Dropper capacitor safety

[Boater Sam]

I'm pleased that finally the consensus is that type X are not suitable as droppers. I have held this belief for a long time but was told I was totally wrong and that they were the best thing since sliced bread.
Thank you all for restoring some sanity.

[Herald1360]

They might well be the best thing since sliced bread- for the manufacturers of cheap mains powered gizmos like remote control socket switches etc etc. Built in obsolescence (life of a few years) and "It has to be that way for safety compliance, guv.".

[John G8MWF]

Well this is just an idea but what if I try this out by shorting the cap?
I already have a couple of bad valves with good heaters and if I collect a few more, in time, and try this out not much would be lost and perhaps something would be gained!


AC supply is nominally 240V.

SO, I have a DAC90A heater chain of 117V AC @ 100mA.

It has the big dropper resistor in series with it of 2 x 150 + 950 ohms and then the lamp and shunt resistor that have a nominal value of 30 ohms so I make that a total 1280 ohms?
Take out the 950 ohms, fit the fusible resistor and put a short in place of the where the capacitor would be in place of the 950 ohms.

What value of fusible resistor would be best for this situation?

[julie_m]

The fusible resistor will be passing 0.1A, so it will have a power dissipation of 0.1 * 0.1 * R = R / 100 Watts. This wants to be as close as possible to its power rating, so the resistor is living life on the edge; so if using an NFR25H, which is rated 0.5W, try 47Ω.

But that's really only part of the story: you need also to consider the resistor's pulse characteristics, which can be found in its datasheet. In the worst case, peak mains is going to appear across the whole series chain at switch-on, when the capacitor is discharged and therefore appears as a dead short-circuit in series with the fusible resistor and the cold filaments. You may need to use multiple resistors in series, so as the pulse energy is distributed among them.

[G6Tanuki]

But that's really only part of the story: you need also to consider the resistor's pulse characteristics, which can be found in its datasheet. In the worst case, peak mains is going to appear across the whole series chain at switch-on, when the capacitor is discharged and therefore appears as a dead short-circuit in series with the fusible resistor and the cold filaments. You may need to use multiple resistors in series, so as the pulse energy is distributed among them.

Or do it classically and use a Brimistor [or modern equivalent] for inrush protection?

Must admit, I always liked those 'safety' ceramic power-resistors you often found in valve TV-sets that had a spring-loaded metal strip along the side, held closed with low-melting-point solder. If power dissipation exceeds design limits, solder melts, *ping* and the contacts open.

There was another design I came across once: a wirewound resistor on a cylindrical ceramic former. One of the leads passed the length of the former, to the top where there was a compressed conical coil-spring held to the central wire with LMP solder. Whole thing mounted on-end on the PCB, spring uppermost. The only downside of this was that when it 'fused' the spring flew off and invariably lodged itself in some obscure place so you couldn't find it - risking a new and unexpected short-circuit when you fixed the fault/replaced the resistor.

[trickie_dickie]

http://https://www.alibaba.com/produ...924120515.html Its a Motor Run Capacitor. Not X or Y rated.

[julie_m]

No capacitor bigger than about 10nF can be rated for Class Y working, because the earth leakage would be too great. And a motor run capacitor is intended to be connected in series with an inductive load, so doesn't need to withstand transients on the mains directly -- it's not used in Class X.

['LIVEWIRE?']

Clicking the link in Trickie Dickie's post (46) only produces an error message 'sorry we couldn't find 'https'!!'

[John G8MWF]

Ok I will give this a try when I have the parts together which may take a while.

I am starting to find this a bit confusing - so WHAT is a motor run capacitor?
And do I want to design such a precision protection circuit for a valve radio?

[radiotechnician]

Common electric motors use two types of capacitors.

Starting capacitors furnish torque current when the motor is started from a standstill , The capacitors are rated for very short duty.

Run capacitors furnish a current on a continuous basis after the motor has reached operating speed. This current greatly improves the power factor.

Ceiling fans have capacitors for each speed. Capacitors used outside of their common use need much study.

[Herald1360]

Common electric motors use two types of capacitors.

Starting capacitors furnish torque current when the motor is started from a standstill , The capacitors are rated for very short duty.

Run capacitors furnish a current on a continuous basis after the motor has reached operating speed. This current greatly improves the power factor.

Run capacitors are not the same as power factor correction capacitors. They effectively act as continuously rated start capacitors in that either is used with a single phase induction motor to energise a second stator winding out of phase with the main run winding to create a rotating field when the rotor is stalled so that the rotor will actually start to turn rather than just sit there vibrating. If anything they will likely make power factor worse, but they will smooth out somewhat torque variations in the rotor output.

A power factor correction capacitor would be connected across the motor external connections to take enough capacitive current to balance out the inductive component of the motor running current. This reduces the actual line current drawn by the motor to the minimum consistent with load on and losses in the motor.

[radiotechnician]

The query I was replying to, 'what is a motor run capacitor', led me to answer as an electrician would.

Two value capacitor motors are part of work, and we would carry capacitors to the job.

C.G Vienott's , Sub fractional and Fractional Horsepower Electric Motors, is at hand here and has been since 1974, so finding the type of motor that was in my mind was simple.

I hope this helps somewhat.

[GMB]

I think there have been some misleading comments so here are a few points I have gleaned from manufacturers that I hope might clarify things a bit:

Metalised film capacitors generally self heal whether they are X or Y or motor capacitors or anything else. X is just a very particular rating and there is more than one kind. If you are paranoid then X1 capacitors are better than the common X2 type as X1 are rated for 3-phase industrial use.

Y class capacitors are in effect two X class in series, and because of their application they are limited to small capacitance values.

Modern motor run capacitors are typically more fire-proof than X class and also have the extra feature than they usually live in a sealed metal can used to generate pressure if they fail - this pressure distorts the case and makes them go open circuit - eventually.

Old motor capacitors will be electrolytic with all the problems that can bring!

Manufacturers suggest that failure mode of most capacitors depends on age. They are generally expected to fail short circuit when new (due to manufacturing defects) and open circuit when old (unless made by RIFA of course, when they catch fire).

A capacitor in series with something resistive is effectively protected from fast transients and so is much less likely to need to self heal. This is because of the transients being fast, so high frequency, so most of the voltage appears across the series resistor. This is good news for capacitor droppers and suggests that this is not such an onerous application.

Always make sure a dropper capacitor is rated for a/c use. This is not the same as the DC rating since it is more about thermal effects. The X class is a package of ratings so the suggestion that they are not appropriate for droppers does not look right to me.

[Sideband]

Originally Posted by GMB

A capacitor in series with something resistive is effectively protected from fast transients and so is much less likely to need to self heal. This is because of the transients being fast, so high frequency, so most of the voltage appears across the series resistor. This is good news for capacitor droppers and suggests that this is not such an onerous application.

That is exactly what I thought which is why I never worried about using X caps as droppers.