- Joined
- Nov 21, 2002
- Location
- Colorado, USA
Ok my fan draws 5.76W, so I need a 10W one. But what about ohms I was going to get a 25ohm one, but they either have 20ohm or 35ohm. Which should I get and what's the difference?
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Another Variable Resitor Questing
- Thread starter DeathONator
- Start date
OP
- Joined
- Nov 21, 2002
- Location
- Colorado, USA
- Thread Starter
- #2
I need a 6W (minimum) variable speed resistor (rheostat) w/ 10-25 Ohms. I can't find one! Anyone?
use a small low wattage pot with a TIP121 power transistor.
here is a schematic i made, just exclude the segment driver an stuff but use the TIP121 wired like that.
http://www.maxvla.com/host/rheobus.pdf
the TIP121 can handle ~60 watts.
here is a schematic i made, just exclude the segment driver an stuff but use the TIP121 wired like that.
http://www.maxvla.com/host/rheobus.pdf
the TIP121 can handle ~60 watts.
DeathOnator! you don't need a 5 or 6 or 7 W rheo to operate a 5.76W fan..
What you need is a 3W rheo. But... it has to handle the currrent.
5.76W / 12 V = I ..... I = 0.48 A
Why 3W ?
Well... if you hook it up in series with the fan you'll be reducing the current, right?
What happens if you have the same voltage but the current is lowered? That's right, the power(the Watts) goes down.
Then you have two devices in series, that means that they will SHARE the power load. The Watts will be distributed between the two units, meaning it doesn't have to be as big as the load.
Actually the MAX load on the rheo will be close to two watts.
But, it has to withstand the current 0.48A.
Good luck huntin' /Paxmax
What you need is a 3W rheo. But... it has to handle the currrent.
5.76W / 12 V = I ..... I = 0.48 A
Why 3W ?
Well... if you hook it up in series with the fan you'll be reducing the current, right?
What happens if you have the same voltage but the current is lowered? That's right, the power(the Watts) goes down.
Then you have two devices in series, that means that they will SHARE the power load. The Watts will be distributed between the two units, meaning it doesn't have to be as big as the load.
Actually the MAX load on the rheo will be close to two watts.
But, it has to withstand the current 0.48A.
Good luck huntin' /Paxmax
OP
- Joined
- Nov 21, 2002
- Location
- Colorado, USA
- Thread Starter
- #5
Paxmax, I actually already have a 3W one! Now what do you mean by in series? This is how I am planning on wiring it up:
I have a female molex connected to a male molex (w/ the 12V and 12V ground only). Then I was going to chop the 12V line, and wire each end of it to the rheo. The ground will stay. Then plug my PS into the female and the fan into the male.
I was going to use the 3W but then I was thinking I should use a larger one. But if you know it will work I'll get it going right now!
I have a female molex connected to a male molex (w/ the 12V and 12V ground only). Then I was going to chop the 12V line, and wire each end of it to the rheo. The ground will stay. Then plug my PS into the female and the fan into the male.
I was going to use the 3W but then I was thinking I should use a larger one. But if you know it will work I'll get it going right now!
Last edited:
- Joined
- Jan 29, 2003
PaxMax,
While you are giving useful information, I must disagree with one of your points:
Current is constant in a series circuit, voltage is what is dropped or "used up"
You can't put a rheo in series with a fan and then say that it will use up some of the current and alter your calculations accordingly.
Your fan is going to draw .48A no matter what, all the rheo does is to create a voltage drop therefor supplying the fan with a lower voltage.
you can use the equation V=IR to determine what rheo you need.
For example, you probably want to be able to control your fan in the range of 7-12V that is a 5V drop, so plug and chug:
5V = .48 R
R = 10.4 Ohms
Now, you would be tempted at this point to say that your fan is only running at 7V and .48A so the power is 3.36W but you are ignoring the fact that your supply voltage is still 12V, the rheo is merely dropping that voltage from 12 to 7
Now, if you have the rheo set down and it is dropping 5V (making your fan 7V) then it is at that point dissipating 5*.48 = 2.4W of power. You would then think that the 3W rheo would be plenty, however it is ALWAYS the best bet to allow for a significant margin of error especially if you are going to get a rheo rated with higher Ohms. That 2.4W of power is a best case scenario nominal power rating. It doesn't take into account startup power requirements or possibly fluctuations. Also if the rheo begins to heat up, its resistance will increase thus causing it to dissipate more power and heat up more.
