Charging ultracapacitors with computer PSU

[drummerdimitri]

I see your point. Any idea on how to do this though?

The most efficient option would be to build a simple DC-DC switching power supply and set its soft-start ramp to provide the 3-4A/µs load slew rate.

A much less efficient option would be to use a pair of op-amps: one integrates a fixed current to generate a ramp signal while the other is wired as a a voltage-controlled current source following that ramp.

In both cases, you need to shut down the regulator (short the soft-start cap on the PWM chip or the integrator cap on the op-amp) so current starts at 0A instead of max when you hook up the next cap.

I am a newbie with electronics so forgive me if all that sounded Gibberish to me. Guess I need to buy a higher powered battery charger for this purpose.

[timb]

I see your point. Any idea on how to do this though?

The most efficient option would be to build a simple DC-DC switching power supply and set its soft-start ramp to provide the 3-4A/µs load slew rate.

A much less efficient option would be to use a pair of op-amps: one integrates a fixed current to generate a ramp signal while the other is wired as a a voltage-controlled current source following that ramp.

In both cases, you need to shut down the regulator (short the soft-start cap on the PWM chip or the integrator cap on the op-amp) so current starts at 0A instead of max when you hook up the next cap.

I am a newbie with electronics so forgive me if all that sounded Gibberish to me. Guess I need to buy a higher powered battery charger for this purpose.

That's the safest option, yeah.


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[ovnr]

So what I did was use a 14 year old PSU and to my surprise, it was able to charge the cap at 25 amps initially with a direct connection from 0 to 2.5 V while the current slowly decreased to 8 amps eventually and poof went the PSU.

(...)

Now the question to ask is why can a 10 year old 350W PSU charge a cap at 25 amps but a new 750 W PSU can't?

Really? But you just said that your 350W PSU couldn't finish charging the cap because it blew up. The newer one just gives up instead of damaging itself.

Also, I have serious doubts about your measurement technique, and take the values you report with a grain of salt. For instance, your pencil lead resistor of 2.5 ohms cannot pass more than 5 amps at 12V, and it will also probably catch fire (Pd at T=0 of 60W).

And if you buy a battery charger: Please don't be surprised if it blows up, blows fuses, or refuses to work - they're not meant for charging supercaps either. Your best bet is, as others have said, a rather large CC LED driver that's specced for low voltages - but they're really not very common.

If you're more adventurous, just build a simple buck converter that ramps from 1% PWM to ~20% (assuming 12V in and 2.6V out) over a period of a couple seconds, and cut it off with a comparator when it reaches the target voltage.

[DanielS]

And if you buy a battery charger: Please don't be surprised if it blows up, blows fuses, or refuses to work - they're not meant for charging supercaps either.

I would be more worried about blowing up the super/ultra-cap since battery chargers usually work fine on a near-short: the cheap chargers are current-limited by their transformer's impedance while fancier chargers operate in current-limit until they reach the constant-voltage top-off phase. The problem is finding a charger that will stop at the correct voltage for a super/ultra-cap to avoid blowing up the cap from over-charge.

I agree the buck converter is the simplest way to go about it: with a PWM controller that does per-cycle current limit and soft-start, you can do everything with one chip and a few external components.

[drummerdimitri]

Do you think this one is up to the job: http://www.icharger.co.nz/Products/308-Duo.aspx

I will need a 1500W psu with a voltage of around 24-27V to feed the charger but I believe it will do a great job of charging the supercaps at high speeds.