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

[Salvo]

ktmrfs wrote:

Salvo wrote:
Looking at 3 different converter manufacturers surge current limit resistance:

Parallax: 2 ohm
Progressive Dynamics: 1 ohm
Iota: No thermistor, ac source resistance and converter input resistance is adequate.

None of these are failing! Their surge current limit is perfect in protecting all parts.

Tell me again why do you want 5 ohm?

Sal

without schematics it's hard to determine how input current is limited in each design.

What a cop-out. Look at the PD patent schematic. All the converters have similar design.

The PD and Iota are not power factor corrected, the boondocker is.

Power factor correction is made after the capacitor bank. It has no effect on turn-on surge.

The PD and Iota have lower max output current.

So what? The 55A Iota doesn't need a thermistor. But you're telling us this 100A converter needs 5 ohm?

Instead of wimping out and saying "I'm conservative and I think we need 5 ohm", why not provide some real analysis showing the need for 5 ohm. So far we've seen nada. And don't use the excuse there's no schematic. Use the PD schematic. Let's see what you're capable of.

Sal

[BFL13]

ken white wrote:

BFL13 wrote:
How about this sort of looking one, under the big heat sink overhang on the right by the fan, marked, "D25XB 60 B 7508"

https://www.google.com/search?q=diode+bridge&rlz=1C1CHFX_enUS511US511&espv=210&es_sm=122&tbm=isch&tb...

That is a pretty durable device, your other part.

For a 25 amp part, it looks like it can handle about 50 amps for 100 cycles and I would probably stick with 50 amps as a conservative number.

Have fun...

Getting back to this, where did that 50a come from anyway? (I see the 60 number for part is not a rating just an identity)

If I want to use my 2R thermistor, then if I get my amps to go with the 170 from where that 50 came from, then I would need to use 85a.
Why can't I use 85 instead of 50? If I did, would I get 60 cycles instead of 100 before it failed? Cycles of what anyway?

The 170v comes from peak volts RMS on 120v line voltage, ? , so that means every time you do a cold start, the voltage could be anything from peak to zip which means in rush may be small or great each time.

So whatever thermistor I pick, I could do several starts and it would work every time but that proves nothing, because it may be that I never caught it at peak inrush, but the next time I could and it could blow? So just because it works great after a repair, that proves nothing?

Meanwhile we do see my thermistors blowing but no harm to the diode bridge, or at least not so you would notice. Which brings us back to the spec about handling stress for 100 cycles. A cycle of what? Does that mean you get 100 starts and then anytime after that your diode bridge might fail? Converters are supposed to last years and years so that can't be right.

(I have not forgotten the comments that the production 100amper design for the straight converter version may not work quite right for the same thing used with variable voltage where perhaps higher inrushes might occur. They might need two different designs, no idea. Might hear something about that from them or not. )

[ktmrfs]

Salvo wrote:
Looking at 3 different converter manufacturers surge current limit resistance:

Parallax: 2 ohm
Progressive Dynamics: 1 ohm
Iota: No thermistor, ac source resistance and converter input resistance is adequate.

None of these are failing! Their surge current limit is perfect in protecting all parts.

Tell me again why do you want 5 ohm?

Sal

without schematics it's hard to determine how input current is limited in each design.

The PD and Iota are not power factor corrected, the boondocker is. The PD and Iota have lower max output current.

And, there are multiple ways to limit inrush current in a design, an NTC thermistor is just one very common method, but not the only way by any means.

Kinda like saying since my mercedes needs only a small battery, no reason to use two big ones in my duramax, just put one small one in and it should work fine.

[ktmrfs]

BFL:

I believe (course your future use may prove me wrong, but I'll say it anyway) that if you use the part proposed by powermax and "change" your use scenerio to the following that your failures will go away.

a) Have the converter off for 3 minutes minimum
b) connect the battery(s) to the converter
c) power on the converter

and your failure will go away.

With a battery connected at power on, the input caps charge, the output circuit starts a ramp up towards 12V. At some point in time the output voltage is above the battery voltage and it starts to supply charging current, but it won't supply 100A instantly, it will ramp current up as the control loop keeps the output voltage rising. How long? don't know. Depends on how it was designed, may take fractions of a second, may take a few seconds till it can supply the full 100A. But the control circuit determines that. And thats likely how it was tested.

