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how to reduce 12v to 9-10v

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voigts

Member
Joined
Feb 11, 2005
Location
Tennessee
I just got two of those monster high speed triple blade Deltas from Sidewinder along with a 25w 10ohm ohmite rheostat. The rheostat takes them down to about 7 volts from what I can tell, but I really want to run them at around 5volts through the Rheostat so that I can still adjust them. These things still pull a lot of air even at 5 volts. What do I need to wire in to reduce the 12v in to about 9v to feed the Rheostat? As much air as these things pull, I don't see myself needing them above 9v or so.
 
I think what he's asking for is to lower the voltage range, from 12-7v to 10-5v, if so just run a diode or three inline from the 12v line to the rheo. Model 1n4001, should cost you about 50 cents a piece tops at any electronics shop.
 
You are right about what I am asking Eobard. I don't exactly get what you mean by running a diode or three inline. I am assuming that you mean taking three of the diodes that you mentioned and somehow putting them in series? Sorry, but electronics is not my forte.
 
voigts said:
You are right about what I am asking Eobard. I don't exactly get what you mean by running a diode or three inline. I am assuming that you mean taking three of the diodes that you mentioned and somehow putting them in series? Sorry, but electronics is not my forte.


Yes he means putting the diodes in series. For every diode you put in series, it will have a subtract .7 volts from the fan voltage. Because that .7 volts will now be across the diode. If you use 3 then 3 X .7 would be minus 2.1 volts.

You could also just buy a fixed power resistor at Radio Shack and put it in series with your rheostat or even another rheostat in series with the one you have now. That way you can dial in exactly what you need for resistance to make the fan run at 5 Volts with the second rheostat. The best bet would be another rheostat because you could still get 12 volts if you want it. If you use a fixed resistor or diodes you will never be able to get 12 volts again because it will always be 12 volts minus the voltage drop on the diodes or fixed resistor. With another rheostat you can turn it to 0 Ohms so it has no voltage drop.

Another option is using a switch with rheostat. You could have one postion on the switch connected to the 5 Volt line on the molex and the other position would be using the rheostat for 7 - 12 volts. You could also have the center position if the switch has one to be an OFF position.

The only problem you might have is some fans do not like a low starting voltage. They need a certain amount of voltage to get them started. If the voltage is too low when you turn the computer on the fan will not spin. You have to start the fan at a high enough voltage first and then turn it down to the voltage you want. You would have to do this every time you turn the computer on.


The three position switch is how I control my Tornado fan on my CPU except for one change. On my switch I have a Thermally Controlled fan position, an OFF position and a manually controlled Rheostat position. The reason I use the thermal controlled position on my switch instead of 5 Volts is because my fan starts at a low voltage using it. The fan only thermal connection from my PS does something different to get the fan started a low voltage, it must put out 7-12 volts when the computer first gets turned on and then drops it quickly. If I set the manual rheostat to anything lower then 6 volts the fan will not start.

Most of the time the thermal control position works fine from my power supply so I use that because it's quiet. On really hot days I might flip the switch to manual mode to speed the fan up with the rheostat.
 
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Thanks for the clarification Crull. I did think of the dual rhoestat idea, but that would just take up more space, and I am not really concerned with getting 12 volts. These Deltas blow more air on 12volts than I ever plan on using! They start with very low voltage. I put the Rheostat on 7v in and turned the Rheostat all the way down (3-4v is my guess) and the fans start fine. These fans are really slick. I just got them. Even at 5volts they pull lots of air and are fairly quiet at that voltage. At 7 volts, they are louder but pull plenty of air. Man, at 12 volts, I could use them as room fans! Of course they are ridiculously loud at 12v. I will take a look at Radio shack to see about the fixed resistors that you mentioned. thanks
 
The particulars are about volt modding at this point, not cooling. *nudge*
 
Ok, I looked at Radio Shack's website for fixed power resistors, and the options that they have are a smorgasbourg of different ohm ratings. How do I know what I need?
 
voigts said:
Ok, I looked at Radio Shack's website for fixed power resistors, and the options that they have are a smorgasbourg of different ohm ratings. How do I know what I need?

