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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
it depends upon your attitude; the resistors are rated for 500 V intermittent so you can use that figure reliably, intermittently or you could de-rate them to 100 V, requiring five times more resistors, is the extra cost & effort worth it ? and consider what may happen if one fails in an mmc balancing situation, plus heat dissipation will be 1/5 per resistor. (Read a resistor datasheet to realise that it is not advisable to use resistors near their rated power dissipation)
personally I use VR37 resistors because they have never failed me and I bought a lot e.g. but I'm sure others are similar.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm glad someone asked, I'm getting ready to do the same, but maybe with carbon composistion for 30 to 60 seconds at a time. I just need a temporary load to pull the HV down to its real value.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
If you size your bleeder/balancer resistor right for a MMC you shouldn't end up worrying too much about dissipation - voltage rating will be the biggest issue. I'm not sure how much you can push them - for a MMC bleeder you'll be solidly in the intermittent operation regime, so start with that figure.
Different resistor technologies are more or less resistant to pulse loading - I think carbon composition is particularly good actually. The internet will tell you all about it.
This video is worth watching, points out some pitfalls of pulse loading resistors:
Finding pulse energy ratings can be a pain, but some brands publish them (normally energy rating up to a certain duration, then a multiplier of continuous rating for longer pulses), so keep looking if at first you don't succeed :P
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Standard metal film resistors, how do they cope in HV applications
