# SSR Relay

#### TAP

##### New Member
Hello, new here and just retired, so have time now to play, still broke but have more time, funny how that works.
Anyway, have been doing metal casting for a few years and the kiln works fine except for these FOTEK SSR relays I buy, Its a big pain the have a relay burn out in the middle of the operation, just like what happened again this morning.

Was reading about fake SSR's and thought I would carefully take this one apart.

Its a 40amp, thats what I have been using for my 120v-1400 watt kiln.

I'll post some pics here but the triac has this # on it, (BTA16, 1000, B, 41, 139)
If I did my research correctly it is a 16 amp part, not a 40 amp as the relay says. Am I Correct?

I enjoy building stuff so I was thinking of building a SSR, one that I could depend on.
I know I could buy on, but the point is the building is the satisfaction of doing it myself.

I have a schematic here (https://www.theprojectasylum.com/electronicsprojects/triacssrelay/triacssrelaycircuitdrawings.html) and the capability to make printed circuit boards

Any input would be great..
Thanks
T

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#### dknguyen

##### Well-Known Member
Using a back to back MOSFETs where their sources are connected and having a floating gate driver that drives both gates in parallel relative to that source terminal would work. Could build in a little isolated floating supply if you really wanted to to power the gate driver and also isolate the signal to the gate driver to give you isolation like a real relay.

You'd need that approach for DC because triacs and thyristors would only work for AC since once you turn them on, you can't turn them off. THey turn off on their own when something else causes the current to go to zero. With AC, this is the zero crossing every half cycle but you don't get that with DC.

You need back-to-back MOSFETs becuase the construction of real MOSFETs produces an internal, parasitic diode which results in the MOSFET only being able to block current flow in one direction and freely pass it in the other. You use them back to back to block current in both directions (for AC or in applications where DC can flow in either direction). What controls a MOSFET is the voltage between it's gate relative to the source pin so if you connect them back to back with source pins connected and use a gate driver that floats on the source, then you can switch both MOSFETs on regardless of the other voltages in the circuit. If the source isn't isolated from the rest of the circuit then you become more restricted.

Triacs can be thought of a (but are not actually ) anti-parallel thyristors but suffer from some quirks due to their assymetrry which leads to assymetrical switching behaviour so using two actual thyristors is a more expensive, but better performing option.

#### JonSea

##### Well-Known Member
Do you have a large heatsink on the units that burn out?

#### MikeMl

##### Well-Known Member
Another thing to consider is that the cold resistance of your kiln heater could be much lower than you think at the instant you first apply power, so what is killing your SSRs is the in-rush current surge.

#### rjenkinsgb

##### Well-Known Member
I'd use a module thyristor such as one of these:

They are extremely reliable, assuming you use a suitable heatsink.

You can use basically the same circuit as you one you link to.
Use terminals one & two as the main power connections, with terminals two and three linked to put the two thyristors in inverse parallel.
Then connect the optotriac between the two gate terminals, or use two optos with the LEDs in series and one triac section between gate and anode on each thyristor.

#### TAP

##### New Member
Do you have a large heatsink on the units that burn out?
Yes, a heat sink and a small fan out of an old computer keeps everything very cool to the touch.

#### TAP

##### New Member
Another thing to consider is that the cold resistance of your kiln heater could be much lower than you think at the instant you first apply power, so what is killing your SSRs is the in-rush current surge.
There is really no set occurance as when they burn out, the last one was just when I was ready to pour, about an hour from start to finish, others were at any time that I recall.

#### TAP

##### New Member
Using a back to back MOSFETs where their sources are connected and having a floating gate driver that drives both gates in parallel relative to that source terminal would work. Could build in a little isolated floating supply if you really wanted to to power the gate driver and also isolate the signal to the gate driver to give you isolation like a real relay.

You'd need that approach for DC because triacs and thyristors would only work for AC since once you turn them on, you can't turn them off. THey turn off on their own when something else causes the current to go to zero. With AC, this is the zero crossing every half cycle but you don't get that with DC.

You need back-to-back MOSFETs becuase the construction of real MOSFETs produces an internal, parasitic diode which results in the MOSFET only being able to block current flow in one direction and freely pass it in the other. You use them back to back to block current in both directions (for AC or in applications where DC can flow in either direction). What controls a MOSFET is the voltage between it's gate relative to the source pin so if you connect them back to back with source pins connected and use a gate driver that floats on the source, then you can switch both MOSFETs on regardless of the other voltages in the circuit. If the source isn't isolated from the rest of the circuit then you become more restricted.

Triacs can be thought of a (but are not actually ) anti-parallel thyristors but suffer from some quirks due to their assymetrry which leads to assymetrical switching behaviour so using two actual thyristors is a more expensive, but better performing option.
I know you mean well, but 90% of your post hit me in the forehead and continued way above my head. Wish I did understand..

#### TAP

##### New Member
Another thing to consider is that the cold resistance of your kiln heater could be much lower than you think at the instant you first apply power, so what is killing your SSRs is the in-rush current surge.
You guys are to smart for me, I really feel out of place here..

