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Patric

Sun Jun 14 2015, 05:58PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 254

I have a a811, heater voltage 6.3V, 4 ampere. I have several transformers from 12V, 50 Watt and I cannot rewind the secondary... Can I use such a transformer with a diode in series with the tube without risk? I tried it with an auto lamp 12V 21 watt, and it worked, but I am not 100% sure, because measuring the voltage is rather difficult in that case... The current is half, so I think it might work... But please, help because I have only one a811...

Ash Small

Sun Jun 14 2015, 06:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If I only had a 12V transformer, I'd rectify and smooth it, then put something like a 6V 21W bulb in series with the heater filament.

Maybe no need for the rectifier and smoothing cap, maybe just a suitable load in series with the heater filament?

(If the current measures a bit low, just add a resistor in parallel with the bulb)

mister_rf

Sun Jun 14 2015, 11:10PM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

1. There is no point in unwinding and rewinding the secondary, I'm thinking you can improvise another tap off the secondary coil to pick up the right voltage.

2. Do you have two identical transformers? Why not wire the primaries in series and just use one of the two secondary? This connection would make the primary voltage to each transformer half of the mains. Therefore each transformer output voltage would be half what it should be andâ€¦

3. ...for the ideal solution complete the above idea by connecting the two secondary windings in parallel and in phase.

Patric

Mon Jun 15 2015, 06:14AM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 254

I can not unwind or make a tap, the transformer is cast (sealed).
No place for 2 transformers, to big.
A bulb in series gives heat and light...
Still wondering if a diode could be trusted, because she cut the sine in half so there is no loss in power and the a811 gets only half of the voltage. My main question is: it it not bad for the filament over longer time? No one with any experience?

PS: it is for a Tesla coil.

Dr. Slack

Mon Jun 15 2015, 07:42AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Don't diode load the output of a transformer, the rectified DC will quickly saturate the core and make it do things you didn't want.

The ideal, from all sorts of points of view (cost, convenience, adjustability, self-indicating, rated to get rid of heat) is a low voltage filament lamp (12v auto bulbs are great for this purpose, readily available, cheap, range of wattages) in series with the heater, increase the number in parallel until the requisite current flows.

Bulbs generate heat and light, well ain't that a b1tch!. I guess that as TCs are completely stealthy, you wouldn't want to blow your cover.

One step down in convenience is to use a capacitor in series, no excess heat and light. Either use a very large low voltage cap in series with the output (construct a non-polar one with two anti-series electrolytics, each shunted with a diode to charge it up), or my favourite which I've used on all sorts of projects, a low value mains-rated cap in series with the transformer input. Increase the capacitance until the right current flows.

The series cap and the series lamp configurations are essentially current sources, that will deliver a nice soft-start to your filament. While good, this will mean a longer warmup time, don't draw cathode current until it's fully hot. With a current source supply, you can monitor the heater temperature by observing the heater voltage.

I've just re-read Mr_RF's suggestion of the two transformers. #2 could be very clever, except that it doesn't work. The transformer with the unused secondary looks just like a high value inductor, and collapses the output of the loaded transformer. The only way this will work is to go to #3 and connect the secondaries in parallel, primaries in series. 6v is probably just fine for 6.3v rated.

Do you have two 811's? Two of those in series from 12v would work!

Signification

Mon Jun 15 2015, 08:04AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Dr. Slack wrote ...

I've just re-read Mr_RF's suggestion of the two transformers. #2 could be very clever, except that it doesn't work. The transformer with the unused secondary looks just like a high value inductor, and collapses the output of the loaded transformer. The only way this will work is to go to #3 and connect the secondaries in parallel, primaries in series. 6v is probably just fine for 6.3v rated.

I've seen those series/parallel configurable transformers (made by: Signal Transformer)--with dual primary and secondary windings. Some can be wired in this manner (primarys in series).
Is your reasoning for failure with two identical discreet transformers because there is no common core?
Perhaps a light load on the unused secondary will work?

Patric

Mon Jun 15 2015, 08:18AM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 254

Dr. Slack wrote ...

Don't diode load the output of a transformer, the rectified DC will quickly saturate the core and make it do things you didn't want.

Are you sure about that? Most transformers secondary are rectified...

Dr. Slack

Mon Jun 15 2015, 08:46AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you draw current in *both* directions from the output of a transformer in a *balanced* way, so getting a net zero DC transformer current, using a bridge rectifier, a voltage doubler, a centre-tapped 'push-pull' type full wave rectifier (which although each winding only has a single diode, the two windings are magnetically coupled so they compensate each other), then all will work fine.

If you try to draw uni-directional current using a single diode in series with a single secondary winding, then it will go pear-shaped real fast.

The two key observations are that

a) a transformer secondary is a short piece of copper wire, so you cannot develop a DC voltage across it

b) the core of a transformer has a high permeability, so only a tiny DC current is needed to saturate it

=> do not do anything with either the primary or the secondary that will cause a net DC on the transformer.

One thing I did wrong with the input of a transformer many years ago was to try to sense its input current for a smart HiFi I was building. I put three diodes in series to develop enough voltage to turn on the input of an opto-coupler, but then only put one diode in shunt to let current flow the other way, because I didn't know better at the time. The imbalance of just two diode drops in 240VAC was sufficient to saturate the transformer, and it hummed like mad. Output current will mess it up just as easily as input current.

If you look at the literature for various single-ended converter topologies, many have compensating windings to stop the transformer flux from saturating, which it would with an unbalanced drive or load.

Dr. Slack

Mon Jun 15 2015, 09:03AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Signification wrote ...

Is your reasoning for failure with two identical discreet transformers because there is no common core?
Perhaps a light load on the unused secondary will work?

No. It is because a transformer draws input current to meet the requirements of its load (plus some magnetising current which we can neglect to first order). A single current will flow through both transformers, so the output voltages will be whatever ohm's law makes them. If the other load is light, so high resistance, it will develop most of the voltage across it, 'collapsing' the voltage across the load output.

Bored Chemist

Mon Jun 15 2015, 07:18PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

While there is some hysteresis in magnetic materials, transformers are not noted for their memory. (They deliberately use materials with low hysteresis)
Imagine, for the sake of discussion, you set up the transformer feeding the filament through a diode and you throw the switch just as the voltage at the o/p of the transformer is zero and it's in the direction so that the diode will conduct.
As far as the transformer is concerned the diode acts like a short length of wire. it conducts.
For that cycle of the mains the transformer doesn't even "know" the diode is there.
After 1/100 or 1/120 of a second the voltage changes direction and, from the point of view of the transformer, it is feeding an open circuit because the diode won't let any current flow.
These two states of affairs- neither of which bothers the transformer, repeat in sequence.

How can running a resistive load via a diode harm the transformer?

( the valve may hum a bit more because the filament temperature is cycling half as fast)

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