Two general questions on Transistors and Capacitors

Thread Starter

ranatungawk

Joined Oct 30, 2008
188
1. Why do transistors with same part number numbers (maybe from different manufactures) show different hFE values ? at that case , which one is the best , Low hFE or High hFE ?

2. When replacing a capacitor, as I know somewhat higher voltage (same capacitance) ones are preferred. but what is the max V level could go for ???

ex: can i replace 10uF/6.3V cap with 10uF/250V cap ..... without any problem ?????? (capacitor Size is not a matter here )
 

dl324

Joined Mar 30, 2015
14,700
Why do transistors with same part number numbers (maybe from different manufactures) show different hFE values ? at that case , which one is the best , Low hFE or High hFE ?
Even transistors from the same manufacturer and lot can have different beta values. They give a minimum and typical value and most will be closer to the typical value than the minimum.

To avoid issues with beta variation, designers will design a circuit so that only minimum value guaranteed by the manufacturer is required for the circuit to function.
When replacing a capacitor, as I know somewhat higher voltage (same capacitance) ones are preferred. but what is the max V level could go for ???
When replacing a component, unless you know something the original designer didn't, you should use whatever specifications they used. Higher voltage ratings will also have a higher price, so at some point, you'll look silly if you specify parts that greatly exceed the parameters the circuit needs.

Electrolytic capacitors are formed at a voltage greater than their maximum rated voltage. In theory, you could operate them at the stated maximum, but most designers will de-rate for improved reliability and fewer calculations.
 

LowQCab

Joined Nov 6, 2012
2,171
On high-volume "Jelly-Bean-Parts",
quite often the manufacturer will make a billion of them,
then after doing some random lot-checks / measurements ......
they will decide what generic-part-number to put on them,
that is "close enough to a particular standard part-number",
so check ALL of the specifications and find where any of the trade-offs may be,
you won't get a bonus without trading it for some other rating / value.
Certain other specifications may be more important to your particular application.

Don't use a Motor-Start-Capacitor instead of a proper Electrolytic.
Capacitors have other very important specifications besides Voltage and Capacitance,
"Equivalent-Series-Resistance" (ESR),
and "Ripple-Current" at a given Frequency,
are just 2 other Specs that will "make or break" your project.
.
.
.
 

Papabravo

Joined Feb 24, 2006
18,806
Thanks for the replies, could please answer the underlined parts of my two questions as well?
There is no best value for hFE. You don't ever want to design a circuit that depends on a particular value of hFE or even a particularly narrow range of hFE values. The way specifications are written you should consider them like normally distributed random variables with a mean and a standard deviation. For example, let's say a typical hFE value is 150, and the standard deviation is 25. This means that in a sample lot you would expect 99.5% of the sample lot to have an hFE in the range 150 ± 75. They would spec the minimum as 75 and the maximum as 225. You can and should be able to successfully design any circuit knowing that range.

With capacitors it might be presumptuous to assume that a change in working voltage does not change anything else. It might hurt if you are working on a Switch Mode Power Supply where the ESR of a capacitor is critically important to the performance. I'm not really an expert on capacitors and electronics repair. I do know to be suspicious about what else might have changed when anything important changes. Your mileage may vary.
 

MrChips

Joined Oct 2, 2009
26,504
1. Why do transistors with same part number numbers (maybe from different manufactures) show different hFE values ? at that case , which one is the best , Low hFE or High hFE ?

It is standard engineering practice to derate manufacturer's specifications.

Transistor current gain is not constant. Transistor circuits should be designed to work every time independent of actual hFE.
Assume the hFE is 50% of what is spec'd.

2. When replacing a capacitor, as I know somewhat higher voltage (same capacitance) ones are preferred. but what is the max V level could go for ???

ex: can i replace 10uF/6.3V cap with 10uF/250V cap ..... without any problem ?????? (capacitor Size is not a matter here )
The voltage rating of the capacitor should be determined by the voltage in the circuit application.
Again, always derate the voltage rating of the capacitor by 25%-50%.
For example, if the voltage in the application is 300V, use a capacitor rated for 450V.
For a 5V circuit, use capacitors rated for 10-16V. Nothing gained by going higher. The frequency response of a 250V capacitor will not be the same as that of a 16V capacitor. The ESR (effective series resistance) will be higher in the higher voltage capacitor.
 

Ya’akov

Joined Jan 27, 2019
6,033
For capacitors, practical concerns with considerably higher V(max) would be size and cost. As the V(max) increases the capacitors become larger and more expensive.

This is in addition to the concerns when using them in high frequency circuits. In theory and mostly in practice, if you carefully specify the other critical parameters (e.g.: ESR and ESL) and meet them, it should make no difference to their function in the circuit.
 

