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Old 28th Feb 2017, 10:46 pm   #41
turretslug
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Default Re: Rectifier Flashover and open circuit anode resistors

In the context, it would make more sense if "^2" meant "squared", the 1.1 being a correction factor for waveform shape.

Whilst the data for those aluminium block resistors usually includes dissipation without additional heatsinking, I still think that it's a good idea to provide them with thermal inertia, even if it's just bolting them to a chassis/rear panel to limit the rate of temperature change and consequent repetitive stress/fatigue. Also, I'm not convinced that the resistive element-to-aluminium case heat transfer efficacy is that good, who knows how hot they get internally!
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Old 1st Mar 2017, 12:17 am   #42
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Default Re: Rectifier Flashover and open circuit anode resistors

A^2 = A*A i.e. A squared.

The power in a resistor with constant current is I*I*R
If the current passes for say one fifth of the time, then the voltage drop will be five times greater as will the resistor dissipation. The time and waveform will be complex but you need to multiply by a considerable factor, possibly estimated from the mean voltage drop.
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Old 1st Mar 2017, 12:40 am   #43
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Default Re: Rectifier Flashover and open circuit anode resistors

So the 1.1 is basically a "flog factor" for the waveform and the rest is just basic ohms law for the power equation i.e. W=VI?

That comes out at just over 1.2 watts which aint much and those resistors get pretty darn hot. that means using 10Watters i should be in a big safety factor.

So whats causing them to fail on a depressingly regular basis? My meter measurements do not show any untoward values in the steady state and as the amps are biased heavily into class A there shouldnt be any really big transient draws.
All I can think is at switch on there must be a very large transient current draw causing these cheap resistors to give up the ghost?

A.
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Old 1st Mar 2017, 9:48 am   #44
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Default Re: Rectifier Flashover and open circuit anode resistors

Second page, second para:

http://www.r-type.org/pdfs/uy41.pdf

Lawrence.
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Old 1st Mar 2017, 11:06 am   #45
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Default Re: Rectifier Flashover and open circuit anode resistors

Its square of 2, I got told off for not considering this when working out power ratings last week.
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Old 1st Mar 2017, 11:25 am   #46
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Default Re: Rectifier Flashover and open circuit anode resistors

I looked at #1 to see where the resistors are connected, it is not clear. I assumed they are in the anodes of the rectifier, not parasitic suppressors in the PA anodes.
Lawence says a factor of three the power should be used for the resistor dissipation, I gave an example of five. It will depend on the impedance of the supply.

You have calculated 1.2W, with a factor of five this comes to 6W, no wonder they are hot. There should be no large transients on switch on as the rectifier valve warms up gently. It is a very different situation with a silicon diode.

My inclination would be to try a bodge diode in series with the rectifier anode to stop back current if there is arc in the valve.

Trevor
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Old 1st Mar 2017, 1:15 pm   #47
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Default Re: Rectifier Flashover and open circuit anode resistors

To work out the power dissipated in the limiting resistor is quite complex, I've never tried to work it out from scratch but I came across this formula, the circuit shows SS rectifiers:

PR = (Io/square root(2*t*f))squared*R.

Where PR = power dissipated in the resistor in Watts
Io = current out from the rectifier in Amps
t = the charge current pulse width in Seconds
f = line frequency in Hertz
R = limiter resistor on Ohms.

If I've got that right....

Link to the book (not all pages are displayed):

https://books.google.co.uk/books?id=...attage&f=false

Which is all well and good but t needs to be calculated as, I think that's in the book as well.

For what it's worth.

Lawrence.

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Old 1st Mar 2017, 5:11 pm   #48
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Default Re: Rectifier Flashover and open circuit anode resistors

That sits well with the Mullard spec for the 5-20 at 6 watts per GZ34 anode. but doesn't leave much room for the rating.
Lawrence, I wonder if the various "rule of thumb" formulae exist because of the difficulty in calculating the charge time and so a "one size fits all" approach is used.
I reckon my approach of ordering up some 14Watt resistors isn't so wrong then.
And goes a way to explaining why certain commercial designs, in particular HJ Leak stuff, didnt use anode limiting resistors but I imagine used the resistance of the transformer plus that of the rectifier in conjunction with lower capacity PSU capacitors to assuage the load on the GZ34. (32 uF on the 20 watt Stereo20, 16uF on the 50W Stereo50 and a mere 8uF on the 50 watt but high HT TL50 monos.). It wasn't just the cost of higher capacitance high voltage electrolytics I think.

And all of it a good argument for Silicon rectification. If there had been cheap high voltage silicon rectifiers in the 50's he'd have used them.

