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Will this relay work?

 
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Scut Monkey



Joined: 03 Feb 2011
Posts: 4



PostLink    Posted: Thu Feb 03, 2011 5:33 am    Post subject: Will this relay work? Reply with quote


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So I have been looking at the prices of for the DPDT relays needed and they aren't cheap. Even more so the price from the online vendor linked to on Kals site has recently increased their pricing ~$20 for three relays. After a search tonight I found this supplier. I was wondering if the 240v version on this page would work for our needs? If so would using this slightly different version require me to change my wiring? At $4 each these seem like a steal if they work for our needs.

Proposed Vendor:
http://www.solarelectricinc.com/DPDT-12V-24V-120V-or-240V-30A-Relay-DPDT-30A-12V-24V-120-240V.htm

Only ebay vendor I can find:

http://rover.ebay.com/rover/1/711-53200-19255-0/1?icep_ff3=2&pub=5574906687&toolid=10001&campid=5336652980&customid=&icep_item=400172865326&ipn=psmain&icep_vectorid=229466&kwid=902099&mtid=824&kw=lg

Link modified by Kal to support the forum
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rosenjm



Joined: 21 Dec 2010
Posts: 249
Location: Ballston Spa, NY


PostLink    Posted: Thu Feb 03, 2011 11:42 am    Post subject: Reply with quote

You really need relays with 120V coils (which are $48 on that site). Using the 240 V coils would require you to pretty much make the entire controller 240 V vice 120V. The coil is what actuated the relay. When you apply 120 V to the coil, it energizes and creates an electromagnet which draws the contactor arm down and shut the contacts allowing current to flow across the main contacts. the 30 A rating refers to the continuous capability of the main contacts, the 120 (or 240V) coil refers to the voltage required to energize the coil and shut the contactor.

Kal's design used 120V everywhere except for the elements, which require 240V. Trying to use 240V coils would require you to use 10 gage wire in almost all applications (which is more expensive and much harder to work with) and upgrade your lights and switches to 240V. You would also be putting 240V on the front components making it a tad more dangerous. You would have to create an entirely separate 240V circuit to power the coils (which would require the use of more relays). You are better off with 120V coils and sticking with Kal's design.
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crush



Joined: 28 Dec 2010
Posts: 709
Location: Telemark, Norway


PostLink    Posted: Thu Feb 03, 2011 1:01 pm    Post subject: Reply with quote

rosenjm wrote:
Kal's design used 120V everywhere except for the elements, which require 240V. Trying to use 240V coils would require you to use 10 gage wire in almost all applications (which is more expensive and much harder to work with) and upgrade your lights and switches to 240V. You would also be putting 240V on the front components making it a tad more dangerous. You would have to create an entirely separate 240V circuit to power the coils (which would require the use of more relays). You are better off with 120V coils and sticking with Kal's design.


Just to give a different perspective, I'm building mine in Europe, where only 240v is the standard supply voltage. I sourced all the components from the same ebay seller, just made sure they were 220-240v versions. You can use the same gauge wire as when building with 120v, since it's the amount of current that requires a thicker wire, and by going to 240v you actually use less current, so it's fine to use the same wire gauge in this application. 16 gauge is used in europe to supply most electronic equipment drawing at most a few amps.

I don't understand how 120 is safer than 240v - since it's not the voltage that kills you, but the current. 100mA flowing across the heart can be fatal, which is why GFCIs rated for human protection have to activate inside of 30mA leakage. The PIDs are double insulated, so there is no danger there, and the chassis is grounded, and the whole thing protected by GFCI, so I feel there is in practice any real increase in electrocution hazzard.

For me, building this in 240v has made it simpler, since I don't have to worry about different voltages in the panel. I only need to use appropriate thickness wire to accommodate the current being drawn by each circuit. I've no idea if it's more expensive or not to build 240v, since I didn't make a point of comparing the prices of the 110v components, but I don't remember seeing any significant difference.

The 30A power relays to cost a fair bit, but when you consider the whole brewery costs upwards of $3000 then it's really not a lot overall. They provide peace of mind by truly shutting off the unused element. There are alternative designs that would not require the relays, using instead relay logic on the SSR control signals to ensure only one element is active, but they are inherently less safe.

