Classic powers up, but dead short at input

[MadScientist267]

Hi all.

The equipment:
Classic 150 running 12V nominal

The situation:
I am in the process of building the solar RV conversion, and have been giving the classic rectified input from a transformer, powered by the grid to maintain the batteries, test consumption, etc, yada. This is in lieu of 800W of PV that is to go up top side soon.

The classic is set for hydro mode, manual MPP. Voc is ~110VDC according to the classic, with the MPP around 95V or so.

I had it limiting input @ 3A, as the transformer runs a bit warm even with a fan on it above that.

In addition, I've been limiting the output current at 15A, so the input limiting wasn't tripping.

All has been going along well for a few months now like this... There have been no excessive heat events, and only on rare occasions had the fans cycled during charging. Everything was working exactly as expected... until last night.

To exercise the batteries for whatever reason, I have a switch that kills the grid going to the transformer, at which point the classic would go into resting, and I run from battery.

When I turned the switch on last night, the display changed to bulk MPPT, but the relay did not engage. I had seen it do this before once or twice, where there was a brief delay, and then it engaged, but this time, nothing.

After attempting to wake it up by cycling the grid power a couple of times and seeing the same results, I reset it by turning off the battery disconnect. Upon reengaging the batteries, it came up as it always had, so I again applied power to the transformer. Again, nothing, with one exception... A faint periodic "chirping" about once every 2 seconds or so could be heard coming from within, likely the cores of the inductors as the buck circuitry attempted to start and restart.

At this point I removed the cover, looked for anything obvious, saw nothing that appeared out of order, no heat anywhere etc, so I reconnected the front panel and put the screws back in.

Cycled the battery one more time, same behavior, chirp, chirp, chirp, etc and no output. The display this entire time showing "resting", and no relay clicks.

I figured that there was nothing to lose at this point, so I again disconnected the power input, cycled the battery, and did the hard reset on it (the two arrow key function). It came up, I again set it for 12V, hydro, etc... then applied the grid.

*Bang* - I heard the MPPT relay engage and then the fuse on the primary side of the transformer (6A) vaporized immediately.

I checked the rectifier bridge suspecting possibly the worst, but there were no shorts. Working my way down stream, I found the input connection is dead shorted going into the classic.

The classic still powers up and runs on the battery, the only noticeable difference (other than it obviously won't charge anything) is the "IN" voltage now reads 0.0 instead of floating around between 6 and 8V when in rest mode with no input.

So, how bad is it you think, and what's it gonna take to get it back up and running?

I'd also like thoughts on whether or not I caused it, if so, what it was, as I'd rather not repeat it.

Thanks,

Steve

[Westbranch]

MS 267, don't know on what you did but I am thinking the stepdown is a hefty one and may be too much for the transformer... 
In the mean tme do you have an automotive charger you can use to keep you batteries up. Not necessarily the best but will do in the pinch...

[MadScientist267]

MS 267, don't know on what you did but I am thinking the stepdown is a hefty one and may be too much for the transformer... 
In the mean tme do you have an automotive charger you can use to keep you batteries up. Not necessarily the best but will do in the pinch...

Hi Westbranch

I do indeed have an alternate charging system in place as part of the design that goes around the classic... using the transformer with the classic is really just a temporary thing as it does a much better job. The "backup" charger is meant primarily for grabbing a desperately needed charge when the need arises and doesn't do an "absorb", but definitely is better than nothing.

I too wondered about such a step down, but the manual says it should do it... and the currents are nowhere near the limit... so I dunno. That's why I come to you guys

When the PV goes up, the voltages will be lower... set up as strings of 72 cell/24V nominal.

If the voltage is indeed too much from the transformer, I can change it over to center tap instead, it's just that it runs cooler at higher power levels at the higher voltage.

Steve

[Vic]

Hi MadScientist,

First,  this type of input power for MPPT CCs  is generally not recommended by the CC manufacturer.

On this Forum,  it has been stated that it is not a very good idea,  due to the risks of damaging a MidNite Classic,  for example.

As you may know,  almost all MPPT CCs will be damaged,  if the PV + input becomes grounded to the PV in -- .  Believe that this blows up some transistors.

It has also been stated,  here I believe,  that adding a series resistor at the output of the external power supply,  to increase its output impedance,  limiting the current available should something go wrong.

