Toroid power transformer - good idea?

[KC2TAU]

I've been planning to replace the power transformer for the LV supply of my ART-13. The fellow who built it did something to the effect of combining the current transformer's 5v winding with the secondary of a 28vdc transformer to help boost the primary of the LV supply's transformer to help get the voltage up to around where it should be. The problem is that at 117vdc I'm just barely seeing 390vdc under load. This really isn't enough not to mention I'm sure the transformer isn't very pleased having so much being thrown into its primary winding.

I was thinking of replacing it with a toroid transformer such as this:
http://www.antekinc.com/details.php?p=90

There's a spec sheet PDF on that page under "Files and Specs". It seems to sit at 470vdc unloaded and drops to 430vdc at 1.3A. Considering how the ART-13 needs about 300-400ma for its LV this should be plenty of current capacity. I also need to increase the voltage capacitor of the filter capacitor network for the LV network. Currently it looks to be a single 60mfd 450vdc capacitor. I recently purchased two 120mfd 420vdc capacitors to run in series.

Do you think I'll need an equalizing resistor for the two capacitors? Following this formula I get the following results for the resistor value:

For 2 capacitors in series:      R = (2Vm - Vb) / (0.0015 C Vb)

where      R = resistance in megohms
Vm = max voltage you'll permit on either capacitor
Vb = max voltage across the entire bank of two (or N) capacitors
N = number of caps in series
C = capacitance in microfarads

(2 (500) - 840) / (0.0015) * (120) * (840)
160/151.2 = 1.058201058201058

Does that seem right to you?

[W7TFO]

A toroidal power transformer will appear to be smaller than an EI type, by eye.  They also have a footprint requiring but one convenient hole to mount them.

A marked reduction in EM hum field as well.

With the caps each so close to operating voltage and only two, I'd say the balancing resistors would be optional.  However, a bleeder is a good idea and you could use two in that function to also do the balancing trick.

73DG

[KC2TAU]

What value would you recommend for the bleeder? The caps will be receiving about 440vdc in total hence why I am doubling up for a total voltage capacity of 840vdc.

[KM1H]

The balancing resistors help when the caps age and develop slight leakage and value changes. A 100K is more than enough wheras Ive seen commercial rigs where a 470K caused problems and one of the 2 caps to actually open.

Carl

[WU2D]

I used a big old TV transformer that I cleaned up for the MV supply. All I used was the HV windings and some diodes in a conventional pi choke filter with a stiff bleeder and she has survived in the rack since 1992.

[gerry_w1id]

Keep in mind they specify 115V primary voltage. Most places run much higher than that. Here I am now reading 122V line voltage. You should recalculate using this figure. Also, I would decrease the value of what you call equalizing resistors so that they effectively become bleeder resistors. This is a needed safety feature and should be calculated to draw 10% of the total load. Power transformers are generally designed to deliver some maximum current value with 10% secondary voltage drop. Toroidal transformers are highly efficient, self shielding, and have a much smaller footprint than E core lamination transformers. At $50, I'd say go for it.

[W7TFO]

Keep in mind they specify 115V primary voltage. Most places run much higher than that. Here I am now reading 122V line voltage. You should recalculate using this figure. Also, I would decrease the value of what you call equalizing resistors so that they effectively become bleeder resistors. This is a needed safety feature and should be calculated to draw 10% of the total load. Power transformers are generally designed to deliver some maximum current value with 10% secondary voltage drop. Toroidal transformers are highly efficient, self shielding, and have a much smaller footprint than E core lamination transformers. At $50, I'd say go for it.

Another classic example of someone posting without ever reading the previous ones.

Ditto, ditto, ditto.

73DG

[Steve - K4HX]

Consider it a second opinion (like at the docs office).

[KC2TAU]

Keep in mind they specify 115V primary voltage. Most places run much higher than that. Here I am now reading 122V line voltage. You should recalculate using this figure. Also, I would decrease the value of what you call equalizing resistors so that they effectively become bleeder resistors. This is a needed safety feature and should be calculated to draw 10% of the total load. Power transformers are generally designed to deliver some maximum current value with 10% secondary voltage drop. Toroidal transformers are highly efficient, self shielding, and have a much smaller footprint than E core lamination transformers. At $50, I'd say go for it.

The load test is carried out at 120vac according to the spec sheet. The line voltage in this area is around 121V but switching on the ART-13's filaments means it certainly doesn't stay that high.

[gerry_w1id]

Sorry for my input. I posted it after looking at the transformer specs. It won't happen again.

[N0WEK]

Keep in mind they specify 115V primary voltage. Most places run much higher than that. Here I am now reading 122V line voltage. You should recalculate using this figure. Also, I would decrease the value of what you call equalizing resistors so that they effectively become bleeder resistors. This is a needed safety feature and should be calculated to draw 10% of the total load. Power transformers are generally designed to deliver some maximum current value with 10% secondary voltage drop. Toroidal transformers are highly efficient, self shielding, and have a much smaller footprint than E core lamination transformers. At $50, I'd say go for it.

The load test is carried out at 120vac according to the spec sheet. The line voltage in this area is around 121V but switching on the ART-13's filaments means it certainly doesn't stay that high.

To be fair, the tests were carried out at 120 vac but the specs specify 115/230 vac!

[KC2TAU]

Before I make my decision on which transformer to purchase roughly what would the change in voltage be from going from AC to DC given a full wave choke input rectifier with 60mfd of capacitance and a 10Hy choke? I've read that the rule is the output DC voltage of such a supply is 90% of the secondary AC voltage of the transformer, correct?

[W4NEQ]

That is a good rule of thumb, but will vary a just a bit with current.

Chris

Before I make my decision on which transformer to purchase roughly what would the change in voltage be from going from AC to DC given a full wave choke input rectifier with 60mfd of capacitance and a 10Hy choke? I've read that the rule is the output DC voltage of such a supply is 90% of the secondary AC voltage of the transformer, correct?