I tested my foam cutter (120VAC -> fuse -> Lutron Dimmer -> stepdown transformer 25.2V -> cutting bow) and found the dimmer reduced the transformer output by about 35%, which is not satisfactory. I guess that's caused by the time-slicing power reduction method of the dimmer. I may need all the 28V+ the transformer actually puts out so I want to lose as little as possible. I looked at a rheostat, but since I'll be using very low amps, <1 on input, <2 on output, it looks like that will waste quite a bit of power. I've been unsuccessfully googling replacements for the dimmer, so I'd like to know if any of you can recommend a relatively inexpensive (<$20), relatively efficient item or kit I can use to regulate/lower/adjust AC power going into the transformer.
Thanks.

 

A dimmer should not cost you 20%. It's probably broken.
See also this thread I did yesterday. Post #8
or Google, "dimmer".

http://forum.allaboutcircuits.com/threads/understanding-nichrome-heating-elements-pictures.106967/

 

I did not like the idea of controlling the input side to the Transformer, I wired it on the output side.
You also need to have a transformer of the correct Va otherwise the voltage will collapse or decrease under load.
The circuit I built is a little more than the average Triac/SCR controller but even the 120v dimmer type should work on the output side.
Max.

 

Good point. Most people underestimate the power needed to heat things.

 

Thx for the response 12. I tried two identical dimmers and they both yielded yhe same results. Both bad
.!?!

Regarding the links in your thread link below:

The low end of the output range of the first is too high for my needs. FYI, I looked at all the similar options on ebay and found none would work;

The Lutron dimmer probably won't work - I spoke to Lutron tech support who gave me two possibilities that I'll investigate;

Third option breaks budget.

Skimming the thread below I found nothing to answer my question. However I do use the Jacobs site and bought my wire from them. Not nichrome, but rene 41, which is a better choice for me.

Thanks for your response.

A dimmer should not cost you 20%. It's probably broken.
See also this thread I did yesterday. Post #8
or Google, "dimmer".

http://forum.allaboutcircuits.com/threads/understanding-nichrome-heating-elements-pictures.106967/

 

DOH! WHY DIDN'T I THINK OF THAT?! Must STILL be in the box.
Thanks for yor response, Max. I'll try your suggestion.

I did not like the idea of controlling the input side to the Transformer, I wired it on the output side.
You also need to have a transformer of the correct Va otherwise the voltage will collapse or decrease under load.
The circuit I built is a little more than the average Triac/SCR controller but even the 120v dimmer type should work on the output side.
Max.

 

You also need to have a transformer of the correct Va otherwise the voltage will collapse or decrease under load.
Max.

Forgot to respond to an important part of you msg. My transformer is rated 25.2V 2A. I might get close to 2A but not reach or exceed it, so I hope I'll be OK.

I did try to get an understanding of the difference between watts and VA, Max, but my non-tech understanding is that for resistive loads there's no difference. If that's not correct please enlighten me.

 

That is only (25x2) 50va so quite a small transformer, so a 100Va min. would give you some margin.
Max.

 

I did not like the idea of controlling the input side to the Transformer, I wired it on the output side.
Max.

Putting the dimmer on the output side is a mute point, Max, as some would say. Both dimmers passed full voltage when turned on, regardless of the dial setting. So the search continues.

 

I tested my foam cutter (120VAC -> fuse -> Lutron Dimmer -> stepdown transformer 25.2V -> cutting bow) and found the dimmer reduced the transformer output by about 35%, which is not satisfactory. I guess that's caused by the time-slicing power reduction method of the dimmer. I may need all the 28V+ the transformer actually puts out so I want to lose as little as possible. I looked at a rheostat, but since I'll be using very low amps, <1 on input, <2 on output, it looks like that will waste quite a bit of power. I've been unsuccessfully googling replacements for the dimmer, so I'd like to know if any of you can recommend a relatively inexpensive (<$20), relatively efficient item or kit I can use to regulate/lower/adjust AC power going into the transformer.
Thanks.

Not sure - but I think you need to use either a snubber or PFC capacitor to drive a transformer with a dimmer.

The transformer primary is predominantly inductive, so current lags voltage - the end result apparently; the triac doesn't conduct for the full phase angle that was intended.

If you use just a PFC capacitor - watch you don't choose the resonant capacitance, otherwise it can generate some wicked voltages!

 

The transformer primary is predominantly inductive...

Hmmmm...interesting.

Since a predominantly inductive load stores energy (as opposed to dissipating it), you are suggesting I can power a resistive load on the secondary for, essentially, free!

