Author Topic: Standard electric kettle as a DC dummy load ? (Read 5281 times)

lordvader88 · « **on:** July 13, 2018, 02:57:08 am »

I have a standard 120VAC electric kettle, how much current would that pass at 12VDC do u suppose ? Right now I'm playing with a 12V 2A DC-DC converter, and I have a 0.3A fan. I and another 4 or 5 fans, but I'm not trying that now.

Soon I really need big dummy loads up to test some ATX PSUs including a 1200W 1 I have. I need a kill-a-watt meter too. And some proper power resistors. I'm going to get some lo/hi-beam bulbs.

Then there's the $150, $250 load testers on ebay, I think a 300W unit for $250.

I have some 3/16" or 1/4" steel plates, 1'x1' (30cm x 30cm), what kind of current would 1 of them pass in a bucket of water.

I'm in my bedroom so I can't do experiments like I should be doing.

Shock · « **Reply #1 on:** July 13, 2018, 04:22:59 am »

Use ohms law to calculate what type of resistance you will need.
You also will need to check if that is 1200W on one 12V rail

edpalmer42 · « **Reply #2 on:** July 13, 2018, 05:35:09 am »

The kettle might be okay, but remember that you have to fill it with water to keep it from overheating. That means you're going to end up with a very soggy, steamy room. And, of course, you'll run out of water at the worst possible time, the kettle will overheat and the thermal protector will trip and cut off your load! Maybe not such a good load after all.

The high-low beam headlights are designed for 12V which is convenient. But they're only 50 or 60 watts which might not be enough. Also, when you turn them on, they're almost a short circuit which might cause your power supply to either not start or even blow up! Maybe not so convenient after all.

Practical alternatives are automotive battery testers or big resistors. I have a cheap 50 amp battery tester that's made of a couple of wire coat-hanger size steel wires in parallel. If you DIY something with actual coat hangers, remember that they're covered with lacquer which will smoke or burn. Either remove the lacquer or do the initial test outdoors.

Mike's Electric Stuff did a video on big loads that might give you some ideas.

Another good load is heating elements from an oven or stove, but they won't get you to 1200W at 12V.

mtdoc · « **Reply #3 on:** July 13, 2018, 05:41:57 am »

Quote from: Shock on July 13, 2018, 04:22:59 am

Use ohms law to calculate what type of resistance you will need.
You also will need to check if that is 1200W on one 12V rail

This.

The kettle resistance is very unlikely to be low enough. Use a DMM to measure it.

viperidae · « **Reply #4 on:** July 13, 2018, 07:04:11 am »

1200W at 120V is 12 ohms. That's 12W at 12V. Maybe a bit more as the resistance will be lower when it's cold

Zero999 · « **Reply #5 on:** July 13, 2018, 09:45:58 am »

Quote from: viperidae on July 13, 2018, 07:04:11 am

1200W at 120V is 12 ohms. That's 12W at 12V. Maybe a bit more as the resistance will be lower when it's cold

That's true. I doubt the lower temperature would make much difference to the resistance, since we're only talking about a difference of 80^oC or so, rather than >2500^oC for something like an incandescent lamp.

A car headlamp seems like a good idea. Another option is a small kettle, designed to run off a car cigarette lighter socket, which will be rated to 120W.

Kalvin · « **Reply #6 on:** July 13, 2018, 09:50:39 am »

Ten kettles in parallel will give 1.2 ohms ...

Brumby · « **Reply #7 on:** July 13, 2018, 03:06:45 pm »

I've got a high power 27R (at ambient) resistor that looks a lot like this:

mtdoc · « **Reply #8 on:** July 13, 2018, 03:55:50 pm »

Quote from: Kalvin on July 13, 2018, 09:50:39 am

Ten kettles in parallel will give 1.2 ohms ...

Right, but you need 0.12 ohms to get 1200W from 12V. 100 kettles?

But actually we don't know what the resistance of the kettle element is, I doubt it is a 1200 Watt kettle! It's likely that resistance of the element is even greater than 12 ohms.

Cheap water heater elements can be used for a dummy load but even they have a high enough resistance (the one I have is 8 ohms) that many in parallel would be needed to fully test a 1200W 12V supply.

