Can you use an AC water heater element with a DC circuit?

[makenzie71]

I need a resistor that can actually bring my Heli 2.0 or i2000 to a stop.  I'm attempting to test out a Chinese 3-phase 2000w grid tie inverter and the resistor it uses is absolutely not up to the task.  Once this inverter gets above 90v on the input the stupid dinky little resistor it came with is blazing lol...the camera dimmed it a lot.  When I had it hooked up to the Heli it was almost white.

I need a resistor than can handle 90vdc and possibly get up there around 120~130vdc if the turbine runs away.  I was hoping maybe an inexpensive 120v water heater element would be worth trying out.

Opinions?

[makenzie71]

And another thing...would a heating element even be a good idea?  I want to being able to bring the turbine to a stop when it's running away, but would a heating element be too little resistance?  They don't have a lot.

[SparWeb]

What's the resistance of the thing you're burning up there, now?

Here's an approach to choosing the right resistor:

What's the maximum current that the wire in that Heli can tolerate before it melts into slag?
Pick a number out of the air, or from a Heli datasheet, or from a website like this: https://www.powerstream.com/Wire_Size.htm

Divide 120V by that current and you'll get the resistance of the total circuit that, when connected, will slag the windings.  Call this resistance R_slag.
Now that you know that, you need to measure the actual resistance in the windings, R_actual.

R_slag - R_actual = The minimum resistance of your load resistor.  You should probably choose a resistance higher than that, but definitely not lower.

Once you have that value then your next calculation is:

Resistor * Max Current ^2 = maximum power released by the resistor.

Then go shopping at Digikey/Mouser/Ebay for the resistor with those specifications.

If you can't find one, yes, consider heating elements from a kitchen oven or water heater. 
If fact, many are rated for 120V anyway so you won't have to stretch your imagination to figure out how much power they will dissipate.
If it's a water heater, it only works in water (which may be a useful thing, if you can use the hot water, or a nuisance, if you can't).

[Mary B]

And another thing...would a heating element even be a good idea?  I want to being able to bring the turbine to a stop when it's running away, but would a heating element be too little resistance?  They don't have a lot.

Why bring it to a stop with heating elements? They are there to keep it in a safe speed range by keeping a load on the turbine, could double them in parallel and really slow the turbine before shorting the phases with a switch... if you really want it stopped. I have seen guys on here say shorted phases won't always keep a WT stopped, and of it takes off you can burn it up.

Would be glowing really bright here right now, winds are steady at 30mph with peak gusts over 45mph! Making the house vibrate LOL and shake all the dust out of everywhere!

[makenzie71]

There are times when I need to bring the turbines to a complete stop.  This controller has no means of stopping the turbine other than shorting the stator through the dump load.

[makenzie71]

Resistors...hwo do they work?  Are they even real? lol

So I'm playing with a 1500w 115v heater element.  Measures 14.2ohm across the leads.

If I connect it to 12v it draws 10w and never gets warm

If I connect to 24v it draws 35w and becomes warm but not hot

If I connect it to 48v it draws 280w and becomes uncomfortably warm.

At 96v  it's drawing 1200w and is exactly like you'd expect it to be....hot

Is that normal operation for a resistor?

[SparWeb]

Yes, that's normal. Do the math and you'll see.

[Adriaan Kragten]

The power is the product of voltage and current. The current increases linear to the voltage if the load is a resistance. So the power increases quadratic to the voltage.

[makenzie71]

Math?  Quadratic?!  No one told me I'd have to be doing rocket science here!

To make this work I need 2000w of absorption at 60v.  At 60v it's pulling something like 700w...which means I'd need three of these things.  Would I run them in parallel or in series?  I seem to remember from high school 143 years ago that resistors in series reduces resistance...or something like that.  Trying to look this up now everywhere that tries to explain resistors starts with "in the beginning there was only darkness" lol

I'm trying to figure it out with these specific resistors not because I think they're ideal, but because they're $10 new at Home Depot and they are plentiful.

[OperaHouse]

The nice thing is there are ohms law calculators on the internet to figure out parallel resistance.

[makenzie71]

The nice thing is there are ohms law calculators on the internet to figure out parallel resistance.

If you wouldn’t mind pointing out the online calculator that calculates how much wattage at what voltage the resistors consume i would be much obliged.  Adding up resistor VALUES is not a problem.  I need to sort out how those values relate to how much wattage the resistors consume.

[MagnetJuice]

Hi Mack,

Here is a link to a simple online calculator.

https://ohmslawcalculator.com/ohms-law-calculator

Better yet, you can find your values using this wheel.



Not rocket science, just simple multiplication and division.

Ed

[Mary B]

The nice thing is there are ohms law calculators on the internet to figure out parallel resistance.

If you wouldn’t mind pointing out the online calculator that calculates how much wattage at what voltage the resistors consume i would be much obliged.  Adding up resistor VALUES is not a problem.  I need to sort out how those values relate to how much wattage the resistors consume.

