give it a try
Thanks to you all
Jeremy

The schematic is temporarily (I hope) missing. I tried to edit it, and I kept getting an error message when I reposted it. I'm waiting for a reply from the admin.

 

[MrAl]

Hi,


This was the original circuit. However, the drop out current might be a little high so that it might drop out about 50ma charge current which seems a little high.
A diode in parallel with the 10 ohm sense resistor will help this and then the 10 ohm can be increased to 40 ohm or something like that. Unless you want less charge current then you dont need the extra diode.

 

[Roff]

Here is the schematic and .asc file that I could not attach in post #14.

 

[MrAl]

Hi Roff,


It was nice that you did a simulation. Did you notice that the shut down current is kinda highish? I noticed that with the last circuit too. Maybe a little adjustment would bring it down a little. I was thinking of throwing a cheap comparator in it just to get a nice accurate shut down point and be able to get it down lower like 25ma.

 

[Roff]

Hi Roff,


It was nice that you did a simulation. Did you notice that the shut down current is kinda highish? I noticed that with the last circuit too. Maybe a little adjustment would bring it down a little. I was thinking of throwing a cheap comparator in it just to get a nice accurate shut down point and be able to get it down lower like 25ma.

Is that detrimental, or at least suboptimal?
Also, the original article never says what the battery chemistry is. Are there any rechargeable AAs that work well with a simple constant current charge?