The Voltage Doubler

[Radio Fixer]

Well, from the Hammond chart (there is a similar one in RS component catalogs) if it is used full wave into a capacitor I/P filter then its 1:1, the best configuration with the DC components canceling out in the core (is this right?). Intuitively I think that it is, and from deep in the memory, but I cant find anything on the Web that says this from a quick look. Good this Forum, gets me brushing up on theory...

So DC can be 100mA but I wouldnt push it that far and I would like it to be under 75mA.

I'm not sure that Hammond, or anyone else, derived these ratings from calculation but suspect it was from temperature measuements; how hot their Xformers got, in the various configs?

For full wave, from a centre tapped Xformer into a resistive load, this Ref: http://www.visionics.a.se/html/curriculu...fier1.html you would think that the max would be 81mA as that is the efficiency. But just to confuse things Hammond would have you believe the DC can be 127 mA in this configuration.

I'm expecting this to be a trick question as you must have looked at the chart

Gary

[pwdrive]

Hi Gary I looked at that Hammond chart you provided the link too.
No, not a trick question regarding my last post, I was just imagining a scenario such as: I want to build a PSU, I find an old transformer in my junk box, it has the secondary HT volts and current marked on it, eg: 250v 0 250v @100ma or 0v 250v @ 100ma, would those ratings be for a resistive load or a typical rectifier/filter situation?

Lawrence.

[Radio Fixer]

Those ratings are into a resistive load, the only one that the Xformer maker can be certain about. What a user does with it, as we know, can vary though 1/2 dozen rectiifer configs, with different Form factors and thats what makes the Hammond chart to me so valuable. But as always with good design we wouldn't want to work to the max.

How hot can a transformer be run? If you cant keep your finger pressed on it until you get bored its over 50 C and too hot for me. They do seem to survive at quite warm temps though. Nice when they are only warm or slightly hot.

Gary

[pwdrive]

The PSU I knocked up had a transformer HT rating of 350 0 350 @150ma I fed that through a series pass valve (6080) the most I have seen drawn from my receivers was around 60 to 70 ma, the said transformer runs warm, warm is good! I have an old Hammarlund receiver and the transformer runs quite hot in that, no apparent faults though.

Lawrence.

[Radio Fixer]

I dont know if this adds anything to the discussion but I like to try to prove things for myself ...

Some while ago I did a project that could use half wave or bridge rectification with capacitive filter. I wondered about the de-rating figures for the rectifier configurations, as given by Hammond, and made some measurements.

The transformer was wired, for three configurations, to supply 100V into a resistive load of 480 Ohms (two large mains droppers in series. The configurations were pure AC (RMS), half wave DC and bridge rectified DC (with capacitor input for the rectifier circuits). A Variac was used to adjust the supply voltage to give 100V across the load in each case. A True RMS (TRMS) digital multimeter was used to measure the transformer input current. These compute the TRMS of non-sine waveforms by sampling and digital signal processing.

The results were AC 113mA, HW DC 416mA and FW Bridge DC 189mA.

Interestingly 113 / 416 = 0.27 and 113 / 189 = 0.6 which are close to the Hammond figures.

Gary

[pwdrive]

Yes, practical experiments are very reassuring.

Lawrence.

[Skywave]

Transformer running temperatures & related issues. Recently I had to do yet another repair on my Tektronix 475 oscilloscope .  Post-repair, during a 'soak-test' with the sleeve cover removed, I noticed how hot the mains transformer got: all the indications were (and are) that that temperature is normal: no malfunctions were detected later when the case was re-fitted and a much longer 'soak-test' was run. However, what was even more alarming was just how hot some of the encapsulated bridge rectifiers got (they're not mounted on heat-sinks). For one of those you wouldn't leave your finger on it for too long! Again, that appears to be 'normal' .  I did some research into this (mainly via the Internet) and it does seem that the PSU section of the 475 is the really weak area in the design: typically the PSU filter capacitors lose capacitance. Now considering the age of the 475, that is fair enough: electrolytics deteriorate with age (especially when fitted next to things that get really hot) - so we replace them: and that's the problem with the 475: component replacement and general maintenance can be extremely difficult. The saving grace with the 475, of course, is that when it is set up, calibrated and working properly, it is a very good 'scope. Just a shame that Tek. crammed a quart into a pint pot - even though a (small) fan is fitted (which runs fairly slowly), this scope still runs remarkably warm - thus shortening component life.

Now compare that to the HP 1715A: a 200 MHz 'scope whose general spec. and performance is comparable to the Tek. 475. Comparing the HP 'scope to the Tek, for the HP, the enclosed volume is 50% greater, the mains transformer is larger; there is a lot more space between the components & pcbs. It runs a lot cooler - and it doesn't have a fan fitted. As a bonus, it is somewhat easier to do maintenance on. The vintage of each is about the same: mid-1970's. My 1715A has proved to be much more reliable than my 475: each have been used in the same environment with about the same usage duty-cycle. For the 1715A, the weight is about the same as a Tek. 475.

Incidentally, both of these 'scopes are displayed in the Gallery.

Al.