Selecting transformer (s) for an isolation bloc to supply a 24V Power Supply - Page 1
Forum section: electronics / beginners

author: Birckcmi
author_rank: Newbie
timestamp: June 26, 2023, 04:14:12 pm
content: That sounds complicated. Here's my issue: I need to power a nichrome wire at about 24 volts & 2-8 Amps There are some choices for how exactly I do that, but my first concern is: if I build an isolation transformer system out of two identical transformers, what does their wattage need to be to deliver 8 Amps at the other end? The isolation transformers should deliver about 100-105 VAC, but to achieve an 8Amp output will the isolators need to be 8Amps themselves?

author: themadhippy
author_rank: Super Contributor
timestamp: June 26, 2023, 04:34:16 pm
content: forget about watts,transformers tend to be measured in VA,8 amps at 24v is 192VA , add a bit for losses, something around the 250VA with the voltage rating you require should do

author: Birckcmi
author_rank: Newbie
timestamp: June 26, 2023, 07:17:39 pm
content: Thanks for the reply. I may not have provided enough information. Once I get the isolation transformer(s) set up, I will have 120VAC coming into either a dedicated power supply with a 24VDC output and 15A capability(based on 120 incoming), or 120VAC coming into a hefty dimmer switch. I need to be able to vary the current going to the nichrome wire. 8Amps would be the maximum I would need, but I would also need lower amounts of current. Same answer? Thank you for taking my question seriously.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 29, 2023, 12:22:22 pm
content: It depends on how you wire the two transformers. If the secondaries are in series then they both need to be able to mange the full current, if in parallel the each transformer only has to provide half of the current. So at a minimum, a pair of 12V 96 VA types. Or a pair of 24V 96VA types, No surprises there.You should only wire transformer secondaries in parallel if the transformer types are truly identical.Series or parallel you have to take care to keep them in phase. See the dots, they matter.For resistive loads like your hot wire the transformer VA's and the Watts are the same thing. For loads like motors, welders and such your VA needs are a different kettle of fish.You dont need rectifiers. 24V 8A DC will heat the wire to the same temperature as it will on 24V 8A DC. AC electricity 1.01For AC the actual power is P=V.A.cosØ for resistive loads cosØ=1. Impress your pals!Unless you're getting the transformers at zero or very low cost... A 10A 24V variable power supply (eg, see link at end of post) will cost about the same or less. Say $50.For a 200W load you need at least a 200VA transformer.In the real world, assume that your transformer is only able to transfer 95% of the power from one side to the other.1.05*192=201VA or 2x100.5VA minimum. Thats not really good enough, you want at least a pair 120VA types.https://www.aliexpress.com/item/1005005444830662.html?spm=a2g0o.productlist.main.35.3d847ccfmmhVRJ&algo_pvid=88e1e8a3-f0d0-41ab-ba34-6ef8a2d4775a&algo_exp_id=88e1e8a3-f0d0-41ab-ba34-6ef8a2d4775a-17&pdp_npi=3%40dis%21GBP%2191.19%2137.78%21%21%21%21%21%402100b88516880408530871137d07b4%2112000033107172648%21sea%21UK%210&curPageLogUid=F580Q0n9k6t9

author: ledtester
author_rank: Super Contributor
timestamp: June 29, 2023, 01:16:46 pm
content: Quote from: Birckcmi on June 26, 2023, 04:14:12 pmand in another thread (title: Power Isolation) you wrote:Quote from: Birckcmi on June 26, 2023, 11:02:17 am1) Assuming the AC-DC 24V power supply is at least 85% efficient you'll only need around 2A from your isolation transformers.2) There are a lot of AC-DC power supplies which provide an isolated output and would obviate the need for the isolation transformers.What exactly is your application? Why are you concerned about isolation?