Power Resistor - Grendals or ????

[hooterville75]

Im current converting a ATX power supply to a lab bench power supply so I can get power to my bilge blower motor being used to power the exhaust on the spray booth I'm building. I need to obtain a 15 Ohms 25 Watts Power Resistor to make the power supply feel as if there's a load going to it. Ive been to Radio Shack and different places that sell power resistors but noone has anything that big of wattage. Ive located the Resistor I need on Evilbay but have ran upon another problem Im hoping Grendals or even someone else that may know the answer too.

Ive ran across the 15 Ohms 25 Watts Power Resistor but am seeing that they come in a Green or Yellow Aluminum casing. My question is what is the difference between the two? Is there a difference at all or just a different color of casing on the Resistor ? Ive tried feverishly to research this on google and find nothing about the difference in color casings however can find tons of info on colors of regular resistors. HELPPPPPPPPPPPPPPP. Thanks lol.

 

[Grendels]

I am not sure I have never dealt with a resistor of that size before. I suspect that since you can't find anything that the color doesn't matter. This link seems to echo that idea:

http://www.csgnetwork.com/resistcolcalc.html

From the site:

Color bands are usually found on resistors that have a wattage value of 1/8 to 2 watts; though it is rare, there are some 5 watt resistors that are banded. There are also some capacitors that are color coded. See our Capacitor Color Code Calculator.

 

[letsgobrowns82]

I have dealt with resistors quite a bit as an electrician. We use them a lot in fire alarm installations and control circuits. I've only ever known them to use the color bands to represent their resistive value. I need a little more info though as to what you are trying to accomplish. I'm assuming that your blower motor is not rated for the voltage or amperage that your power supply is putting out and your trying to bump down what you are delivering to the motor. What is the output of the power supply, what is your desired load to the motor, and what is the circuit AC or DC?

 

[hooterville75]

I need a little more info though as to what you are trying to accomplish. I'm assuming that your blower motor is not rated for the voltage or amperage that your power supply is putting out and your trying to bump down what you are delivering to the motor. What is the output of the power supply, what is your desired load to the motor, and what is the circuit AC or DC?

Letsgobrowns, what I'm trying to do is get power to this bilge blower motor I purchased to exhaust the airbrush spray booth I'm building. The bilge motor requires 12 Volts 5 Amps to run. I'm converting a ATX Power Supply from a computer to a Lab Bench power supply to do this. Going to use the 12 Volt rail which runs at like 18 Amps which is plenty. I need the 15 Ohms 25 Watt Power Resistor to fool the Power Supply into thinking it actually has a load going to it. Without the Power Resistor to give a fake load to the Power Supply it will turn on but it wont stay running. I need this size of resistor (is what I was told) so it doesn't overheat while in use. I would assume that I can go smaller in resistor if I would purchase a axial fan to mount on the top of the Power Supply but feel the bigger power resistor is the way to go. It is DC which is the entire problem that I need to convert the Power Supply lol. Other wise I have a plug I could just plug and go haha.

 

[letsgobrowns82]

I see, all you need is a load to fool your power supply. Well then as far as your resistor goes I dont think color of the casing is indicative of the resistance or power handling capabilities. The only thing I would say is to be carefull what you go with, wire wound resistors create a ton of heat.

 

[7165jay07]

Hi Hooterville75,

The colour of the outer aluminium casing does not specify anything as it is manufacturer and range dependant so will not affect the actual resistor.
As it is aluminium cased i assume it is a chassis mount wirewound type and as such lgb82 is correct when he says it will create a ton of heat when running at load so you will need to securely mount this to a solid metal plate ( i have always used a thick slab of ally) or heat sink to help dissapte some of this heat.
My company use this load system to test power amplifiers and they generate an extreme amount of heat.
I would test out how much heat is generated when running at load for a prolonged spraying session and this will give you some idea of how much you need to get rid of. (there is a formula for working this out but it's far too complicated.)

Hope this helps
Jay

 

[7165jay07]

Forgot to say, it's not a good idea to mount it to the ATX power supply as this is already working hard to keep cool.

Jay

 

[Elm City Hobbies]

Not really sure why you don't just save your pennies and buy the proper DC transformer for your motor rather than jerry rigging something that sounds like will be more of a potential fire hazard.

 

[steezy]

I agree with Agree ELH and 7165jay07. You shouldn't use that power supply with the blower. Keep it simple and just use a DC transformer.

 

[hooterville75]

My reasoning behind it all is this. Ive watched a ton of You Tube videos and many many people use these successfully. I already have everything but the Power Resistor which costs $3.00 minimum. With the regular DC transformer that I need with the Amperage I need is well over $30. You may find them offered for pennies on ebay but till it comes time to purchase them in the last few seconds of the auction watch it go from $.99 to $30 in two seconds.

