Note the very very (lfe savingly) important aspect of HRC = "High rupture capacity" fuses, discussed at the end.
You have done a fairly good job of summarising both the reasons and the dilemmas involved.
Fast blow are used where possible, where the fuse can be sized such that typical faults will always cause it to blow but nuisance blowing is rare. Suh situations have little or no startup surges or large occasional current excursions.
Slow blow are used where large short term transients are known to occur and if sizing of the fuse to accommodate the transients will result in inadequate protectionm against typical faults.
Where neither fast or slow blow fuses offer adequate protection (transients are very high but faults may be relatively low compared to maximum usual) then a cicuit breaker can be used, whose characteristics can be mapped accurately to a desired time/current profile.
Fast blow is the "more ideal" where possible.
Circuit current is well defined within known limits,
Start up transients are not so large compared to typical current that allowing for them is going to cause problems.
Fault currents are liable to be much much larger normal operate current and much larger than expected transients.
Slow blow is a compromise that allows protection while accommodating expected transient behaviour.
Startup or other transients may occur which cause much higher than average currents but for short periods.
Sizing a fast-blow fuse to allow the transients would result in a fuse which may not provide protection during some expected fault conditions.
The ideal may be both a fast and slow blow fuse in series (very unusual and possibly also illegal for regulatory reasons) or a circuit breaker with a well defined current versus time "envelope".
Regulatory requirements often make it clear which sort of fuse must be used.
________________________-
HRC / High Rupture capacity.
In some situations fault conditions can develop which can result in fault currents vastly in excess of the normal operating current and so high that massive destruction to property or loss of life may occur. An excellent example is a multimeter intended or measuring AC mains voltages of 230 VAC or higher. A meter measuring nominal 230 VAC mains voltages may easily be exposed to over 330 VDC peak, and transients on the waveform may cause much higher voltages to occur. A domestic range/stove/oven may be supplied with two phases with phase to phase voltages of 400 VAC or approaching 600 VDC peak to peak.
In either case above, if these voltages break down circuitry in the meter, an arc may occur followed rapidly by carbonisation of components, PCB, nearby case etc and a relatively low resistance across mains short may occur. The mains may then be supplying a high energy load vastly in excess of what is expected or designed - at least kilowatts with ease and tens of kilowatts in some cases. The onset of arc formation and generation of heat can be so rapid as to cause an explosion of th equipment with debris being ejected violently and with electric shock hazard also increasing.
Standing in the gap to this happening is "the fuse".
Edit: Actually, the fuses in multimeters are used to protect the current measuring circuitry. The voltage measurement stuff is protected by MOVS and PTCs.
If the fuse is able to blow and stay functionally blown when such a fault occurs the meter etc 'just stops working". if the fuse holder arcs and the PCB carbonises or the fuse otherwise fails to interrupt current, then the above scenario can occur. And does.
People have died due to this scenario and will die in future
An answer is the use of an HRC fuse which is designed to "rupture" in suh a way that a damaging arc does not form and the circuit is cleanly broken.
HRC fuses are usually ceramic bodied, usually white.
Not all white or ceramic fuses are HRC.
Not all HRC fuses are white or ceramic.
Image below shows fuses said by makers to be HRC. note that one is glass bodied.
( From here.)
Many HRC fuse images and links here.
Test equipment intended for AC mains use will usually specify HRC fuses. DO NOT SUBSTITUTE inferior types.
I have only ever had one meter fail under high voltage high energy conditions.
That was on a 1000 VDC range with a 1200 ior so VDC transmitting power supply being measured.
Very impressive.
A good lesson.
Long long ago.
Cheap multimeters often have their high end ACV ranges marked "not for mains use" or similar. That's why.
If you use them on mains you usually won't die.
But if you do, you won't be able to say that you weren't warned.
Remember that before you can't !!!
I must have missed this when it was asked in January.
This is a well described question and Al's answer to part of his own question was very good. He subsequently deleted it, but hopefully it will get undeleted sometime soon.
I'll address the core questions first and then come back and talk about some clever circuit aspects.
Q: So now I have one old 15uF, and one new 22uF [in series]. ...Will there be problems?
A: Probably not.