You have to figure it out using ohms law. You also need the fan specifications such as wattage, fan resistance, and current

Fan Voltage = 12 volts X Fan resistance / Fan resistance + rheostat + fixed resistor

Another way is doing this below.

Connect the first rheostat temporarily and turn it so the fan is running at full or 12 volts. Then connect another rheostat in series with the first rheostat. Turn the second rheostat until you get the maximum fan speed you want which I believe you want to be 10 volts. Disconnect the second rheostat and measure it with an Ohm meter and that will be the fixed resisitor value to need to add to your first rheostat in series. Radio Shack sells cheap meters if you don't have one.

The resistor has to be of a high enough wattage to handle the fan. Radio Shack sells a few resistors that can handle enough wattage in the values you will probably need. I used some of them before with my Tornado. I made a switch that ran the fan at high, med and slow using the high wattage fixed resistors.

If you give me a link to the fan you have I might be able to tell you what value resistor to try. Your sure your rheostat is 10 Ohms also?
You might also try the diode idea that was posted but you would need to use more then one.

Another place to get power resistors is here and they are cheap.

http://www.allelectronics.com/matrix/Power_Resistors.html
 
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Thanks for all of your help. This is the link for where I purchased the fan from:
http://sidewindercomputers.com/de12trblhisp.html

The Rheostat I have now is definitely a 25w 10ohm Rheostat (Ohmite p#rhs10r). I looked up the specs to make sure.

I looked at Delta's site and I don't see anything that mentions fan resistance in their charts.
 
voigts said:
Thanks for all of your help. This is the link for where I purchased the fan from:
http://sidewindercomputers.com/de12trblhisp.html

The Rheostat I have now is definitely a 25w 10ohm Rheostat (Ohmite p#rhs10r). I looked up the specs to make sure.

I looked at Delta's site and I don't see anything that mentions fan resistance in their charts.


Your fan is around 15 Ohms using Ohms law.

Ohms = Watts (9.60)/Amperes Squared (.8 Amps X .8 Amps)

http://www.the12volt.com/ohm/ohmslaw.asp

When your 10 ohm rheostat is set to 10 Ohms the voltage for the fan is 7.2 Volts.

Fan Voltage = 12 volts X Fan resistance / Fan resistance + rheostat + fixed

If you add a fixed resistor of 8 ohms in series with your rheostat you will get a range of around 5.45 volts to 8 volts. If you want higher then 8 Volts you have to use a smaller size which Radio Shack does not have in high wattage. I used 8 ohms because that is around the size you need and it is of a high wattage for your fan.

http://www.radioshack.com/product.a..._name=CTLG_011_002_014_004&product_id=271-120

Another option is buy two 8 Ohms fixed resistors like above and put them in parallel to each other and then wire the parallel group of both of them in series to the rheostat that will give you 4 Ohms instead of 8 ohms which should give you around 6.2 volts to 9.5 volts.


Using the diode method suggested seems to be a good idea in this case because they would take up less room then one or two power resistors. You would need 4 of these in series with your rheostat. That would give you around 4.4 volts - 9.2 volts. Just make sure you wire the diodes with the proper polarity. I would solder 4 of them in series and then slide a piece of shrink tubing over all of them and shrink it.

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=276-1101

If you lack space on the front of the case. What I did was mount everything to a spare slot cover on the back of the case.
 

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Circuit using diodes. The diodes can actually go on any wire as long as the stripes on the diodes are facing the PS on the black wire of the fan or facing the fan on the red wire. If the stripes are facing wrong on any of the diodes the fan will just not work.
 

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Crull, you are freaking incredible. Thank you so much for taking the time to work the numbers, research the parts, and even draw a chart! This is why I take time in this forum because people like you are willing to explain things, which I also try to do with what I know and am learning. I will try to go to Radio Shack this morning and make this happen. Again, thank you. I think your info should be in a sticky as I could not find anything that would explain how to do this, and putting a few diodes or fixed resistors in the power line is a nice, clean option to getting the voltages I am after.
 
voigts said:
Crull, you are freaking incredible. Thank you so much for taking the time to work the numbers, research the parts, and even draw a chart! This is why I take time in this forum because people like you are willing to explain things, which I also try to do with what I know and am learning. I will try to go to Radio Shack this morning and make this happen. Again, thank you. I think your info should be in a sticky as I could not find anything that would explain how to do this, and putting a few diodes or fixed resistors in the power line is a nice, clean option to getting the voltages I am after.