As a result input current stays in check and rises predictably till it supplies the needed current to sustain output.


In your previous case as I understand it,
The discharged battery connected to the converter that had no load on it and a voltage in the 15 range with a battery voltage in the low 12V range that looks almost like short circuit. This likely caused a very rapid discharge to the input caps, the ouput voltage drops way down, feedback circuit goes into fits trying to keep the output voltage up, may even oscillate or go in and out of current limit for a while and it takes a sustained high current draw, maybe well in excess of the nominal 15A for a while (seconds?) till the output recovers.

[BFL13]

Salvo wrote:
Looking at 3 different converter manufacturers surge current limit resistance:

Parallax: 2 ohm
Progressive Dynamics: 1 ohm
Iota: No thermistor, ac source resistance and converter input resistance is adequate.

None of these are failing! Their surge current limit is perfect in protecting all parts.

Tell me again why do you want 5 ohm?

Sal

I don't care what R it is, I just want one that won't blow! 🙂

BTW the spec here for that 350a was at 50hz, not 60, and I don't know what it means for half cycle or whatever that is, where you are not talking about real cycles or whatever---arghhhh!

http://www.datasheetcatalog.com/datasheets_pdf/D/2/5/X/D25XB60.shtml

I have some 325R020s left over and some 322R025s but no 355R25s, so I keep waiting for someone to say (besides just Salvo (ha ha) ) that I can use either and it won't matter. since the 5R020s do blow at times, I would like to try the 2R025s I have, but am scared to when PowerMax has gone to the 5R.

[Salvo]

Looking at 3 different converter manufacturers surge current limit resistance:

Parallax: 2 ohm
Progressive Dynamics: 1 ohm
Iota: No thermistor, ac source resistance and converter input resistance is adequate.

None of these are failing! Their surge current limit is perfect in protecting all parts.

Tell me again why do you want 5 ohm?

Sal

[BFL13]

Salvo wrote:
Yes, if this is your typical capacitor peak charge circuit, then the diode bridge is the only part at risk. You must keep the surge current less that the diode rating. If the diode has a 350A surge rating then you need a current limiting resistor greater than:

R = 170V/350A = 0.5 ohm

Sal

BFL13 wrote:

Please remind me, on a cold start, with the capacitors unloaded, is the diode bridge at risk at all?

Ok so these are the diode bridge specs. (D25BX60) I can't follow the argument here what to use for amps and the 170v. Ken wants to use 50a , back a bit from the 60 number--but is that 60 amps??? and Salvo wants to use the big number threshold to arrive at a way lower R.

ISTR the actual thermistor requirement is to hold back a certain amount of amps to the diode bridge out of the total inrush, so the bridge stays under its limit. But we don't know the total inrush so how can we subtract the diode bridge's limit, whatever that is really, from the total, to see what the thermistor's minimum R must be to hold back that much current?

http://www.fagorelectronica.es/images/download/semiconductor/d25xb.pdf

[BFL13]

westend wrote:
BFL, the diode bridge is in series with the capacitors.

BTW, I was going to mention this since we are talking about inrush current and a possibly faulty converter. Do you have or know about a dim-bulb tester? It might be a good tool to use in a case like this where components are failing. You can build one for less than $10 and has saved my bacon a couple of times. I'll try to fond a link to a well made one.

Not a clue, sorry. I can understand there might be other components causing my problem , except it has happened with two different 100ampers. Also they work just fine with cold starts even with broken thermistors as long as not too badly broken it seems. This makes me think the other components are ok.

The 100amper with the broken MS35-5R025 thermistor that would not even start, fired right up with a new thermistor (it has an SL32-5R020 at the moment--might have to change that, we'll see) so I don't think the rest of it is damaged.

There remains the issue that if I only do cold starts, the thermistor will eventually fail from accumulated overheating from:

-running steady state with too high an R value, or
-too many inrush hits where it holds ok for a while then can't take any more

This is a really good discussion, with differing opinions, so I am now well armed to ask questions of tech support. They are working on a reply, so that might clear things up--or not! 🙂

[Salvo]

Yes, if this is your typical capacitor peak charge circuit, then the diode bridge is the only part at risk. You must keep the surge current less that the diode rating. If the diode has a 350A surge rating then you need a current limiting resistor greater than:

R = 170V/350A = 0.5 ohm

Sal

BFL13 wrote:

Please remind me, on a cold start, with the capacitors unloaded, is the diode bridge at risk at all?

