Converter In-Rush Thermistors etc, UPDATE -4

[BFL13]

UPDATE on 17 Nov, Update 2 on 22 Nov, Update 3 on 16 Jan 14,
Update 4 on 19 Oct 14.
---------------

In a recent thread there was mention of how you can wreck your gizmo (in my case a converter) by turning it off and restarting it again before the capacitors have finished unloading----if I was understanding that discussion.

So I am worried I am going to wreck my nice PowerMax 100 amp converter doing that.

Background is that last year I had one of these and had the top off so I could adjust its voltage with the internal pot they have. Somehow at one point after some weeks of using it, the in-rush current thermistor burst into flames and that was that.

PowerMax sent me a new "beefier" thermistor which they said would also be used in the newer 100 ampers. I got that in and had that going for a while and then I think when starting and stopping and restarting right away (maybe that was it, not sure) that thermistor over-heated and fell to bits.

I am wondering if it was operator error in starting/ stopping sequence that did for both thermistors.

I do see on the voltmeter that after you shut off the converter (when disconnected from battery), its voltage tapers down for a good time rather than drops straight to zero, and from the discussion mentioned above, I understand that is the capacitors "unloading." So now I am wondering if I blew the new in-rush thermistor last year by restarting the converter under full load of 100amps too soon?

The in-rush thermistor seems to be on the 120v input side while the capacitors that unload are on the DC side some distance from the thermistor on the circuit board so I am not sure how they relate.

BTW PowerMax was kind enough to help me out replacing my ruined converter and I now have a fancy new prototype ( I guess) version with an external voltage pot this time. I promised not to take the top off and poke around inside it 🙂

However, the way I do my recharge, it can happen that say the Honda pops its breaker due to other loads added besides the converter, and I restart the Honda before the converter capacitors unload. Even with a normal start, the converter sometimes makes a snap sound like my big inverter will when disconnecting and connecting, which is capacitors according to folks here who have commented on that. (except how can the converter ones snap on start-up if they are already unloaded?)

I have been using this new 100amper for a while now and nothing has gone wrong. But I am scared to death of screwing up my nice converter by some sort of mystery operator error if that is what it was. I can't avoid "doing it again" when I don't know what I did!

Thanks

[BFL13]

BFL13 wrote:
I already had put a new 5R020 in the newer unit where its big 5R025 was, so I will leave that there and see how that goes instead of hauling that out before even trying it. Meanwhile the older unit I was using last time camping with the now a broken (but still working!) 5R020 in it from that episode, will get a new 2R025 to try. Edit-- it is installed, unit works.

I couldn't decide what to do, but seeing that 1R030 in the PD9280 has tipped the scales on which way to go for me. I will have the two 100ampers, one with the 5R020 and the other with a 2R025

Unfortunately they aren't quite the same front end design, so don't know what that might mean, but we can't keep dithering, so that's the Grand Plan. The newer unit has one less emi inductor and some other input differences, so its thermistor might have to do more of the work. So I might have lucked out having the 5R in that one, no idea.

I have not opened up the two 100ampers since then till now, Oct 2014.

The older unit with the 2R025 has been used many times and its thermistor looks fine.

The newer unit with the 5R020 has not been used as often and worked ok last time. I don't think I did anything weird with it like an accidental hot restart. Its thermistor has a big crack in it but no pieces have fallen off (yet!) and no obvious burn spots.

That unit came with a 5R025 but that thermistor failed with hot restarts and not clamping on first as reported way back in this thread from last winter. I have spare 5R025s so I will use one of those when/if this cracked one fails completely. See how that goes when the time comes 🙂

Hard to know what is the critical factor here. The 020 failed and the 025 is still good, but the good one is also 2R not 5R and the old 5R025 did fail, although with hot restarts. The PD 80 amper has that 1R030

No need to get this long thread going again and spinning wheels, but thought an update was due to those who got interested in all this last winter.

[BFL13]

I agree not all the mysteries have been solved. Not my job to solve them! 🙂 Now is the time to leave well enough alone and hope for the best.

[Salvo]

I doubt the hot-switch is the problem. You can easily put that issue to bed by doing 20 cycles. You got enough thermistors. Run the test with the lower resistance thermistor. No guarantees with the 5 ohm device.

Sal

[wa8yxm]

BFL13 wrote:

I am wondering if it was operator error in starting/ stopping sequence that did for both thermistors.

From time to time I mention in posts that I'm much amused (And occasionally scared) by some of the advice given in these forums.

YES, the thermistor is designed to pass different amounts of current depending on how long it's been doing it (It heats up when current flows and since it is a Negative Coefficient device the resistance thus goes down.) If you "Hot switch" (turn off and quickly back on) too much current can flow, this can both cause the thermistor to overheat (it never gets to zero) and damage down-line stuff as well.

The 3 minute rule (used for air conditioners) is a good one here too, though it may be a bit longer than needed.

As I stated in another thread.. I have designed and built power supplies (These are basically what we call single stage converters) for use with some of my radio gear in the past. So I've done the hot switch thing, Though since I over build all it cost me was a fuse or two.

[mena661]

Mine will be permanently hooked up once I get one but I'll have to remember the no hot restarts too. I'm currently setting aside cash for a solar controller. 🙂 Thanks.

[westend]

Good deal, thanks for posting the outcome.

It is possible that the original thermistors were substandard or faulty in some way. Anyway, no hot restarts is probably going to work well for you. Best of luck.

[BFL13]

Success (so far)

Camping again this week similar to last November's effort, so a chance to try out the two 100 ampers with their new thermistors.

