Home Member Services Content Areas Tools Info Center WC Partners Shop Help
Search for:

Welcome to the WetCanvas forums. You are currently viewing our boards as a guest which gives you limited access to view most discussions, articles and access our other FREE features. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload your own photos and access many other special features. Registration is fast, simple and absolutely free so please, join our community today!

If you have any problems with the registration process or your account login, please visit our help center.

Go Back   WetCanvas > Explore Media > Glass Art > Let's Talk > Glass Technical Forum
User Name
Register Mark Forums Read

Salute to our Partners
WC! Sponsors

Our Sponsors
Thread Tools Display Modes
  #1   Report Bad Post  
Old 04-16-2008, 11:45 AM
Jill_'s Avatar
Jill_ Jill_ is offline
New Member
Join Date: Apr 2008
Posts: 49
Moly Glass Furnace - SCR & temp control

Thanks to everyone who has commented so far.

This thread is a discussion of what we have learned about the SCR and temperature control for a Moly glass furnace. The SCR is one of the differences between a Moly glass furnace and a Wire Melter glass furnace which some of us on here have built and are using. The transformer and elements are also different and are being talked about on those threads.

The SCR is not difficult to understand once explained.

This thread is linked to these other threads about Moly glass furnaces –

Moly Transformer Step Down Explained

Mosi2 element sizing/configuration

Moly Hybrid Furnace (Moly & Wire Coil or Gas)

I wanted to list these so people can find them later on down the road.

Zero-Cross SCR's -

To begin, there are two basic types of SCR’s. The first is called Zero-Cross. It is known as “time proportional” because it turns full on and full off for specific periods of time.

For example, if it receives a 50% signal from the temperature controller, then a Zero-Cross SCR will turn on for 5 seconds then turn off for 5 seconds. Likewise for a 25% signal, then on for 2.5 seconds and off for 7.5 seconds, and so on (depending on the TCU cycle time setting).

When building a Moly furnace, it is good to know what Zero-Cross is, however we will not be using this style. We will be using a different style, called Phase-Angle SCR.

It is good to know the difference, because if you decide to buy an SCR on eBay (like we did), then you must know which one you are buying. They sell both Zero-Cross and Phase-Angle there.

The only real way to know for sure, is to check the part number of the item on eBay with the manufacturer’s literature found on the manufacturer’s website.

Phase-Angle SCR's -

Now about Phase-Angle SCR’s and why you have to use them for a Moly furnace. At room temperature, 68 F, the Moly element is about 1/10th the resistance than it is at high temperature for a glass furnace, 2100 F.

As an example, let’s look at a 100 pound Moly glass furnace running at 11,500 Watts, at 60 volts and 192 amps (from a 4:1 ratio transformer), then the high temp resistance is 0.3125 ohms. But at room temperature, the resistance is around 1/10th, or around 0.03125 ohms.

What this means is that at room temperature, when applying the same 60 volts of electricity out of the transformer, the elements would want to draw 1920 amps! (60/.03125 = 1920)

Remember, Volts = V = I x R and Power = P = V x I

In order to keep the maximum amps at 192, we need a way to limit the amps and keep them from going above 192. This is done by what is called a “current limit” device on the SCR. It is an option which is only available on Phase-Angle SCR’s.

The reason this is important is because this “current limit” feature is not available on a Zero-Cross type of SCR. Therefore if buying an SCR for a Moly glass furnace on eBay, you have to check the manufacturer’s specs and buy not only a Phase-Angle SCR, but one with the “current limit” option.

These types of SCR’s (Phase-Angle plus Current Limit) come up on eBay about every month. There are some on there now. We bought 2 of them over the last 3 months. One is an 80 amp unit by Control Concepts. The other is a 200 amp unit by Spang. One has a housing, the other doesn’t. We bought the Control Concepts SCR for around $60. The Spang SCR we bought about two weeks ago for $30.

See the pictures –

Below is an 80 Amp SCR with Current Limit & Soft Start Options -

Below shows the control connections for TCU -

Below is a 200 Amp phase-angle SCR with the current limit and soft start. It is very similar to the above unit, except it does not have a housing -

These types of units receive a control signal from the temperature controller (TCU). This signal is not an on/off control like most TCU’s provide. This signal is a variable signal and is known in the heating industry as a “process signal” or a “process control signal”. Depending on what model of TCU and SCR you have, this signal can be either 0 to 10 volts, 0 to 5 volts, 0 to 20 milliamps, 4 to 20 milliamps.

Tricky Part #1 – you have to make sure you match and/or buy the TCU output to match the input of the SCR. This is because a 4 to 20 milliamp output from the TCU will not work with an SCR which needs a 0 to 10 volt input. You have to match the one with the other.

