Element operating at lower power

[st_brewer]

Hey guys

So I'm trying to hook up the elements to my HLT and hop back but the perfect element I want (due to thread size and shape is 4kw which mauls my home electrics so I want to restrict the power going to it so it doesn't use all 4kw but my question is, will it even heat up?

 

[abarnett]

Probably. People run their 240v elements on 120v, and they still heat up.

 

[raouliii]

How do you plan to "restrict" the power going to it? An SSR with PWM control or something similiar?

The element is simply a resistor. Any current allowed to flow through it will create heat. The issue, of course, is to control that current.

To build on abarnett's comment: A 4kw/240vac element draws about 17amps and calculates to about 15ohms of resistance. That same element, operated on 120vac, should draw about 8amps and would be equivalent to about a 1kw element.

 

[st_brewer]

Thanks for the reply guys, an ssr will be used, but ssr is simply a switch that would allow the full power to flow I'm not totally sure how to reduce the power, I was originally going to use resistors

 

[raouliii]

Correct. An SSR allows full voltage to be sent to the element.

However, an SSR switches on and off very fast, so something like Pulse Width Modulation can be used to control the SSR output. The PWM controller varies the ON and OFF time of the SSR with the resulting effect much like varying the voltage applied to the element.

 

[Bobby_M]

You cant use resistors other than elements. Either run it at half voltage or you can also run two elements in series to double the resistance and quarter the power.

 

[st_brewer]

I'm thinking about these replies I can see where pwm comes in but the moment the ssr goes on then the full 17 amps will go through which will trip my fusebox.

Wiring multiple smaller elements in series is an idea but when hooked up to the mains electricity surely the first element will receive the full brunt of the current momentarily and melt, or at the very least both will receive all available amps shared between them?

I read that people use transformers to step down the voltage but a 1kw transformer costs a very pretty penny

Or have I completely missed the point

 

[grumpywolf]

I use two 2.4 kilowatt elements in my kettle that I run from differant power points that are also on differant circuits in the house.

 

[onthekeg]

A phase angle controller allows you to run less than full power like a PWM (555 timer) does. You can set if for any variable amount of voltage from 0 to max wattage of your resistor.
PM me if you are interested.

 

[raouliii]

...Wiring multiple smaller elements in series is an idea but when hooked up to the mains electricity surely the first element will receive the full brunt of the current momentarily and melt, or at the very least both will receive all available amps shared between them?....

That's not really how it works. The current drawn is a function of the resistance of the element and voltage applied to it.

If you placed two of the 4kw/240vac elements in series, you would end up with about 30ohms of resistance. If this is wired to 240vac it would draw about 8amps and be equivalent to about a 1.9kw element. Not coincidently, equivalent to placing 120vac across two discrete elements as in my example above in post #3.

The resistance and voltage are the constants.
The current drawn is a function of the voltage and resistance. I= V/R
The power consumed and converted to heat. P=V*V/R

 

[Bobby_M]

I didn't say to wire smaller elements in series. You'd wire two 4kw elements in series (which is like adding another really big resistor in series with one element). This is really easy ohms law type stuff.

What happens with one element:
4000 watts / 240 volts = ~17 amps
240v / 17a = 14 ohms

Now two in series:
Two 14ohm loads in series are simply additive so they would present 28 ohms.
240v / 28 ohms = ~8.6 amps
240v * 8.6 amps = 2064 watts (which are spread across two different elements for pretty low watt density)

Your electrical circuit and panel will only ever see a peak of 8.6 amps.

I don't recommend messing with electricity.

 

[tjpfeister]

I can assure you that if you use a PWM (to control a SSR) with a fast enough cycle duration, like the ones for dimming LEDs, that the amperage will be controlled in a linear fashion proportional to the voltage. I have two of these: http://www.amazon.com/Dimming-Controller-LED-Lights-Ribbon/dp/B003L4KKF2/ref=sr_1_1?ie=UTF8&qid=1342964766&sr=8-1&keywords=pulse+width+modulator on my rig for manual throttle.

