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> Limiting Current Draw From Power Supply
PIC
Posted: November 26, 2011 06:10 pm
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Hi

I'd like to use a mains power adaptor (wall wart) to charge a capacitive load, whilst limiting the amount of current drawn from the adaptor.

Suggestions on an opamp/discretes -based circuit which does this?

Thanks.
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telomere
Posted: November 26, 2011 06:13 pm
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Resistor, lm317, op-amp + mosfet, ptc resistor, there are tons of options. We need more details.


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PIC
Posted: November 26, 2011 07:11 pm
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I'm using a 9V/1A adaptor, and want to limit current draw to 500mA.
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telomere
Posted: November 26, 2011 08:02 pm
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How much voltage drop can you handle?


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PIC
Posted: November 26, 2011 08:11 pm
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Less than 1V. I need the capacitors to charge up to 9V.
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telomere
Posted: November 26, 2011 08:53 pm
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An op-amp, sense resistor, and MOSFET would work. I don't know if it is the best, but dropout can be tiny.

There is a tacked thread somewhere on driving LEDs, most all of the circuits are constant current, it may be worth looking at.


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Sch3mat1c
Posted: November 27, 2011 12:21 am
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Also, current mode buck regulator if you like high efficiency (but unless you're doing ultracapaitors, that's not going to be a problem).

Tim


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tekwiz
Posted: November 27, 2011 12:31 am
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Yep, unless you're filling supercaps, a magnetic wall wart's inherent current limiting will take care of it. If it's a switchmode wall wart, it'll have active limiting. Charge away.
The only way to limit charging current with zero voltage drop when you're done is with a resistor. If you're using a 9V WW, then 18Ω will give you 500ma at the start of charging. But this is complicated by the fact that both WW output voltage & capacitor voltage will be constantly changing. So will the currents. Magnetic WWs also put out a lot more than rated voltage with small loads. Full voltage is 1.414X the nominal voltage.


BTW: With repetitive cap charging, it's the average current that counts. wink.gif


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telomere
Posted: November 27, 2011 02:19 am
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BTW, Here is the op-amp/mosfet setup that I mentioned.



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PIC
Posted: November 27, 2011 01:40 pm
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Thanks, I'll check it out.

I've seen the schematic of a cheap WW, and it's nothing more than transformer-rectifier-cap. Does this provide some inherent current limiting?
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CWB
Posted: November 27, 2011 02:00 pm
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"impedance protection"
between the dc resistance and ac reactance of the secondary and the primary , a "shorted secondary" condition (windings or on the leads as in a shorted bridge rectifier) the worst case current flow in the primary will be limited to a "it's going to take a long time to burst into flames" amount ... this heat rise is slow enough to allow the internal thermal fuse or external fuse device to do it's thing .

has anyone ever seen this statement on those little shaded pole motors ?


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PIC
Posted: November 27, 2011 05:57 pm
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QUOTE (CWB)
... this heat rise is slow enough to allow the internal thermal fuse or external fuse device to do it's thing .


In the case of a short I want the current to be limited to 500mA, rather than having the current totally cut off.

Will the adaptor alone still do the job?
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CWB
Posted: November 27, 2011 06:17 pm
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the thermal device requires the temp to get high enough to open it .
i imagine that a direct short of the secondary would allow at least one minute before the temp of the windings got high enough to open it .


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telomere
Posted: November 27, 2011 08:32 pm
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QUOTE (PIC @ November 27, 2011 05:40 am)
Thanks, I'll check it out.

I've seen the schematic of a cheap WW, and it's nothing more than transformer-rectifier-cap. Does this provide some inherent current limiting?

Most wall-warts these days are switchers, 60Hz transformers are too expensive with all of that copper. laugh.gif

The difference is easily discernible by picking it up, switchers are much lighter than their old-school counterparts.


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CWB
Posted: November 27, 2011 11:12 pm
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"The difference is easily discernible by picking it up, switchers are much lighter than their old-school counterparts."

sort of opposite of your girlfriend back in high school and today ?
laugh.gif tongue.gif


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