Avantages of proper bench power supply vs. alternatives?

Hello,

I want to make a hot wire foam cutter and looked up general purpose power supplies for that and eventual future projects. They cost around 100$ so I looked at ways to have something cheaper and since I have a laptop cord with a decent ac/dc adapter I planned to get something like this

or this http://www.ebay.ca/itm/12A-200W-Adjustable-DC-DC-Step-Down-Converter-Buck-module-4-5-30V-to-0-8-32V-/310792739982?pt=LH_DefaultDomain_0&hash=item485cb0dc8e

I'd like to then wire the adapter to the regulator to have an adjustable "power supply".

However, when researching I've found pages saying proper power supplies work much better than alternatives like my hack.
I didn't find the underlying reason for that, so basically if you could explain why or point me to documentation where this is explained, I would be grateful.

Thanks!

I don't agree totally... on them being better than using the components you suggested, infact you'll probably find similar components inside the cheaper chinese bench power supplies...

But ...

The bench supply gives you protection against shorts, the transistors, switching regs are all or hopefully should be mounted to the case to act as a heatsink, or have much better fanned heatsinks attached. The bench supply should also give more accuracy dialing in voltage or current and of course a built in power supply..

Add the missing components up and 80 $ - $100 works out more economical.

I don't know how much it can be trusted, but one of those I linked says "Short circuit protection: Yes (please try not to short-circuit)"

I admit the parenthesis is worrying.

Also it apparently is a 200w unit when heatsinked, which is overkill for me , so a good safety margin.

Well, thanks for the answer!

I've not needed one, I bought a stack of switching voltage regulators, if i require more than 5amps (more than a laptop switch mode charger) I'd opt for a computer power supply for about 30 bucks.

And current limiting can be annoying on workbench power supplies, you may have it set to 100ma to protect something but next time you plug something in, you look directly at voltage set it to 5v or whatever, and then connect the device, the 5v reading drops voltage! duh! current limiting readjust, So i have a feeling if i used a labbench not the method you propose, a screwdriver on a pot on these switching buck regs, i'd forever be adjust the damn current... So i prefer to not bother with current regulation, let the device pull whatever it requires.. rely on the circuit not external devices to limit it's own current then current regulation can just be set at a fixed value.

So in simplified terms, current limitator= bad idea? if so I'll avoid using something with current regulation. Anyway, my first link has current regulation since on the picture there's two pots, but I don't think the second one does (but I might be wrong). The reason the first link caught my eye is because of the display, but I'd now prefer the second one. If I ever need precise voltage in the future I'll buy a multimeter.

Foam cutting requires between 2 and 5 amps voltage depending of length and thickness of the wire (and voltage should be adjustable) so a laptop brick should be sufficient, but I assembled my new pc myself with a 700w psu and have the old one laying around.

I'll see what I can do... heck since I have an arduino laying around I could forget about voltage totally and try to pwm the whole thing instead just for fun (I don't even know if that's possible since that just spouted from my head, depends on how costly sufficiently strong transistors are, something I have no idea about)

Anyway, thanks for taking your time to explain stuff for me because I'm a cheapstake! :stuck_out_tongue:

Cool, and no, current limiting is important.... - eg if you wanted to protect a laser diode, a bench supply would be perfect for quickly testing and diagnosing faults, with el cheapo method (unless you buy current regulated one) it's trickier using resistors or a current limiting circuit..

Buy whatever suits you, i'm just saying try not to rely on current limiting or you'll eventually blow something :slight_smile:

Hi, before you do anything.
What is the load, the wire cutter will consume energy, how much?

Current limit is essential, if you can get it, IT PROTECTS THINGS, even though some might think they are annoying.

And current limiting can be annoying on workbench power supplies,

Tom................... :slight_smile:

Hi, look at this link.

Read about using constant current supply instead of constant voltage, it protects your wire and gives better heat regulation.

Tom...... :slight_smile:

PS, this site looks good for this topic.

TomGeorge:
Hi, before you do anything.
What is the load, the wire cutter will consume energy, how much?

Current limit is essential, if you can get it, IT PROTECTS THINGS, even though some might think they are annoying.

And current limiting can be annoying on workbench power supplies,

Tom................... :slight_smile:

One day you'll get complacent and forget while everyone else relies on the circuit...

Check this DIY power supply on Instructables.
It's like what your thinking of doing, may give you some ideas.
I'll be finishing one as soon as the V/A meter arrives.

Im not sure about the required control for a hot wire cutter.

Current or voltage control.

Constant voltage, if wire gets too hot resistance increases and current drops.
Constant current , wire could overheat if not moving fast enough ?

Whew, this blew up!

Sorry about not responding sooner, I expected this thread to have pretty much ended so I didn't look it up often.

Anyway...

I got an answer form somewhere else that corrected me a on how amperage works with power supplies.

"Im not sure about the required control for a hot wire cutter.

Current or voltage control.

Constant voltage, if wire gets too hot resistance increases and current drops.
Constant current , wire could overheat if not moving fast enough ?"

Nichrome wire cutters cut foam at 300c I think. However, the wire can get red hot before it fails. So if overheating happens the wire's not going to be the problem. On the other hand, based on what you say voltage control seems to sorta stabilise itself, so it might be better.

As far for the load, ideally it should work for different types, so that I can convert it if needed. The website I use says nichrome from 20 to 26 gauge, up to 2 feet long.

Ill look up the links you provided and I won't intervene in your feud about benchtop current regulation. :stuck_out_tongue:

Didn't know passing current in a wire would be so complicated! Thanks for the help.

A bench top supply would get you either withe the advantage of other uses..

Psus specificaly for hot wire cutters can be had cheaper or used to, dont know where to get theses days though.

I suppose the heating elements work with AC ?

Arman5592:
I suppose the heating elements work with AC ?

Either ac or dc can do the job, since the way a foam cutter works is simplistic. Personally dc will probably be more convenient in my specific situation, but I don't have any strong favoritism for one or the other.

And yeah a real power supply might be more versatile, but given what I have on hand I might be able to make something for 5 bucks. I'll probably have to pick between paying more for shipping than the actual components or waiting 3 weeks, however... c'est la vie.