240 to 120 volts - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: November 03, 2018, 01:15:45 pm
content: hi all i just got an old heathkit 4802 dual channel pc scope,its 120 volt,ive no manual or schematic,the transformer has about 6 taps out,if i get a transformer thats 120v primary and wire that in series with the 120v primarys of the heath,should that work?,cheers m3vuv.

author: langwadt
author_rank: Super Contributor
timestamp: November 03, 2018, 01:33:27 pm
content: no it won't work because the load on the two transformers are not the same, you need a transformer with two 120V windings

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2018, 02:14:25 pm
content: Or a transformer with a 240V primary and 120V secondary.The transformer only needs to be small. Here's one with two 55V secondary windings, which can be connected in series to give 110V, which is close enough.https://www.rapidonline.com/vigortronix-vtx-146-050-155-toroidal-transformer-230v-single-primary-50va-0-55v-88-5196

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: November 03, 2018, 02:26:24 pm
content: the other and dangerous way is using a capacitive dropper. It is cheap but doesn't provide isolation. There are some travel adapters for doodling

author: m3vuv
author_rank: Super Contributor
timestamp: November 03, 2018, 02:43:56 pm
content: just to clarify the heath primary has no taps its one single 120v winding,ie no center tap.

author: rstofer
author_rank: Super Contributor
timestamp: November 03, 2018, 03:24:41 pm
content: Treat it the same way Americans do when we travel to Europe. We wind up buying a 240/120V step down transformer. I don't know how big (VA rating) the transformer needs to be but smaller is bad. If there is a label with Amps or Watts, you can work it out.The Rigol DS1054Z draws 50 Watts so a 100 VA transformer would be about right (guessing at power factor and all those kinds of things).

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: November 03, 2018, 05:18:13 pm
content: quick search on the internet reveals that thing has a max power rating of 100 W , so what you need is step down converter with min. rating of 100W.You can probably get one for around 100$ or you can use variac if you have one avaliable.You can also buy variac for around 100$https://www.ebay.com/itm/APS-500W-0-5kva-Variac-Autotransformer-Voltage-Regulator-Powerstat-2A-0-250V-UK/163168079902?hash=item25fd93501e:g:iAkAAOSwofFbkKIg:rk:1:pf:1&frcectupt=true

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: November 03, 2018, 10:28:31 pm
content: I have one of these on my TV to prevent mains be refenced to the chassis., but i also used to power an older stereo from U.S.A.TRANSFORMATOR 230V AUF 110V 300W KONVERTER STEP UP/DOWNhttps://ebay.us/nSUfYL

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 03, 2018, 10:38:59 pm
content: or any suitable transformer with primary 120 and a 240, connect 240 , you ll get 120 from primary 120 pin; as well, and the power rating are better ( i assume for 100w consumed you need 50w tranf. ) , as it auto-transformer kinda

author: Zero999
author_rank: Super Contributor
timestamp: November 04, 2018, 09:30:39 am
content: Quote from: Karlo_Moharic on November 03, 2018, 05:18:13 pmI wouldn't recommend using a variac to continuously power a device designed for 120V, because it's easy to changing the voltage setting. Use a transformer with a fixed 120V secondary. A variac could be used for test purposes, but not as a permanent solution.If the device takes 100VA, then the transformer I linked to previously will be too small. The transformer linked to previously will be more suitable.Quote from: malagas_on_fire on November 03, 2018, 10:28:31 pm

author: Wolfgang
author_rank: Super Contributor
timestamp: November 04, 2018, 02:14:12 pm
content: Quote from: m3vuv on November 03, 2018, 01:15:45 pmTry an isolation transformer. There are a lot around that have a 230V primary and two 120V secondaries.