You are probably wondering, what about if my TCU has an SSR output, a switched DC or a Relay output? These types are strictly PID on/off control and will not work. They are an example of time proportional control mentioned above.

Fuji makes the process variable output style TCU and so do Watlow and Red Lion.

As an example, on Watlow’s Series 96 controller, it has a “universal process output”. It can be programmed to select and output any of the ones I listed above here. Therefore, you can set it to match any type of SCR you buy. Caution – not all TCU’s can do this. And when buying a Watlow 96 on eBay, you have to check the part number against the Watlow 96 brochure to see if the TCU has this option. We have bought two of them with this option on eBay and they work great. One we paid ~$55 for and the other about $70. A new 96 TCU with the universal process output costs over $500 new.

Here is one -

The last thing about the SCR is that it must have a “soft-start” feature. This feature is offered as an option, so not all Phase-Angle SCR’s have it. The reason this is needed is because the SCR goes between the sub-panel in your hot shop and the transformer. It is on the “primary” side of the transformer, rather than on the “secondary” side. Let’s use the above example to explain why the SCR goes here. For an 11.5 KVA Moly glass furnace, as mentioned the volts are 60 and the amps are 192 on the secondary side of the transformer. On the primary side of the transformer, the volts are 240 and the amps are 48. Since SCR’s go up in price as they get larger, you can find a smaller SCR a lot easier on eBay, plus it will be a lot cheaper.

The tricky part is that a transformer is just like a motor, it has a lot of wire windings. At start-up, there is a huge in-rush current, just like a motor. Sometimes in-rush amps can jump as high as 3-times the full load amps. Therefore the SCR has to limit it with the “soft-start” feature. Without it, the 48 amps on the primary from above could go up to 150 amps and destroy components in the SCR.

Tricky Part #2 – you have to find an SCR with phase control, and “current-limit”, and “soft- start”.

The above two SCR’s we purchased meet this specification. Believe me, I looked at a lot of SCR’s on eBay which I thought would do the trick, and pulled up and checked a lot of mfr’s brochures and/or users manuals. And I turned a lot of them down because they did not meet the spec. But it is worth it because these types of SCR’s sell for over $800 new.

Hope this helps.

Many of you on here can understand this info. If any needs clarification or if you have questions, please ask. Again, if you don't understand this material and you still want to build a Moly furnace, please talk with an electrician. He/she will understand the above and can help you out.


PS - there are other items about SCR's we can talk about, such as power factor, proper mounting, etc. Feel free to mention anything about SCR's & TCU's here. Thanks.

Last edited by Jill_ : 04-16-2008 at 12:05 PM.
Reply With Quote
  #2   Report Bad Post  
Old 04-16-2008, 12:52 PM
Cheng076's Avatar
Cheng076 Cheng076 is offline
Veteran Member
South of Seattle
Join Date: Feb 2004
Posts: 764
Hails from United States
Re: Moly Glass Furnace - SCR & temp control

I have been interested in your "Moly" posts and would like to suggest that you put them all in one thread. Later on finding all the threads may be a problem. Looking for only one that has "Moly" in the title is so much easier.
Regards, PJH
Federal Way, WA
[email protected]
Reply With Quote
  #3   Report Bad Post  
Old 04-16-2008, 02:03 PM
Jill_'s Avatar
Jill_ Jill_ is offline
New Member
Join Date: Apr 2008
Posts: 49
Re: Moly Glass Furnace - SCR & temp control

We agree with you. The problem is the one thread will get hundreds of posts, on everything from transformers to SCR's to insulation to door design to furnace top and TCU, etc. This is what happened over on Mark's private forum that we have been on for the last three weeks. And then if anyone who comes on here in the future wants to know anything about Moly, then they have to sift through pages and pages of posts.

What we have done is put out three individual posts on the SCR, the transformer and elements. We think that they will disappear in a couple days from the front page. BTW, each of these threads have links to the others.

Only one of these will carry on - it looks like it will be the transformer thread.

This way, if anyone wants specific info on the SCR, transformer or elements, it will pop right up with a search of the forum.

I know it probably looks like we're hogging the site, but we're not. In three days there will only be one thread and maybe not even that one. But this info will pop up with a search of "moly furnace".

Reply With Quote
  #4   Report Bad Post  
Old 04-19-2008, 09:31 AM
Jill_'s Avatar
Jill_ Jill_ is offline
New Member
Join Date: Apr 2008
Posts: 49
*** Videos Which Explain Phase and Zero SCR's

These Videos Explain Phase and Zero SCR's -

A couple of months ago when Jimmy started talking about Phase and Zero SCR's with me, I didn't have a clue what he was talking about.

