What's this surge protection component?

Dave,

The downside is unnecessary stress on the components - diodes, xformer, caps. You could jumper it and leave it that way and run it for the next 50 years with no problems - or you could have premature component failure and be back inside the amp in the near future replacing something else. A second reason: generally speaking, if an equipment manufacturer decided to spend more money to add a part, it is probably a good idea for us to spend it when we repair the thing.

On Fri, 14 Nov 2003 6:05:41 -0800, DaveC Gave us:

It is called a thermistor in american circles, if that's what you meant.

They vary in size and functional parameters, so it would be hit and miss without knowing what was removed or the circuit particulars.

Mouser, and digi-key, among others, have pages where thermistors re offered.

OK, so the original NTC thermistor is toast. The reason, probably: I upgraded the capacitors in this amp when the originals went bad. Originals: four

19,000 uF. New ones: four 54,000 uF ("hey, they fit!"). I presume that the inrush current increased in proportion to this additional turn-on load.

So how do I determine what the new thermistor should be? Can I determine the specs of the old one (p/n is still partially visible, although the company is defunct) and just triple that value (based on tripling the capcitance turn-on load)?

I've put another NTC in there, which is also too small in value, and it's sparking (quite an effect at turn on to see sparks spit from this thing). But it continues to work, sparks and all. I measured the inrush current with this one in the circuit and it is right about 48 amps. But since this is measured with an unknown value of thermistor in the circuit, I'm not sure if the measurement is at all useful. The slow-blow 8A fuse has held up through all this, although I'm not sure that's a good thing...

Is my only choice to install a jumper in place of the the thermistor and measure the inrush? I fear doing some damage to the rectifiers or something else in the circuit.

Suggestions?

Thanks,

I read in sci.electronics.design that DaveC wrote (in ) about 'OK, so how do I choose a replacement thermistor?', on Sun, 16 Nov 2003:

No. The peak current is limited by the resistance in the circuit. This may have gone down a bit if the new caps have lower ESR (Effective Series Resistance) than the old ones, but probably not by much. However, the *duration* of the current pulse will have increased. What matters is the RMS value of the current pulse, and it's not possible to determine how much it has gone up. It is most unlikely to be three times what it was.

No, whatever you mean by 'triple that value'.

How did you measure it? It's not by any means an easy thing to measure. But if you can measure it meaningfully, it will enable you to pick a replacement NTC thermistor. However, even translating the measured current into an NTC sec is not simple.

No, don't do that. You might explode the new caps, which you wouldn't enjoy at all.

On Sun, 16 Nov 2003 23:53:48 -0800, John Woodgate wrote (in message ):

Thanks, John.

New caps are approx triple value of old (54K vs 19K uF). I was going on the presumption that this triples the inrush current, but you say not so...

Fluke model 43 Power Quality Monitor. It has inrush current setting which measures current flow (using current clamp) over time. I get a nice waveform that peaks at turn-on and degrades over milliseconds' (?) time. Max is shown as 48 A.

And so here I am, with this measured inrush current value (with an unknown thermistor installed) so what to do with this knowledge? I agree, from what I see in the application notes of some NTC manufacturers, selection is indeed not a simple "look-up table" task.

Suggestions?

Thanks,

I read in sci.electronics.design that DaveC wrote (in ) about 'OK, so how do I choose a replacement thermistor?', on Mon, 17 Nov 2003:

Ask the NTC manufacturers if they have any design guides? I've only studied this purely theoretically, for IEC/EN 61000-3-3, and haven't had to do a specific design job, especially without full data on the total resiance in the circuit. I don't even know what (if any) transformer you have there.

The maximum permissible surge current of the rectifier diodes and the maximum permissible surge current of the capacitors may give you a lead as to what maximum surge current you can allow. 24 A on a 230 V supply is not untypical of a high-power unit (and meets the standard), so I think your 48 A on 120 V supply is in the right ball-park. This current is equal to the 120 V divided by the total resistance in the charging circuit. I don't have enough data to even estimate that.

If you have a "Variac" (variable autotransformer) Try creeping up on the current while measuring it with an ammeter.

If the NTC thermistor is to limit inrush current you might just need one with higher dissipation. Thermistors are rated for power dissipation as well as Delta R.

You don't say what the resistance of the original thermistor is. From what I understand. Thermistors are rated in delta R for delta T, and dissipation in watts.

You power up the amp and the thermistor has a relatively high resistance (10 ohms?), the power dissipated in the thermister(it heats up) causes it to drop in resistance (.1 ohm?) thereby letting full power (minus whatever normal HOT state of the thermistor is), into the power supply.

If your thermistor can't handle the power it opens (smoke and sparks)

You can parallel thermistors - to some extent, but one with a high enough dissipation will work better.