Watts = voltage x current

Parallel resistors of the same size are easy, 2 divide the resistance by 2! 3 divide it by 3! It is when you have unequal size parallel resistors that the math gets more difficult.

[mab]

The nice thing is there are ohms law calculators on the internet to figure out parallel resistance.

If you wouldn’t mind pointing out the online calculator that calculates how much wattage at what voltage the resistors consume i would be much obliged.  Adding up resistor VALUES is not a problem.  I need to sort out how those values relate to how much wattage the resistors consume.

Power= V^2 / R

If using off the shelf heating elements with standard power and voltage ratings, then it's often easier to convert (approximately) to your actual voltage without thinking about resistance, using the following method:

Say your element is 120V 1500W, but you want to use it at 60V:- because you have halved the voltage the power will be 1/2 of 1/2 of the rated power - i.e. quartered, so 375W (the voltage squared relationship in the above equation). If you need 1500W at 60V the you're going to need 4 of these elements in parallel.


If you use them at 40V then that's 1/3 voltage, so 1/9 power: 1500/9 = 170W (170W-ish actually comes out at 166.66W but the resistance of elements usually goes up a little when at full power so don't be surprised if they work slightly above the calculated power when under-running them).

[JW]

something at least one of us should have.

https://www.amazon.com/Engineers-mini-notebook-formulas-circuits-Siliconcepts/dp/B00072TU8Y

[makenzie71]

The table above works for me!  Thanks!

something at least one of us should have.

https://www.amazon.com/Engineers-mini-notebook-formulas-circuits-Siliconcepts/dp/B00072TU8Y

My hobby budget can't afford that much match   

[JW]

Think there is an e-book.

I know the Sensor projects has one.

[JW]

Here it is 

https://www.walmart.com/ip/Engineer-s-Mini-Notebook-Formulas-Tables-and-Basic-Circuits-Cat-No-276-5016A-Pre-Owned-Paperback-B0014B3J6Q-Forrest-M-Mims-III/511601359?wmlspartner=wlpa&selectedSellerId=1719&&adid=22222222227000000000&wl0=&wl1=g&wl2=c&wl3=42423897272&wl4=aud-430887228898:pla-51320962143&wl5=9013362&wl6=&wl7=&wl8=&wl9=pla&wl10=131366457&wl11=online&wl12=511601359&veh=sem&gclid=EAIaIQobChMI6cSd-qu89wIVkr7ICh1lpAt7EAQYBSABEgKsBvD_BwE&gclsrc=aw.ds

5 bucks

[JW]

Just bought this book, figuring a way we can display it here on Fieldlines

[tanner0441]

Hi

With a resitance element running that hot that close to a wooden particle board the risk of something catching fire would worry me. It looks like a standard electric heater element and well within its limmits but not doing what you want.

Brian

[Mary B]

The table above works for me!  Thanks!

something at least one of us should have.

https://www.amazon.com/Engineers-mini-notebook-formulas-circuits-Siliconcepts/dp/B00072TU8Y

My hobby budget can't afford that much match   

How about free as a pdf? Print it and put it in a binder! It comes in handy! I have an original that is getting dog eared from use. I still refer back to it even after working on electronics for 40+ years

http://www.zpag.net/Electroniques/Kit/Radio_Shack_-_Engineer%27s_Mini-Notebook_-_Formulas_Tables_Basic_Circuits_.pdf

[Mary B]

Just bought this book, figuring a way we can display it here on Fieldlines

PDF for free http://www.zpag.net/Electroniques/Kit/Radio_Shack_-_Engineer%27s_Mini-Notebook_-_Formulas_Tables_Basic_Circuits_.pdf

[JW]

Cool good job guys, im glad I only spent 5 bucks for it.

[Ungrounded Lightning Rod]

I need a resistor that can actually bring my Heli 2.0 or i2000 to a stop.  ...

I was hoping maybe an inexpensive 120v water heater element would be worth trying out.

Opinions?

Sure you can use a water heater element - if it's kept immersed in water and the water isn't allowed to all boil away.  They burn out almost immediately if run dry at their rated voltage.

If you don't have water handy that can deal with that energy (e.g. the water heater element is actually in a water heater full of water that can be allowed to warm up enough to eat the energy), you might want to consider an air heater element - a glowbar or coil type - maybe in an actual air heater housing or otherwise mounted so it can air-cool without setting things on fire.

When playing around with ohm's law on a heating element, you should bear in mind that the element's resistance will typically vary substantially with temperature.  At the rated voltage (once it heats up) it will be at the expected resistance.  But at a lower voltage, or when you first start driving it and it's still cold, its resistance will be substantially lower and the resulting current higher than what you'd expect when extrapolating from its ratings.

This is much more pronounced with something like an incandescent light bulb - which gets white-hot - than with a glowbar or toaster, etc. which only gets red.  If I were fooling around with this approach, I'd probably consider a ceramic heater element, which typically is designed to not get hot enough to set paper on fire.  (But I'm not sure if they have some oddball resistance vs. temperature curve that would foul up the calculations differently.)