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Old 6th Feb 2013, 12:53 pm   #81
TrevorG3VLF
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi dinkydi #75

I have analysed the circuit you have shown. Unfortunately, I have lost the method of outputing the .emf file so I printed it and scanned it in b&w to get a small .jpg file.

The upper curve is the output of the rectifier and the lower curve is the voltage across the 600 ohm load.
My mind boggles and I do not see the advantage of this circuit.

I have never met the 'cooking on gas' phrase, please interpret.

Trevor
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Old 6th Feb 2013, 1:03 pm   #82
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by G8HQP Dave
The WA2ISE site seems to have been 'got at'. Let's hope the site owner soon gets it cleaned up and back in use.
Further information here. It seems someone has inserted some JavaScript into the site.
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Old 6th Feb 2013, 1:42 pm   #83
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Default Re: Use of Class X2 Capacitors as Droppers.

"Cooking with gas" simply means things going well. I suspect it has its origins in the very slow response of early (and indeed some current ) electric cookers. If you are cooking with gas you have instant control which is much easier to use. In control terms a slow responding electric cooker is a bit like a control loop with a long delay in the feedback path- very prone to instability!

A capacitor in the feed (ac line) side of a half wave rectifier circuit sees only the same dc pulses as on the output side of the rectifier- it is a series circuit so the current is the same wherever. When you turn on the system, the series capacitor simply charges up to the peak voltage of the supply, give or take a little bit that escapes through any resistive load on the far side of the rectifier.
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Old 6th Feb 2013, 3:15 pm   #84
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Default Re: Use of Class X2 Capacitors as Droppers.

If you run LTSpice, control/left click on a trace name gives average and RMS values. Keep an integer number of cycles in the 'raw' window for this.
 
Old 6th Feb 2013, 3:31 pm   #85
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Default Re: Use of Class X2 Capacitors as Droppers.

It appears that my last post #77 may have been misinterpreted by some.

The post is a simple statement of fact with a starting point that separate droppers can exist. The post makes no recommendations as to whether this is desirable or not. In addition, no practical implementations are canvassed.

Peter
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Old 6th Feb 2013, 4:15 pm   #86
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi Trevor,

Thanks for producing your transient analysis. The vertical axis labels are not quite right, however, the important R2 voltage curve looks good (voltage across the heaters). I did not explain that the voltage of interest is that across R1 (not the voltage from the diode to the V1 negative input).

If you have any energy left, curves for R1=3k and 5k would also be worthwhile.

Thank you Chris for your explanation about the response-time of cookers

Peter
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Old 6th Feb 2013, 5:57 pm   #87
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Default Re: Use of Class X2 Capacitors as Droppers.

Livewire, I would stick with your original thought of using X2 capacitors as a combined HT and heater dropper, but do use the bulb-in-the-heater chain trick to ensure the RMS heater current is about right (assuming you have a suitable power supply (batteries) and a current meter to "calibrate" the setup). Then just check the DC value of the HT for gross error.

Spice simulations are only as good as the models used (for components and equivalent circuit) and the interpretation of the results. It's great for complex circuits where the need detailed investigations justify the effort of generating and proving the models etc, but for a one-off simple application such as this, then you could have a value for the cap by experimental methods long before you had a Spice simulation you had any faith in.

S
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Old 6th Feb 2013, 8:54 pm   #88
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi Peter.

Your wish is my command. I do not have a gas cooker here, perhaps that accounts for my instability!

I attach a graph of the voltage across R2 with R1 = 1k, 3k and 5k.
The upper curve is 1k, the lower curve is 5k.

Trevor
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Old 6th Feb 2013, 10:12 pm   #89
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi Trevor,

Great work Genie - it looks good! What we need to go with it are the corresponding graphs of the voltage across R1 for the same values 1k, 3k and 5k. Go forth!

Actually, it is rather interesting on the internet to find out the expressions that others use (in a similar work/hobby context), in different parts of the world. For example, no one here uses the word "kit" (except as a pile of parts for assembly).

Peter
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Old 6th Feb 2013, 11:32 pm   #90
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by LIVEWIRE?
I'm a little puzzled as to why a capacitor can't(shouldn't?)be used to drop the HT. It will be in the Mains side of the rectifier circuit so will not be in a DC path (or am I missing something here?)
It can be used, provided you provide a path for DC to go. AC/DC sets always use a half-wave rectifier so the rectifier-smoother-radio circuit loop has to carry DC. That DC current can't come from the mains, so it has to come from something else connected on the set side of the dropper capacitor. Options are a diode (to make a voltage doubler so peak voltages can be too high), a resistor (inefficient unless it doubles as a heater chain) or a choke (theoretically best but big and heavy)
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Old 7th Feb 2013, 12:44 am   #91
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Default Re: Use of Class X2 Capacitors as Droppers.

