Modification at vacuum cleaner power supply

[2PAC Mafia]

Hello,

I have a vacuum cleaner which works with batteries and I use it for cleaning inside my car. The problem is that the yield is too short only around 10 minutes with fully charged batteries.

Now I would want to connect a plug for the 12Vdc car´s connector and use this source as power supply. I have checked the power when my car is running at this plug and there are 14,4Vdc.

I aslo checked that the vacuum cleaner uses a motor which consumps 7A at 3,6Vdc (3 batteries in serial) so I have thought about the possibility of installing a power transistor which takes the 10,8Vdc at its Vce or Vds. I think is necessary to use a MOS transistor but I don´t work usually with them so I´m not sure if that is possible to do.

For example I found a IRFZ44R which I think has enough properties to do it but I would like to confirm the way of connecting it. Can you help me?

Thanks and best regards.

 

[stevez]

I have seen numerous descriptions of 3 terminal voltage regulators such as the LM317 with a pass transistor to handle larger currents. As I recall, the datasheet for the LM317 has just such an arrangement shown. This might be one way of solving the problem.

 

[Oznog]

The heat generated by burning up 10.8v at the amps used by a vac is very high. This is 70W of heat! It would need a big transistor (actually several transistors) and heatsink. Much simpler to just get a 12v vac. The cost of doing this is quite high.

 

[Someone Electro]

Looks like you need an switch mode powersuply.

 

[Sebi]

The cheap and dirty solution: apply serial an appropriate car-bulb seriel with cleaner. Also give nice light for work

 

[2PAC Mafia]

Hello Stevez,

I have seen numerous descriptions of 3 terminal voltage regulators such as the LM317 with a pass transistor to handle larger currents. As I recall, the datasheet for the LM317 has just such an arrangement shown. This might be one way of solving the problem.

I checked the maximum Iout from this LM317 and it´s only 1,5A. I know there are some voltage regulators which could do this function but they use to have drop voltage at output and they are more expensive than a transistor.

Hello Oznog,

The heat generated by burning up 10.8v at the amps used by a vac is very high. This is 70W of heat! It would need a big transistor (actually several transistors) and heatsink. Much simpler to just get a 12v vac. The cost of doing this is quite high.

I already checked that we would have so much heat at the transistor but this IRFZ44R I already checked it supports at least 100W and I found some more which supports more than 100W. The cost of installing a MOSFET there should be cheap, it´s around 2 or 3 euros and the vacuum I have I got from fuel station BP card points, so it´s like free. I also do it to learn I little bit more.

Hello Someone Electro,

Looks like you need an switch mode powersuply

I think it´s not necessary, I only want to excitate the MOSFET with some resistor and the 14,4 Vin to have the 10,8V at Vds and then get the 3,6V / 7A at the motor... It seems easy.

Hello Sebi,

The cheap and dirty solution: apply serial an appropriate car-bulb seriel with cleaner. Also give nice light for work

In fact, it´s not a bad idea but the problem is that the motor needs exactly 3,6Vdc and 7A to have the maximum yield and I don´t think I can find a bulb so accurate to the final values I need.

You can see the vacuum cleaner, I don´t have a lot of place inside, just enough to fix there some MOS transistor and resistors to excitate it... I think that is the best option but I still need to know the way of calculate and connect the MOS transistor to the resistors and Vin to get this 10,8V at its Vds. Any idea???

Thanks for your answers.

 

[Someone Electro]

If you try to regulate it linear you will have to get rid of 75W of energy.

So if you use an transistor or sotming on an 1 C°/W heatink(Such an heatsink is already quite big and heavy)The transistor wod be runing at about 90 to 110 C° and this is very hot.

An swicthmode powersuply is from 80 to 90 % eficent wich means there is very litle heat made and so it can be smaller,lighter and cooler.

 

[Sebi]

The dissipation of analog supply exactly same as bulb disipation. Without really big heatsink cannot work. With heatsink impossible to build into this case. The SMPSU is a solution, but maybe also not enough place in case for it. Don't worry about 3,7V, it will happy with 3,5...4,2V. I know only this type for 10A:

 

[chemelec]

I would use a "Pulse width modulation" circuit to control the power from the larger battery.

 

[Someone Electro]

Yep this cod also be a solution.

And you can use power MOSFETs.

 

[chemelec]

Yes, Just go to the Projects on my Website.

 

[2PAC Mafia]

Hello,

now I have a doubt, I checked that the motor used nearly 7A when was running from its 3,6Vdc battery source but when I simulated the power with my power supply and I put 4,4Vdc it consumps nearly 8A and the propeller turns faster, more rpm!!!!

As Sebi said this motor has a range and I think the interesting thing would be to know the maximum voltage I can supply it. Then I would get maximum rpm, maximum cleaning vacuum (very important to remove the dirty at the car) and less problems to connect it to the car´s power supply. The motor is a Johnson with numbers 62855 and 3F1113. I already tried to find the data sheet for this one using those numbers but I couldn´t find anything.

