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Manufacturer using part over maximum ratings, is it safe?
Manufacturer using part over maximum ratings, is it safe?
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Old 2nd August 2019, 09:26 PM   #21
MarcelvdG is offline MarcelvdG  Netherlands
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Mistakes can definitely be made, also by big companies; Philips also used to be a big manufacturer of consumer equipment, and they used 300 mW-rated transistors that dissipate 450 mW in the amplifier section of their FR675 receiver:

Philips FR675 repair: replace 2SC2240 with ZTX458
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Old 2nd August 2019, 10:21 PM   #22
nigelwright7557 is offline nigelwright7557  United Kingdom
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The old 1980's Maplin 225WRMS disco power amp was used well outside the 2n3055's spec. I think the 2n3055 was max 70v for CE but they used it on +/-55 volts rails. So 110V across CE when output was on full swing.
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Old 3rd August 2019, 11:10 AM   #23
davidsrsb is offline davidsrsb  Malaysia
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Quote:
Originally Posted by Mooly View Post
These CMOS type switches, and it could even be the same part number, were used extensively in mid price music systems from the likes of Aiwa and many others in the late 80's and 90's and they were a fairly regular failure item. Fwiw, these systems used nothing like -/+20, more like -/+9 from memory.
My experience of amplifiers with cmos input switches is that even when the supplies are below limits, that they tend to go into latch up when connected to source with the power on. This is a result of RCA plugs mating centre contact first while floating at half mains supply in Class II equipment. I have learnt my lesson a few times, but still sometimes forget to switch off first.
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Old 3rd August 2019, 12:04 PM   #24
Mooly is offline Mooly  United Kingdom
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Manufacturer using part over maximum ratings, is it safe?
Interesting. I think the situation of 'centre contact first' is responsible for a lot of problems, particularly when we see dead mute transistors in CD players and the like.
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Old 4th August 2019, 05:24 AM   #25
MarcelvdG is offline MarcelvdG  Netherlands
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That could be a reason to use a supply with a very tight current limit and not too much decoupling, for example resistor - Zener diode - 10 nF. When the short-circuit current is less than the holding current of the parasitic thyristor and the energy in the capacitor is small enough, it should recover from latch-up without damage.
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Old 4th August 2019, 06:28 AM   #26
tomchr is offline tomchr  Canada
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Originally Posted by simon7000 View Post
A manufacturer can get parts tested for a particular specification. That looks like what was done here.
How would you design a test to screen out the parts that can survive the higher supply voltage? Run all parts through at the required supply voltage (plus some margin) and sell the ones that make it hoping that the test won't have affected the reliability? That seems like a rather sketchy proposition.

Parameters such as noise, operating frequency, supply current, etc. that can be measured in a non-destructive way can be screened for. That say, screening is typically rather expensive. If there is sufficient market for a higher performance part, the manufacturers will much rather design a higher performance part - and can be willing to do so for a key customer willing to buy millions of them.

Absolute maximum voltage ratings tend to be dictated by the semiconductor process. The products are guaranteed to last for at least 10 years of continuous operation at the worst case operating conditions. As we can't delay product launch by ten years so see if the parts fail within that timeframe, we put the parts through accelerated lifetime tests. These test take place in autoclaves where the parts are operated under worst case conditions at 100% humidity and elevated temperature (150 ļC junction temperature). This test typically lasts 1-4 weeks. While the test does not guarantee that the parts will last 10 years in the customer's hands, it does at least give enough confidence that such a lifetime can be expected.

Tom
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Old 4th August 2019, 06:45 AM   #27
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by simon7000 View Post
No anecdote having real experience in the field both with the designers and service folks. It is a very common practice as noted by others here.

Do you really think running an 18 volt nominal part at 23 volts would get past a design review? Even make it past the warranty period?

Yes there are actual products out there where parts are used in an off data sheet parameter method. Not surprisingly high warranty failure rates and quickly redesigned. Almost an unknown issue from major manufacturers.
I've come across many instances of "designed-in" obsolesence in the shop. Un-heatsinked regulators, under-wattage resistors, you name it, I've seen it.

And this practice goes back decades....into the "tube era' as well. Milking a extra few watts from an amp to make advertising more impressive. And all with the hopes that once the thing breaks down, the owner might buy a "new one" to keep the cash flow going for the manufacturers and sellers.
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Old 4th August 2019, 08:45 AM   #28
simon7000 is offline simon7000  United States
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Tom

It was standard practice for Crown amplifiers to use Motorola/On power transistors screened to run at higher voltage. These parts did have house numbers but started life as standard parts. I thought they screened them by testing at higher voltage with a current limited supply.

As Crown effectively no longer exists for repair use the stock parts are often just swapped in and run for a burn in to see if they will work. Not reasonable for manufacturing, but the seems to be best way to repair things when manufacturer parts are no longer available.

As the breakdown mechanism probably does not change with age, higher voltage current limited tested may just be adequate.
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Old 6th August 2019, 04:25 PM   #29
Jsixis is offline Jsixis  United States
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Originally Posted by simon7000 View Post
No anecdote having real experience in the field both with the designers and service folks. It is a very common practice as noted by others here.

Do you really think running an 18 volt nominal part at 23 volts would get past a design review? Even make it past the warranty period?

Yes there are actual products out there where parts are used in an off data sheet parameter method. Not surprisingly high warranty failure rates and quickly redesigned. Almost an unknown issue from major manufacturers.
oh most definitely, why do you see so many items of everything die a year after the warranty expires. They design things to eventually fail because they want you buying more.
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Old 6th August 2019, 04:38 PM   #30
wiseoldtech is offline wiseoldtech  United States
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Indeed, read my posting #27
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