Using 12V power cord as resistor to dissipate heat

[soldar]

I have a hand-held car vacuum cleaner which works on 7.2 (nominal) rechargeable batteries which are long dead.

I just want to use the vacuum plugged into the car's 12V.

I started another thread about using a switching step-down converter but that did not work out and I am looking at the simple and expedient solution of just dropping some volts in a resistance.

All the numbers are round, quick and loose because no precision is required. The car voltage will fluctuate and the vacuum motor has wide latitude.

So let us say I want to drop 4 volts at 6 amps. That would require 0.666 ohms and would dissipate 24 watts.

I suppose I could make a little box with the resistor but I am thinking of just making the electric cord be the resistor.

If I make the cord 2.4 m long it would be dissipating 10 W / m. Is that reasonable? Is that doable? How hot would it get? Are there cables made for this purpose?

I remember some old radios has a resistor cable built into the power cord used to limit the filament current.

I think a cord of 2.5 m length is reasonable so it can reach all corners of the car. Maybe a bit longer. But I think 5 or 6 m although would dissipate heat better and have longer reach might be too cumbersome to use and get in the way.

Copper cable would need to be quite thin and subject to mechanical breakage. Here maybe that infamous copper-clad iron might be better.

So the basic idea is to use the power cord at (say) 6 A to drop (say) 4V and dissipate (say) 24 W.

If it turns out to be too complicated I guess I can just make a little box with ventilation.

[Gyro]

Just a personal opinion, but I would say 10W/m is pushing your luck, for PVC insulated at least. Of course, it depends on the duty cycle.

[mariush]

Not worth it.

Just get a bunch of diodes and screw them down to a small heatsink?

FR806 will drop nearly 1.3A at 8A : https://www.digikey.com/en/products/detail/smc-diode-solutions/FR806G/21705614

2.2v at 10A  : https://www.digikey.com/en/products/detail/smc-diode-solutions/SDURK1060/21705492

2v at 20A : https://www.digikey.com/en/products/detail/smc-diode-solutions/SDURK2060/21705412

around 1.1v-1.2v for the above 2 at 5-6A

0.75v at 16A (~0.5v at 5-6A) : https://www.digikey.com/en/products/detail/smc-diode-solutions/MBRF1660/21705371

[soldar]

Just a personal opinion, but I would say 10W/m is pushing your luck, for PVC insulated at least. Of course, it depends on the duty cycle.

Yes, it looks like it. So it would either have to be a much longer cord or a special cord with heat resistant insulation.

I think something like half that, 5 W/m should be OK.

Or I could even split, half on the cord and half in a resistor.

[soldar]

Not worth it.

Just get a bunch of diodes and screw them down to a small heatsink?

FR806 will drop nearly 1.3A at 8A : https://www.digikey.com/en/products/detail/smc-diode-solutions/FR806G/21705614

2.2v at 10A  : https://www.digikey.com/en/products/detail/smc-diode-solutions/SDURK1060/21705492

2v at 20A : https://www.digikey.com/en/products/detail/smc-diode-solutions/SDURK2060/21705412

around 1.1v-1.2v for the above 2 at 5-6A

0.75v at 16A (~0.5v at 5-6A) : https://www.digikey.com/en/products/detail/smc-diode-solutions/MBRF1660/21705371

No need for diodes, resistor is good enough. The issue is concentrated heat. I suppose a little box with good ventilation should do.

[Gyro]

Put the resistor inside the vacuum? There should be plenty of airflow to get rid of 24W.

You could probably increase the motor voltage a bit too for better performance.

[soldar]

Put the resistor inside the vacuum? There should be plenty of airflow to get rid of 24W.

You could probably increase the motor voltage a bit too for better performance.

That is really a brilliant and obvious idea that should have occurred to me! Thanks!

I took out the battery pack so that is the perfect place for it. Plenty of space and with strong air circulation.

Now I need to find or build a suitable resistor.

[Gyro]

The obvious is not always obvious.

As with all power resistors, the bigger the body size the better, to expose the maximum area to the airflow and keep the surface temperature down, especially in a plastic housing. EDIT: Those gold colored 50W Aluminium clad ones are pretty cheap on ebay and you could mount one and stand it off the side of the case on a couple of long screws and a few nuts.

[Ian.M]

Alclad resistors *require* a heatsink to get anywhere near their power ratings.  Without a heatsink normally you'll be pushing your luck much over 10% of their nominal rating, though with forced air, you may get to 20%.

[Gyro]

I would have expected a bit better than 20% in a fast airflow. If it's replacing a battery pack, it doesn't sound as if it would be difficult to add some sheet Aluminium... or even a scrap finned heatsink.

[soldar]

I am thinking in terms of building a rectangular prism, the shape and size of the battery pack, using "cordwood build". Two parallel PCBs with a bunch of resistors connected in parallel. That will allow air circulation. I can mount 5W or 3W resistors or I may look into making them myself because I have plenty of nichrome wire.