Know that you were using Hydro Mode,  and some Hydros might have a fairly low output impedance ...  and so on.

I do wonder if the output of the external power supply became shorted,  and that this might have toasted some transistors in the Classic.

Another thought,  regards weather you have filter capacitors on the output of the rectifier.  The Classic (and most other MPPT CCs)  have capacitors on the Vin terminals to help the stabilize the PV input voltage.

FWIW,  Guessing over for now.     Vic

[Watt]

Hello Vic,

I'd like to mention my own results using a transformer ( not the best of the best home-brews but effective ) and the classics.   I was using one transformer and two classics at one time and the only issue I had was using two classic with one power source.  In an IRC which boB is a contributor in on occasion, we discussed using the classic on a transformer and he explained why I was having trouble trying to use two classic with the one power source.  That was not a good idea but diodes would have eliminated the issue I was having.  I chose not to connect the two classics to the one power source and the problem was solved.  However, he never mentioned it being a bad Idea to use such a power source and in fact just commented about it being an interesting use of the classic.  I was tying to condition an older forklift battery and was somewhat successful.  I'm only mentioning this because I use this setup quite regularly.

How can this be any different from rectified power from a wind turbine?  He limited his input to ~350w

[Vic]

Hi Watt,

Believe that we have had a conversation about this ... 

I have used a Home Brewed AC-in DC charger with the Outback MX-60,  for many years as an auxiliary charger.  Have never tried using a Classic instead of the MX.

Also use a different  one that uses a Variac (c)  on the input,  and Auto Jumper Cables on the output,  so a single battery or cell can be charged/EQed.  It uses NO CC.  I am the CC,  by varying the Variac output to control current/voltage. This does work quite well.

Perhaps,  I mis-spoke when saying that it was described,  here,  as a bad thing to do.   At the least,  it was said to be not recommended,  due to a risk to the CC's health.   The series R  was mentioned as something that could help reduce some of the risk.  Believe boB did say that if one did DC PS into the Classic thing,  that having the DC input voltage not much above the battery voltage,  should help reduce the risk to the CC   ...   from distant memory.

And,  way back when,  there was a MN team member that was using rectified AC line voltage,  to a Classic 200,  IIRC.  This was described here,  but then,  probably due to added risks on having no isolation (transformer) from the AC line,  it was noted as not a good idea.

The one thing about using a CC with a DC power supply on its input,  is that many DC PSes  can deliver a lot of fault current.  PV modules cannot.  CCs were primarily designed to work with PVs.   Some CCs can be used to charge a battery from another higher voltage battery,  consult the CC manufacturer before doing these types to things,    and so on.

FWIW,  YMMV.    Vic

[MadScientist267]

Hi all

Just to clarify:

This shouldn't be seen by the classic as anything other than a single phase hydro alternator with full wave rectification, running at a constant speed.

There are no caps externally connected

The grid is fully isolated via the transformer, and fault current is(was) limited to 6A, on the primary side, yielding a theoretical 720W of input.

The rectifier bridge rectifier still checks out as good, so there was never an AC component to the input.

Whatever happened appears to have been a cascade event... power on was briefly clear (the small delay between application of power and when the classic changes modes), then the sound of the relay engaging, followed by the fuse immediately popping.

There had been no sign of any input related issues before this when it simply wasn't changing modes (eg it was showing the usual input voltage etc), and nothing wiring-wise was messed with during this time.

After it was all said and done, I did then disconnect the classic from the rectifier to determine which one was the source of the short - it was indeed the classic.

Hopefully this helps with any thought processes along these lines.

Steve

[Watt]

Thanks Vic, for the clarification.

I'd like to add too about another member of this board and others, who direct charge batteries from Rectified AC voltage from a small diesel generator ~170vdc through a classic 200.  This has been done for a year or so at around 3KW into a 48vdc battery bank. This is mainly done for proper equalize charge once a week as that battery bank has proven too big for proper care from solar and wind available. 

I'm sorry I don't remember that topic regarding previous talk of this setup.  If what you say is true, we may have to come up with a bit of filtering to continue using these methods of maintenance charges.  I'm certainly open to following proper use of this classic. 

