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Old 03-02-2011, 04:06 AM   #1
BenS
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Default Ekeggle amperage question

I'm making a basic ekeggle and I've got a question pertaining to the size of the SSR needed. I've been searching through HBT for ideas from other ekeggles and it seems that the general view on the board is to use an oversized SSR. I'll use myself as an example. I am installing a 5500W/240V heating element into the keggle and using an auber PID connected to a RTD sensor to monitor the temp(not needed for this question, just giving you a complete view of what I'm doing). Watts=Amps*Volts. Watts/Volts=Amps. 5500W/240V=22.917Amps. Why do I see everyone on the board purchasing and using 40A SSR units? One SSR can only control one mechanical object so why go overboard? If there is a valid reason, I would like to know before I purchase my SSR/heatsink. Thanks.

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Old 03-02-2011, 04:22 AM   #2
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You're at almost 23A, the e-bay SSR are commonly found at 25A and 40A. If you're using the 25A for your heating element you're above 90% of it's capacity. If you use the 40A you're barely over 50% of the capacity. If you use a 25A SSR that close to it's capacity it will fail much more quickly than the 40A SSR. Cost and size difference between the 40A and 25A are negligable, so it's really a no brainer to use the bigger one.

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Old 03-02-2011, 04:38 AM   #3
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If it's life expectancy is 25% shorter when running at 90% capacity compared to 50% capacity I still don't see why you would go bigger than you needed. SSR's are built for continous use applications and homebrewers have their units running for a couple hrs a week, maybe half that time at max amperage. I prefer not to go with the "bigger is better" attitude unless there is a reason to go bigger. Is there a reason for me to go bigger with my application?

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Old 03-02-2011, 05:29 AM   #4
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When SSRs fail, they typically fail closed - so you're element will be ON. That's reason enough for me to be careful. I'm not sure I understand where you're coming from, what do you stand to gain/save by using the smaller SSR?

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Old 03-02-2011, 12:22 PM   #5
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I'm an engineer by trade and believe there is a proper tool for every job. Doesn't make sense to me that they would produce a 25A SSR that isn't capable of handling a 25A continous load for the duration of its shelf life.

I apologize, I think I just talked myself into using the 25A SSR. I hope I did not come across as rude bruin ale.

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Old 03-02-2011, 12:55 PM   #6
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IMHO the 25A SSR can and will do the job. However, you need to be mindful of the one problem that can cause it to fail. Heat dissipation. When you drive the SSR it produces heat. High current that is close to its rating produces higher levels of heat. The use of a heat sink is critical. Also the heat sink must be place so that it can dissipate the heat into the surrounding air. (In other words, do not mount it inside an enclosure.)

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Old 03-02-2011, 01:44 PM   #7
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Heat is IMHO the biggest reason. You'll be generating more heat by running it at or closer to max capacity. You may need to use a fan or a larger heatsink than you see most of us using due to this.

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Old 03-02-2011, 02:18 PM   #8
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Quote:
The use of a heat sink is critical. Also the heat sink must be place so that it can dissipate the heat into the surrounding air. (In other words, do not mount it inside an enclosure.)
Quote:
Heat is IMHO the biggest reason.
I agree completely. The SSR/heatsink and PID will be mounted in a toolbox with a computer fan mounted on one side to draw air out of the box. Do either of you know the difference in heat generation between a 25A SSR running at 90% capacity compared to a 40A SSR at 50% capacity? (More out of curiosity than anything else)
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Old 03-02-2011, 02:27 PM   #9
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Not a clue, it would really depend on how often you're switching that SSR.

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Old 03-02-2011, 05:26 PM   #10
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Quote:
Originally Posted by BenS View Post
I'm an engineer by trade and believe there is a proper tool for every job. Doesn't make sense to me that they would produce a 25A SSR that isn't capable of handling a 25A continous load for the duration of its shelf life.

I apologize, I think I just talked myself into using the 25A SSR. I hope I did not come across as rude bruin ale.
No offense taken, do what you want. I'm an electrical engineer, though mostly focused on digital design.
I'm just curious why you'd use the 25A over the 40A? Just because it's capable? I spent $8 each for my 40A SSR on e-bay, I maybe could have saved a dollar or two if I went to 25A. I'd rather get the higher rated SSR and not worry as much about heat dissipation, but that's just me.
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