Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:I suggest adding a resistor in parallel with each 450v cap to help equalise the voltage across each cap, say 1M/1W. Then the voltage across each one should be less than their rated voltage. They also may take some time to 're-form', a process that an electrolytic cap requires when not exposed to a voltage for a long time.
If you have a source of 110/115v start off with that for a few hours, then apply 220/240V. If you have a Variac, even better still...but not many do.
Steve A.
Klaas Robers wrote:This is impossible. You should get about 600 volts with this circuit. So:
- or the 400 volt AC is much more, say 1200 volt,
- or your misread the voltmeter(s),
- or you have used a different circuit than you have been drawing.
Steve Anderson wrote:If the transformer is really 900V AC on the output one would expect about 1270V from a simple rectification arrangement as shown with no load. You're quite correct in ensuring the caps are rated for the job, but check the diode(s). They need a reverse rating of at least 3kV! Not that many around like that!
With a load (which probably was very light originally) that figure could come down significantly. If the transformer is quite small, i.e. it fits in the palm of your hand it definitely would have been a light load, but a CRT with a suitable circuit is a light load too.
Steve A.
Steve Anderson wrote:One way around the 3kV rating required of the diodes used in a half-wave rectification circuit is to use a bridge rectifier arrangement, then in this case they would only need a 1500V rating. It also improves regulation and reduces ripple...for the cost of only three extra diodes. However, it isn't as easy to produce both positive and negative supplies at the same time...if needed.
Steve A.
Steve Anderson wrote:With meters it's always best to buy the best you can afford. It's also said of time, "A man with one one clock knows what the time is, the man with two clocks is never really sure." The same applies to any test instrument. Buy the best you can afford and throw the rest away. Seriously, I only have one meter that I trust 100%...and it's not a high-priced Fluke - as attractive as they may be.
Steve A.
Klaas Robers wrote:In our technology we seldom need a better precision than 10%. So all meters that you have show 800 volt, that is between 720 and 880 volts. A Delft-collegue of mine always said: "the first number and the place of the decimal point". But I have read on your drawings your 9 (900 volt) to be a 4 (400 volt). That is a problem of hand writing...... For me a nine should be closed at the top.
Harry, when you do things with high voltages on, ALWAYS work with ONE HAND. Place your other hand behind your back, I always grip the back edge of my throusers firmly. And see that you wear shoes with rubber or plastic soles. Then you only can be shocked at your working hand, which is painfull but not too dangerous. And if possible switch off the circuit before you are going to do things and wait long enough to discharge the capacitors. In several tube equipment I mounted a small neon lamp on the mains input and on the high voltage, with a resistor in series of course, say 220k. Then you see that something isn't safe, when the little lamp is on.
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