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Scut Monkey



Joined: 03 Feb 2011
Posts: 4



PostLink    Posted: Thu Feb 03, 2011 6:46 pm    Post subject: Reply with quote

Hmm. Well I didn't know all of this. I am by no means an electronics guru but I'm not dumb with them either. I'm glad some people are better with electronics than me and I can ask. Thanks.
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rosenjm



Joined: 21 Dec 2010
Posts: 249
Location: Ballston Spa, NY


PostLink    Posted: Fri Feb 04, 2011 12:40 pm    Post subject: Reply with quote

crush wrote:

Just to give a different perspective, I'm building mine in Europe, where only 240v is the standard supply voltage. I sourced all the components from the same ebay seller, just made sure they were 220-240v versions. You can use the same gauge wire as when building with 120v, since it's the amount of current that requires a thicker wire, and by going to 240v you actually use less current, so it's fine to use the same wire gauge in this application. 16 gauge is used in europe to supply most electronic equipment drawing at most a few amps.


I didn't realize that you guys only had 240V. You are correct about the wire size, only the element circuits would need the higher current capability. How are you powering your PIDs and Timer?

crush wrote:

I don't understand how 120 is safer than 240v - since it's not the voltage that kills you, but the current. 100mA flowing across the heart can be fatal, which is why GFCIs rated for human protection have to activate inside of 30mA leakage. The PIDs are double insulated, so there is no danger there, and the chassis is grounded, and the whole thing protected by GFCI, so I feel there is in practice any real increase in electrocution hazzard.


I hesitate to say that any voltage is safe. While it is the current that kills, it is the voltage that produces the current. Assuming a constant body resistance (~300 ohms) to ground, the higher the voltage the more current you get. I=V/R. My point being that with 240 V the potential to receive a fatal shock is higher that it would be with 120 V.

Insulation, grounds and GFCIs are methods to mitigate the risk of electrical shock, they don't prevent shocks. I'm not trying to insinuate that this build or design is unsafe. If everything is done correctly, the risk is minimal but there is still a risk espically considering the environment (wet). All (at least in the US) electrical appliances are tested and verified prior to being sold to ensure the grounds are proper and safety components are working. This controller is not.

crush wrote:

For me, building this in 240v has made it simpler, since I don't have to worry about different voltages in the panel. I only need to use appropriate thickness wire to accommodate the current being drawn by each circuit. I've no idea if it's more expensive or not to build 240v, since I didn't make a point of comparing the prices of the 110v components, but I don't remember seeing any significant difference.


Usually, the higher the voltage, the more dielectric strength is required to insulate that component. Typically, this requires those components to be larger and more robust, typically increasing the cost. For example a 240V switch's contacts typically need to be larger to ensure there is sufficient gap between the open contacts to prevent arcing. Again, I haven't priced 240V components myself, so it could be moot at the component level we are working with.


It seems to me, without doing a lot of research into pricing, that unless you are building the entire controller at 240V, using 240V relays isn't worth the cost difference between the 240V and the 120V ones. My $0.02.
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crush



Joined: 28 Dec 2010
Posts: 709
Location: Telemark, Norway


PostLink    Posted: Fri Feb 04, 2011 12:51 pm    Post subject: Reply with quote

rosenjm, you make good points, and I agree what you say above. So a body is 300 ohms, I didn't know that! But I do see now how the higher voltage could be more dangerous. I was going to mention the dielectric, which is also a factor with the cables, but thought I'd said enough already! Since there is no choice of voltage here, I've not thought through the advantages in terms of smaller components that 110v might bring.

Quote:

It seems to me, without doing a lot of research into pricing, that unless you are building the entire controller at 240V, using 240V relays isn't worth the cost difference between the 240V and the 120V ones. My $0.02.

I agree. Using the 240v relays when 120v will do doesn't seem to be worth the trouble. As to the price of the 240v relay on the website posted, I think it's a simple mistake on the website. They're surely not selling 240v 30A power relays for $4, and the 120v versions for $48?

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mschippr



Joined: 09 Feb 2011
Posts: 2
Location: Australia


PostLink    Posted: Wed Feb 09, 2011 6:21 am    Post subject: Reply with quote

Hi,

I'm in Australia and we also only have 240 volts supply. How did you go with these relays for $4? has anyone received them yet and tried it out? $4 with $26 USPS shipping (im using a mail forwarding service) is pretty good.

Thanks,
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Scut Monkey



Joined: 03 Feb 2011
Posts: 4



PostLink    Posted: Wed Feb 09, 2011 11:39 pm    Post subject: Reply with quote

I did not purchase the relays listed and I don't really plan on it at this point. I simply don't have the knowledge or time to change the layout based on what people are telling me it would include. That dosen't mean it's not possible I simply don't trust myself to pull it off. One of the main reasons I decided to take on Kal's build is simply because he explains it so clearly and straight forward. With a modification like this I would be doing the opposite and confusing the myself with the build. Best of luck!
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