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.

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.

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?

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?

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.

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.)

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.

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.)

Click to expand...

Heat is IMHO the biggest reason.

Click to expand...

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)

Not a clue, it would really depend on how often you're switching that SSR.

BenS said:

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.

Click to expand...

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.

I have been using 25A SSRs (Crydom) for 2 years. 240VAC, 5500W elements. No problem. I have an excellent heat sink, though, not one of those smallish chinese black ones. 0.7°C/W.

When I was building my system, ebay didn't have as many options as it does now.

I could have gotten a 25A SSR+heatsink for $15 or a 40A SSR+heatsinck for.... $15.

I decided to get the 40A since it was literally the exact same price, and if I ever wanted to go over 5500W, I would be able to do it without necessarily getting new SSR.

BenS said:

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.

Click to expand...

Go on and run that race car in the red. :rockin: Oh and speaking from experience these bad boys only last a short time without proper ventilation. The fact that you will be pushing the 25a to near maximum operation everytime you use it will most likely decrease its life span. Like Walker and others said before the price is nearly the same for either why not take the more robust one?

bruin_ale said:

No offense taken, do what you want. I'm an electrical engineer, though mostly focused on digital design.

Click to expand...

Me, too... and I used to live and work in Campbell. I work for a company headquartered in Santa Clara now, but we have an office in Durham, NC, and I wanted to get out of silicon valley.

Go on and run that race car in the red.

Click to expand...

Haha! Woohoo!

I'll do it. It's going to be blazing!, perhaps literally....

What you guys are saying does make perfect sense, and I do agree with you, just makes me disappointed in industrial products when I have to "overbuy" something just to make sure it will work for an extended period of time. Also disappoints me that I have to buy said ****ty product from the chinese and that we can't make it here at home. But that is a rant for a different forum on HBT.

IDK if US ever manufactured SSRs here I know there are some German/Swiss/Japanese models that cost 5x - 10x as much and perform better (less noise). For what we are doing with brewing we can live with noise it does not matter. For what I do at work we have to use the pricey ones.

I also am an electrical engineer. A few of my designs include SSR for military applications. For various reasons, (most false turn on due to high dv/dt spikes) we build our own SSRs. Do you need to derate the SSRs? It depends. If you are using a avalanche turn on device (think triac or SCR) with zero crossing turn on circuitry, you can push the limits. No zero crossing, start to derate. It is a bi-polar transistor, I derate to 10% rating, ie. If I need to switch a 6 amp load, I use a minimum 60 amp rated transistor.

For me, bigger is better.

PS - generally, the voltage drop is the same for all SSRs. The higher current units use a larger silicon wafer so the current density is the same.

Charlie-I should clarify, I'm a mech egnr. I know of what you're talking about, but thats about it. And while I'm glad that you derate so heavily when designing for the DoD(I know how things get used and abused in the field), I was just wondering if a 25A SSR would work for my brewing(not exposed to elements or jostled or run in extreme temps) for an acceptable shelf life.