Parts help for a power supply project

Thread Starter

bartjet

Joined Jan 17, 2024
7
I am building a power supply for a project that uses a transformer. The transformer will be 120 volts AC in but can do 3 volt AC @ 50 amp or 6 volt AC @ 25 amp out. I am looking for a DPDT on-off-on switch that can handle this. Does anyone have any ideas?
 

LowQCab

Joined Nov 6, 2012
4,064
"" ........... but can do 3 volt AC @ 50 amp or 6 volt AC @ 25 amp out. ""
Well which is it ?????

Hammond Toroidal Transformer ~300VA
~$100.oo
You will have to add-on your own Secondary-Winding
if You really expect to get ~50-Amps out of it without smoking some Windings.
This is simple and easy to do, just ask if You don't know how.

This Transformer model comes with 2 - 12-Volt Secondary-Windings,
but the maximum Current for those Windings in parallel is only ~25-Amps,
which would meet your 6-Volt requirements, but not the 3-Volt requirements.

You can also make a Custom-Transformer from a used MOT ( Microwave-Oven-Transformer ).
It's substantially more labor to go this route, and has some quirks that You need to consider.

You'd get a much more useful answer if You would explain, in detail,
exactly what your project is supposed to do.
.
.
.
 

Thread Starter

bartjet

Joined Jan 17, 2024
7
"" ........... but can do 3 volt AC @ 50 amp or 6 volt AC @ 25 amp out. ""
Well which is it ?????

Hammond Toroidal Transformer ~300VA
~$100.oo
You will have to add-on your own Secondary-Winding
if You really expect to get ~50-Amps out of it without smoking some Windings.
This is simple and easy to do, just ask if You don't know how.

This Transformer model comes with 2 - 12-Volt Secondary-Windings,
but the maximum Current for those Windings in parallel is only ~25-Amps,
which would meet your 6-Volt requirements, but not the 3-Volt requirements.

You can also make a Custom-Transformer from a used MOT ( Microwave-Oven-Transformer ).
It's substantially more labor to go this route, and has some quirks that You need to consider.

You'd get a much more useful answer if You would explain, in detail,
exactly what your project is supposed to do.
.
.
.
I already have the transformer, what I am looking for is a dpdt switch, the Transformer that I have has 2 secondaries and if I wire them in series I get 6 volts @25 amps, but if I wire them in parallel I get 3 volts @ 50 amps
What I am trying to do is use the dpdt switch where ON (3 volt) - OFF - ON (6 volt)
The biggest problem I am having is finding the switch that can handle up to 50 amps.

As far as the project, it is a power supply for multiple "tools" ranging from multiple styles and sizes of hot wire cutters, plastic bending tools, and resistance soldering tools. So it serves multiple purposes
 

LowQCab

Joined Nov 6, 2012
4,064
It will be very difficult and expensive to come up with a DPDT Switch that can handle that type of Current.

Exactly what type and configuration of Transformer do You have to work with ?

The best solution would be to install a Variac on the Input of your Transformer,
but they are really expensive, unless You luck-out and find a used one.

You could use a Triac-Motor-Speed-Controller on the Input to your Transformer,
but You may run the risk of smoking your Transformer under certain circumstances,
and it would also limit You to ~25-Amps on the Output.

Does you Transformer have Dual-Primary-Windings ?,
that would simplify things a lot, but would limit You to ~25-Amps on the Output-Side.
.
.
.
 

Ya’akov

Joined Jan 27, 2019
9,145
Welcome to AAC.

@MrChips solution—using a relay—is the most sensible. The relay is just an electromagnetic switch that allows you to use a small, inexpensive switch to operate its coil, switching the much higher current while offering a lower cost (and probably longer life) for both items than a manually operated switch with sufficient capacity to do the job.

You might have already noticed that switches are rated separately for AC and DC with the DC capacity noticeably smaller. This is because of the greatly increased arcing caused by DC. If the switch isn’t designed for the high DC current it can be damaged by the arcing—sometimes failing by welding itself on which can be a serious safety concern.

The relay route is cheap and easy, and the suggestion of automotive relays takes advantage to the cost engineering done for car parts to drop the price even further. If you need any help working to how to use one, don’t hesitate to ask.
 