I have a stereo amp that has two switches - one or the other - but not both. Switch on one amp, wait five seconds switch on the other. Toroid power transformers, and .5 farad capacitors in the power supply. Switch on both at once and the computer dies, the microwave dies, the TV dies . . .

When you are flying by the seat of your pants (don't know the max permissible inrush) you can try to get around that by using 4 NTC thermistors. Pick thermistors that can handle at least 125% of the steady state current. Wire them in series/parallel (put 2 in series, put the other 2 in series, and then put the two series strings in parallel).

That's better than a jumper, and better than a single thermistor that sparks. No way to tell if it limits the inrush to the maximum permissible, since we don't have that spec, but it's a step you can take while you're doing more research.

NTC devices will not work in parallel. They are effective in series, however.

Thermistors have an energy surge limit. This is related to their material quality, rather than their body size - however for a constant material quality, larger parts with a larger continuous power rating (and larger self-regulated constant power loss) will have higher surge ratings.

By more than doubling the capacitors in an otherwise unchanged circuit he has increased the input surge power level by the same factor, if R is constant. If R was reduced, the surge energy will have been increased further as it is related to the square of the peak current.

A larger capacitance will also increase the RMS input current, if no other changes are made, due to the increased peak-to-average ratio of the rectified input current. NTC inrush limiters have a maximum safe continuous current rating.

Check out typical ratings for more common parts before determining the part suitable to the new stress levels.

The original poster did not indicate the location of the original thermistor, however as one alone is used, it is likely in series with the line. Considering the size of the capacitors mentioned, these are likely on the secondary-side, after a rectifier that is 'likely' more complicated than a single full-wave circuit. He will have to reflect the equivalent capacitance back to the primary, in order to evaluate the equivalent capacitance seen by the limiting circuitry.

RL

Can you go into a little more detail on this? "Will not work" could mean many things. For example: Say you add an identical NTC in parallel with an existing one. Do you degrade the function, improve the function, or have no effect on the function of the NTC?

I ask because I don't know for sure. But the way I would describe "will not work" in this context is as follows: It seems to me that if you parallel 2 equal limiters you'll increase the inrush current over what it would be with just one, by halving the limiting R. That would hurt, rather than help the protected circuit during inrush. However, the NTC's would have similar delta R as they heated, so they would very likely both reach their low R points. Thus there would be an improvement in steady state current. It seems to me that the improvement in steady current handling is not a factor of interest - you should choose an NTC that can handle the steady state current with the proper hot R.

To that end, I recommend that, if you want to parallel

2 NTC thermistors, use 4 in series/parallel. The series part doubles the inrush limiting, and the parallel part restores the steady state hot R. Obviously, the better and proper approach is sizing the NTC to the max inrush and steady state specs of the device, but when you're operating in the dark, the series/parallel approach is better than strictly parallel, which will do more harm than good.

They are effective in series,

On Mon, 17 Nov 2003 17:06:47 -0800, snipped-for-privacy@bellatlantic.net wrote (in message ):

E, Your logic sounds... well, sound, to me. Let's see how L. Legg responds.

I read in sci.electronics.design that snipped-for-privacy@bellatlantic.net wrote (in ) about 'OK, so how do I choose a replacement thermistor?', on Tue, 18 Nov 2003:

The one with the lower initial resistance, or the one with the steeper fall with temperature, will hog nearly all the current.

On Mon, 17 Nov 2003 9:17:01 -0800, John Woodgate wrote (in message ):

Thermometrics' applications engineer, on the phone, was quite taken aback by my requirements. Their largest NTC thermistor can handle a maximum capacitance load of 6000 uF (while this is not an absolute limit, "it is an experimentally determined value beyond which there may be some reduction in the life of the inrush current limiter" -- maybe that's why my thermistors have been sparking and fragmenting...). He was surprised that with the original total capacitance (

On Tue, 18 Nov 2003 9:10:43 -0800, John Fields wrote (in message ):

Hmm... so it seems. But why do many techs use it as a troubleshooting tool to limit max current if the initial surge will fry/blow the fuse/component?

Sure!

Thanks,

I read in sci.electronics.design that DaveC wrote (in ) about 'OK, so how do I choose a replacement thermistor?', on Tue, 18 Nov 2003:

I recommend that solution. It's much more flexible in design. The appropriate lamp may not exist.

I read in sci.electronics.design that John Fields wrote (in ) about 'OK, so how do I choose a replacement thermistor?', on Tue, 18 Nov 2003:

That was my initial reaction, but it's not true. The cold resistance of the lamp needs to be high enough that initially almost all the available voltage appears across it, so it heats up very rapidly and its increasing resistance limits the inrush. When the caps are charged, the lamp goes cool and its resistance falls.

A lamp that will do that for a given design may not actually exist.