Anyways, back to my original point in your other thread.
Your 5W rheo might cut it, however it might also get hot and screw things up. I merely said to be careful with it and watch it closely when you first hook it up and also said if you can get a rheo rated for higher W, you probably should.
I use 15W ceramic rheos and have no problemo whatsoever. Call me over-precautious, but it's better than the alternative.
PS Deathonator, when he says "series" it means that the current should flow through one path through the rheo then the fan. So you could hook up the 12V line to the side post on the rheo, then wire from the middle post to the 12V line on the fan, then from the GND line on the fan back to the ground wire in your PS, thus making a series circuit. so the current would flow to the rheo first, then to the fan, then back to ground where it's supposed to.
While you are giving useful information, I must disagree with one of your points:
Current is constant in a series circuit, voltage is what is dropped or "used up"
You can't put a rheo in series with a fan and then say that it will use up some of the current and alter your calculations accordingly.
Your fan is going to draw .48A no matter what, all the rheo does is to create a voltage drop therefor supplying the fan with a lower voltage.
you can use the equation V=IR to determine what rheo you need.
For example, you probably want to be able to control your fan in the range of 7-12V that is a 5V drop, so plug and chug:
5V = .48 R
R = 10.4 Ohms
Now, you would be tempted at this point to say that your fan is only running at 7V and .48A so the power is 3.36W but you are ignoring the fact that your supply voltage is still 12V, the rheo is merely dropping that voltage from 12 to 7
Now, if you have the rheo set down and it is dropping 5V (making your fan 7V) then it is at that point dissipating 5*.48 = 2.4W of power. You would then think that the 3W rheo would be plenty, however it is ALWAYS the best bet to allow for a significant margin of error especially if you are going to get a rheo rated with higher Ohms. That 2.4W of power is a best case scenario nominal power rating. It doesn't take into account startup power requirements or possibly fluctuations. Also if the rheo begins to heat up, its resistance will increase thus causing it to dissipate more power and heat up more.
Anyways, back to my original point in your other thread.
Your 5W rheo might cut it, however it might also get hot and screw things up. I merely said to be careful with it and watch it closely when you first hook it up and also said if you can get a rheo rated for higher W, you probably should.
I use 15W ceramic rheos and have no problemo whatsoever. Call me over-precautious, but it's better than the alternative.
PS Deathonator, when he says "series" it means that the current should flow through one path through the rheo then the fan. So you could hook up the 12V line to the side post on the rheo, then wire from the middle post to the 12V line on the fan, then from the GND line on the fan back to the ground wire in your PS, thus making a series circuit. so the current would flow to the rheo first, then to the fan, then back to ground where it's supposed to.
Because fans are what they are (mechanical things), they actually take less current when their voltage is lower so calculating what resistance is needed is difficult without measuring current at the desired voltage. I didn't know that originally, but when I tested my 120mm fan, I found out that at 12V the current is 0.42A and at 7.11V the current was only 0.18A. That's a major difference in wattage.
Lord_Mil, current is dependent on resistance in a series circuit and is given by the formula:
I = E/R.
So, you can see if you increase the series resistance in the circuit the current flowing to ground will be reduced.
Deathonator if you wire the rheostat in series in the 12 volt line to the fan your 3W 25 ohm rheostat will work fine.
I = E/R.
So, you can see if you increase the series resistance in the circuit the current flowing to ground will be reduced.
Deathonator if you wire the rheostat in series in the 12 volt line to the fan your 3W 25 ohm rheostat will work fine.
Last edited:
OP
- Joined
- Nov 21, 2002
- Location
- Colorado, USA
- Thread Starter
- #9
Where can I find a larger rheostat? The one I thought was 5W I now think is 3.3W. Where can I find one that is somewhere around 10W?
- Joined
- Jan 29, 2003
Irlucas,
You are absolutely correct, I don't think I said anything to the contrary. I merely said that current is a constant in a series circuit. Current isn't dropped across components as voltage is.
Whatever current is drawn from the PSU will flow through all the components. I was merely clarifying something that was said earlier about the rheo "using up" current, which is not the case.
Breez, I was unaware that the fans would draw less current at a lower voltage. I've never looked into it and just always took it for granted that their current draw was always the stated value. Thank you for that info.