 

I'm not sure regular dimmers and transformers like each other.

How about this:

http://www.allelectronics.com/make-a-store/item/ps-2428/24vdc-2.8a-switching-power-supply/1.html

http://www.ebay.com/itm/12V-40V-10A...552?pt=LH_DefaultDomain_0&hash=item5d44d25ce0

 

I'm not sure regular dimmers and transformers like each other.

How about this:

http://www.allelectronics.com/make-a-store/item/ps-2428/24vdc-2.8a-switching-power-supply/1.html

http://www.ebay.com/itm/12V-40V-10A...552?pt=LH_DefaultDomain_0&hash=item5d44d25ce0

Both of these are DC devices. I already have a DC-based solution if I have to go that way. I'm trying to get the AC project to work.
Thanks for your response.

 

Give this a read. Maybe some ideas there.
http://www.ilight.co.uk/downloads/iLIGHT Binder-HowDimmers.pdf
Most dimmers have a diac to reduce EMI, but I would have thought that would have kept it from turning on when you put it in the secondary.
You should be able to make your own easily enough if your so inclined.
Edit:
Add diac info.
http://en.wikipedia.org/wiki/DIAC

 

I still suspect the transformer isn't big enough. Can you plug it all together with no dimmer and measure the output then?

 

Give this a read. Maybe some ideas there.
http://www.ilight.co.uk/downloads/iLIGHT Binder-HowDimmers.pdf
Most dimmers have a diac to reduce EMI, but I would have thought that would have kept it from turning on when you put it in the secondary.
You should be able to make your own easily enough if your so inclined.
Edit:
Add diac info.
http://en.wikipedia.org/wiki/DIAC

ronv, my dimmers have triacs, and, according to Lutron tech support, those won't work with transformers well. When dimmers were put on the output side it seemed they weren't there at all. Max voltage passed regardless of dial setting.

I still suspect the transformer isn't big enough. Can you plug it all together with no dimmer and measure the output then?

12, with only the transformer connected to AC its output was about 28.7V.

++++
I think it's time to bag this one and use my DC solution. I've spent enough time searching for a seemingly nonexistent cost-effective solution. Maybe I should just get out my old Lionel train transformer and see how much power that thing puts out!

Thanks to everyone who responded to this thread.

 

Hmmmm...interesting.

Since a predominantly inductive load stores energy (as opposed to dissipating it), you are suggesting I can power a resistive load on the secondary for, essentially, free!

I never suggested anything of the sort - but you can cheat the electricity meter with way out of phase VI.

 

I am using a 16vac a simple PUT/SCR unit I had on hand on the secondary side, I am using .6mm Nichrome, I adapted it slightly from this page.
http://www.mi.mun.ca/users/rmatchim/assign/e1100/triggers.pdf
I get more than enough power at halfway on.
The PUT version.
Max.

 

I tested my foam cutter (120VAC -> fuse -> Lutron Dimmer -> stepdown transformer 25.2V -> cutting bow) and found the dimmer reduced the transformer output by about 35%, which is not satisfactory. I guess that's caused by the time-slicing power reduction method of the dimmer. I may need all the 28V+ the transformer actually puts out so I want to lose as little as possible. I looked at a rheostat, but since I'll be using very low amps, <1 on input, <2 on output, it looks like that will waste quite a bit of power. I've been unsuccessfully googling replacements for the dimmer, so I'd like to know if any of you can recommend a relatively inexpensive (<$20), relatively efficient item or kit I can use to regulate/lower/adjust AC power going into the transformer.
Thanks.

what I have found is that AC based foam cutters do not cut as cleanly as variable DC power supplies, so you should look into getting a variable DC power supply for your project. you can get a really inexpensive one like this (0-30V and 0-3A): http://www.volteq.com/volteq-regulated-variable-dc-power-supply-gps-3030d-30v-3a.html

If you wish to stick to AC based foam cutter, then try to get a variable transformer, or variac, which allows you to lower the voltage if you need less output, and you don't waste any electricity. Volteq has small ones starting at $44.50: http://www.volteq.com/variac-auto-transformer.html

If you still want to continue your own way, I will do the following:

1) make sure without any dimmer, your power is enough to heat up the foam cutter
2) once you are sure of that, you can put in a variable resistor to reduce the voltage on the foam cutter. you need to make sure that the resistor can handle the current you are putting through without getting hot.

 

The one I showed actually uses variable DC due to the SCR instead of Triac, I was unsure about the half wave effect but it works fine.
Another reason I used it on the secondary.
Max.