Put another way, Livewire, a capacitor is a component which blocks the flow of direct current. A very great number of them are used to do just this, and they do it very well.

If you want DC coming out of your rectifier diode, then you should se that there must be a DC component entering the anode of the diode, and ask yourself where it came from.

If the only thing at the anode end is a capacitor, then DC is blocked, and none could have come from anywhere, and the rectifier doesn't work. There will be a turn-on surge, then a total lack of DC output into the HT system.

As Dave says, there has to be a route provided which can carry DC to complete the circuit.
A choke would be best, but these sorts of power supplies were done just to avoid the need for heavy lumps of iron, and there are reasons which could make the choke larger than a proper mains transformer for the set. Once upon a time live chassis power supplies were used to handle DC mains, but DC ended to all intents a long time earlier, and firms used them simply for cheapness. The manufacturer wasn't paying for the electricity thrown away in hot droppers, and a choke wuld also have been expensive.

A resistor could be used, though it is lossy. If the radio's own valve heaters are used, then at least part of the loss goes into doing something which would have been necessary anyway.

For current to flow there has to be a complete circuit

For DC to flow, there has to be a complete circuit of things which will carry DC

David
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Old 7th Feb 2013, 1:03 am   #92
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Default Re: Use of Class X2 Capacitors as Droppers.

Hi Peter,

Here are the voltages across R1 for R1 = 1k, 3k, 5k.

If I go forth and multiply, then I could calculate power.

Trevor
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Old 7th Feb 2013, 1:15 am   #93
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Default Re: Use of Class X2 Capacitors as Droppers.

We've got an awful lot of angels dancing on the head of this pin, now!

Several people, myself included, have concluded that just feeding the lot (heaters and HT rectifier) via about 3uF will likely work just fine. A tweak of capacitance (perhaps using the grease spot photometer method) to get the heater current just so should leave the HT volts somewhere usable- the actual voltage is not particularly critical unless it ends up significantly high (possible capacitor rating issues) or low (LO won't start, distorted audio).

Getting back to part two of the OP's original question, I reckon the most overlooked consequence of asking here is 92 and rising replies
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Old 7th Feb 2013, 1:39 am   #94
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Default Re: Use of Class X2 Capacitors as Droppers.

Um, while commenting about 92 replies, you pumped it up to 93... and I've just... Oh rats!

I'm unhappy about the effort of trying to work out how to do a bodge to the Nth degree of precision, when the safe, elegant, accurate and legally safe method is a proper transformer, and they aren't even expensive.

It's gone on a lot too long and all value has been squeezed out of it. I've been expecting the Monty Python flying foot to come whistling down for some while, and the raspberry sound-effect to accompany it.

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Old 7th Feb 2013, 9:37 am   #95
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Default Re: Use of Class X2 Capacitors as Droppers.

Thanks to Trevor's excellent graphical analyses, we can now get some insight into the capacitive-dropper design problem. This allows us to compare two different methods of implementing a dropper: traditional design (adding a capacitor in series with the power input) and Casey's design (adding a capacitor in series, together with a reconnection of the HT leads).

Trevor chose component values for his simulations to model a typical valve radio with direct ac-mains connection. The capacitive dropper for the traditional design was 3uF and for Casey's design was 2uF. To aid the reader I have re-uploaded Trevor's relevant graphics.

The results for the traditional design are given in transient-1.jpg. This shows that when the HT rectifier turns on, the ac signal after the dropper capacitor is clipped. Unfortunately, this means that the HT output voltage is low (below 50V for HT load = 1000ohm). The cause of this problem is the capacitive voltage divider formed by the dropper capacitor (3uF) and the smoothing capacitor (47uF). (Just like the capacitive divider with a high-impedance probe.)

The results for Casey's design are given in casey2.jpg and casey3.jpg. Casey2 shows that when the HT rectifier turns on, the ac mains input is coupled via the smoothing capacitor (C1 = 47uF) directly to the heater string (R2 = 600R) resulting in a "blip" on the heater voltage (the heater is supplied current via the HT circuitry). This is desirable because the dropper capacitor can be made smaller. Casey3 shows the HT voltage for various loads. With an HT load = 1000ohm, the voltage is 85V. For lighter loads the HT can rise to a very respectable 150V.

