Quote from: Tassieviking on Today at 11:31:58 AMI would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

That's really cool damn I wouldn't have thought about that in a million years 

Unfortunately I would receive another slap upside the head guessing the secondary would be >24v   

I really like these 🥼🧪 experiments

I'm curious to see the outcome though seems like I should be able to calculate it 

I'm not gonna try it but I wonder what would happen if I connected mains to the secondary  (600V ?) ¹

Yes sir thanks guys 

I put the big ugly 120 (140) volt variac together as an enthusiast but it has gathered dust.  Even though it has two fuses in it I'm looking for a smaller safer tool.

I'm not that guy rebuilding amps and bringing up full voltage on a variac so it's pretty useless for me...or I'm useless to it 

I was using the big variac one time and it threw my breaker  I was surprised to see both fuses still good and checked all my wiring for shorts 🩳 to no avail and have used it since.....idk.

I just plan to use this for my breadboard textbook experiments when I need some ac like 15v or whatever 

Btw I have read ad nauseum about isolation transformers and floating etc and it makes my head spin     

I would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

The other way is to use the variac to supply the transformer so you could regulate it that way
Cheers
Mick

It depends on what will that variable AC feed.

If very little current (a few mA) a rheostat (heavy duty wirewound potentiometer) will do, but for any significant load, a transformer is better.

IMPORTANT: A Variac does NOT isolate you from Mains, so secondary is still very dangerous/deadly (pick one), **even if you set it to low voltage**

Now through a 120 to 24 Vac *isolating* transformer as shown, it becomes safe.

I'm thinking that using my ugly 120v variac to get like ten volts AC is a pretty bad idea 


So here's my latest plan: 

I have another autotransformer and a few transformers

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So how about mains to transformer (24v) and transformer to autotransformer 

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However my other question(s) is could I not just run the transformer output to a potentiometer to adjust voltage?

(Yeah dirt daubers are bad here)

After work I hooked up a simple circuit and checked it out

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Input was 10v and everything jived 

Next I made the same measurements between the series resistor and diode

The scope showed I lost half of my voltage which I presume makes sense but if I do the math on the rms meter reading I come up a little short I don't get 5 I get about 4.64 Vpp

So maybe I should account for voltage drop across the diode like the meter caught it but the scope did not....idk maybe the scope did catch it and I wasnt precise enough reading graticules

 

Also I was like oh hey the analog meter on my ugly variac must be rms 

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But alas I learned a bit more about X+Y mode it's just volts sideways and volts up and down.

 

Quote from: saturated on May 23, 2025, 08:39:54 AMMight be time to read up on half wave voltage and rms etc 

Yes.  If there is any AC involved, meter can only give you RMS value.  Scope will not display RMS (except as calculated numeric readout on newer scopes).

Also I guess the one volt division for the horizontal is for VCE

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Because when I changed the volts per division the line got longer and shorter

 

At least my evening tonight is already planned 

I'm going to run some low voltage ⚡ alternating current through a diode and compare measurements with the meter and scope.

I went back and played with this some more

With VBB at 3.6v Ib should be about 30 ua

Looking at the scope the collector current is showing five division at 0.1v setting so 5x 0.1 divided by 100 ohm should be about five milliamps
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If I change volts per division to 0.5 I get the same thing so that's cool
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The problem is if I use my meter to measure the voltage drop across the resistor I only get about half what I want (0.244 dc and 0.288 ac) 

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Next I figured ok I can measure the collector current manually and then I only got 2.83 milliamps 

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Summary....idk why my meter is showing about half of what my scope is

Maybe it's got something to do with that diode 


Might be time to read up on half wave voltage and rms etc