Your Welcome,

I made a change to the drawing. That is how I would do it. Solder two diodes to each connection of the Rheostat like in the picture. Soldering two together on each connection will be stronger then trying to put four in a row. You don't even need to use wire to solder the diodes to the Rheostat. Just clip the first diode so the wire is short and solder that to the rheostat then solder on another short one then add a piece of wire. I would slide a piece of shrink tubing over the soldered diodes to give them some extra strength and protection so they don't touch anything inside your case. Radio Shack sells shink tubing and it's good stuff to always have for projects like this.
Only using three diodes might be better which will be 5.1 volts to 9.9 volts.

Make sure you get 1N4001 diodes or diodes that can handle at least 1 Watt.

These are the correct ones to use at this link.

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=276-1101

Also remember if you set the voltage too low when you turn the computer on the fan might not start.

Lets us know how it works out for you when your done or if you have any problems let me know. Good Luck!
 
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I went to Radio Shack today and bought the diodes. My Rheostat however is already prewired on the negative wire instead of the hotwire. Is this the correct way to wire these in given this difference?
 

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Well, I soldered the diodes together and decided to make it a separate connector so that I could easily remove it if I later choose to do so. I soldered the diodes according to the diagram above, shrinkwrapped the wires individually and then together to make a connector. It works as far as the voltage like a charm. It sounds like I am getting the correct voltage range expected. My only concern is that the diodes do get pretty toasty. I can't hold on to them for more than about 5 seconds without it getting hot in my hand. I am running it with my two Deltas for a while just to make sure. Here are some pics:
 

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Its not the correct way to put switches, rheostats, resistance..etc...etc on the ground wire. Same goes for home wiring. Everything should go before a light on the hot wire, not on the return. In this case I don't think it matters but I would still fix it.

It is easy enough to fix so you don't have to re-solder anything. You need to switch the wires on the rheostat, and you need to switch the wires with the diodes and flip them so the stripes on the diodes are facing the rheostat.

You need a very tiny screwdriver, if you look in the molex connectors there are small metal tabs that keep the connectors from coming out. Take the small screw driver and gently push the tabs down. There are 2 tabs on each connector. Then the connector will slide out of the plastic molex piece. Once you slide it out, gently bend the tabs open again and then slide it back in the connector where you want it to go. The metal tabs will hold the connector in place when you have pushed it in enough.

Look at the pictures I posted, because I edited them to show you what I mean a little better.

The color of the wires will not be correct anymore, but it doesn't really matter as long as you know. A wire is a wire, the color does nothing.

By changing the position of the wires on the rheostat. If you turned it clockwise before to speed up the fan, now it will be clockwise to slow down the fan. The only way to fix this would be to switch the wires on the rheostat.

Switch everything around first and then see if the diodes are still getting hot. I think they will still be hot, but it is normal. If you are using the correct diodes that I posted. They are able to handle 1 Amp, your fan pulls .8 so even though they get hot they should be fine.

Just follow the pictures I posted exactly and everything should be OK.

FYI. A Rheostat does not really have a true negative or positive wire even though the wire they used is black. Just because it came prewired with a black wire does not mean it goes on the black wire of a circuit. I think they used black because a rheostat does not have a polarity. It does not have a positive or negative side out of a circuit. So it can go in a circuit in any direction, but in this case it should always be installed on the hot or positive wire even if the wire they used is black.
 

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Make yours look like this by moving things around.

***Anyone else the color of the wires in this picture are the wrong colors. So keep that in mind if you use this picture as a guide. Electrically it will work fine, but usually only the color black or green is used for ground wires. ***
 

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Thanks for the details. I will change things around. Thanks again for all of your help.
 
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