When you charge two capacitors in series so that the same current fklows through both capacitors, as happens here, the larger capacitor will experience a smaller voltage rise. This will be very approximately in inverse proportion to their capacitance. The two capacitors are close in nominal value (15/22 =~ 0.7) Electrolytic capacitor values may vary more widely than this (depends on specification). The older capacitor has probably lost some capacitance with age. So, the older small one will probably have a higher voltage to start when charging finishes. This will offset the capacitor voltage midpoint.
However, as you rightly note in your deleted answer (please undelete), when the capacitors discharge they will be electrically in parallel bu=t behind diodes so that the somewhat higher voltage capacitor will start to discharge first and when the output voltage gets down to the voltage of the lower voltage cap the second cap will "join in" seamlessly.This will have some effect on capacitor ripple currents and the higher voltage MAY stress the old cap more, but overall it should work OK. Arguably, a new cap that is not the same as the old one should be at a somewhat LOWER capacitance so that it takes more of the stress. BUT should be OK.
This is Al's picture of the discharge process. Whichever capacitor is at higer voltage will discharge first.
Q: Those caps are surrounded by a lot of diodes. I expect that normally the potentials around and between those caps are -162V, 0V, +162V. When I replaced one of them by a different one, I probably moved the center potential out of ideal zero. Does it matter here?
A: As above. This is the heart of the Valley Fill circuit. The caps charge to ABOUT Vinpeak/2. All should be well enough.
Q: Note that the reason why there are two strange capacitors instead of one 400V one is probably just the space.
A: No. As above. this provides passive power factor correction by very substantially spreading the conduction period of the input diodes. It also provides Vsupply at half Vin peak during the valley period.
Q: The 0R5 resistors on emitters of each transistor are now 0R56. I don't understand ... if it's dangerous change or not.
A: This is OK. The emitter resistors are current sense resistors which provide voltage drive via the diode D1 D2 to trigger SCR1, which terminates the current switching half cycle via D3. I'd have to spend more time on this circuit to get all the nuances and I'm pretty sure it's not 100% correct, but it gives a reasonably good idea of what happens. Increasing the resistors to 5R6 from 5R increases the voltage across them by a factor of 5.6/5 ~= 12% so they will cause the circuit to turn off at very slightly lower currents causing very slightly lower brightness. You would be very unlikely to see the difference visually.
Valley Fill Circuit:
A Valley Fill Circuit is a piece of brilliant black magic from the beginnings of time that allows surprisingly good power factor correction into a resistive load - which a constant brightness high frequency inverter tends to provide.
Rather than continue to sing their praises - here are some references to basic and more clever versions and some discussion. Well worth acquainting oneself with if you have not met them.
IR (amongst market leaders) AN1074 - New valley fill circuit -
A new Circuit for Low-Cost Electronic Ballast
Passive Valley Fill with additional Control Circuits for Low Total
Harmonic Distortion and Low Crest Factor - passive magic refined.
+____________________________
A very clever circuit that appears to offer substantial gains over the traditional circuits Improved Valley-Fill Passive Current Shape - 1997
- The original valley-fill current shaper permits input current
conduction from 30° to 150°, and then from 210° to 330°. Due to the
discontinuities from 0° to 30° and from 150° to 210°, substantial
amount of harmonics were introduced into the input current
waveform. This article presents an improved version of the valley-fill
circuit which extends the conduction angle to near 360°, thus
lowering unwanted harmonics as well as improving power line
current waveform. Improvements are made with passive components.
SPICE simulations compare original circuit with different improved
versions of the circuit. 98% power factor is achievable with this new
circuit.
Useful EDAboard discussion
IEEE abstract - of interest]The circuit with valley switching technique
And again High power factor correction circuit using valley charge-pumping for low cost electronic ballasts
Related
Best Answer
The lights are gas discharge tubes, so have a struck voltage usually between 60-80 V. So on 110 V mains you won't have enough to keep them alight in series, but at 220-240 V you would have enough voltage. If you have a simple inductor/choke limiter, and glow starters this may be your solution:
With electronic ballasts it will depend on the connection complexity they support, but there are plenty that support 2, 3 and 4 tubes with heater activation and no starters. Here's a pointer to a 2 tube electronic ballast.