[Haenk]

Really I would check if the motor is OK with 13V, the voltage given is to match the battery pack, but: no battery pack, no matching needed.
Though doubling the voltage might be a bit too much...

[Zero999]

Why not just PWM it? Put a fast diode in reverse parallel with the motor and a MOSFET in series. Drive the MOSFET with a 555 timer set to 60% duty cycle.

[soldar]

Why not just PWM it? Put a fast diode in reverse parallel with the motor and a MOSFET in series. Drive the MOSFET with a 555 timer set to 60% duty cycle.

Well, that was my first idea and then I thought an eBay module does the same but cheaper and simpler, and then it turns out the module doesn't work quite right and then I realize a resistor in series is extremely simple. reliable, cheap...

Years ago I had a design with an H bridge which worked fine with a resistive load on the workbench but would go totally wonky with a slightly inductive load and the transistors would fire at the wrong moment.

You cannot beat the simplicity of a resistor in series. This is for a vacuum cleaner which will only see very occasional use so power efficiency is not a concern.

At this point my general idea is to fit a 3m cord to dissipate about 12 W and another 12W in a resistor inside the vacuum.  Drop 2 V in the cable and another 2 V in the internal resistor. 0.333 ohm the cable and the same the resistor.  Couldn't be simpler.

[Seekonk]

I have an off grid camp and have picked up a number of older 7.2V tools for cheap or free.  I corded them and operated them on 12V with no issues. 

[soldar]

I have an off grid camp and have picked up a number of older 7.2V tools for cheap or free.  I corded them and operated them on 12V with no issues.

If it were for my own use I would have no problem doing that but it is for a friend and I would rather not take the risk of starting a fire or blowing a fuse. 

Also, I know to monitor the heat and the duty cycle, etc. but give the vacuum cleaner to someone and theyŕe liable to leave it running while they answer a phone call.

But, yes, I do realize things with motors, like drills, etc. can work with a wide range of voltages as long as you abide by a duty cycle that prevents overheating.

[Xena E]

Add a thermal fuse, in case anything goes wrong?

I think Gyro's idea of putting a resistor in the airflow of the fan is good, think of the tiny motors in line powered cleaners that are being cooled like that.

X

[Zero999]

Why not just PWM it? Put a fast diode in reverse parallel with the motor and a MOSFET in series. Drive the MOSFET with a 555 timer set to 60% duty cycle.

Well, that was my first idea and then I thought an eBay module does the same but cheaper and simpler, and then it turns out the module doesn't work quite right and then I realize a resistor in series is extremely simple. reliable, cheap...

Years ago I had a design with an H bridge which worked fine with a resistive load on the workbench but would go totally wonky with a slightly inductive load and the transistors would fire at the wrong moment.

You cannot beat the simplicity of a resistor in series. This is for a vacuum cleaner which will only see very occasional use so power efficiency is not a concern.

At this point my general idea is to fit a 3m cord to dissipate about 12 W and another 12W in a resistor inside the vacuum.  Drop 2 V in the cable and another 2 V in the internal resistor. 0.333 ohm the cable and the same the resistor.  Couldn't be simpler.

It might be more parts, but is simpler in other respects, because not so much attention needs to be paid to thermal design.

[bostonman]

I think diodes or resistors is a great solution, however, I have a question.

Would using resistors cause the motor to starve for power at turn on and possibly damage it?

Also, where are you getting 12v at 6a (not including startup current)? A cigarette lighter may work. Those are rated (I just looked on Google) for approximately 10A, but this is a resistive load; not an inductive load that has high current at turn on.

[soldar]

It might be more parts, but is simpler in other respects, because not so much attention needs to be paid to thermal design.

I like your circuit with 555 astable and I will keep it in mind for similar applications but for now I am inclined to go with the simplicity of dropping voltage in a resistor.

I am thinking of dropping a couple volts in the supply cable and another couple in a resistor inside the vacuum where it is in the air stream. It seems quite simple and easy.

If for any reason I run into any trouble with this then I will try the astable.

[soldar]

I think diodes or resistors is a great solution, however, I have a question.

Would using resistors cause the motor to starve for power at turn on and possibly damage it?

Also, where are you getting 12v at 6a (not including startup current)? A cigarette lighter may work. Those are rated (I just looked on Google) for approximately 10A, but this is a resistive load; not an inductive load that has high current at turn on.

The motor has no trouble starting with low voltage as it only moves a fan.

If it takes 6 or 7 Amps when running it will take more when starting and could blow a 10A fuse. A resistor helps limit the current but a diode does not. In this sense a resistor is better.

I have a tire air compressor which also draws about 5 or 6 amps at 12 V but has already blow quite a few car fuses. Now I do not take the risk and I just use an unregulated 12 V PSU plugged into the mains.

[mariush]

The reason I suggested diodes was because the voltage drop on resistors is proportional to current, so i was thinking without the device actually working and pulling current it will have 12v or more on the terminals and maybe the charger chip or something else inside may not be able to handle that high voltage. 
The diodes will have a voltage drop on the diode even at just a few mA going through them.