Transformer protection

[XSoph]

So, I'm a noob to electronics. I am trying to build an AC/DC circuit for metal etching and marking. I was using a doorbell transformer that has output terminals for 8v, 16v, and 24v operation and a bridge rectifier with a couple dpdt switches to create seperate ac/dc loops. I had everything soldered together and working and was in the process of adding an lighted arcade switch to the circuit. I had my multimeter hooked up to the output leads and the circuit switched to AC when I saw a puff of magic smoke and pulled the plug. I placed my hand on the transformer and it was warmer than normal. Ten seconds later, the transformer was getting really hot. I had only applied power for a few seconds, so I'm guessing the transformer shorted and fried itself. Should I have added a resistor to each, the AC and DC loops or would that drop my voltage too low? I'm looking for output from 10-16v and 20-24v. How do you guys protect your transformers? Btw, the load on the circuit will be the 5v led in the arcade button and the steel bar or metal piece to be etched (typically no bigger than .25" x 2" x 8").

Thanks for any help

 

[fezder]

hmm, could you post a schematic how you hooked things up? i might be the wrong person to help out, but there are others too here. And, welcome to ETO!

Edit: as for transformer protection, fuse and/or thermal fuse. Also, whatever i test, i use lamp-cord first, but it has some issues depending on in what it is used.

 

[ericgibbs]

So, I'm a noob to electronics. I am trying to build an AC/DC circuit for metal etching and marking. I was using a doorbell transformer that has output terminals for 8v, 16v, and 24v operation and a bridge rectifier with a couple dpdt switches to create seperate ac/dc loops. I had everything soldered together and working and was in the process of adding an lighted arcade switch to the circuit. I had my multimeter hooked up to the output leads and the circuit switched to AC when I saw a puff of magic smoke and pulled the plug. I placed my hand on the transformer and it was warmer than normal. Ten seconds later, the transformer was getting really hot. I had only applied power for a few seconds, so I'm guessing the transformer shorted and fried itself. Should I have added a resistor to each, the AC and DC loops or would that drop my voltage too low? I'm looking for output from 10-16v and 20-24v. How do you guys protect your transformers? Btw, the load on the circuit will be the 5v led in the arcade button and the steel bar or metal piece to be etched (typically no bigger than .25" x 2" x 8").

Thanks for any help

hi,
What is the VA or current rating of the transformer.? and how much current do you require to etch the metal.?

E

 

[XSoph]

Wow! You guys were fast! Here is a link to the transformer description (I'm at work, so don't have the little bugger in front of me):
**broken link removed**
It should be 10 va on the 8 and 16 volt terminals and 20 va on the 24v terminal.

I am also using a power inlet with built-in 120v indicator led and 250v 20amp replaceable fuse upstream of the transformer:
https://www.amazon.com/gp/product/B0050HH70E/ref=oh_details_o02_s05_i01?ie=UTF8&psc=1

 

[XSoph]

Here is the button I was wiring up at the time of the transformer's (I think ) demise:
https://www.amazon.com/gp/product/B008MU3UM0/ref=oh_details_o02_s01_i03?ie=UTF8&psc=1

Here is a diagram:
**broken link removed**

 

[XSoph]

hi,
What is the VA or current rating of the transformer.? and how much current do you require to etch the metal.?

E

I am unsure about the current required. Should I have tried to pass current through a piece of target metal with my multimeter attached to see what the current draw would be?

 

[ericgibbs]

I am unsure about the current required. Should I have tried to pass current through a piece of target metal with my multimeter attached to see what the current draw would be?

hi,
You should limit the spark current to less than 1 amp on the 24V terminal, say a 27Ohm 50Watt resistor or a lorry 24V , 24Watt lamp bulb
E

For the DC side, you could use a 27 ohm 50W resistor to charge a 10,000uF cap 50V working,
discharge the cap thru the spark

 

[KeepItSimpleStupid]

My thoughts. 10 VA isn't going to cut it. At 24 V, that's P = V*I or I = 10 / 24 amps.

You probably also need to use a constant current supply.

The size of a plating supply is generally sized by area. Can't help you with the actual requirements.

 

[unclejed613]

when i was a teenager i did a lot of experimentation with electroplating and other things requiring some form of electrochemical reaction. i used a 100VA transformer, and somehow it seems even that wasn't enough. what you need is low voltage/high current from most of what i've read on the subject.

 

[XSoph]

I was thinking the arcade button would allow me to use the device in short bursts. Unfortunately I didn't get to try that out. It's not designed for long periods of sustained current. I would probably only use the 8v and 16v loops as I've heard anything higher tends to eat up etch stencils. I think my mistake was to throw in a fuse at the inlet that passed more current than the transformer was rated for. Pretty silly, really.

 

[KeepItSimpleStupid]

Yep!