I connected already a MOSFET (IRFP350A) with 2 resistors without heatsink to do the test but as you said the heat is too much and in some seconds the component gets so hot that the soldered point I had at the drain pin was removed. This MOSFET supports 17A Ids as maximum and it´s not so big but the problem is to cool it as you said, not possible to install anything inside the case. Maybe the cheapest option would be to make a box where I put this MOSFET circuit with a heatsink. The box with car´s plug male output cable and normal plug male output cable to connect at the vacuum cleaner, where I should also install a female normal plug and a switch, or even the switch could be at that external box.
What do you think about this solution?

I don´t know how big should be the heatsink to dissipate the heat properly...

 

[2PAC Mafia]

Hello,

I would say this would be enough to dissipate the heat (1,5ºC/W):

http://www.amidata.es/cgi-bin/bv/se...chType=quick&viewType=text&SearchText=2342514

 

[Someone Electro]

get an MOSFET that can suply more curnet.If its high enugh it will be just warm whithout an heatsink.

the IRF3205 can suply 110A at 55V and its in an TO-220(This is like most power transistors and voltage regulator) Over here you can get it for $2.

 

[Optikon]

get an MOSFET that can suply more curnet.If its high enugh it will be just warm whithout an heatsink.

the IRF3205 can suply 110A at 55V and its in an TO-220(This is like most power transistors and voltage regulator) Over here you can get it for $2.

Beware Sebi,

The 110A spec is a marketing spec. The TO-220 pins CANNOT support 110A of continous current.

 

[Someone Electro]

It has an resistance of 0,008 Ohm wich means it will make very litle heat.And an TO-220 has an termal resistance to a heatsink from 0,5 to 0,75 °C/W.

This shod take 3V at 7A and make 0,4W of heat.And the termal resistance to air in an TO-220 is 62 °C/W so this shod get to about 45 °C to 55 °C whith no heatsink.

Yea i know it cant suply 110A. The thin pins of it wod get hot.But it has a very very very low resistance wen on.

All you need then is an simple 555 timer to drive it.

 

[Oznog]

I already checked that we would have so much heat at the transistor but this IRFZ44R I already checked it supports at least 100W and I found some more which supports more than 100W. The cost of installing a MOSFET there should be cheap, it´s around 2 or 3 euros and the vacuum I have I got from fuel station BP card points, so it´s like free. I also do it to learn I little bit more.

The IRFZ44R can dissipate 100W as a maximum IF the back is kept very, very cool. If you dipped it in cool oil for example the back might be kept at 25C but the internal junction will still melt. If the backing is hotter, the maximum power rating goes down.

Keeping a TO220 this cool under 100W isn't going to happen, not for 3 euros. A cheapo folded aluminum clip-on heatsink can allow a TO220 to dissipate something in the 5W range or so. The modest sized heat sinks with milled fins that fit within a few cubic inches can allow a TO220 to go into the tens of watts. 100W requires a HUGE heatsink, lots of fins, skillful thermal mounting, and a fan.

You can keep the heat in the temp-sensitive MOSFET low by adding a resistor. For example if you put a 1.2 ohm resistor in series with the 12v supply it would burn up 8.4v, thus 58.8 watts of the heat. 5-20W resistors are relatively cheap and building a network of them will bring the MOSFET heat down to a realistic level.

Still, off the shelf, I'd say this project can easily run 30-40euro or more, and the results will be less than "satisfactory" due to the size and heat generated. It will also run down your battery pretty fast! That's why I suggest getting a 12v vacuum which is quite common.

I take it you want a learning experience here. First thing to learn is nobody will ever design a linear regulator like this, it's impractical. When we have to step down voltages like this we use a DC/DC converter- a switching reg- which will do the job cleanly. Cool, efficient, and cheaper than huge FETs and heatsinks. It would be difficult for us to communicate how to do this to you because it doesn't sound like you're ready for advanced circuits like this.

 

[Sebi]

Beware Sebi,

The 110A spec is a marketing spec. The TO-220 pins CANNOT support 110A of continous current.

Yes, i know this...

2PAC Mafia

When You thinking about independent box, i think my idea a with car-bulb better. Try with 120W bulb (or two 60W parallel) Johnson motor is very strong and bullet-proof: my hand-drill specified for 12V 6A and i use this device about 20 years with self-built thyristor RPM controller from 24V. When the 3mm diameter drill go through a metal plate, the current over 20A! ...and work fine. :twisted: I know, this is a barbarism, but i purchased two spare motors and no needed.

 

[zachtheterrible]

If you're so concerend about correct voltage, how about using a light bulb to dissipate most of the wattage and then use a voltage regulator (or a couple in this case)

 

[williB]

I dont think that motor could possibly draw 7A ..
0.7 is more like it..
that motor is only about an inch in dia.!!