Procedure is clamp on first, no hot restarts. Three recharges in three days with batts low enough to accept 100amps for a time. One charger starting and run briefly, then shut down and the other charger started to finish the recharge. Taking turns each day which one starts first.

Both ran perfectly, no smoke. After each did three runs took their lids off to inspect their thermistors. Like new, no cracks or pieces fallen off like before.

Conclusion has to be that within reason for specs, it doesn't matter so much which thermistor is used. (Different ones in each charger remember, and none the same as in the latest production 100s---as covered in the thread earlier.)

The problem must have been the hot restarts and also by not clamping on first, which acts like a hot restart in a way. Only slight worry left is that there is no way to know if any of the starts in this series was at the point in the cycle where you get the max in-rush, so it may be they have not been really tested hard yet. Ignoring that 🙂 I am declaring the problem solved. Thanks again everyone for the good advice and education on this topic.

[BFL13]

I already had put a new 5R020 in the newer unit where its big 5R025 was, so I will leave that there and see how that goes instead of hauling that out before even trying it. Meanwhile the older unit I was using last time camping with the now a broken (but still working!) 5R020 in it from that episode, will get a new 2R025 to try. Edit-- it is installed, unit works.

I couldn't decide what to do, but seeing that 1R030 in the PD9280 has tipped the scales on which way to go for me. I will have the two 100ampers, one with the 5R020 and the other with a 2R025

Unfortunately they aren't quite the same front end design, so don't know what that might mean, but we can't keep dithering, so that's the Grand Plan. The newer unit has one less emi inductor and some other input differences, so its thermistor might have to do more of the work. So I might have lucked out having the 5R in that one, no idea.

[Salvo]

As I said many times before, turn-on is not the problem. The Joules rating is only for turn-on.

Sal

BFL13 wrote:

DryCamper11 wrote:

BFL13 wrote:
What size is that thermistor on the 80amper?

SL321R030
1 ohm
30 A
160/270 Joules recommended/instantaneous actual failure
11, 114 uF at 120 VAC

(For the PD9280) That's fewer Joules than the 5R020 and much fewer than the 2R025.
I wonder what the point of using 30a is? Of course it is not the same design as my PowerMax 100amper, but even so I feel better about maybe using the 2R025s now without putting two in series to get 4R.

All very mysterious!

[BFL13]

DryCamper11 wrote:

BFL13 wrote:
What size is that thermistor on the 80amper?

SL321R030
1 ohm
30 A
160/270 Joules recommended/instantaneous actual failure
11, 114 uF at 120 VAC

(For the PD9280) That's fewer Joules than the 5R020 and much fewer than the 2R025.
I wonder what the point of using 30a is? Of course it is not the same design as my PowerMax 100amper, but even so I feel better about maybe using the 2R025s now without putting two in series to get 4R.

All very mysterious!

[BFL13]

IMO almost anything will not fail with only cold starts as long as steady state can do over 13a. We'll see.

I do have some 2R025s and some 5R020s on hand to keep things going, now I am an expert at replacing thermistors with my handy-dandy 3/64 drill bit and cheapo soldering gun.

The trouble is, you can't prove a negative, so if nothing breaks from now on it proves nothing whether a different thermistor would be better or worse. Also I am not clear on this, but it could be a******shoot just where you happen to be on the voltage sine wave cycle when you turn the unit on what stress the thermistor gets.

If we camp four days in a row each month needing the 100amper each day, till solar charging re-starts in April, beginning end Jan/early Feb, that would be say 12 recharges with 100ampers and then maybe a few in the summer, depending on cloudiness.

So it could be a year before anything goes wrong, or two years, or never, or the very first time out next February. IAW don't hold your breath waiting to see how this all comes out. 🙂

Don't forget I just did four days last month and the 100amper worked every day even with its broken 5R020 from the first hot restart event the first day. If I don't take the lid off and peek, I could go for a long time with a broken thermistor and never know it.

(Perhaps the low failure rate on these is due to that too, who can say? Could be lots of broken ones out there that still work so nobody knows they are broken, or maybe none of them is broken just mine 🙂 There is no way to know. )

EDIT: I don't know why rv.net thinks that game of dice is a swear word, but it did a re-work on that all by itself in this post, so don't think I did it.

[Salvo]

It's pretty obvious the 2.5ohm, 9A device won't work well at 13A. I can see why that failed. If you have a 2 ohm, 20 or 25A device, then try it. Worst case, it will fail like the others. It won't subject the converter to any high stress. And best case, problem solved and it works perfectly!

Sal

BFL13 wrote:
The thermistor is sure protecting the diode bridge--it acts as a fuse!

[BFL13]

The thermistor is sure protecting the diode bridge--it acts as a fuse!

Nobody is protecting the thermistor 😞

so far I have gone through:
-2.5R 9a
-5R 20a
-5R 25a

PowerMax will no doubt find a better way, but so far the complaints just aren't coming in ( except from that crazy guy -that would be me) Even so they are going to see what they can figure out.

The diode bridge is a "25a" size (in my 100amper) and the PD patent one was a "20a" (not sure what amps size converter that was) I have no clue what is right over the dispute which diode bridge specs matter for calculating thermistor Minimum R. I will let PowerMax worry about that.

I am happy with my way forward ( with a lot of good help in this thread )

[Salvo]

Those are interesting questions. One problem is that we don't know the resistance of the thermistor once the converter starts operation.

My 300W inverter doesn't like to get powered up when ramping from 0V dc. I guess I could connect the converter to the battery to check loaded turn-on. That entails a lot more work than the un-loaded bench testing I've done.

Sal