|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
4th Feb 2013, 10:17 am | #61 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: Use of Class X2 Capacitors as Droppers.
The link on your most recent post works, Chris, but the one on your post # 58 still doesn't, yet I can't see any difference between the two!!!
|
4th Feb 2013, 10:33 am | #62 | |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
|
Re: Use of Class X2 Capacitors as Droppers.
Quote:
Although the two links look identical, placing the mouse over each link reveals the problem with the first link. In Firefox you can see the full web address appear in the bottom left of the screen. The first link is missing quite a few characters. Regards Symon. |
|
4th Feb 2013, 11:17 am | #63 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
|
Re: Use of Class X2 Capacitors as Droppers.
The forum software alters the displayed version of links, inserting three dots to replace characters from the middle in order to make the link displayable even on small monitors; while leaving the machine-readable version intact.
If you highlight the human-readable version and middle-click, you will get an invalid link. You have to either right-click the link and choose "copy link location"; or middle-click it to open it in a new tab (thus proving it works), highlight it in the address bar, then tab back and middle-click in the editing area to insert the highlit text.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
4th Feb 2013, 1:31 pm | #64 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
|
Re: Use of Class X2 Capacitors as Droppers.
That explains it. I just highlighted the visible link in another post then ctrl C copy followed by ctrl V paste in the new post.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
4th Feb 2013, 3:09 pm | #65 | |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
|
Re: Use of Class X2 Capacitors as Droppers.
Quote:
This device would seem to be ideal to measure the effective current in a valve heater string. |
|
5th Feb 2013, 9:48 am | #66 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Use of Class X2 Capacitors as Droppers.
Thermocouple ammeters are getting a bit thin on the ground. They are rather fragile and haven't been made for a long time.
More easily obtained is a "True RMS" multimeter. Their AC current ranges use the same rectifier/processor so the current ranges are true RMS also. But even with a capacitive dropper in the circuit, causing the current to be out of phase with the applied mains, the current and voltage on each valve heater will still be in phase with each other, and will be sinusoidal unless the mains is distorted. So an ordinary AC meter with the usual peak/RMS fiddle factor intended for sinewaves should be OK. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
5th Feb 2013, 11:52 am | #67 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
|
Re: Use of Class X2 Capacitors as Droppers.
A simple current or voltmeter will do if the capacitive dropper is driving the heaters only.
If the dropper supplies the HT as well, there will be a DC contribution in the heaters. I have used 5spice to look at the effect of varying HT curent but the version I have will not calculate power. Perhaps LTspice or TINA will do it. |
5th Feb 2013, 11:58 am | #68 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
Re: Use of Class X2 Capacitors as Droppers.
With low pass and high pass filters it might be possible to separately measure the AC and DC components of the heater current, and so calculate the total power. However, given that the capacitor, by design, is dropping some voltage there may also be a contribution from the 'hole' left by the rectifier charging pulses. Only a true RMS meter will cope with that.
|
5th Feb 2013, 3:39 pm | #69 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
|
Re: Use of Class X2 Capacitors as Droppers.
I have done a transient analysis, using 5spice.
I used 3uF driving 600R, a diode and three different HT loadings, 1k, 3k, 5k with a 47uF bypass capacitor. As can be seen the positive side of the heater waveform is clipped. I am surprised that the negative side appears to be unaffected even though there must be a DC component. As said previously, I cannot get the heater dissipation. |
5th Feb 2013, 6:40 pm | #70 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
|
Re: Use of Class X2 Capacitors as Droppers.
Yes, there's a DC component. The wave shape of the negative half cycle looks unaffected (as expected), but there is a negative bias.
This again is as expected - the HT rectifier takes current in one direction, so the heater current must be offset in the other direction to maintain zero average current through the capacitor. |
5th Feb 2013, 7:20 pm | #71 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
|
Re: Use of Class X2 Capacitors as Droppers.
To equalise the effect of the rectified-HT-component is why quite a few half-wave power rectifiers designed for AC/DC radios had a tapped heater - you connected the dial-lamp bulbs across the tap to provide an extra path for the 60-odd milliamps of HT current.