A.
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Old 1st Mar 2017, 5:23 pm   #49
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Default Re: Rectifier Flashover and open circuit anode resistors

I'm also thinking the maximum voltage rating of the resistors was exceeded.
I have no data sheet for these resistors in particular but it would seem its only the 10W and over that generally have 500V or better ratings. I guess the nature of a WW means that high voltages could "flash over" between the coil windings of resistance wire if closely wound?
Ok you may argue that once warmed up theres only 10V across the resistor and in its specs it may have a short overload rating etc. But that isnt strictly true is it? these resistors are on the AC side of things, and effectively they are having 400+ Volts switched on and off 50 times a second so theres a possibility over time that they get over stressed and fail? Ok I know its a sine wave and a gradual in relative terms rise and fall but if the valve is only switching on at the top of the wave to put a burst of charge into the reservoir then for the majority of the time the resistor has a quite large potential difference across it?
Or have i got the wrong end of the stick again?
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Old 1st Mar 2017, 5:33 pm   #50
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Default Re: Rectifier Flashover and open circuit anode resistors

The resistor only has a voltage drop across it when the rectifier conducts, when the rectifier doesn't conduct then no current flows through the resistor, no current flow means no voltage drop.

Lawrence.
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Old 1st Mar 2017, 5:39 pm   #51
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Default Re: Rectifier Flashover and open circuit anode resistors

If the rectifier arced over A to A, then the resistors most probably were over voltaged, over currented and over powered all at the same time. It looks like they opened up and maybe saved your mains transformer. If so, they did a good job.

David
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Old 2nd Mar 2017, 6:36 am   #52
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Default Re: Rectifier Flashover and open circuit anode resistors

Wouldn't it be a good idea to put a thermistor on the primary side, especially as Andy's going to use a big toroid? That and a few MOV's wouldn't go amiss, I'm sure the original designers would have used them if they'd had them.

Andy.

I
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Old 2nd Mar 2017, 2:17 pm   #53
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Default Re: Rectifier Flashover and open circuit anode resistors

That indeed is the plan "other Andy". When the big toroid is used in anger Joe B will be keeping me right and the use of MOV etc will be definitely on the agenda.

Andy.
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Old 2nd Mar 2017, 11:03 pm   #54
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Default Re: Rectifier Flashover and open circuit anode resistors

Slightly off topic "other andy", but where does one find inrush limiting thermistors ?
I have had zero joy in finding them in Australia, and not even eblech has anything I could understand.

Joe
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Old 3rd Mar 2017, 1:04 pm   #55
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Default Re: Rectifier Flashover and open circuit anode resistors

Puzzled that they're difficult to find, Joe- unless there's a particular vernacular for them in Oz, search engine-wise. Here, there's a good spread of choice in NTC devices as far as operating current, cold resistance and so on is concerned from Epcos and Ametherm in particular and no doubt others. The SMPSU world has provided us with lots of tough, currently approved primary-side NTCs, as well as high performance, compact HV electrolytics,

Colin
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Old 3rd Mar 2017, 5:07 pm   #56
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Default Re: Rectifier Flashover and open circuit anode resistors

For very little time and trouble lots of inrush limiters with manufacturers part numbers can be found. Use the part number of what you want in a local internet search and the job will be done.
http://uk.farnell.com/in-rush-curren...tc-thermistors
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Old 5th Mar 2017, 5:27 am   #57
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Default Re: Rectifier Flashover and open circuit anode resistors

Surprised they can't be had in Oz, don't Mouser and Digikey or even Element 14 ship to your side of the planet Joe? Drop us a PM or email if I can help.

Apologies Andy BHN.

A.
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Old 5th Mar 2017, 7:41 am   #58
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Default Re: Rectifier Flashover and open circuit anode resistors

Hi Gents, I may have a good few of these, NOS from SMPS, drop me a PM if you need any.

ED
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Old 5th Mar 2017, 4:52 pm   #59
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Default Re: Rectifier Flashover and open circuit anode resistors

I would be interested in a couple Ed for this big toroid I have. I am assuming they are to control inrush current and/voltage spikes on switch on/off that can punch through the insulation and wreck the transformer?

Andy.
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Old 5th Mar 2017, 4:55 pm   #60
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Default Re: Rectifier Flashover and open circuit anode resistors

Ok quick update, I fitted some quality 14 watt WW 100R from RS online (Great service and free postage! why have I never used them before?) I measured all the circuit criteria AC and DC and all seems in order. They do run hot but so far stayed healthy. I am now sure those white cement box jobs from maplins are just shoddy and to be avoided.
I'll keep an eye on things, fingers crossed.

A.
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