I do realize staying closer to battery voltage helps with charge efficiency.  I also realize that with a larger input voltage to battery voltage spread heat at the classic is greater but, I'm also under the impression that this range is well within the tolerances of the classic.  Honestly, that spread is what has made me lazy and gives me the comfort of just throwing near what I want at it so long as I stay within current limit, under Hyper VOC and above the highest battery voltage required for eq.  I also realize some of the difference in charging from a wind turbine and from the mains.  I've just never had issue at this point and the one Classic that has been used the most on this mains transformer of mine has been clicking along fine for years on solar and transformer.

Thanks for your input Vic.

[Vic]

Hi Mad ..

Regarding the fuse;  fuses usually are not installed in circuits to protect equipment,  but to protect wiring from excessive heating.  And in the case of your power supply,  perhaps to help protect the transformer from being toasted.  Am sure that you have looked at the curves for fuse opening a circuit plotting time and the amount of current in excess of the fuse's rating.

The device that probably limited the current into the Classic input was most probably the transformer.   And depending on its design,  the Classic could have seen many times the fuse's rating for a short period of time.

Regarding the discussion on this Forum,  regarding using a Classic connected to a DC power supply  ...  Believe the stated reason for working to keep the DC supply output voltage only a small amount above the battery voltage,  was to try to limit the Classic's output current,  should the Classic be a bit slow in regulating the output voltage/current (from distant memory).

Watt,  perhaps you were not in the previous discussion on the topic of using a DC PC on the input of a Classic to charge a battery from the AC line or generator.

The thing about adding filter capacitors to the output of the rectifier on such a setup,  is to try to keep the Classic's input filter capacitors from being "worked" excessively -- from the 120 Hz (usually)  ripple voltage on the output of a simple rectifier.  This working can cause heating of the caps,  and might not allow the Classic to find a Max Power Point,  as the voltage is fairly rapidly changing.  PV modules have a nice pure DC output,  and a constant load current,, unlike the output of a full wave rectifier without its own fairly large filter capacitor bank.  Such a large filter,  could create its own problem for the MPPT CC,  as sequencing might be an issue ...

I am no expert on MPPT CC design.   And,  I may not be characterizing what the MN crew,  including boB,  had to say on the topic.  Just my recollections.

FWIW.   Good Luck,   Vic

[Watt]

Hi Mad ..

Regarding the fuse;  fuses usually are not installed in circuits to protect equipment,  but to protect wiring from excessive heating.  And in the case of your power supply,  perhaps to help protect the transformer from being toasted.  Am sure that you have looked at the curves for fuse opening a circuit plotting time and the amount of current in excess of the fuse's rating.

The device that probably limited the current into the Classic input was most probably the transformer.   And depending on its design,  the Classic could have seen many times the fuse's rating for a short period of time.

Regarding the discussion on this Forum,  regarding using a Classic connected to a DC power supply  ...  Believe the stated reason for working to keep the DC supply output voltage only a small amount above the battery voltage,  was to try to limit the Classic's output current,  should the Classic be a bit slow in regulating the output voltage/current (from distant memory).

Watt,  perhaps you were not in the previous discussion on the topic of using a DC PC on the input of a Classic to charge a battery from the AC line or generator.

The thing about adding filter capacitors to the output of the rectifier on such a setup,  is to try to keep the Classic's input filter capacitors from being "worked" excessively -- from the 120 Hz (usually)  ripple voltage on the output of a simple rectifier.  This working can cause heating of the caps,  and might not allow the Classic to find a Max Power Point,  as the voltage is fairly rapidly changing.  PV modules have a nice pure DC output,  and a constant load current,, unlike the output of a full wave rectifier without its own fairly large filter capacitor bank.  Such a large filter,  could create its own problem for the MPPT CC,  as sequencing might be an issue ...

I am no expert on MPPT CC design.   And,  I may not be characterizing what the MN crew,  including boB,  had to say on the topic.  Just my recollections.

FWIW.   Good Luck,   Vic

Thanks Vic for the update. 

Can you explain how the classics work so well with wind turbines as input sources?  I understand your comment on the frequency but a 120hz ripple out of a wind turbine would not be hard to duplicate.  Just trying to see the difference Vic, not trying to be complicated.  I don't want to trash such great charge controllers.  At the same time, I have some experience with this scheme and also have not had issue. 