I'm also told that back in the 50s and 60s in some rural areas fed by small substations, during TV-viewing hours there was often a noticeable DC component on the mains because of the asymmetric effect of half-wave rectification. |
6th Feb 2013, 12:44 am | #72 |
Retired Dormant Member
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
|
Re: Use of Class X2 Capacitors as Droppers.
Hi Trevor,
Thank you for your instructive transient analysis. If you have a spare moment, for comparison it would be very informative to have a companion analysis of Robert Casey's preferred configuration. Casey's configuration uses a smaller capacitor, let's choose 2uF, but otherwise keep the other component values as you have chosen. For Casey's circuit the capacitor is in parallel with "a diode and three different HT loadings" including the bypass capacitor, and that combination in turn drives the 600R. I imagine that the new transient waveform becomes magnified/extended in the place where it was previously clipped and this is offset by a lower amplitude of the fundamental. Peter |
6th Feb 2013, 12:49 am | #73 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
|
Re: Use of Class X2 Capacitors as Droppers.
I reckon you'd get near enough to the power by treating the two halves of the waveform separately. The bottom half will simply be half the power of the corresponding "complete" sine wave. The top can be treated either as half a sinewave of lower amplitude or perhaps as half a simple square wave, depending on the degree of clipping. Maybe someone more analytically inclined could work out the error doing it that way?
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
6th Feb 2013, 1:00 am | #74 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
|
Re: Use of Class X2 Capacitors as Droppers.
Hi Peter,
Yes I can do what you want but am not too clear on the circuit. Could you post a sketch? I have also done a simulation using a 600 ohm resistor insted of the input capacitor, simulating a resistive dropper cable. I have even managed to find out how to convert the strange output file to jpeg! Trevor |
6th Feb 2013, 2:16 am | #75 |
Retired Dormant Member
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
|
Re: Use of Class X2 Capacitors as Droppers.
Hi Trevor,
Sounds like you're cooking with gas! Here is a sketch of Casey's dropper configuration. Peter |
6th Feb 2013, 9:25 am | #76 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Use of Class X2 Capacitors as Droppers.
Yes, the HT rectifier current certainly mens that a true RMS meter is essential, and this would also be true with resistive droppers.
With the spice simulation, it would be interesting to also plot the rectifier current. This issue gets worse with larger smoothers. The voltage drops less through the dead part of the cycle, and then the rectifier turns on a bit later on the rising edge of the mains, and less time is left to re-charge the smoother back to the peak. Consequently the current through the rectifier becomes a shorter pulse, of greater current. I use LT Spice, which is a freebie and was originally created and tweaked for power supplies so it handles fast transients and slow control loops together very well. I use it for GHz microwave work as well. There are internet user groups and lots of guidance on importing 'foreign' libraries. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
6th Feb 2013, 9:53 am | #77 | |
Retired Dormant Member
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
|
Re: Use of Class X2 Capacitors as Droppers.
Quote:
Peter |
|
6th Feb 2013, 12:01 pm | #78 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
Re: Use of Class X2 Capacitors as Droppers.
If you use a capacitive dropper for the HT alone then you need something, either another diode or big resistor, to deal with the impossibility of passing DC through a capacitor. A diode turns it into, roughly, a voltage doubler. A resistor simply replicates the heater chain problem, but simplified (because you don't care too much about the resistor current) and wasteful (the resistor heat is just thrown away instead of heating cathodes).
You can't pass DC through a capacitor! |
6th Feb 2013, 12:30 pm | #79 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Use of Class X2 Capacitors as Droppers.
If you went for seperate droppers, as Dave says, the HT one couldn't be a capacitor. You don't want a voltage doubler so that means a resistive dropper, and if you're doing a resistor in the HT path, why not put it in the DC side of things between a reservoir and a smoother. OK the reservoir has to handle full rectified mains, but the dropper and smoother give you a good bit less hum on the final DC. The capacitor dropper for the heaters saves you more energy than the HT dropper handles, so you've got most of the savings.
But I really don't like live chassis. Give me a proper transformer any day! David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
6th Feb 2013, 12:42 pm | #80 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: Use of Class X2 Capacitors as Droppers.
David(and others) I am still thinking of going down the Class X2 Capacitor route for a combined Heater/HT dropper for reasons stated in my other posts, but 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?) I have worked over 40 years at repairing radios, etc., but admittedly have never before converted a 120v radio using capacitor droppers.
|