Using an isolation transfer to step down mains to 115v to run a Hot Air gun - Page 1
Forum section: electronics / beginners

author: bsodmike
author_rank: Regular Contributor
timestamp: August 22, 2022, 12:39:50 pm
content: Hi all,I live way out in the sticks and cannot order a step-down transformer easily. It's simpler just to build one.I plan to order this particular isolation transformer (as I have used a similar model for my Variable AC Isolation Power Supply)https://export.farnell.com/schneider-electric/abl6ts40g/transformer-400va-1-x-115v/dp/2070201The device I need to run a Metcal HC-900 which has is a 230W load on 115vac. I'm in a 240vac location and given even 300VA would suffice, can step this down to 115VAC as per the correct taps (see Datasheet at the above link).I would still tie the Metcal's green Earth line the mains earth, even though L/N would be an "isolated" pair (essentially Live-Live).Only means of electrocution would be to hold both L/N, but since it's isolated there's no way it can conduct to earth.For safety reasons, I would wire in order:an NTC, I could try using this since I have a few on handhttps://export.farnell.com/epcos/b57213p0330m301/icl-ntc-thermistor-33ohm-3a/dp/3407656?ost=3407656/ 33 Ohm, 3A capable4A breaker (Schneider), I have a few spares around.Fuse: 5A / 5 x 20mm (https://export.farnell.com/multicomp/mc000889/fuse-cartridge-5a-quick-blow/dp/2461220?ost=2461220)Neon indicator (chassis mount) and a large enough resistor to protect the neonLet me know if any of the above are over kill, or if there's anything else I should add?You'll see some of the same components used here:https://youtu.be/_VV9mvuu--QThanks!Cheers, Mike

author: Monkeh
author_rank: Super Contributor
timestamp: August 22, 2022, 12:49:27 pm
content: Have you considered simply fitting the 230V element instead?

author: bsodmike
author_rank: Regular Contributor
timestamp: August 22, 2022, 01:13:33 pm
content: What do you mean? Checkout the video below, he takes it apart and the paddle pump seems to be driven straight off 115VAC mains?

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2022, 05:59:05 pm
content: Toroidal is cheaper, lighter and more efficient.https://export.farnell.com/multicomp/vtx-146-625-155/transformer-625va-2x-55v/dp/1675103

author: bsodmike
author_rank: Regular Contributor
timestamp: August 23, 2022, 09:52:06 am
content: Thanks for the tip re. the torroidal transformer but there are not isolation specs; at least the Schneider version lists the primary/ground and primary/secondary isolation voltage ~= 4kV etc.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2022, 05:32:51 pm
content: Quote from: bsodmike on August 23, 2022, 09:52:06 amWhy do you need isolation? An autotransformer (no isolation) would be perfectly find for powering a soldering iron station.Here's another even cheaper transformer.https://export.farnell.com/vigortronix/vtx-146-625-255/625va-toroidal-transformer-2x55v/dp/2817718?MER=sy-me-pd-mi-alte

author: Robert763
author_rank: Super Contributor
timestamp: August 23, 2022, 06:49:13 pm
content: The transformer you have linked to is expensive and excessive for your application.You do not need an isolation transformer to run the Metcal station. An auto transformer is fine nad will be smaller and cheaper.In fact you can useany250VA or larger transformer that has a dual 115/230V (or 120/240) primary. It does not matter what the secondary voltage(s) is (are).You just connect the primaries in series for 230V as you normally would. Make NO connection to the secondary. Connect the live of the socket the Metcal will plug into to the center connection of the two primary windings and the neutral to the neutral of the 230V supply. Earth goes to the earth of the supply.This will now act as an autotrnsformer. The primary is fully capable of carrying the load. In fact as the Met call does not draw 230W all the time a slightly smaller transformer could be used. A good quality 220VA EI transformer would be fine and for a toroidal a 180VA would be OK unless you are using the station continiously for hours.Robert.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2022, 10:32:53 pm
content: Quote from: Robert763 on August 23, 2022, 06:49:13 pmThat will work, but it's better to use a transformer with a 115V primary and 115V secondary. If the soldering iron draws 230W, then the transformer only needs to be rated to half the power, if it's connected as an autotransformer, so 115VA will do. Connect the primary and secondary in series, first via a small incandescent lamp to check the phasing. If it's right, the lamp shouldn't light. If it does, swap the connections to either the primary, or secondary. Connect the load between the where the primary and secondary are connected and neutral.

