ordering custom toroid transformer: idle current and other parameters - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: March 26, 2019, 12:00:45 pm
content: Hello,I want to order a custom transformer. One of parameters is idle current. I'm not sure what to put there.General desired parameters: 160-240VA, 230V input, outputs: 2x14V, 2x5VThere is also an option to add varnish, what do you think about it?

author: mariush
author_rank: Super Contributor
timestamp: March 26, 2019, 12:31:10 pm
content: Wouldn't it be cheaper to buy a ready made 2 x 14v or 2 x 15v transformer and ADD your own 5v windings over the existing windings of the transformer?Also how much power do you need on your 2 x 5v? It may be cheaper to use a separate pcb transformer.Here's some results for 2x14v or 2 x 15v , 160-300 VA :TME.EU linkThey're as cheap as 25$ for a 160VA 2x14v AC ... not sure you'll get a cheaper CUSTOM transformer* 225VA 2x15v is 34$ :https://www.tme.eu/en/details/55161-p1s2/toroidal-transformers/talema/* 300VA 2x14v is 42$ (58x118) :https://www.tme.eu/en/details/tst300w_14v/toroidal-transformers/indel/tst-300003/And for low voltage, you may want to go with 2 x 6v , way more options, more mass produced transformers for that voltage .. here's list :TME.eu listFarnell also stocks transformers but they're usually more expensiveHere's 230v 2 x 5v or 6v AC :https://uk.farnell.com/w/c/transformers/isolation-transformers?secondary-voltages=2-x-5v|2-x-6v&sort=P_PRICEHere's 2x14v or 2x15v AC 160VA or higher :https://uk.farnell.com/w/c/transformers/toroidal-power-transformers/prl/results?power-rating=160va|225va|300va&secondary-voltages=2-x-15v|2-x-15v-30v&sort=P_PRICE

author: exe
author_rank: Unknown
timestamp: March 26, 2019, 12:58:14 pm
content: I wanted to add an electrostatic shield, that's why I wanted a custom solution. It seems it won't be much more expensive than what TME offers if ordered from the right country.Current design indeed uses a separate psb transformers. But if it's not much expensive to add a few more windings, then why not?.PS the load current on 5V windings is less than 100mA. It's just to supply opamps and voltage reference of my power supply.

author: Benta
author_rank: Super Contributor
timestamp: March 26, 2019, 05:10:13 pm
content: I'd buy a standard 2 x 15 V transform and add an off the shelf isolated DC/DC module for the +/- 5 V. At 100 mA it's small, it's only 1 W.Varnish? If it's for a ship or jungle application, yes. Otherwise leave it.

author: Jwillis
author_rank: Super Contributor
timestamp: March 26, 2019, 06:13:06 pm
content: These guyshttp://www.antekinc.com/don't make custom transformers but removing the plastic covering and removing some of the winding's on a 15 volt and adding an additional secondary of 5 volt you can achieve what your looking for.They even have steel cases to fit .This would be the least expensive way of getting the parameters you desire. Modifying transformers is not a hard procedure at all because the primaries are wound first because they all have a common voltage then the secondaries are wound according to stock demand. Then if you wish they can be varnished.

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 27, 2019, 01:30:34 am
content: Quote from: exe on March 26, 2019, 12:00:45 pmAt that power rating (160-240VA), an idle current of 5% of the full load current on the primary is reasonable. For example, let's suppose that at full load your transformer will be drawing 1A on the primary. Specify 50mA as an idle current.If the insulation used between the winding layers is going to be paper, then varnishing is imperative. Paper attracts moisture and becomes a conductor instead of an insulator with time. Varnish is used to prevent this process.