The proper transformer I would get off ebay is a maximum of 5 Amps which is exactly whats needed to run. Spose I ever fall below the 5 Amps ? What happens ? Does it blow up ? With the ATX Power Supply, I have the Voltage I need, I have well over the Amperage I need 2 X over again not to mention with the 3.3 and 5 Volts Rails I can charge anything I need to charge with a USB port Im putting in the front of the Power Supply.

If I was to pay $30 for the proper transformer Id have well over $55 into this and can buy a Dayton Squirell blower which is AC. I originally screwed up and purchased a blower motor that is DC. Too late to fix it now. So Im just sticking with the ATX Power Supply again since I already have everything but the PR. Is thinking I could go with two 5 watts 11 ohms Ceramic Power Resistor and mount them to a piece of prototyping board.

 

[steezy]

It can't fall below 5 amps.

I found one for $13.49 on amazon.

Why do you need 18 amps?

Can you link to the videos your watching? I don't understand why you need to trick it to think there is a load on the supply.

 

[hooterville75]

Because if you dont fool it to thinking there is a load on it it will turn on and stop.

Converting Computer ATX Power Supply to Lab Bench Power Supply

El Cheapo 20A Lab Power Supply from ATX PC Supply

Again, it all boils down to not wanting to spend $14 when I have everything I need minus a $3 Power Resistor. I can buy a few items I need for modeling wise for $14.

 

[steezy]

That's pretty neat.

To answer your question. The guy in the first vid used two 5ohm resistors in parallel, which equals 2 1/2 ohms. So you could just get one 2 ohm 10 Watt resistor.

Now don't take this next part the wrong way. I'm just concerned for your safety, I don't think you understand how dangerous it is to mess around with electricity without the proper education/ training. Watching a youtube video doesn't make you an expert.

The switches, connectors, L.E.D.s, heat shrink tubing, and pc board needed to make the supply into a desktop supply would cost well over $14. Not to mention all the tools.

It makes more sense to me to have a permanent supply for the spray both. Something you could just plug in when it's needed.

Going through all the trouble to make your own cheap desktop supply makes sense if your going to be using it a lot with different projects, i.e. prototyping, testing circuits, etc.

That's just my opinion, and you seem to have already made up your mind. So good luck to you and don't electrocute yourself please.

 

[hooterville75]

Well I appreciate your concern but having all the supplies already other then the power resistor I figured why not. It has the 3.3 and 5 Volts rails which I could use for alot of other stuff besides the main reason I want it, not to mention will work to charge my nephews RC chargers, daughters Ipod, Ipad, etc.

In regards to hooking it up, I figure if I can move my washer and dryer from down in the basement, upstairs to the laundry room we built, hooking that all up working with 220 and not get shocked, I should be able to accomplish this smaller task. In regards to the plug and play AC transformer. I wish I could do that but I feel Im already too far into this to not stick with my original plans were.

Im pissed I didnt look into the Bilge Blower Motor more before I purchased that and just got a Dayton Squirell Cage or equivalent. The caps in my ATX PS arnt charged. I think Im going to give it a shot. If this fails or doesnt work right, then Ill save my pennies and buy the AC Transformer I need to plug and play. Hoping I can the ATX to work as it will be beneficial more ways then one.

 

[steezy]

Well good luck then, sounds like a very neat project. I'm almost tempted to make one myself.

I'd be very careful charging ipods and stuff. There is a limit to how fast they can be charged before blowing up. A typical USB port puts out .5 amps give or take. Those devices were designed around that spec, so charging them at higher amps can be dangerous. When lithium ion batteries overheat they explode, and charging it causes it to heat up and charging it very quickly won't give it time to cool, causing it to overheat.

RC stuff should be fine, but tred lightly, you could reduce the operating life.

It occurred to me that the reason the guy in the video was using two resistors in parallel was to dissipate the heat. So, if needed, you could run four 10 ohm 30 watt resistors in parallel and still get 2 1/2 ohms, and a better heat sink. Another reason may have been that those were the only resistors he had laying around.

 

[7165jay07]

Hi Hooterville,
Sorry if you thought i was implying not to do it, i just meant you need to be careful.
I do know on certain ATX power supplies you can join the sensing leads and have it running constant without any problems as it is just the same as running a PC 24/7 so it will eventually burn itself out.
We use these power supplys all the time as benchtop supplies and they are actually really useful as they have multiple 5v and 12v feeds.

As a thought if you have an old Intel i core CPU heatsink this will work with the Resistor to cool it.

Hope this clears things up.
Jay

 

[hooterville75]

Na man its all good. With the size of Power Resistor Im going to be using I dont see how it could possibly overheat. Thats the reason for the bigger size of Power Resistor. I;m considering using more smaller ceramic power resistor on the prototyping board. Not sure yet. Thanks for all the info. Much appreciated.

 

[7165jay07]

Ok, if you need any help just pm me and i'd be glad to help.
Jay

 

[hooterville75]

Thank You Jay!!!! Much appreciated.