Recent content by BruceM

Ampster raised a good point on the typical failure of DC SSRs of ON. Relays do occasionally fail with welded contacts, but I expect that the $25 EV relays rated for 900V that A.Justice linked to at BatteryHookup.com would be so overkill for this application that that would never happen. I...

A solid state DC relay, with plenty of headroom for open collector voltage and current rating seems the obvious solution, as DThames suggests. They are readily available and cheap. Mechanical relays are ill suited to your high current DC application. This is an understatement. My home is...

Yes, it's only the case of the PV string voltage being higher than the maximum rated input voltage of the BMS where the reported cascade of failure (after MPPT failure) occurs. It would be redundant and unnecessary to add this to a low voltage PV array. I'm glad to help you with your op amp...

If you contact me via a private conversation on this forum I'll try to help. It's unclear what you are trying to accomplish, but it's not related to this thread, since there's no need for an external disconnect for a low voltage PV application with BMS. This thread was about high voltage PV...

That first NASA paper listed is obsolete, and was about failures in using late 90's and 2000's switching only Mosfets with very low Rds in linear mode. Yep, that's a well known failure, even happening with too low power gate drivers when operating near max rated power. The dated 2010 Nasa...

Thanks, Hedges. I agree with your analysis for the intended application. That's an interesting (!) finding regarding NASA linear Mosfet failures, I'd love to see the report for the NASA finding linear failures at 1/100th of the SOA; in particular who the vendor was that flat out fabricated...

Hey Davismitc, I applaud all those learning electronics, concur with the good advice to abandon the crappy simulator and develop on a solderless breadboard. Learning basic op amp and comparator circuits is the best place to start. Some tips: Your reference voltage via Zener shunt regulator...

I'm a gray haired EE/CS myself. Formerly in military flight simulation development and some work in ICE and related development tools at Motorola. Thanks to 30 years of MS and resulting epilepsy I'm not the sharpest anymore. Simple things take a lot more time and effort. Oddly, I now find...

Fun to read about your competent and creative work, Crossy. Bravo and thanks.

Nicely done, Crossy. If the line voltage is sensed BEFORE the BMS, and BEFORE the DC SSR, then this could catch the voltage going ABOVE the normal BMS battery full level when the charge controller has failed as a short and after the BMS has opened it's input Mosfet(s). The high voltage sense...

Fshan, Crossy's nice solution meets your requirement; the SS relay is kept on until voltage gets too high, then the on signal is removed. That fits your safety philosophy, right? A small latching relay or 4000 series logic latch could break the signal to the SS relay optically isolated input...

Yes, fshan, to avoid oscillation the triggering over-voltage event must be "latched" by either the control electronics or the device...thus the use of a latching relay, or SCR type crowbar, or a cmos flip flop gate to maintain a disconnect signal to a DC SSR, or microcontroller to do the same.

Yes, if the relay trip voltage is lower than the BMS, the the relay only sees a modest rising voltage as it starts to open. It should be OK. I didn't grok that the relay would be the first to open, thanks for pointing that out. The monster relay is not my preferred solution because it's...

That's a good approach and the small voltage drop on the high voltage side is an insignificant loss. Just make sure the heat sinked DC SSR is not too hot at the end of a deep bulk charge cycle. Some of the current ratings on China products are laughable, and they don't generally provide the on...

If the charge controller fails as described here or as a typical switching Mosfet failure (melted short between source, drain and gate), the full PV voltage is sent to the battery BMS. When battery voltage gets above 14.4, then the BMS disconnects the PV from the battery via its Mosfet switch...