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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I would have thought that zero ohms would have been sufficient to prevent damage to a properly operating converter. To prevent damage to an improperly operating one, then a fuse might be the thing to go for.
As ever, read the specs. Does it have an inrush I, or a protection I2t specified? If not, then you're on your own with a MK1 wet finger in the air. 24 watts at 240 volts input needs 100mA, so perhaps a 250mA fuse, with time delay to cope with inrush? That'll give you a reasonable ballpark.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
So you want to drop 11KV to 600v ? To avoid damaging the DC to DC converter with more than 600v, A potential divider arrangement might be an idea. That's a ratio of 18.333 : 1
Now if the load allowed on the dc to dc is 0 - 50% (or up to 12w), ( I in) at 300v = 40ma RLoad = 7.5k making R2 equal to this value 7.5k. Now ( I R1) has to be 80ma (half to the load and half in R2) unloaded volts is then 0.08 * 7.5k = 600v which is max
Registered Member #5323
Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104
The 959uf cap will provide 600vdc maximum to the input of the DC-DC converter (the cap will be permitted to charge only as high as 600vdc). My concern is whether a direct connection between the cap and the DC-DC converter will damage the converter (the converter listed above) or if a more prudent approach would be to place a resistor in line. If so, what value resistor.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Oh right totally misunderstood what you meant. The capacitor is a bit overkill then?, The amount of energy it can hold is mad.
Only issue left then could be inrush current, if you plan on connecting the converter to a charged capacitor. The dc-dc may already have a NTC thermister or low value resistor in series internally, Difficult to say. (is is common in ac switch mode psu's.).
It probably wouldn't do any harm if you wanted to put a few tens of ohms in series for the sake of caution, something like 10 to 22 Ohm, but choose large one of a couple of watts so it dosn't burn up with the surge.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Usual DC/DC converters are not affected by the amount of capacitance at the input, as they are designed to work from a voltage source. The main problem is to charge this large capacitance safely. A rectifier going to this capacitor may require some form of current limitation. This is usually done with a thermistor, that when cold presents relatively high resistance and reduces it when hot. Look at the input circuit of a PC power supply. There is always one there.
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HV resistor input for DC-DC converter
![Back to top](http://www.mouser.com/ProductDetail/Tamura/TCDC7001/?qs=%2fha2pyFaduivNY6r%252b1W3v3kO%252bAUMTgUoLSr40itWybPkdwt9GnNUP4J6xvF2TdaA<a class='bbcode bbcode-link' href='hyperlink"' rel='external' >https://4hv.org/e107_images/generic/bbcode/link2.png[/img]</a> url]</a> which accepts up to 600vdc input and outputs 24vdc. I plan to provide 500-600vdc to the DC-DC converter using this 959uf cap<br /><a class='bbcode bbcode-link' href='../../e107_files/public/1426953803_5323_FT0_capacitor_image_cropped_2.png' ><img src='/e107_files/public/1426953803_5323_FT0_capacitor_image_cropped_2_.png' width='200' height='138' alt='1426953803 5323 FT0 Capacitor Image Cropped 2 ' title='1426953803 5323 FT0 Capacitor Image Cropped 2 ' class='img-rounded rounded bbcode bbcode-img' /></a><br /> The challenge is to select a resistor value large enough to prevent damage to the DC-DC converter, but not so large as to waste energy unnecessarily. Resistor value suggestions? (yes... I'll be careful).
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