a 30-40 amp triac

[MrDEB]

need to turn an electric hot water on and off controlled by an op amp/ thermisistor circuit.
looking at either a relay or triac. need a 30 amp per leg (220v) so two triacs or an alternistor
https://www.electro-tech-online.com/custompdfs/2010/03/E4AlternistorTriac.pdf
but if I am reading it right I need 200ma gate current?
any suggestions or just go with a DPST relay?

 

[Hero999]

Use an opto-isolating triac such as the MOC3041?

 

[crutschow]

You could go with a solid-state relay such as this. If you need to break both sides of the line you could use two of them, but I'm not sure that's necessary, unless it's required by code.

 

[MrDEB]

I plan on using an opti coupler.
Have opti-couplers MOC3023 I think (have a bunch for my Christmas Light display)
I kinda like the idea of a pre built SSR. I will check out that website further, as long as its in the USA (postage from europ is way out there.
If I build one it will be hard wired on a perf board with terminals for 220 etc.
I like the SSR over a relay anyway.

 

[mneary]

Just keep in mind that any solid state switch requires a substantial heat sink, as the TRIAC or SSR needs to be getting rid of 48 watts (30A * 1.6V).

At this power level, the relay can have advantages.

 

[Dick Cappels]

"but if I am reading it right I need 200ma gate current?"

Yes, you have to supply the required gate current once each cycle to trigger the Triac, but after that, it will stay on by itself for the rest of that half cycle.

 

[MrDEB]

The water heater is 4800 watts which equates to about 19 amps plus 125% as recommended for resistave applications. thats why #10 wire is used for electric water heaters. Plan to include a large heat sink as well as a muffin fan just to be sure.

 

[MrDEB]

Will just go with a relay

Relay rated for 30 amp w/ 12v coil 86 ohm
555 to turn on relay for 30-40 miniutes .
keeps triping until water temp is 60 degrees or higher
can't add png file for upload???

 

[MrDEB]

lets try this again

to upload schematic in png format
won't upload??

 

[Hero999]

Try, GIF format or saving using a different picture editing program, for some reason the forum software doesn't like PNG files which were created by certain programs, for example, Eric Gibbs had a problem with attaching files saved using Paint Shop Pro.

 

[MrDEB]

It used to work??

 

[MrDEB]

try again in GIF

It used to do png
lets see if it will post a gif
no can do
I tried ememptying the uploaded pictures , all 28 of them but ???
the form administrator may need to look into why?

 

[Hero999]

Don't delete every attachment you've uploaded.

Use a hosting service such as TinyPic if you're that desperate.

 

[MrDEB]

I think its something to do with the home network I just set up.
I can't post to Photobucket either?
I posted a question on the geek link that was posted on connecting two computers together.

 

[RODALCO]

Just use a 1 or 2 pole relay / contactor.
For heating you want all your power in the elements instead of wasting power in an external heat sink.

 

[MrDEB]

Thats my plan a DPST relay so I break both LINE voltages
Problem I am working on is a 555 (a 7555 will go for 30 miniutes) that is triggered by a 741 op amp driving an NPN transistor to trigger the 7555 as a mono stable 30 minute timer.
The output of the 741 stays high thus the TRIGGER pin 2 stays LOW. Can't get (LT SPICE) to stay on (mono) while pin 2 of 7555 stays LOW.
doing something wrong. Wish I could post a schematic but I haven't figured out this network I connected two computers together via a hub.. It prevents me from uploading any files.

 

[tcmtech]

Why do you want to cycle your heating elements on and off with a timer?

 

[MrDEB]

This is for an electric hot water heater. We installed an ON DEMAND electric hot water heater. Problem is it requires a min of 60 degree water input. Then the unit is capable of outputting 2 gal per minute. Plan is to monitor the cold water supply (I have the 50 gal water heater still plumbed but not on.) At present the cold water with out the 50 gal tank is 38 degrees. The 50 gal tank tempers the water to about 50 + but not 60..
I plan on sensing the 50 gal tanks output, if not 65 (741 op amp w/ thermistor) degrees then the 50 gal tank heats for 30 minutes.If still not hot enough then it turns on for another 30 minutes. This app is only used on weekends (church hall) so if the water is not flowing then the heater should not even turn on.
And yes I tried lowering the temp on the tank but it only goes to 130. The ON DEMAND heater max temp input is around 80 degrees.
Now just need a timer to run for 30 minutes. A 7555 is supposed to be stable enough if Tant caps are used but My problem is the trigger pin 2 is held low until the op amp senses warm water. All the 555 circuits I find require the trigger pulse to be very short. Could have sworn I have seen a schematic showing otherwise.

 

[MrDEB]

After searching etc I think I will just get an electric automotive temp sensor and pipe in a Tee. Then figure out what the resistance is at 70 degrees and be done with it. This way the water remains at 70 degrees instead of when water is needed the 50 gal water heater starts heating.
Another way would be to put a flow switch so the only time the 50 gal heater comes on is when hot water is wanted.

 

[tcmtech]

How about one of these? Selco - WKF20.6E-007-020 - Sensors - Sensors, Switches & Relays - Allied Electronics

$14.99!