Casey's dropper design is preferable to the traditional series-capacitor dropper because it gives higher HT output. Another advantage is a lower dropper capacitance. Both dropper configurations power the heater string with non-sinusoidal waveforms, as noted in previous posts.

Peter
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Old 7th Feb 2013, 1:07 pm   #96
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Default Re: Use of Class X2 Capacitors as Droppers.

I got involved with this thread since I am curious as to how things work. Obviously, a transformer is the simplest and most inconvenient way to go. You may need a large transformer to take account of the DC or use a bridge rectifier.

A capacitive dropper can probably be mounted inside the set, making the result quite neat. The capacitors I have seen have quite wide tolerances so the circuit may need to be trimmed.

I used 5spice since I found that the drawing of diagrams was easy but the free version I use does not allow power to be calculated. Merlinmaxwell says that LTspice will do this and when I have the inclination I may decide to bash my head on that program.

My guess is that the HT current will not greatly affect the power in the heaters but a simulation would help here.

I did look at an attenuated voltage doubler for the HT but think it impractical.

We should all go out and buy a true RMS voltmeter.
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Old 7th Feb 2013, 1:45 pm   #97
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
Originally Posted by Radio Wrangler View Post
.. a capacitor is a component which blocks the flow of direct current...
If you want DC coming out of your rectifier diode, then you should se that there must be a DC component entering the anode of the diode...
We need a circuit diagram of what is proposed! Capacitive droppers for HT are causing me to scratch my head somewhat.
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Old 7th Feb 2013, 3:17 pm   #98
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Default Re: Use of Class X2 Capacitors as Droppers.

Quote:
We should all go out and buy a true RMS voltmeter.
Sometime around early 1984 Tektronix issued a recall of all Series-2200 oscilloscopes. Many tens of thousands of units were involved, probably way over 100,000 units total. For this Series the CRT filament voltage was incorrect. The issue was resolved by modifying the filament transformers, and all CRTs were replaced just in case there was any degradation.

From my limited perspective of the recall, I believe most of the replaced CRTs were actually undamaged, but Tektronix was doing the right thing and they deserve full marks. One hates to think what this little exercise actually cost them, and I wonder how many companies would show such integrity today.

What led to the recall? The filament voltage in the Series 2200 was a square wave and, when the prototypes were built and tested, a standard voltmeter was employed to measure that voltage, instead of a true-RMS model! What an expensive lesson it was.

Peter

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Old 19th Feb 2013, 10:05 pm   #99
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Default Re: Use of Class X2 Capacitors as Droppers.

I have now fitted 3 x 1uf 275v Class X Capacitors in parallel, shunted with 340kOhm(2 x 680k in parallel)resistors. This assembly, wired in series with the mains supply, allows the radio to work fine, although the voltages, measured with a 20k/v analogue meter are about 25% lower than those shown on the circuit diagram. Examples are:-
(1) 100vac at 'load' end of capacitive dropper (2) 25vac across 35Z5GT(rectifier)Heater
(3) HT(or should I say B+?)at rectifier Cathode 90v (4) V4(50L6GT)Cathode 5v
The Mains supply here (still)measures 240vac. The radio works quite well, despite the low readings above, and will receive several stations at good strength via it's built-in loop aerial, notably '5 Live', Talksport. and Absolute Radio. I may try and get the voltages up to what they should be, by adding some capacitance to the dropper, or maybe by increasing the value of the 'bleeder' resistor. Other refinements, such as adding a surge limiter, or an internal fuse may follow. Incidentally, the Mitchell isn't strictly a 'live chassis' set, since the mains neutral/DC Negative line is isolated from the metalwork via an 0.22uf capacitor in parallel with a220k 0.5w resistor.
With reference to Kalee 20's post above, the radio works in exactly the same way as if it were supplied directly from the mains, in that the rectifier anode is connected to the (AC)supply, and outputs DC from it's cathode. The series capacitor 'drops' the voltage from the UK 240 supply down to nominally 120vac for the U.S./Canadian radio, whose circuit remains otherwise as it was designed.

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Old 20th Feb 2013, 12:28 am   #100
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Default Re: Use of Class X2 Capacitors as Droppers.

Sounds like a good result.

We've all had a good chat about it too!
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