MrChips

Joined Oct 2, 2009
26,504
I also failed to mention that the choice of capacitor voltage will depend on other factors besides circuit operating voltage:

  1. value of capacitance, e.g. below 1μF vs greater than 1μF
  2. polarized or non-polarized capacitor
  3. material construction of capacitor
  4. application function of the capacitor
 

MisterBill2

Joined Jan 23, 2018
12,424
1. Why do transistors with same part number numbers (maybe from different manufactures) show different hFE values ? at that case , which one is the best , Low hFE or High hFE ?

2. When replacing a capacitor, as I know somewhat higher voltage (same capacitance) ones are preferred. but what is the max V level could go for ???

ex: can i replace 10uF/6.3V cap with 10uF/250V cap ..... without any problem ?????? (capacitor Size is not a matter here )
THere will certainly be problems when replacing a 6.3 volt cap with a 250 volt cap, because of the physical size. And usually size matters. End of advice.
 

MisterBill2

Joined Jan 23, 2018
12,424
The "best " transistor for an application depends on the application. Aside from hfe and HFE, there is power handling capability, noise factor, and frequency response. And many of the characteristics vary in production. So there is a range published in the specifications data, and most of the devices with that number will fall inside that range, or close to it. There is variability.
Usually, the smaller the range the higher the price, because of the additional testing and sorting required.
 

tindel

Joined Sep 16, 2012
889
Higher

All other parameters being the same ( same application ), The max V you could go in for is what is that available.
Wrong on both accounts. In both instances "best" depends on the application. In power supplies a larger voltage cap of the same type also comes with a different equivalent series resistance and parasitic inductance, which may make the power supply unstable. Similarly, in some circuits high beta is better, and in others low beta is better. Depends on what the transistor is doing.

@dl324 had it right in post #2, try to replace parts with similar specs to the originals. If you don't know the original specs, replace as closely as you can with fit, form, and function.
 

MisterBill2

Joined Jan 23, 2018
12,424
Capacitor types matter, some are much more stable with time and temperature, some have a higher "Q" factor, some have less dielectric losses, There can be enough difference so that some circuits will not function correctly with the wrong kind of capacitor.
 

Ramussons

Joined May 3, 2013
1,274
Wrong on both accounts. In both instances "best" depends on the application. In power supplies a larger voltage cap of the same type also comes with a different equivalent series resistance and parasitic inductance, which may make the power supply unstable. Similarly, in some circuits high beta is better, and in others low beta is better. Depends on what the transistor is doing.

@dl324 had it right in post #2, try to replace parts with similar specs to the originals. If you don't know the original specs, replace as closely as you can with fit, form, and function.
The questions are specific. hFe and Capacitor Voltage rating.
Your opinion is different from mine. :)
 

MisterBill2

Joined Jan 23, 2018
12,424
There is more to a capacitor than just value and voltage rating. Those are critical, though. ten percent over is totally OK, though.
For transistors, aside from HFE there are voltage and current ratings that matter a whole lot.
 

Bordodynov

Joined May 20, 2015
2,995
I'll throw in my five cents. Not always more static gain is better. This is especially true for high-frequency circuits. All transistors have a technological variation in the physical size of the structure. Take for example the base thickness variation. If you increase it, the gain decreases, but the base resistance of the transistor decreases. For low impedance signal sources the base resistance has a great effect on noise. The base resistance also affects the time constant of the collector circuit RbCc. So it may be that a transistor with less gain will perform better in amplifiers and oscillators.
 

MrAl

Joined Jun 17, 2014
9,169
1. Why do transistors with same part number numbers (maybe from different manufactures) show different hFE values ? at that case , which one is the best , Low hFE or High hFE ?

2. When replacing a capacitor, as I know somewhat higher voltage (same capacitance) ones are preferred. but what is the max V level could go for ???

ex: can i replace 10uF/6.3V cap with 10uF/250V cap ..... without any problem ?????? (capacitor Size is not a matter here )
When replacing capacitors you should have some idea what the capacitor is being used for and what kind it is.
For example, in a regular circuit with a ceramic cap you can often get away with just another ceramic cap of the same value around the same voltage spec.
For an electrolytic cap though it can be a little more involved. If it is on the output of a switching converter you should get a cap with around the same voltage spec as the old cap as well as the same capacitance value, and try to match ESR values too if you can. Those kind of caps often have low ESR ratings so they can smooth the output properly.

There are some applications that require a certain range for the ESR also because with the wrong value they can oscillate even if the capacitance and voltage rating are the same. These circuits are a little more rare but they do come up from time to time. An example would be a linear voltage regulator with a PNP output stage. These can sometimes be a pain because of the ESR too high or too low.

There are many applications that have to have the same type of capacitor and wont work well with a different kind. An example is a sample and hold circuit with a polystyrene capacitor. This type of capacitor has low leakage and high stability and also has low 'bounce' and 'droop" relative to voltage so it holds it's voltage for a decent amount of time when disconnected from a circuit. If you replaced this type with a regular ceramic, the circuit would probably not work right at all as it would provide output levels that are not as correct as with the right capacitor type (and value of course). In this kind of application you cant replace it with a cheaper cap with higher ratings either because of other circuit constraints.
 
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