Shock · « **Reply #9 on:** July 13, 2018, 04:27:20 pm »

Quote from: mtdoc on July 13, 2018, 03:55:50 pm

Cheap water heater elements can be used for a dummy load but even they have a high enough resistance (the one I have is 8 ohms) that many in parallel would be needed to fully test a 1200W 12V supply.

Do you have a part number for your 8 ohm one? Is it a constant resistance?

mtdoc · « **Reply #10 on:** July 13, 2018, 04:38:31 pm »

Quote from: Shock on July 13, 2018, 04:27:20 pm

Quote from: mtdoc on July 13, 2018, 03:55:50 pm
Cheap water heater elements can be used for a dummy load but even they have a high enough resistance (the one I have is 8 ohms) that many in parallel would be needed to fully test a 1200W 12V supply.

Do you have a part number for your 8 ohm one? Is it a constant resistance?

No part number. It's a cheap one I picked up a a local plumbing store. It's stamped 8 ohms. And that's about what I've measured it to be at room temp. I've not tested it further.

Kalvin · « **Reply #11 on:** July 13, 2018, 06:14:56 pm »

How about some thick wire or beefy resistor(s) submerged into mineral oil? Probably requires a pump to keep the oil moving and transferring the heat from the wire/resistor(s).

SeanB · « **Reply #12 on:** July 13, 2018, 07:02:12 pm »

Does not really need a pump, the oil or water will convect naturally. Just keep the hot surface of the resistor off of the plastic walls and it will be fine.

I did make a few quick and dirty dummy loads using a regular cup of cold water, a sacrificial TO3 power transistor ( I used 2N3773's that had survived the detonation of the UPS units they were part of), some wire and a resistance decade box to supply base current to them. They would run fine, boiling the water, till it was below the device, so just had to have some extra water handy for long load testing. Solder to base and emitter, a M4 nut and bolt to connect the collector, or just solder the wire to the top of the steel can. Worked from 1A to 15A, up to 100W of power dissipation.

CopperCone · « **Reply #13 on:** July 13, 2018, 07:05:51 pm »

I highly recommend that you make a spacious box made of steel mesh to house any kind of loads like this.

I have seen dummy loads like this meant for this using light bulbs, they put the bulbs in a steel mesh box basically to eliminate risk of electrocution and burns. This was meant for either parallel or series light bulbs with the circuits. Also stores nicely and looks good and protects it from falling screw drivers and other things.

Having blazing hot light bulbs, heat sinks, wire in the open is asking for trouble and possibly fire.

Home depot sells material that would work well.
https://www.homedepot.com/b/Hardware-Metal-Sheets-Rods-Sheet/N-5yc1vZc27v

If you use oil as a coolant and it gets real hot I would recommend you weight the platform down with some lead or stone and add some kind of splash guards (i.e. make baffles), and if it is not meant to get hot you should put a thermostat on it to break the circuit if it shorts out and starts to over heat so you don't accidentally deep fry yourself. A drain valve on the bottom would be nice to if you make some kind of oil testing tank so it can be stored drained.

Shock · « **Reply #14 on:** July 13, 2018, 08:20:31 pm »

One idea is connecting up a 12V mains inverter and sticking the load on the inverter. No idea how accurate that would be but you could really punish a 12V DC supply that way if you wanted to.

Doctorandus_P · « **Reply #15 on:** July 13, 2018, 08:34:53 pm »

Some length of lacquered ("enameled") copper wire in a bucket can serve as a temporary quite nicely.
MikesElectricStuff did a youtube about that some time ago:

Seekonk · « **Reply #16 on:** July 14, 2018, 12:06:12 am »

As far as water heater elements, I use 2,000W 120V that are about 7.5 ohms (regardless of what they are stamped) for heating water at my off grid home. There are "DC" elements for solar use that I believe are at least 300W at 12V, could even be 600W. These are more expensive. I bought two 2,000W elements for $15 shipped.

tkamiya · « **Reply #17 on:** July 14, 2018, 04:52:50 am »

If I needed a quick and dirty load for 100 amps continuous, I'd use a pair of heavy duty jumper cables, a bucket of water, and some resistive material. I might try hardware type steel wire or something. I probably will test the setup OUTDOOR and have a quick way to disconnect. That's enough power to cause some serious issues.