[boB]

The problem with running MPPT charge controllers in this way is that using the grid and especially with a step down transformer,
the MPP voltage can be too close to the Voc and this is where the CC has trouble....  It expects the input source to be
a limited source but to the Classic (or other MPP CC's), it can seem like Hoover Dam.  A wind or hydro turbine
is not the same !  They are current and power limited and max power occurs far enough below the Voc that
the CC has a bit of a "buffer" to adjust and limit the current by raising the input voltage when it reaches the max
current (or user limit).  Some users get lucky and have a "soft" input source.  I bet your source (Steve) is
just too "stiff".  It may not be and you just got UN-lucky after a while ?

What I recommend is to add a series power resistor of a couple of Ohms between the rectifier output
and the input of the CC.

Others at Midnite will say don't do any of this.  I would say that you just better know what you are doing
in these cases.  An added resistor can make it work by giving it that input buffer voltage from Voc (0 amps)
and MPP voltage (max current).  Some are lucky enough to have soft source impedance.

Also, I'm not sure what tracking mode you were using but solar is not the mode for these situations...
Legacy or Hydro mode is usually better.

Sounds like you are going to need an RMA for your Classic.  Sounds like it is now dead.  Gone to meet
its maker... Or, needs to go to visit its maker again.

boB

[Watt]

The problem with running MPPT charge controllers in this way is that using the grid and especially with a step down transformer,
the MPP voltage can be too close to the Voc and this is where the CC has trouble....  It expects the input source to be
a limited source but to the Classic (or other MPP CC's), it can seem like Hoover Dam.  A wind or hydro turbine
is not the same !  They are current and power limited and max power occurs far enough below the Voc that
the CC has a bit of a "buffer" to adjust and limit the current by raising the input voltage when it reaches the max
current (or user limit).  Some users get lucky and have a "soft" input source.  I bet your source (Steve) is
just too "stiff".  It may not be and you just got UN-lucky after a while ?

What I recommend is to add a series power resistor of a couple of Ohms between the rectifier output
and the input of the CC.

Others at Midnite will say don't do any of this.  I would say that you just better know what you are doing
in these cases.  An added resistor can make it work by giving it that input buffer voltage from Voc (0 amps)
and MPP voltage (max current).  Some are lucky enough to have soft source impedance.

Also, I'm not sure what tracking mode you were using but solar is not the mode for these situations...
Legacy or Hydro mode is usually better.

Sounds like you are going to need an RMA for your Classic.  Sounds like it is now dead.  Gone to meet
its maker... Or, needs to go to visit its maker again.

boB

Thanks boB  for your answer and explanation.  Got my confidence back.... 

[MadScientist267]

Bob,

You most certainly hit the nail on the head with the stiff supply thing methinks... it's only a small-ish transformer, a little bigger than my fist, and doesn't really like to pass more than about 300W without getting on the warm side... BUT... it's incredibly close to the classic... about 3 ft as the electrons flow... by your description, things certainly make a lot more sense than they did...

I was indeed running in hydro mode, manual MPP... I was rather excited to find that combination and found it very useful... maybe a little TOO useful..? I can confirm that for anyone else wanting/needing to go this route, that "solar" is indeed not the setting to use; you'll witness squirrelly things that even made me squirm (not always easy to do LOL).

Best theory that had come across any table in my mind involved inductive kick, even tho there were a couple holes in it. Thank you for clarifying. I appreciate your response, middle of the night on a weekend and everything... You guys have more ethic than the collective that I've worked with over the course of my entire life. It doesn't go unnoticed.

I guess now the burning questions of the hour...

What happens next, and how bad is it going to hurt? 

Thanks again,

Steve

[boB]

Hmmmm.......   If it is a small-ish transformer, then I am wondering how much power you were able to get out of the
whole thing ?  And how far below the Voc did you get that power ?

Seems that with a small transformer that you SHOULD be one of the luck soft ones.

00

[MadScientist267]

During a brief curiosity thing, I was able to pull about 375W out of it, around 90-95V, with the Voc being somewhere in the 110V range... both fluctuated a little because of line variations (I'm at the end of a 100 ft extension), as well as some shifting that appeared to be associated with temp. The latter isn't entirely clear.

Steve