transformer questions

[W8ACR]

A few questions about transformers:

1. Is it always possible to use a transformer with a center tapped secondary with a bridge rectifier, or are there situations that one must use only a full wave center tap rectifier circuit? I assume that if a bridge rectifier is used, the center tap is not connected to anything - correct?

2. Suppose a transformer secondary is rated 1000VCT@200mA. Using a fullwave center tap rectifier with a choke input, one would have a DC output of approximately 450V@200mA. With a bridge rectifier, one would obtain 900V@200mA - correct? I assume that there would be 2X the primary current, but other than that, it seems to me like one is getting something for nothing. I seem to be missing something here.

3. What is a typical percentage increase when going from CCS to ICAS ratings? in other words, if a transformer secondary is rated 200mA CCS, what is a safe level of current draw for intermittent use?

4.  I sometimes see filament wiring as a twisted pair, and sometimes see one side grounded. What are the pros and cons of these two options?

Thanks ... Ron ..... W8ACR ........... ignorant in Montana

[AB2EZ]

Ron

Simple answers (given more details about a specific transformer, and how it will be used, there might be  somewhat different answers to some of your questions... but not that different):

1. Yes, you can use a center tapped winding to drive a full wave rectifier or a bridge rectifier. In most cases, you would not use the center tap.

[Exception: if you want to build a supply that provides both B+ and 0.5 B+, then you would use two capacitors in series to serve as the output filter capacitor; and you would connect the center tap of the winding to the point where the two capacitors are connected. By doing this, you would be able to draw current from the 0.5B+ output of the supply (which is also where the two capacitors connect together) without disturbing the split of the voltage between 0.5 B+ and B+]

2. No, you cannot produce twice the full wave configuration power rating of the transformer by using a bridge configuration... but may be able to produce somewhat more, depending upon lot's of details.

A. In a full wave rectifier, current flows during only half of each cycle in each half of the secondary winding. In a bridge rectifier, current flows during both halves of each cycle in each half of the winding.

B. In a full wave rectifier, current flows in both halves of each cycle in the primary winding, and this primary current is equal to the current flowing in either half of the secondary winding x the turns ratio N. In a bridge rectifier, current also flows in both halves of each cycle in the primary winding, but that current is the sum of the currents that flow in each half of the secondary winding x the turns ratio N 

Thus, if you used the same rectified (DC) output current, you would produce twice as much heating in each half of the secondary winding using the bridge configuration, and you would produce four times as much heating in the primary winding using the bridge configuration.

If heating of the secondary winding is the limitation on the rating of the transformer, then you will have to reduce the rectified (output) current by a factor of 1.414

If heating of the primary winding is the limitation on the rating of the transfromer, you will have to reduce the rectified (output) current by a factor of 2

If in doubt, use the factor of 2.

3. I don't know if there is a standard practice for converting the CCS rating to an ICAS rating (particularly considering the old buzzard phenomenon)

4. If you use chassis ground as one of the filament leads (vs. a twisted pair with neither side grounded) you are more likely to have hum problems. This is caused by several physical phenomena (mostly magnetic pickup by the open loop formed by the single wire and the ground return) Even better is to use a center tapped filament transformer, whose center tap is grounded, and a twisted pair to feed each filament.

Best regards
Stu

[WQ9E]

Ron,

Using a center tapped transformer with the center tap disconnected can be a problem if the insulation is insufficient around the center tap area.  On lower voltage transformers this is not a problem but with higher voltage there is a definite chance of breakdown since the center tap using a bridge rectifier will be at half the total secondary voltage instead of ground.  There is a similar concern with putting the filter choke in the lead between center tap and ground which will raise the center tap above ground equal to the peak ripple voltage.

Using the bridge you will get double the DC output compared to the center tap connection but using the same transformer the current output will be reduced to 1/2 so the total power rating of the transformer remains the same.  Using the full wave grounded center tap connection each half of the secondary is providing half of the load current; with the full wave bridge the entire current is drawn across the entire secondary.  Of course the primary power draw is also increased if you try to use the transformer with a bridge connection at double the original power rating and it would likely more than double since the transformer efficiency is less than 100%.

