I hacked an old variac by tapping a range of ac voltages from 6 to 90 AC ( in steps ). I used a rotary switch to select a specific range..
I put an 80 nf across the two terminal out of the ac voltage selector.In small voltage 6-20 volt ac I did not get sparks across the contact points of the selector. But once I go above 20 V when I select I get a spark.

How can I prevent these sparks; they will ruin the contact points of the selector.

I am using

SELECTOR SWITCH(SR-2511)

 

How often do you plan to switch the selector switch? It looks like you could de-energize the variac, change the voltage selector, and then turn the power back on. So, what is this being used for, specifically?

 

1. I will use the selector very "infrequently".
2. I am using it as bench tool whenever I need certain small ac voltage. for purely "educational" purposes. Mostly low current ( Less than 1 Amp)
3. How do I de-energize the Variac ??
4. I was hoping for "snubber circuit" but I do not know how ????

The circuit is very simple. Mains to Variac, tabs to selector,to output.

 

3. How do I de-energize the Variac ??

Turn the power to it off, before switching the voltage range.

 

Got it.......Many thanks for your "KISS" .....It is definitely simple.

 

Your rotary switch is spec'd as being a make-before-break switch.
This is usually recommended to prevent arcing when switching under load.

I suspect what is happening is when you switch, you are shorting across two windings of the variac causing sparks.
Try using a break-before-make switch. However, you will still get arcing if you are drawing substantial current.

The KISS suggestion is still a valid one.

 

The sheet I have on it shows Break before make?
Max.

 

The sheet I have on it shows Break before make?
Max.

This is the datasheet I examined:

http://datasheet.octopart.com/10WW019-Alpha-Taiwan-datasheet-160950.pdf

 

One test I did; was to connect a bridge followed by Two parallel Caps 200Uf / 0.1 uf; then adjustable IRF740 circuit ( see att.).
The 0.1 uf did the trick ( the spark was gone).

I definitely do not want to waist you valuable time in pursuing my "education path" which I admit a weird one.!!!! BUT, here it is.

Why this post you might ask ; At one time; when I switched the whole thing ON (NO SELECTION) the 0.1 uf blew away.

 

Is T1 your VARIAC or does the VARIAC feed T1?

Ken

 

Yes..My circuit is Mains (240v) to Variac To selector Then Bridge to Caps (200uf/0.1 uf ) then the rest of the circuit as shown.
This way I can get adjustable dc from any range of ac I choose. using the selector switch