2
\$\begingroup\$

I am looking for a circuit that will charge 8.1 volt capacitor (3 2.7v 400f in series) using a 24VDC power supply. I want to limit the charging current to no more than 100ma.

\$\endgroup\$
5
  • 2
    \$\begingroup\$ 400 F is a lot. Did you make a typo? \$\endgroup\$ – Brian Carlton Nov 13 '12 at 17:14
  • 2
    \$\begingroup\$ Probably not, they're readily available: 2.7V 400F \$\endgroup\$ – Jim Paris Nov 13 '12 at 17:33
  • \$\begingroup\$ en.kamcap.com/jszc.html THis Korean SuperCap OEM has support for such use in series. DO you intend to do this manually or automatically like a smart battery charger or simply a power fail backup circuit. YOu need to balance the voltage and protect from reverse polarity and over voltage plus current limit. Are you prepared to do this? \$\endgroup\$ – Tony Stewart EE75 Nov 13 '12 at 18:03
  • \$\begingroup\$ It sounds like what you want is a normal current source. What's the problem? \$\endgroup\$ – Olin Lathrop Nov 13 '12 at 18:20
  • \$\begingroup\$ problem is similar to series battery cell charger if not matched cells \$\endgroup\$ – Tony Stewart EE75 Nov 13 '12 at 21:46
1
\$\begingroup\$

KAM appears to be a Korean supplier of superCaps and yes it is 400 Farads. These are used for backup so they have a fairly high ESR.

If they are not matched identically upon charging, the voltage will not be equal, so parallel clamps, other zener like parts are needed to prevent over voltage.

When it is drained in series, there is a risk a lower value of capacitance can be discharged quicker and then be reverse biased. This will accelerate aging effects of high ESR. Reverse diode protection will extend the life.

This might seem overkill, but if you don't the risk of failure is catastrophic.

You are best to consult with KAM on the schematic for specs on series charging. I count 14 parts per Cap including transistors.

enter image description here


If you want to design your own, here are a couple ideas; Using the TL431 precision voltage reference as a programmable zener, you can use it to limit the voltage and current instead of a 240 ohm series resistor. ( simple 100mA start current limiter)

This is my simple Cap triad limiter simulation with Cap ESR of 1ohm (which is SWAG at best)

enter image description here

Replace Zener and current source with TL431 app note schematics. It is a <1% 100mA 2.5V ref so you need 2R's to make 2.7V to program the voltage from 2.5 to 2.6 with R ratios. The TL431 also has a reverse diode to protect the cap but the overall circuit does not (yet) have current limit protection for reverse voltage.

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ Rod is this what you are looking for? \$\endgroup\$ – Tony Stewart EE75 Nov 14 '12 at 18:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.