 

[st_brewer]

Thanks for the replies guys, the confusion was that I thought you meant using smaller elements but it makes sense using the larger elements, I completely understand ohms law but like yousaid electricity is dangerous so I wanted some other ideas brought to the table.

I am seriously considering looking into the use of pwm in the ways described, my understanding was that allowing the ssr to open would be the same as flicking a switch which would have tropes my fuse box but it appears that it's a little more sophisticated then that.

Thanks for all the help guys

 

[Bobby_M]

I can assure you that if you use a PWM (to control a SSR) with a fast enough cycle duration, like the ones for dimming LEDs, that the amperage will be controlled in a linear fashion proportional to the voltage. I have two of these: http://www.amazon.com/Dimming-Controller-LED-Lights-Ribbon/dp/B003L4KKF2/ref=sr_1_1?ie=UTF8&qid=1342964766&sr=8-1&keywords=pulse+width+modulator on my rig for manual throttle.

I wouldn't consider pwm as a safe way of regulating a load down below the current capacity of a circuit whether it technically works or not.

 

[tjpfeister]

I wouldn't consider pwm as a safe way of regulating a load down below the current capacity of a circuit whether it technically works or not.

In this instance, the goal is to maintain amperage levels below the rating of the breaker, so worst case scenario is that the amperage spikes and the breaker trips. I would agree with you if someone was trying to circumvent the breaker for purposes of OVER-loading the circuit, that it would be unsafe. But under-loading it wont hurt a thing.

 

[tjpfeister]

To clarify, the PWM doesn't regulate the load. It regulates the supplied voltage, which was one of the suggestions given earlier. The resistance of the element stays constant so the amperage drawn becomes directly proportional to the supplied voltage. So if one is uncomfortable with the PWM/SSR combo, one could use any other method of voltage control, so long as it can handle the max amperage draw of the element. One of those neat bench testing voltage regulators would do the trick, if you have the coin for one that can handle the amperage.

 

[Bobby_M]

One of us doesn't understand how PWM and SSRs work. When signal voltage is applied to the SSR it opens the flood gates. The element sees 240v and pulls max current per ohms law. In this configuration, the PWM regulates how long the SSR stays open and how long it stays closed. Are you thinking that there is a pulse length short enough that the SSR doesn't let a full 240v sine wave to complete?

 

[tjpfeister]

Yessir. I did extensive small scale testing before I had the funds to build my entire rig. I had concerns about the same thing, and also how the duty cycle of the PWM might not work in conjunction with the 60hz cycle of AC current. To my surprise, with an inductive ammeter and a DVOM set to voltage, I found that the above linked PWM could give you a very smooth, linear voltage supply from 0-100% of your available household supply. If you looked at the line with a lab scope, I'm sure there are slight variations, but for our purposes, it works.

Yes, on a very micro scale you are quite right Bobby but that amperage spike is probably measured in nanoseconds and doesn't have the time required to trip a breaker, heat up a wire, or even register on either of my meters. I have two of those PWMs wired in parallel to the PIDs on my rig. This allows for manual override of the PIDs and also, if I choose to transport my rig say out to my patio, I now have the option of using my 20A 240V air conditioning circuit. (Instead of my 30a service I use inside the apartment) I cannot let the PID control the 5500W element, because the PID cycles too slowly and will trip the breaker every time, but I can use my PWM and keep the applied amperage under 75%.

I wish I had shot some footage with the meters in place, but here is something I threw on the interwebs showing my small scale testing.

Just a heads up to anyone else reading this though, this most likely will not work on anything with an AC motor! You will still want to use a ceiling fan controller or something if you are stirring wort or grinding grain due to that whole cycle thing I mentioned earlier ;-)

 

[Bobby_M]

Pretty crafty. I see what you did there.