[westend]

BFL, the diode bridge is in series with the capacitors.

BTW, I was going to mention this since we are talking about inrush current and a possibly faulty converter. Do you have or know about a dim-bulb tester? It might be a good tool to use in a case like this where components are failing. You can build one for less than $10 and has saved my bacon a couple of times. I'll try to find a link to a well made one.
Edit:Here's a pretty good looking one.

[BFL13]

Salvo wrote:
They are mistaken in regards to needing the thermistor to limit capacitor inrush current. These capacitors can easily take inrush current that's 100 times the steady state current (13A).

The person who wrote the patent was probably not the designer. I've had a patent issued, but I didn't write it.

Most likely the designer did the same calculation I did.

Rmin = 170V / I_diode_surge-max

Sal

BFL13 wrote:

Salvo wrote:
Interesting, the PD patent shows a surge current thermistor of 1 ohm.


Sal

I see they use a 20a, 400v diode bridge, where mine is a 25a 60. Not sure what that means for what the thermistor needs to do to protect the bridge.

However it says the thermistor is to protect the unloaded capacitors at cold start. It has three 850s instead of my two 1200s.

I missed the PD's output DC amps size etc that the specs for those parts goes with. Mine is a 100amper with 13a steady state input.

So what does that all mean wrt the risk to the diode bridge as asked about in my previous post?

[Salvo]

They are mistaken in regards to needing the thermistor to limit capacitor inrush current. These capacitors can easily take inrush current that's 100 times the steady state current (13A).

The person who wrote the patent was probably not the designer. I've had a patent issued, but I didn't write it.

Most likely the designer did the same calculation I did.

Rmin = 170V / I_diode_surge-max

Sal

BFL13 wrote:

Salvo wrote:
Interesting, the PD patent shows a surge current thermistor of 1 ohm.


Sal

I see they use a 20a, 400v diode bridge, where mine is a 25a 60. Not sure what that means for what the thermistor needs to do to protect the bridge.

However it says the thermistor is to protect the unloaded capacitors at cold start. It has three 850s instead of my two 1200s.

I missed the PD's output DC amps size etc that the specs for those parts goes with. Mine is a 100amper with 13a steady state input.

[BFL13]

While everybody is working on the last few questions 🙂

Please remind me, on a cold start, with the capacitors unloaded, is the diode bridge at risk at all?

If not, then why do we use its specs for estimating Minimum R?

If we are only protecting the unloaded 2400uF capacitor bank, then what would be the appropriate Minimum R?

ISTR the diode bridge is only at risk if the caps have not unloaded and the thermistor has not cooled down enough yet as can occur with a hot start.

[BFL13]

Salvo wrote:
Interesting, the PD patent shows a surge current thermistor of 1 ohm.


Sal

I see they use a 20a, 400v diode bridge, where mine is a 25a 60. Not sure what that means for what the thermistor needs to do to protect the bridge.

However it says the thermistor is to protect the unloaded capacitors at cold start. It has three 850s instead of my two 1200s.

I missed the PD's output DC amps size etc that the specs for those parts goes with. Mine is a 100amper with 13a steady state input.

[ktmrfs]

Salvo wrote:
The reason for looking at the smaller thermistor is because it has no, or very little ceramic material surrounding it. The thermistor that we're dealing with has a time constant of 134 s. It's slower due to the attached mass. Still, the small bead thermistor has a sizable time constant.

I could go on with this discussion, but it's really a moot point that we're arguing. The turn-on surge current is not the problem! Regardless if we're using a 2 ohm or 5 ohm device, the energy dissipated in the thermistor is only 35J. The part fails at 1200J. It isn't failing during turn-on. Your argument that the 5 ohm part is better during turn-on is not valid. That is not the problem.

Sal

the 134 second time constant is how long it takes to COOL to 50% of initial resistance with no current flow in free air. I has a very limited relationship to how fast the thermistor heats up. that is an entirely different time constant and is dependent on current and the mw/C thermal impedance. Proper choice of thermistor and it will heat up to the near 200c expected operating temperature in way less than 134seconds. Probably in seconds or less.