Rheostat to make a variable isolation transformer - Page 1
Forum section: electronics / testgear

author: robsims
author_rank: Frequent Contributor
timestamp: October 01, 2020, 04:45:54 pm
content: Hi,I live in Suriname (South America) and mains voltage is just like in the USA, 120 volts ac with a 16 amp breaker. I have a step up/down transformer with an output of 220 volts/1000 watts max. I want to purchase a rheostat to make the output of the transformer variable. That way i have a variable isolation transformer. What i need to know is how much watts the rheostat must be to handle the output of the transformer. Please also recommend a rheostat from amazon or ebay. Many thanks

author: james_s
author_rank: Super Contributor
timestamp: October 01, 2020, 05:49:32 pm
content: A rheostat to do what you want would have to be huge, and it would dissipate a tremendous amount of power. Perhaps you are thinking of a variable autotransformer? Superficially they resemble a large wirewound rheostat but they are not the same thing. You need one which is able to handle as much current as you intend to draw from the isolation transformer.

author: robsims
author_rank: Frequent Contributor
timestamp: October 01, 2020, 06:23:25 pm
content: I have a red chinese 20 amp variac, and i can use the transformer with it. I then have an isolated transformer up to 220 volts, but that setup will also waste a lot of energy i thought. That's why i thought a rheostat would be a better option

author: bob91343
author_rank: Super Contributor
timestamp: October 01, 2020, 07:12:16 pm
content: You can use an isolation transformer as a step up combination with the variac but you lose the isolation. Just plug in the variac and the isolation transformer into the mains and add the outputs in series. You will get variation from 110 to 220 without wasting too much power.Do not use a rheostat. Not only are you wasting power but the voltage will vary with the load.

author: james_s
author_rank: Super Contributor
timestamp: October 01, 2020, 08:04:36 pm
content: Why would you want the outputs in series? That will not result in having any isolation. Is 240V needed? The variac feeding an isolation transformer will give you isolated 0-120V. There will not be much wasted energy, just the core and resistive losses in the windings. A rheostat turns all of the excess voltage into heat, transformers are far more efficient.

author: bdunham7
author_rank: Super Contributor
timestamp: October 01, 2020, 08:13:15 pm
content: Quote from: robsims on October 01, 2020, 04:45:54 pmIf you have a variac you have everything you need already. The variac is connected to the mains, the 120V side of your step up/down transformer is connected to the variac, make sure there are separate fuses for the input and output of the variac connect the secondary 220V side of your step up/down transformer to your outputs. You now have an isolated output from 0-ish to probably 250V. Your isolation will be as good as the isolation of the step up/down transformer, so make sure it is actually isolated! Some of those step up/down units are autotransformers, so make sure you test or check that.

author: james_s
author_rank: Super Contributor
timestamp: October 01, 2020, 08:20:05 pm
content: I've never seen a 120:240V step-up/down transformer that wasn't an autotransformer.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 01, 2020, 08:58:53 pm
content: Quote from: james_s on October 01, 2020, 08:20:05 pmHuh? They’re as common as cat poop at the local animal shelter.First of about a thousand google hits:https://www.automationdirect.com/adc/overview/catalog/power_products_(electrical)/transformers/1-phase_encapsulated_core_industrial_control_transformers/240x480_vac_to_120x240_vac

author: james_s
author_rank: Super Contributor
timestamp: October 01, 2020, 09:03:38 pm
content: Quote from: WattsThat on October 01, 2020, 08:58:53 pmThat's an industrial control transformer. I thought we were talking about the benchtop "foreign converter" things that can be set to do 120:240 or 240:120 for operating appliances.

author: robsims
author_rank: Frequent Contributor
timestamp: October 01, 2020, 10:05:52 pm
content: Thanks bdunham7. Yes you're right. I will connect the variac to mains 120 volt. Then the output of the variac goes into the 110 coil of the transformer. Then i will get a variable voltage from 0 to around 240 at the 220 volt coil of the transformer . The 220 coil of the transformer is fully separate from mains. No ground etc. Only the two cables from the 220 volt coil. That's why i it will be a isolated transformer

author: bdunham7
author_rank: Super Contributor
timestamp: October 01, 2020, 10:16:00 pm
content: Can you measure from one of those 220-volt leads to one of the 110-volt leads and see what the resistance is? And maybe post a photo of this transformer? You may be correct that it is isolated, but I don't want to see you accidentally fried.

author: robsims
author_rank: Frequent Contributor
timestamp: October 01, 2020, 11:46:32 pm
content: Quote from: bdunham7 on October 01, 2020, 10:16:00 pmMeasured the resistance of the leads and the meter is showing OL (overload). So no connection between the leads of the 110 v coil and the 220 volt coil. The transformer is also chinese made max 500 watts and works well. So i guess i have an isolated transformer. I do not always have the 20 amp variac on the bench. I use it for other stuff to. I'm going to order a 3 to 5 amp open type variac, to build it with the transformer in a box.

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2020, 06:58:35 am
content: That all should work just fine then.

author: robsims
author_rank: Frequent Contributor
timestamp: October 02, 2020, 11:16:25 am
content: Quote from: james_s on October 02, 2020, 06:58:35 amYes i think so. Many thanks guys