Old Printer · « **Reply #18 on:** July 14, 2018, 03:15:32 pm »

Murphy's Law states you will burn up the coffee pot, and after paying to replace it you will be back where you started, minus xx.xx dollars (last coffee pot I bought was $60) and still without the components for a proper load. Take some of the coffee pot money and buy some components to make a safe and durable load. You will probably need it again someday.

GigaJoe · « **Reply #19 on:** July 14, 2018, 04:36:25 pm »

when i was a kid , I boil resistors in the water , metal can ...

Now i get a bit older and build a primitive electronic load .... trouble with the load unfort. it I=const , sometimes need R=const. then measure the noise for example
so I bought like this with diff nomination: https://www.aliexpress.com/item/-/32876339506.html
with small heatsink, and in the water it can handle up to 200-300W per a single resistor

heatsink like this https://www.aliexpress.com/item/-/32827258255.html was on ebay much cheaper ...

hope it helps ...

Doctorandus_P · « **Reply #20 on:** July 14, 2018, 08:15:43 pm »

Sorry about double posting Mike's video, I didn't realize it was already linked to.
A kettle made for 120V (ac or DC, does not matter) is going to see 1/10th of the voltage at 12V, which will be 1/144th of the power.

Over time I've come to dislike the gold colored aluminimum resistors such as in Mikes YT vid. These are very heat sensitive, break easily, and are more expensive than ceramic resistors.

Ceramic resistors are pretty cheap, and you can easily mount 2 of them to a chunk of aluminimum. Just tap a single hole in the Aluminimum between them and use a big washer with a screw. More neatly also more effective would be to use a short piece of Aluminimum U-profile with a hole in the middle. The flanges of the U-profile provide extra cooling.
5W ceramic resitors cost about 15ct from Ali / Ebay / China, and I would not be surprised if these can safely & reliably handle a 10 fold overload, if bolted to a chunk of aluminimum.
If you want to go fancy, bolt them to a piece of square aluminimum tubing, and pump water through the tube.

These ceramic resistors can get bloody hot without failing.
A very simple & effective overload cutout would be to solder the wires close to the resistors, and ad a spring. If the thing gets so hot that the solder melts, the spring will pull the wire away and break contact.

Edit:
I just had a peek at a datasheet for a 5W Ohmite resistor, Series TWM/TWW:
https://octopart.com/tww5jr39e-ohmite-1147827
http://datasheet.octopart.com/TWW5JR39E-Ohmite-datasheet-8450738.pdf

These resistors are rated for a temperature of 275 Celcius, and have to be derated lineairly from the ambient temperature. So they can be safely used to temperatures above the melting point of solder, and they have 2 nice flat surfaces to bolt some aluminimum on.

Brumby · « **Reply #21 on:** July 15, 2018, 01:43:56 am »

Quote from: Doctorandus_P on July 14, 2018, 08:15:43 pm

A kettle made for 120V (ac or DC, does not matter) is going to see 1/10th of the voltage at 12V, which will be 1/144th of the power.

(Ahem... Check your math.)

Nusa · « **Reply #22 on:** July 15, 2018, 03:03:10 am »

Quote from: mtdoc on July 13, 2018, 03:55:50 pm

Cheap water heater elements can be used for a dummy load but even they have a high enough resistance (the one I have is 8 ohms) that many in parallel would be needed to fully test a 1200W 12V supply.

Fairly cheap water heater elements can also be had for 12V, 24V, or 48V systems.

Shock · « **Reply #23 on:** July 15, 2018, 04:29:24 am »

Quote from: Nusa on July 15, 2018, 03:03:10 am

Fairly cheap water heater elements can also be had for 12V, 24V, or 48V systems.

12V travel hair dryer with different heat settings might work as well. They go for as little as $20 have built in cooling. Plus you can do your hair.

max_torque · « **Reply #24 on:** July 15, 2018, 10:33:45 am »

If you need to pull really high currents at low voltages, get yourself a roll of 1/4" (or 6mm for people who live in proper countries..... ;-) ) diameter thin wall copper tube, typically used for automotive brake lines. Unroll it into a spiral onto a wooden 'former' and by sliding the contacts up and down the spiral you can change the load. And, using something like an aquarium or car windscreen washer pump, (or just a hose off a kitchen tap) you can pump cold water up the middle of the tube to cool it.

When i was developing a high current / low voltage DC/DC project this is what i used!


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Author Topic: Standard electric kettle as a DC dummy load ? (Read 5281 times)

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