Then he found these videos on Watlow's website which explains it. Even I can understand this stuff now!

This first video explains how a "phase-angle" control SCR works. Like Jimmy mentioned above, this is the type of SCR which is used for a Moly glass furnace.


This next video explains "phase-angle" control with the "soft start" option (soft start is required due to the high in-rush current of the transformer - just like the high in-rush which a motor has). Soft-start limits this in-rush current so that the SCR is not damaged -


This next video talks about "soft start" like above, but it also adds in the "current limit" option. The current-limit feature is necessary because cold Moly (at room temp of 68 F) has a very low resistance. At these low temperatures (at furnace start up) the amps would want to spike way up, and damage things. The current-limit keeps the amps within a safe level.


And finally, here is Watlow's video which explains how a "zero cross" control SCR works. This is the type of control recommended for wire elements, but NOT for Moly (becuase it does not have soft-start nor does it have current limit).


Hope this helps,
Reply With Quote
  #5   Report Bad Post  
Old 04-20-2008, 01:42 PM
Jill_'s Avatar
Jill_ Jill_ is offline
New Member
Join Date: Apr 2008
Posts: 49
200 Amp SCR on eBay for $30 (Costs ~$800 new)

Jimmy just noticed this listing on eBay again for a 200 Amp SCR from Spang.

Here it is -

Spang Power Control Unit 751-D-CBA-2011 48KVA 240V 1Ph

http://cgi.ebay.com/Spang-Power-Control-Unit-751-D-CBA-2011-48KVA-240V-1Ph_W0QQitemZ230243260083QQihZ013QQcategoryZ97184Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

There are 2 days left for this auction. This is the same one we bought which we talked about above. This auction keeps popping up because apparently they have a lot to sell.

This is Spang's model 751 - it is a "phase-angle" control SCR and a link to Spang's info about it is here -


And here is a link to the Spang catalog (in pdf) which also talks about the model 751 (look at page 35) -


And here is a brochure for it -


This is the same SCR we bought about a month ago (see pictures above) from the same eBay seller. At the time, he had 10 for sale and he sold 6 then. Now they have another 10 for sale here, so they must have a bunch of them they need to sell.

Oh, and as a comparison, $30 is about the same amount we paid for a Crydom SSR with heat sink for our wire melter.

Hope this helps,


PS - this is for illustration purposes only. We have nothing to do with this seller. We have listed this information only to show how these very expensive phase-angle SCR's (~$800 new) come up on eBay, and if you are watching, you can get them at a very good price ($30).

It is up to you to pick one for your particular system, one which is sized accordingly. This is NOT a recommendation to purchase this item.

And remember, you must verify from the SCR part number and their published catalogs or by calling the factory, that the SCR is indeed a "phase-angle" and has "current-limit" (due to cold Moly low resistance) and has "soft-start" (due to the transformer).

Last edited by Jill_ : 04-20-2008 at 01:49 PM.
Reply With Quote
  #6   Report Bad Post  
Old 12-04-2016, 08:21 PM
GlassMan777 GlassMan777 is offline
New Member
Join Date: Dec 2016
Posts: 1
Re: Moly Glass Furnace - SCR & temp control

Hello all,
I am new here and this is my first post. Sorry to dig up an old thread but I am attempting to put together a Molybdenum Disilicide powered melter and have a question regarding the controller. I am attempting to purchase a Control Concepts type 1029D SCR controller (this is more or less the same as the Control concepts controller pictured above (1029C)).
I am having a problem with the 5'th letter of the product number.
The product number goes something like

1029c [X]-[xxx]V-[xxx]A-IL[xxxx................... etc

Some of the devices have a V or a P or an I or an A or an E at the location of the red x
What is the device pictured above. I cannot see the full description of the device as the picture is too small. The device number is on a smaller sticker somewhere on the device. See picture attached. It may be internally in the device in which case it will be too big of a job to find out.

Devices with a P are best for Silicon Carbide elements. I don't know what is best for M Disilicide and I think it does not matter much but I would love to know what was used here before I purchase.

V = voltage feedback
P = Power feedback
I = current feedback
This stuff in internal 'stuff' to the SCR controller and nothing to do with the feedback coming from the temperature sensor.

How is the melter going after all these years?

Thanks alot.
Attached Images

Last edited by GlassMan777 : 12-04-2016 at 08:29 PM. Reason: Picture not attached
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -4. The time now is 12:06 AM.

© 2014 F+W All rights reserved.