Isolation transformer.. Math! - Page 1
Forum section: electronics / beginners

author: edmundopt
author_rank: Regular Contributor
timestamp: February 21, 2014, 04:31:56 pm
content: Hello,I have some work to do with a 320W Switching power supply, it is a 240VAC(max) mains input.I want an isolation transformer to work with it, so I need a new one, I am looking at this :https://pt.rs-online.com/web/p/transformadores-de-seguridad/4369808/Wich is BLOCK TIM 500 rated at 230AC with 500VA, two output of 2x115VAC, wich I will join to make 230VACWill it be near the limit ? Maybe a 800VA one will be better?This will be a one time only job, so, saving € is important!What is the math on isolation transformers for power output ?

author: Fsck
author_rank: Super Contributor
timestamp: February 21, 2014, 04:36:00 pm
content: Are you fully loading that PSU? And, do you have the specs of the PSU?

author: edmundopt
author_rank: Regular Contributor
timestamp: February 21, 2014, 04:59:46 pm
content: MeanWell SP-320-24, 24volts output, stepper motor driver up to 8Amps, and resistive dummy load up to 10amps.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 21, 2014, 06:18:20 pm
content: If its any kind of modern PSU, its power factor should be really close to 1.00 (~0.95). In that case it should be drawing something very close to 320VA. 500VA should be just fine:320 / 0.95 / 0.92 = 366 VApower / PF / trans eff = peak VA from wall

author: edmundopt
author_rank: Regular Contributor
timestamp: February 21, 2014, 06:22:27 pm
content: Aren't there any pratical maths for transformers?for example, relays can switch :75% on resistive loads40% on inductive loads20% on motor10% on Incandescent bulbs

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2014, 08:10:39 pm
content: Does the switched mode power supply have power factor correction? If so a 500VA unit will do, otherwise it may need to be larger, depending on the actual current drawn.How long do you intend to operate it for? If it's just for test purposes, it may be fine for a short period of time, say under 10 minutes.

author: edmundopt
author_rank: Regular Contributor
timestamp: February 21, 2014, 08:46:59 pm
content: yes, it has active power factor correction. About Meanwell, they do pretty good PS, this one is 87% or better, 0.95PF.All of their PS beat the specifications mentioned.Testing, periods of 1 to 2 min, 5max.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2014, 09:08:35 pm
content: Then I think a 500VA transformer is ample.

author: Fsck
author_rank: Super Contributor
timestamp: February 23, 2014, 02:38:50 am
content: Quote from: Hero999 on February 21, 2014, 09:08:35 pmI get: (320W / .82 psu efficiency [they spec 87%])/(0.90 PF [their specs are >0.95 for "typical"]) / (0.90 transformer efficiency [again, assuming you may have a slight dud]) = 464VA.Meanwell's specs are incomplete, efficiency and PF appear to be specced at 230VAC and 25C ambient. I think you'll probably be fine unless one or both item don't meet their basic specs.

author: megajocke
author_rank: Regular Contributor
timestamp: February 26, 2014, 09:40:41 pm
content: The transformer is probably rated for output power so its efficency shouldn't matter.