Doorbell Transformer for AC experiments?

Thread Starter

kallanreed

Joined Jan 7, 2023
3
Hi All,
I want to mess around with some AC circuits on a breadboard with my oscilloscope. No real projects in mind, I just want to get some hands on experience with some AC to DC circuits.

The main problem is the Mains problem. I obviously don’t want to plug a breadboard into the wall, so I was looking for ways to get a safe, low voltage, AC power source. I have a doorbell transformer and I was wondering if that would be safe to use as a source? I can’t seem to find any definitive answer on whether it’s “mains isolating”, but I think all converting transformers are just by design.

I’ve never worked with AC before so I want to be safe. Are there any other concerns (e.g. I’ve seen posts about dangerous ground loops) that I need to be careful of when working with transformed AC?
 

nsaspook

Joined Aug 27, 2009
13,570
A doorbell transformer is a safe way to experiment with a AC source, take care to always isolate the main power wires. The doorbell transformer is designed (with high leakage reactance so they usually run hot) for the secondary to be shorted without overheating/destroying the transformer so your AC voltage regulation will be poor under varying loads.
http://electroncoil.com/leakage_transformers.php
Leakage Reactance Transformers provide inherent current limiting, short circuit protection, and peak current reduction. They are commonly used for current limiting in applications such as welding, starting and ballasting gas-discharge lamps, for decoupling output circuits and as anode supply transformers in continuous wave magnetrons.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,829
I’ve never worked with AC before so I want to be safe. Are there any other concerns (e.g. I’ve seen posts about dangerous ground loops) that I need to be careful of when working with transformed AC?
The NEC only considers anything above 60v as dangerous.
And anything above that is normally earth ground referenced.
 

MrChips

Joined Oct 2, 2009
31,088
Welcome to AAC!

Another option is to visit a second hand store and look for an AC wall adapter (wall wart).
Read the label or imprint on the adapter carefully and look for one that states AC output, usually in the 9V-12V range.
A transformer based adapter would have substantial more weight than a direct-to-mains DC phone charger.

It does not matter what output plug/jack is supplied. You can always cut off the plug and use your own type of connection.
 

Thread Starter

kallanreed

Joined Jan 7, 2023
3
Welcome to AAC!

Another option is to visit a second hand store and look for an AC wall adapter (wall wart).
Read the label or imprint on the adapter carefully and look for one that states AC output, usually in the 9V-12V range.
A transformer based adapter would have substantial more weight than a direct-to-mains DC phone charger.

It does not matter what output plug/jack is supplied. You can always cut off the plug and use your own type of connection.
Thanks, everyone for the replies. I might try the second hand store first so I don’t have to hack up a cord for the transformer.

When nsaspook says the doorbells run hot, does that mean literally hot because of internal resistance of the windings?
 

crutschow

Joined Mar 14, 2008
34,844
it’s “mains isolating”, but I think all converting transformers are just by design.
Yes, most main's transformers are isolating except auto-transformer types, such as Variacs or those small travel transformers that convert between 120V⇔240V.
Auto-transformers have only one shared winding between input and output, so have no isolation.
 

nsaspook

Joined Aug 27, 2009
13,570
Thanks, everyone for the replies. I might try the second hand store first so I don’t have to hack up a cord for the transformer.

When nsaspook says the doorbells run hot, does that mean literally hot because of internal resistance of the windings?
What I mean is the doorbell transformer is intentionally (class 2 transformer) inefficient in transferring energy from the primary to secondary. That intentional energy loss to increase safety is mainly transformed into heat.
https://www.foster-transformer.com/products/class-2-transformers/

1673116300630.png
 

dendad

Joined Feb 20, 2016
4,503
Years ago, I made up a display board with plug in parts, diodes,capacitors and resistors, to demonstrate power supply operation for a class. Connecting meters and oscilloscope to show the various parts of the circuit etc. I was fortunate to find a transformer that had a 10V output. It made the math a lot easier!
An old thrift store plug pack is a good way to start. Also, to protect things while you are playing around, add a Polyswitch (resettable fuse) in one line or even a taillight lamp to limit the current. A lamp will cause an extra voltage drop but for small loads it will be ok as when the lamp is cold, the resistance is low. When more current flows, it heats up and the resistance increases so limiting the current to help keep the smoke in.Also, the lamp is a built in overload indicator.
One test power supply I used for years was a 24V transformer with a 24V lamp and a buzzer across the lamp. It worked very well as most board I was testing only drew 100mA or so normally. A lamp lit with buzzer operating indicated a problem.
 

Thread Starter

kallanreed

Joined Jan 7, 2023
3
Well the thrift store here was a bust. Might have to try the bigger one on the other side of town. I cut the one of an old computer PSU cable and wired up the doorbell transformer I had in a drawer. Seems to be ok, no smoke so far. Measured the output and it is reading 22VAC even though it’s stamped 16V on the housing.

Another thing, I tested voltage to the wall outlet ground and it was like 3V. I thought that was strange. I’ll mess around with this one for now. I mainly wanted to build a few little voltage limiting and rectifier circuits to help my understanding of them.
 

MisterBill2

Joined Jan 23, 2018
19,461
Thanks, everyone for the replies. I might try the second hand store first so I don’t have to hack up a cord for the transformer.

When nsaspook says the doorbells run hot, does that mean literally hot because of internal resistance of the windings?
My very first electrical experimenting was done with a doorbell transformer. That is an excellent starting point indeed. I suggest unplugging the transformer from the mains supply when it is not in use, though.
 

MisterBill2

Joined Jan 23, 2018
19,461
No, shorty, I mean when the experiment is done but the wires are still connected to the experiment, or when the intention is to not waste energy by producing not needed heat, or when somebody else might accidentally short circuit the terminals and produce quite a bit of heat.. In general it is appropriate to switch off things when they are not in use.
 
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