As to going from CCS to ICAS the old handbooks discussed around a 50% increase as being reasonable but they were generally using the case of a TV set transformer being used in a CW transmitter where the intermittent service is truly intermittent.  A series of old buzzard AM transmissions would approach CCS conditions pretty quickly; CW and SSB are much more friendly to pushing the ratings.  Also realize that if you go from choke input to capacitor input for you filter you will get more DC voltage but with increased transformer heating due to the higher peak charging current for the capacitor input so you will have a hotter transformer than originally intended and may need to de-rate it a bit if it is rated for a choke input filter.

Twisted pair filament leads are meant to reduce filament hum pickup in other circuits; you will often see along with a hum balance pot in sensitive low level audio equipment.  Using one side grounded simplifies the wiring and seems to be the most common approach in ham gear.  Either approach should be fine for the typical transmitter.

Hammond transformer provides some useful reference materials including this guide to rectifier circuits and current:  http://www.hammondmfg.com/pdf/5c007.pdf   The older handbooks including the Bill Orr handbooks are also good items for your reference library.

Happy powering!

Rodger WQ9E

[W7XXX]

A 1000 volt CT 200 ma transformer with a center tap grounded and using a diode on each side or full wave will put out approx 500 volts (this depends on line voltage, solid state or tube rectifier) at 200 ma. If one needs the full 1000 volts then the CT is left unhooked and one end is grounded and the other end is where the bridge rectifier is hooked. The output is approx 1000 volts, but the ma rating is halved. You have twice the winding and more resistance, thus the amount of current that can be pulled is halved.

The twisted pair filament hookup maybe seen used with tubes with common cathodes and heaters, such as 813. The center tap of the filament transformer is grounded for the cathode return and the voltage needed for the heaters is taken from the other legs. The twisted pair is usually accompanied by capacitors .001-.01 hooked at the tube socket to each side of filament/cathode. Twisted pair for tubes with separate cathode and heaters maybe used with one side grounded as a lower resistance ground and/ or for shielding. It all depends on application. Anytime I use a steel chassis, I use a twisted pair with one side grounded or run the hot wire thru a grounded braided shield.

[W7XXX]

doubled sorry

[KE6DF]

Many of the old UTC CG plate transformers are designed to use with the center tap grounded -- you are warned about this issue in the catalogs. The reason is that the center tap is not insulated well enough to withstand the voltage that the center tap would see in a bridge circult.

In some cases, they have some versions of the transformer, that ARE designed to work in a bridge circuit -- this is indicted by a W as the final letter in the part number (e.g., a CG-303 vs. a CG-303W.

I belive that to run a transformer in a bridge circuit rather than a full wave circuit with the center tap grounded you can draw about 70% of the rated current rather than 1/2. In other words, if your transformer is rated at 1000v CT at 200ma that means you can get 500v at 200ma with a full wave circuit with the CT grounded, or 1000v at 140ma with a bridge circuit. (the output voltage depends on the type of filter used of course)

If you look at the old UTC catalogs, you will find some transformers are rated for either rectification circuit, and indeed, the current rating for the bridge circuit is 70% of the current for the grounded CT full wave circuit.

I posted an analysis a while back of this, but it went over like a lead balloon, but I still feel it's right.

The reason is that when you use a grounded CT circuit, only 1/2 the secondary is conducting at a time. When using the bridge circuit, the whole secondary is conducting all the time.

So if you run the transformer at the same output current in a bridge circuit you would have double the heat generated in the resistance loses in the secondary. But, if you decrease the current to 70%, then you have the same heat loses. The reason is that heat generated is I squared R. So you to get half the heating you need to derate by dividing the output current by the square root of two which is approximately the same as multiplying by 0.7.

Here is interesting link for info on using CCS transformers in various ICAS applications:

http://wb0nni.dakotamade.com/xfmrpwr.html

[KM1H]

Most of those old transformers will not survive in either a FW bridge or doubler when using a high C filter as in SSB/CW amps due to the high secondary winding DC resistance and impedance. Use a choke input and calculate the minimum L required to keep the voltage from soaring and destroying the xfmr via saturation. All the old Handbooks covered this even back in the early 30's.

Carl
KM1H