author: Robert763
author_rank: Super Contributor
timestamp: August 24, 2022, 06:54:01 pm
content: Quote from: Zero999 on August 23, 2022, 10:32:53 pmNot quite right. A 115V to 115V isolation transformer will provide two times it's VA rating in step-up (115V to 230) because the current is the same in both windings. But for step down it will only be the normal isolation mode rating. For example a 250VA 115V to 115V transformer will have a 2.17A secondary winding current. In step up (115 to 230V) the secondary is in series with the 115V supply so that is effectively unlimited current and the winding adds 115V at 2.17A so that is 230 x 2.17 = 500VA.For STEP down ALL the output current flows in the secondary winding which is limited to 2.17A so that is 115 x 2.17 = 250VA.Note however that this transformer would be smaller than one with a 230V 2.17A secondary (e.g. isolation).Available power is limited by core size but the maximum current also depends on the wiresize. An isolation transformer only needs to carry it's rated current. A dedicated autotransformer rated for step up/down will use heavier gauge wire to reduce the resistive (I2R) heating and acheive full VA on both step-up and step-down modes. The transformer is still physically smaller because the required core is still half that required for isolation mode.

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2022, 08:00:10 pm
content: Quote from: Robert763 on August 24, 2022, 06:54:01 pmThat's not true at all.Why would a transformer require a higher VA rating when run in step-down, than in step-up?In both cases, if it has a 1:1 turns ratio, half the load current flows through both windings. The current in the coil which was originally intended as a primary will be identical to the current in the secondary. Half of the primary current is diverted through the load, adding to the secondary current.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 25, 2022, 07:39:43 am
content: Hi Robert,Just to clarify if I understand your numbers. Let's assume we have 240VAC on the primary, 400A transformer, this means a max of I= 400VA/240VAC ~= 1.67ANow, this is stepped down to 115VAC on the secondary, so the secondary is 400VA capable, hence 400VA/115VAC ~= 3.48A; also a turns ratio of 3.48/1.67 = 2.08 turnsDoes this sound right?

author: bsodmike
author_rank: Regular Contributor
timestamp: August 25, 2022, 08:48:25 am
content: Hey Zero-What about this one, it's 500VA (just 125VA less than yours) and even lighter at 4.3kg. Everything looks identical to the one you recommended.https://export.farnell.com/vigortronix/vtx-146-500-255/500va-toroidal-transformer-2x55v/dp/2817713?ost=vtx-146-500-255Primary: 500VA/230VAC = max 2.17A -- does this mean I can use a 2 amp fuse on the primary input?Secondary: 500VA/110VAC = ~4.55A

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2022, 09:50:39 am
content: Quote from: bsodmike on August 25, 2022, 08:48:25 amYes, that's fine, if you don't mind waiting. I did notice it in the search results, but discounted it as it's on back order for the 21stNovember.

author: Haenk
author_rank: Super Contributor
timestamp: August 25, 2022, 04:00:54 pm
content: I got a 230 to 115V a couple of months ago from Amazon, I think it was like 60 EUR, sold by Amazon imported into EU with CE 'n stuff. Nice case, fuse and thermal protection. AFAIR it is 500W, which is plenty enough.Isn't shopping at Amazon an option for you? There are plenty if those transformers readily available...Ebay has them most likely as well....

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2022, 05:39:38 pm
content: Quote from: bsodmike on August 25, 2022, 07:39:43 amWhen wired as autotransformer, the transformer only handles half of the power, so it can be half the size.

author: Robert763
author_rank: Super Contributor
timestamp: August 26, 2022, 08:19:32 am
content: Zero999 you are correct.However the 2:1 advantage of an autotransformer is ONLY for the 2:1 voltage step up/down.The advantage is because the "primary" current also flows in the load for step down as you describe. My mistke was partially based on knowedge of multiple tapped autotransforme that have reduced rating and variacs which have a current rating rather than power / VA.To have the advantage the primary and secondary currents have to be balanced.Consider a 100V applied to a variac wound with 100 turns of 1A rated wire (assume core and wiper are adequate size). This is nominally 100VA as a 2:1 stepdown auto transformer. If the windings were split into a 1:1 autotransformer it would be 50V 1A, 50VA output At the center position position the output is 50V, if we draw 2A then 1A will flow through the "top" (primary) 50 turns from the supply. The magnetic field from this is coupled to the bottom windings providing another 1A of output We have 100VA. There are 50 Amp turns in each section. If we move the tapt to the end of the 1st turn for 99V output and draw 1A we have 1 Amp turn in the primary and 1/99 Amp turns in the secondary Thus the secondary is only supplying 10.1mA for an output of 1.01 VA This is no advantage over an isolation (or fixed ratio autotransformer) which has the advantage of using different size wire for the high and low voltage sections.

author: mariush
author_rank: Super Contributor
timestamp: August 26, 2022, 09:31:03 am
content: You could also just get a 110/120v AC inverter and power it from a laptop adapter or a computer power supply.Computer power supplies are cheap, can get decent ones for under 50 euro/pounds and you can reuse it for other things. An inverter would take 10-14v and produce 115v just fine, and you could also reuse it for other things.Benefit would also be that it does 60 Hz frequency, compared to transformers which will do 50 Hz or whatever frequency you have. Doubt the wrong frequency would affect the pump or the heating element but still...

