Soft start for toroidal transformer

Thread Starter

EliCustom

Joined Sep 16, 2018
19
I need a least 5 amps of an isolated 120V AC power supply to test a circuit. It would seem a toroidal transformer is the way to go, specifically the Triad MD-1000-U. It's a 120VAC, 1000kVA toroidal transformer with a secondary current rating of 8.33 amps.

There should be a product made to take care of the inrush problem but there appears to be none. Is there a circuit I can build that'll take care of the inrush problem? Is there a better solution than a toroidal transformer to get an isolated 120VAC at 7 amps?
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
I have never had to wire a self start on a Toroidal yet.
If really concerned, fit two switches, one to power the primary first, then switch in the DC/Cap circuit second.
Max.
 
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Use a metered Variac as well, so you have an isolated variable supply.

Here's a soft-start ckt http://sound.whsites.net/project39.htm and there are plenty more. All basically for audio amplifiers.

I made a very rudimentary one for my amp. I have 50,000 uF of capacitance at an effective 50V. I do short out the primary resistor when all of the supplies are aproximately 2/3 of 50 V. The audio and the inputs are muted until you reach the 2/3 mark. I have fuses on each power rail which is good and bad. The time constant is set by the 50,000 uF effective capacitor. To make MY circuit better, I would need something to turn-off power if the system did not power up within a certain time frame, otherwise my metal oxide line resistor pops.

it did use a separate power supply and it logarithmically ramped up the audio with an opto-FET in series with the input. The resistor popped as intended when I accidentally swapped the NPN and PNP outputs with no other damage.

When servicing the amp the slow-start needs to be disabled.
 

ebp

Joined Feb 8, 2018
2,332
Are you concerned with the inrush current that can result from residual magnetism in the core or inrush due to charging capacitors on the secondary side? Both are rather hard on switches on the primary side and necessitate time delay fuses. Neither are harmful to the transformer but the former is more severe with toroids because there is effectively zero air gap, whereas E-I core types have an unintentional tiny air gap that helps with this problem.

Toroidal transformers have lots of desirable properties. One drawback is that it is harder to put an electrostatic shield between the primary and secondary for improved high-frequency isolation, but this often isn't necessary. If such a thing is necessary, an "ultra isolationr" transformer may be more suitable. They may be available as toroids but the only ones I've ever seen are E-I core types.
 
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Thread Starter

EliCustom

Joined Sep 16, 2018
19
Thanks for the help everyone. It looks like the DXengineering Ameritron ICP-120 will work. It would be nice if there was something in their specs that stated if it was rated for 1kVA

I would build a project39 circuit board but they give no information on the value of F1. It only says the value 'depends on the amps'. I'm assuming F1 is a switch

I would rather go with an EI transformer but there seems to be none available in 1kVA that's rated for 120VAC. 750 or 800VA would probably be enough but aren't any of them rated for 120VAC
 

ebp

Joined Feb 8, 2018
2,332
Some transformers are made with dual primary and/or dual secondary windings. For 240 V use you connect the windings in series and for 120 V you connect them in parallel. This does mean that you are unlikely to find one with a cord with a standard plug and a standard receptacle. There are "control transformers" (so named because they are commonly used in things like industrial control systems) that very commonly have dual primaries and secondaries and have screw terminal blocks on the larger sizes. I've used Hammond transformers like this. They are conservatively designed, but they use an E stack and an I stack and weld the two together, rather than alternating E and I laminations. This can result in buzzing.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
@EliCustom Have you actually hooked it up a tested it in order to see if there is any need for concern?
I have used many Toroidal's including adding custom over-winds for small aux P.S.
Max.
 

shortbus

Joined Sep 30, 2009
10,045
This has morphed from the first post, from "1000kVA" in post 1, to "1kVA" in post 6. For the 1kVA, wouldn't a simple NTC thermistor work?
 

Thread Starter

EliCustom

Joined Sep 16, 2018
19
Typo on the 1,000kVA. As you already know it's 1,000VA or 1kVA. My mistake

Most people don't think an NTC thermistor is s a good idea for a toroidal past 300VA. I did the math and came up with the NTC thermistor value that would supposedly work. It would be great if such a simple solution would work.

I'm going to go with the ICP-120. This unit has been used in the audio world for some time and is the accepted solution. I'll be building a project39 circuit board in the near future. With my offshore job. it takes a long time to get any project completed. The ICP120 will remove a what-if problem and I can get the ultrasonic tanks completed.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
According to the spec sheet it is a medical grade ready to go unit in an enclosure.
A little strange the primary neutral is also fused?
Max.
 

shortbus

Joined Sep 30, 2009
10,045
Most people don't think an NTC thermistor is s a good idea for a toroidal past 300VA. I did the math and came up with the NTC thermistor value that would supposedly work. It would be great if such a simple solution would work.
Just my thinking on this. When doing "high end audio" there are many things that people seem to be able to hear, that the common man can't. And this leads to applying more and more "improvements" to a system. Many of these "improvements" come from "experts" online. Things that weren't necessary back when the only source of the improvements was print magazines. Now any one with a computer and the right language skills can make their pet project the next "needed improvement" that 'can' be heard. And no one wants to be the guy that can't hear what they are supposed to. All you need to see to prove this theory is the 'oxygen free' wire and the capacitors that make claims to improve the sound.
 
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