 

[crane]

Yessir. I did extensive small scale testing before I had the funds to build my entire rig. I had concerns about the same thing, and also how the duty cycle of the PWM might not work in conjunction with the 60hz cycle of AC current. To my surprise, with an inductive ammeter and a DVOM set to voltage, I found that the above linked PWM could give you a very smooth, linear voltage supply from 0-100% of your available household supply. If you looked at the line with a lab scope, I'm sure there are slight variations, but for our purposes, it works.

Yes, on a very micro scale you are quite right Bobby but that amperage spike is probably measured in nanoseconds and doesn't have the time required to trip a breaker, heat up a wire, or even register on either of my meters. I have two of those PWMs wired in parallel to the PIDs on my rig. This allows for manual override of the PIDs and also, if I choose to transport my rig say out to my patio, I now have the option of using my 20A 240V air conditioning circuit. (Instead of my 30a service I use inside the apartment) I cannot let the PID control the 5500W element, because the PID cycles too slowly and will trip the breaker every time, but I can use my PWM and keep the applied amperage under 75%.

I wish I had shot some footage with the meters in place, but here is something I threw on the interwebs showing my small scale testing. Video Link: http://www.youtube.com/watch?v=ePc3zpyAeoY&feature=plcp

Just a heads up to anyone else reading this though, this most likely will not work on anything with an AC motor! You will still want to use a ceiling fan controller or something if you are stirring wort or grinding grain due to that whole cycle thing I mentioned earlier ;-)

I think your testing results may have been flawed by your test equipment. Multimeters generally average over a period of time to produce the number you see on the screen. They are not good at measuring small pulses. A short pulse to 5V may show up as 2.5V and if you were to halve the time that the pulse is on then you would only see 1.25V. But the fact is you are still applying 5V for some amount of time. To properly conduct this test you would need to use an oscilloscope with a current clamp. Then you would most likely see current spikes of equal amplitude and varying duty cycle as you adjust your PWM going to the SSR.

You are most likely turning on the element for a short enough time to not trip the breaker.

 

[mendesm]

If you're uncomfortable wiring two elements in series, get yourself something like this: http://www.aliexpress.com/store/pro...ostat-Voltage-Regulator/610943_557766931.html

 

[mendesm]

I think your testing results may have been flawed by your test equipment. Multimeters generally average over a period of time to produce the number you see on the screen. They are not good at measuring small pulses. A short pulse to 5V may show up as 2.5V and if you were to halve the time that the pulse is on then you would only see 1.25V. But the fact is you are still applying 5V for some amount of time. To properly conduct this test you would need to use an oscilloscope with a current clamp. Then you would most likely see current spikes of equal amplitude and varying duty cycle as you adjust your PWM going to the SSR.

You are most likely turning on the element for a short enough time to not trip the breaker.

EDIT: I didn't watch the video, no sound here at work.

Agreed.

Or using a meter that measures AC (most will have an AC and a DC measuring mode)

PMW does nothing to regulate voltage/current. It's either on or off, and when it is on, it'll let the full voltage on the line through.

Knowing that, you can make a PMW generators work to control power IF and only IF:

1) Your PMW's frequency is exactly 1/2 the AC frequency
2) AND they are in phase (meaning the PMW ON period starts at the 0 crossing point of the AC wave and ends before the next 0 crossing of the AC wave)

Now you have and over engineered phase angle regulator.

A normal TRIAC based phase angle regulator (AKA dimmer) is a lot simpler and cheaper.

 

[crane]

Or using a meter that measures AC (most will have an AC and a DC measuring mode)

even then it is going to average over a number of cycles before it changes it's value on the screen. I've haven't heard of any multimeter that will update its display 60 times a second to give you instantaneous readings. You wouldn't be able to read the values anyways. A scope or a high speed data acquisition system are the only ways you will truly be able to see instantaneous measurements.

 

[tjpfeister]

Guys guys guys. It works, and I will happily discuss the dynamics via PM so as to avoid any further convolution of this thread. None of us are wrong, but you have to think of things over time. And by time, I of course, mean milliseconds.