Hot Wire Cutter Power Supply

[KrisBlueNZ]

I think it's best if duke37 advises you on this project. I'll answer the questions I can.

I don't know what the item labelled SCHUMACHER is. Fuses normally only have two connections. I can only make out 12V and 12A in the marking, and Googling Schumacher 12V 12A -Michael shows that Schumacher make batteries and chargers, but that doesn't tell us what that component is. If there are any other markings on it, you could try Googling those. Can you show how it is connected in the charger circuitry? Also, does it have any other features apart from the three contacts?

Your schematic is not quite right. You have the capacitor connected in series with the supply to the PWM module. The capacitor and the PWM module should be connected in parallel with each other, across the DC output from the transformer-rectifier combination.

Assuming the rectifier diodes have commoned cathodes, as you've shown, that is the positive output of the transformer-rectifier combination, and the centre-tap is the negative output. Connect the capacitor and the PWM module across those points in the circuit.

How does the 2A/6A switch hook into the wiring? Can you draw a schematic of the charger in its original state?

Yes, a zip tie or two is a good way to secure the capacitor.

 

[duke37]

You should connect the capacitor across the input of the PWM module, it can be connected with the wires from the battery charger. It would be good to insulate as much bare wire as possible.

Make a little plywood box to enclose the module and fix the module and capacitor to the box with hot melt glue. You may need ventilation holes.

A fuse should be connected in series with one of the output wires of the module. It is normal to put it in the positive wire.

Dropping a screwdriver on the bare module could be interesting.

 

[sferrari]

I think it's best if duke37 advises you on this project. I'll answer the questions I can.

I don't know what the item labelled SCHUMACHER is. Fuses normally only have two connections. I can only make out 12V and 12A in the marking, and Googling Schumacher 12V 12A -Michael shows that Schumacher make batteries and chargers, but that doesn't tell us what that component is. If there are any other markings on it, you could try Googling those. Can you show how it is connected in the charger circuitry? Also, does it have any other features apart from the three contacts?

Your schematic is not quite right. You have the capacitor connected in series with the supply to the PWM module. The capacitor and the PWM module should be connected in parallel with each other, across the DC output from the transformer-rectifier combination.

Assuming the rectifier diodes have commoned cathodes, as you've shown, that is the positive output of the transformer-rectifier combination, and the centre-tap is the negative output. Connect the capacitor and the PWM module across those points in the circuit.

How does the 2A/6A switch hook into the wiring? Can you draw a schematic of the charger in its original state?

Yes, a zip tie or two is a good way to secure the capacitor.

I trashed the fuse. I will be trashing the switch. The center tap is negative and the plate is positive.

Wow. This is embarrassing. I didn't get much exposure to capacitors in my training. I'll hook it up in parallel and hot glue it to something. My mom got some hot glue with glitter in it for her Halloween decorating. This project needs some glitter.

My calculations were accurate. The 4ft bow with a 15ft lead(16AWG) has 7.2ohms.



While perusing alliedelect.com for panel mount circuit breakers I discovered they have a few cheap transformers that would work for my application so I've decided I'm gonna order a new transformer. The transformer I have now is sketchy. I want to feel good about this thing when I'm done.

http://www.alliedelec.com/search/productdetail.aspx?SKU=70218265

They also have a $13 6.3VCT 4A transformer that would be good for a tabletop foam cutter.

http://www.alliedelec.com/search/productdetail.aspx?SKU=70181255

6.3V/1.8ohms=3.5A

It would be just about right for a cutter with a 12 inch wire. Now that I have a better idea of what I'm doing and I have a good source for components I kinda want to build this other tabletop cutter from scratch. I haven't decided how I want to do heat control on the tabletop unit. I don't need the infinite control provided by the PWM but it needs to be able to crank up the heat to burn gunk/residue off the wire.

First things first; I need circuit protection. Here are the circuit breakers I picked for the cutter that will use the 12.6VCT 8A transformer. I read on several websites that the standard for circuit breaker sizing is up to 250% rated full load current on the transformers primary side and 167% rated full load current on the secondary side when both primary and secondary protection is being used.

100W/115V=.87A*2.5=2.175A

2A primary - http://www.alliedelec.com/search/productdetail.aspx?SKU=70075438

8A*1.67=13.36A

12A secondary - http://www.alliedelec.com/search/productdetail.aspx?SKU=70075452

Whaddya' think? Do my calculations look right? Do the circuit breakers I linked to look like they'd work?

I'll draw up a new schematic tomorrow after work. I gotta get some sleep.

 

[Fish4Fun]

Honestly you shouldn't need a circuit breaker on the primary side, and you really shouldn't need one on the secondary side either unless you are trying to protect your nichrome wire. A proper transformer will withstand a direct short across its secondary for a fair length of time. The primary should NEVER draw more than the listed maximum unless the transformer primary fails. If I were going to add a safety feature it would be a thermal limit switch on the transformer. But that's just me. Transformers provide what is called galvanic isolation from the mains supply, this means that the mains voltage is never directly connected to anything on the secondary side of the transformer. This is an essential safety feature if you are going to experiment with mains-powered projects, but it also means you shouldn't need to worry too much about over current in either the primary or the secondary.

Fish