Capacitor sizes

[Bob Eager]

I have a Pama handsfree kit that does exactly that....

[PeTe33]

DC Volts + Inductance ?

Not really an issue is it.

Yes, to a DC voltage a Capacitor has infinite resistance because it is open circuit!

I presume the OP was querying strapping 3 capacitors of the same value in parallel i.e. 3 x 22 uF instead of a single 66 uF cap.

[Bob Eager]

His point, I think, is that an electrolytic capacitor does not have infinite resistance. (actually, no capacitor has infinite resistance....)

[Dave Plowman (News)]

Eh?

[The Natural Philosopher]

The issue is RF suppression in the original comment about paralleling capacitors. To preserve a low impedance across a broad range of frequencies it is usual to have up to three capacitors - a large electolytic, which is reaosanbly low impedance in the audio and kilohertz sort of arena then something like a mylar layered capacitor, which will do well up to a Mhz or tow and then something like a ceramic disc, which can absorb frequencies in the 10-100MHz range, or higher.

Beyond that you are into black magic...every mm of wire will do SOMETHING.

[NoSpam]

But it's possible to make some reasonable estimates. Assume that 2V droop is the max acceptable and that the load current is 10mA ... CV=IT will tell you that 66uF will give about 13mS hold-up ... it would be a rare car that starts in 11mS ;-)

As somebody else has suggested the best way is to wire it from somewhere else or take a diode to a small rechargeable battery and power it from that.

Dave

[PeTe33]

My mistake. Your explanation above clears up the error of my confusion. :¬)

[Andy Wade]

But if the 'DC' voltage changes a current will flow, so you can no longer describe it as presenting infinite resistance - although of course energy is being stored or released, rather than dissipated as heat as it would be in a resistor.

For a perfect capacitor i = C * dv/dt, for a practical capacitor there's always some additional loss present which does result in heat being dissipated. This is usually thought of in terms of an equivalent series resistance, ESR.

There's no such thing as DC really, unless you've been hanging around since before the big bang.

[dcbwhaley]

But the main bus voltage can fall below 6 volts when cranking the engine, especially if it is very cold.

[Dave Plowman (News)]

No it can't.

[Nick2]

Why not?

6v gets you max power delivered from the battery...

[Dave Plowman (News)]

I've got an accurate voltmeter sitting across my car battery at all times the ignition is on, and I can assure you it never drops under nine point something volts even on the coldest day when starting. Usually the high

10s. If you think about it, old cold start ignition systems are designed for 9 volts minimum.

[The Natural Philosopher]

you won't even start a modern car on 6v.

I had two batteries die on me this year on different cars. Around 9v was the startable limit. I stuck a meter on them to see.

[The Natural Philosopher]

And enough power absorbed inside it to explode it.

[dingbat]

This is completely wrong. Although aircraft certainly did (and some still do) use rotary converters, they didn't use them to supply the high power loads. The high power system was fed by engine generators (DC, then later AC) at 100V and above and the rotary inverters were used to supply the older 28V standards for existing instrumentation systems. Nor do these rotaries have particularly massive rotors with appreciable stored energy in them.

I posted a summary of aircraft electrical power to this ng a while ago.

[dingbat]

If it changes, then it isn't DC (for the purposes of reactive theory), you've imposed some AC component upon it. It may be of very low frequency, but it's still an AC component and thus the impedance of the capacitor is no longer infinite.

[dingbat]

If the terminal voltage of a lead acid car battery falls below about

9V, the effective internal resistance (for high currents) will also have become so high as well that they're no longer capable of supplying starter-levels of power. Try it and you'll find that voltage at the starter might be 6V when off, but drops to approximately zilch when you pull starting current.

There's also the problem that trying to start at low voltages can burn the starter windings out. The engine is often cold and reluctant to start anyway, so the cranking time goes up. As the efficiency of a DC motor (mechanical power out / heat wasted) drops dramatically at lower voltages, then the temperature rise in the starter is significantly worse when trying to start from depleted batteries - even if they're not even delivering the same power. This is one reason why the Swedes were so keen to switch to permanent magnet starters. In their cold winters it was one less source of failure - it's not that they work significantly better, it's that they don't kill themselves when trying to.

[Guy King]

The message from The Natural Philosopher contains these words:

The neighbour's Kia got upset with me jumpstarting it. Had to charge the battery even though it was cranking over easily fast enough for a petrol engine. I didn't actually measure the voltage, but it was damned annoying.

[The Natural Philosopher]

Not quite true, as those of us who built go karts powered by starter motors can attest. Its a bit of a race as to whether the battery explodes, the start current in the motor burns the windings out, or the kart actually gets up to speed, but for a split second, the voltage is pretty low..

We estimated that from te Kart duration on a stock battery, peak currents were up to a Kiloamp, and normal running was a couple of hundred plus.. About 2-3KW

..of coourse these days you can buy model aircraft motors of less weight and similar power. I think I juts read that someone has a 15KW one in pre production. Bang in a thousand quids worth of lithium batteries and you have something that will take a 20lb plane vertically upwards. Takes all the fun out of it.

[Nick2]

A few hundred watts for several seconds? I doubt that would result in much of a bang.