author: bsodmike
author_rank: Regular Contributor
timestamp: August 26, 2022, 11:09:25 am
content: I'm somewhat skeptical of the various chepo chinese clones in terms of their safety etc. Brands like Meanwell etc are usually OK.Feel free to send me links of anything on Amazon that you'd tried in the past and it has been engineered so as "not" to kill the end userThanks!

author: bsodmike
author_rank: Regular Contributor
timestamp: August 26, 2022, 11:19:29 am
content: For the time-being I'm going to use my bench-top isolation transformer (which isolates 230VAC) + Variac to supply 115VAC.I need to do some re-wiring as right now it's configured to only provide L/L` through a parallel-pair of 60W light-bulbs (acting as a current limiting device, to test for potential shorts in a DUT).I have this isolated rotary switch, which just needs to be wired up to allow by-passing the pair of light-bulbs. Will give this a shot first.https://export.farnell.com/schneider-electric/k10b001uch/rotary-switch-1-pole-10a-400v/dp/3108265?ost=3108265My Variac is 480VA rated but with a max output current of 2Ahttps://export.farnell.com/multicomp/mc2f-1/transformer-variable-2a/dp/1086028?ost=1086028With the Variac stepping the voltage down, doesn't this mean it'll try to push 400VA/115VAC ~= 3.48A through (max)? I suppose this will cause the Variac to heat up excessively, but this all depends on how hard the Hot air station is pushed right?

author: wizard69
author_rank: Super Contributor
timestamp: August 26, 2022, 05:49:53 pm
content: Honestly, I'd buy a large transformer and derive a properly grounded secondary that can support a number of NEMA 15 amp outlets. Place fuses or circuit breakers on the windings according to NEC, maybe even a ground fault device. You then have a number of properly installed NEMA outlets to support any North American configured hardware.This will solve your problem, at the expense of a larger transformer, for years to come. Beyond that you will be secure knowing that it is properly installed and protected. Really 120 VAC really should be treated no different than any other power supply issue a home lab has. If you need a 12VDC battery eliminator you buy or build for that need, same thing for a variable volt DC supply, dim bulb tester or anything else. In this case just build the electrical supply like the engineers expected the soldering station to operate against.By the way yes I know a supply specfic to the soldering station will be cheaper. I'm just looking at the long term here and the possibility that this is not your last 120 VAC class device.

author: Robert763
author_rank: Super Contributor
timestamp: August 26, 2022, 06:25:49 pm
content: Quote from: bsodmike on August 26, 2022, 11:19:29 amNoThe variac will only supply the current demaded by the load. That is about 2A for your Metcal. This is well within it's capabilities.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2022, 08:25:01 pm
content: Quote from: Robert763 on August 26, 2022, 08:19:32 amThat's right.Another thing to note is with an autotransformer, it's the difference between primary and secondary voltages, as well as the currents, which determines the amount of power the transformer needs to handle and therefore its size. The smaller the difference, the smaller the transformer needs to be. For example, consider an autotransformer designed to power a 400V 10A load, from a 230V single phase supply. The difference between the two voltages is 170V, so the transformer only has to handle 170*10 = 1700VA, rather than 10*400 = 4000VA.Quote from: Robert763 on August 26, 2022, 06:25:49 pmThat's true, but bear in mind the Variac's wiper will determine its maximum current rating. It's a good idea to add a fuse to protect it against overheating, if there's a fault.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 27, 2022, 04:44:50 am
content: Quote from: Zero999 on August 26, 2022, 08:25:01 pmThere's a single 2A fuse at the 230VAC input.

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2022, 04:59:30 pm
content: Quote from: bsodmike on August 27, 2022, 04:44:50 amThe wiper current can be much higher than the 230VAC input. It's a good idea to add a fuse to the wiper as well.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 28, 2022, 02:19:58 pm
content: Thanks for the tip!

author: bsodmike
author_rank: Regular Contributor
timestamp: September 01, 2022, 12:59:34 pm
content: Thanks again, it turned out well! If I increase the temp past ~250degC, it started to draw more than 2.6A on the Variac common winding. Will see if I can do some handheld re-flow at this temp.

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2022, 06:06:18 pm
content: I've not watched the video and don't have time to.How long did the current go up to 2.6A for?I generally wouldn't recommend using a variac for this sort of thing. There's too much of a risk of it being accidentally set to too higher voltage.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 02, 2022, 08:26:48 am
content: Under 40-60 seconds, but that’s only if I push the temp higher on the hot air gun.Agreed - I would need to be very careful with the output voltage before plugging it in for sure. I suppose what you mean is that it’s easy for the wiper to be disturbed by accident, either prior or during using?