LowQCab

Joined Nov 6, 2012
4,064
Questions sometimes need repeating to insure understanding.
I've been bitten in the rear way to many times, so I try not to make assumptions.

The only problem with the High-Current Automotive style Relays is that they are generally
not Double-Throw, or, the N/C Contacts are rated for Current far lower than the N/O Contacts,
and, they are all Single-Pole,
meaning that You would have to use a lot more Relays for a series / parallel switching setup.
But it might still be cheaper than using an Industrial-type-Relay.
.
.
.
 

Thread Starter

bartjet

Joined Jan 17, 2024
7
It will be very difficult and expensive to come up with a DPDT Switch that can handle that type of Current.

Exactly what type and configuration of Transformer do You have to work with ?

The best solution would be to install a Variac on the Input of your Transformer,
but they are really expensive, unless You luck-out and find a used one.

You could use a Triac-Motor-Speed-Controller on the Input to your Transformer,
but You may run the risk of smoking your Transformer under certain circumstances,
and it would also limit You to ~25-Amps on the Output.

Does you Transformer have Dual-Primary-Windings ?,
that would simplify things a lot, but would limit You to ~25-Amps on the Output-Side.
.
.
.
It does have dual primary, but my understanding is that it is for 120/240 input setup, this is the one i have. Screenshot_20240117_213513_Chrome.jpg
This is the basic design I am following
Screenshot_20240118_063450_Chrome.jpg
Screenshot_20240118_065342_Chrome.jpg
based on your reply, I guess the possibility of a dpdt switch to go between 3v and 6v is probably not going to happen. My calculated need based on the nichrome wire gauge and length for 2 of the tools I am making is about 40 amps @ 3v. I guess I’ll need to re-wire the connections to switch voltage, unless there is a different recommendation.
 

Thread Starter

bartjet

Joined Jan 17, 2024
7
Welcome to AAC.

@MrChips solution—using a relay—is the most sensible. The relay is just an electromagnetic switch that allows you to use a small, inexpensive switch to operate its coil, switching the much higher current while offering a lower cost (and probably longer life) for both items than a manually operated switch with sufficient capacity to do the job.

You might have already noticed that switches are rated separately for AC and DC with the DC capacity noticeably smaller. This is because of the greatly increased arcing caused by DC. If the switch isn’t designed for the high DC current it can be damaged by the arcing—sometimes failing by welding itself on which can be a serious safety concern.

The relay route is cheap and easy, and the suggestion of automotive relays takes advantage to the cost engineering done for car parts to drop the price even further. If you need any help working to how to use one, don’t hesitate to ask.
I appreciate the information, however this is a step down Transformer not a voltage converter, so everything is AC not DC. I will look at ac relays to see if that would be my best option
 

MrChips

Joined Oct 2, 2009
30,795
You should have stated in your initial post as well as in the subject title that you a building a hot wire foam cutter.
 

Thread Starter

bartjet

Joined Jan 17, 2024
7
You should have stated in your initial post as well as in the subject title that you a building a hot wire foam cutter.
My apologies, however in reply to another response I did mention that this power supply would be used for multiple purposes, hot wire foam cutter, plastic bending, resistance soldering, etc...
 

BobTPH

Joined Jun 5, 2013
8,945
My apologies, however in reply to another response I did mention that this power supply would be used for multiple purposes, hot wire foam cutter, plastic bending, resistance soldering, etc...
You don’t need to apologize. I read through your posts and nowhere did you mention DC.
 

Thread Starter

bartjet

Joined Jan 17, 2024
7
Are you sure you need that much current?
I built my son a hot wire cutter for his model work and it only runs PWM DC at maybe 5 amps.
I build very large props for my Halloween and my kids marching band, as seen in these pictures, for the foam cutter side of things i max need about 30 amps, a little more for the plastic bending and molding tools, and even a little more for the resistance soldering FB_IMG_1633219267688.jpg20201107_153332.jpg
 

MisterBill2

Joined Jan 23, 2018
18,464
OK, for the high current switch that will be needed for the series/parallel switching, I suggest "Automation Direct" as a source. They are by far the least expensive supplier of industrial strength components. They should be able to provide a switch able to meet the requirements..
 
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