#### KeepItSimpleStupid

##### Well-Known Member
In the lab I worked in, they used variacs and triac controlled heaters from Eurotherm to drive "exposed" tantalum heaters in a vacuum.
They did use Phase Angle fired triacs, but kept blowing expensive $25.00 I^t fuses. I can tell you that 25 A SCR units did better than 10 Amp ones. Most heaters were low voltage <40V. The first change I made was to make sure all of the triac units were fitted with currrent imit. Then made sure 25A SCR/TRIAC units were used. Finally, I added a small glass fuse that cost a$1.00 instead of the expensive one.

For our process we had to use variacs, voltmeters and current meters that read wrong and that was labor intensive.

We used phase angle fired control of the SCR unit. Using zero cross with your Kiln would be a lower cost approach to try,.
With zero cross, the surge won't kill lifetime.

So, finally we had to do a technology upgrade from a proprietary dual SCR control to something standard like 0-20 mA, 0-5 v whatever.

I stuck my nose out real far and said let;s loose the Variacs and replace the custom junk with a 1RU (1 rack unit) DC power supply.
So, most were 1500 W, with a max of 40V. We laso had a 120 VDC 6A supply that we used as well for a 200 W heater.

This worked under one condition. The Eurotherm 818 temperature controllers could be configured for an isolated voltage output
The power supply could be bout with isolated inputs//outputs for about \$500 more.

We had at least 7 heaters, so 7 power supplies were 7 Rack Units high and you could put 2 full sized DIN cutouts for the controllers on a panel.

What we had noticed on a pilot system (it was way cooler) but not cheap that a co-worker and I programmed and built that the heaters lasted much longer with DC,

If you want the longest lifetime, use DC Power supply controlled by an isolated controller. The DC supply usually uses the positive voltage post for it's reference and not ground.

You can also use the current limit of the power supply.

We had designed in recipies, voltage, current, power, temperature limits into our controller. We also had a heat-up energy alarm which would detect a thermocouple shorted or out of place. We also had slew limits.

And yes, tungsten (we were using tantalum) has about 13x less resistance at room temperature than at operating temperature.
This does impact lifetime. DC heating will help a lot.

The isolated voltage output would have been nice. IEEE-488 was an option, but you don't have analog too.

#### tomizett

##### Active Member
Surely the point here is that a BTA16 is indeed a 16A triac, which has no place in something claiming to switch 40A. So I suspect that the OP is right, and that these SSR modules are fake.

#### shortbus=

##### Well-Known Member
The heat treat ovens(kilns) where I worked used mercury relays to switch on and off. But don't know the wattage of them. https://en.wikipedia.org/wiki/Mercury_relay Why run this high of an amperage on 120V? That is more suitable for 240V.

#### TAP

##### New Member
The heat treat ovens(kilns) where I worked used mercury relays to switch on and off. But don't know the wattage of them. https://en.wikipedia.org/wiki/Mercury_relay Why run this high of an amperage on 120V? That is more suitable for 240V.
I was going to build a 240v kiln, but decided against it, 120v is more versitle for me, I had already purchased the relays for 240v, so used them.

#### shortbus=

##### Well-Known Member
Is this a kiln or is it a furnace? You mentioned making castings earlier, and can't imagine melting metal at 120V. Even most small ceramics kilns are usually 240V. Where do you find circuit breakers for 40A 120V? Or outlets for that matter?

#### unclejed613

##### Well-Known Member
can't imagine melting metal at 120V
i have a small lead pot for casting bullets (it can also be used to melt solder for tinning wires) that runs off of 120V. there have been a couple of places i've worked that had small kilns like what the OP is talking about. i used one for case hardening steel cams for check processing equipment. the cams were mild steel with 2 working surfaces (around the circumference, and on the top). the top surface of the cam would wear out very quickly (i'm convinced that paper dust is one of the most abrasive substances on the planet). we found that by case hardening the cam surfaces, more than doubled the life of the cam. we used a case hardening compound, coated the working surfaces of the cam, and put the cams in the kiln for several hours.

#### tomizett

##### Active Member
Post #1 says it's 1400W, so only 12A.

#### shortbus=

##### Well-Known Member
i have a small lead pot for casting bullets (it can also be used to melt solder for tinning wires) that runs off of 120V. there have been a couple of places i've worked that had small kilns like what the OP is talking about. i used one for case hardening steel cams for check processing equipment. the cams were mild steel with 2 working surfaces (around the circumference, and on the top). the top surface of the cam would wear out very quickly (i'm convinced that paper dust is one of the most abrasive substances on the planet). we found that by case hardening the cam surfaces, more than doubled the life of the cam. we used a case hardening compound, coated the working surfaces of the cam, and put the cams in the kiln for several hours.
Forgot about the lead pots. When I saw metal casting I was thinking aluminum or bronze. I to used to do case hardening of small parts but always used an acetylene torch to burn off the Casenit.

#### TAP

##### New Member
Is this a kiln or is it a furnace? You mentioned making castings earlier, and can't imagine melting metal at 120V. Even most small ceramics kilns are usually 240V. Where do you find circuit breakers for 40A 120V? Or outlets for that matter?
For that matter this is correct....1400 Watts, 11.667 Amps, 120 Volts
I assume you are trying to make me look like a fool.