Deathonator, go ahead and wire up the 3W rheo. It will probably be alright, but don't just wire it up and pack it away, make sure you watch it for a few minutes to make sure everything's kosher. If not, allelectronics.com has some 15W 15Ohm rheos that look like they should work. I haven't looked all that much, but I found those pretty quickly. If I find anything closer to what you want, I'll certainly let you know.
You are absolutely correct, I don't think I said anything to the contrary. I merely said that current is a constant in a series circuit. Current isn't dropped across components as voltage is.
Whatever current is drawn from the PSU will flow through all the components. I was merely clarifying something that was said earlier about the rheo "using up" current, which is not the case.
Breez, I was unaware that the fans would draw less current at a lower voltage. I've never looked into it and just always took it for granted that their current draw was always the stated value. Thank you for that info.
Deathonator, go ahead and wire up the 3W rheo. It will probably be alright, but don't just wire it up and pack it away, make sure you watch it for a few minutes to make sure everything's kosher. If not, allelectronics.com has some 15W 15Ohm rheos that look like they should work. I haven't looked all that much, but I found those pretty quickly. If I find anything closer to what you want, I'll certainly let you know.
Lord_Mil: You need to re-read my post above. What you claim in your post is not said by me, it's fabricated by your mind for not reading careful enough.
I say that the WATTAGE will be shared if you have two units in series.
I say that the current WILL be reduced IF you add something more in series.
I don't claim that the current will be "shared" anywhere.
The explanation for why the rheo must withstand the same current as the fan is when you are on the LOWEST setting of the RHEO you will get ALMOST the same current as without a RHEO since it MIGHT be just 0.01 ohms in the beginning of RHEO operating range. That scenario that the rheo will work in that region is however really remote, usually the rheo jumps in at a higher value. I.e. it might not start at 0.1 ohms but rather at maybe 1 or two ohms.
Otherwise it seems like you've understood your mistake.
I say that the WATTAGE will be shared if you have two units in series.
I say that the current WILL be reduced IF you add something more in series.
I don't claim that the current will be "shared" anywhere.
The explanation for why the rheo must withstand the same current as the fan is when you are on the LOWEST setting of the RHEO you will get ALMOST the same current as without a RHEO since it MIGHT be just 0.01 ohms in the beginning of RHEO operating range. That scenario that the rheo will work in that region is however really remote, usually the rheo jumps in at a higher value. I.e. it might not start at 0.1 ohms but rather at maybe 1 or two ohms.
Otherwise it seems like you've understood your mistake.
Rheo heats more at the maximum resistance setting and less at the minimum resistance setting. Sure there maybe more current flowing through at the minimum resistance setting, but because it is paired with a fan, the fan eats almost all the voltage. If at some situation, the 3W rheo 'eats' 1V, then the current can be 3A at the maximum.
Terve breez!
A 3W rheo can never eat no 3A when used in a series connection to a fan that consumes 0.48 amps as stated in the first post.
The wattage of a rheo can be 50W but due to manufacturing constrictions or choice of materials it may only be used up to 0.5 amperes, however it will consume 50W if maybe 100 volts are applied and the current lands on 0.5A.
You can't always rule that a rheo which is rated for 3W and 3 ohms can be used with 3 amps at the 1 ohm setting!
It may only be able to handle a maximum of 1 A, which you might get at 3V at 3 ohm setting. If you pass the 1A setting you might burn the wires or the resistive substrate.
So dear ppl, when you buy or are looking for a rheo for some amp-sucking fans, be sure to check the current handling of the intended rheo! That is usually more conservative than the maxium power handling.
Rule of thumb: If the rheo has half of your fans wattage AND handles the same (or more) maximum current as your fan consumes then it will probably work.
A 3W rheo can never eat no 3A when used in a series connection to a fan that consumes 0.48 amps as stated in the first post.
The wattage of a rheo can be 50W but due to manufacturing constrictions or choice of materials it may only be used up to 0.5 amperes, however it will consume 50W if maybe 100 volts are applied and the current lands on 0.5A.
You can't always rule that a rheo which is rated for 3W and 3 ohms can be used with 3 amps at the 1 ohm setting!
It may only be able to handle a maximum of 1 A, which you might get at 3V at 3 ohm setting. If you pass the 1A setting you might burn the wires or the resistive substrate.
So dear ppl, when you buy or are looking for a rheo for some amp-sucking fans, be sure to check the current handling of the intended rheo! That is usually more conservative than the maxium power handling.
Rule of thumb: If the rheo has half of your fans wattage AND handles the same (or more) maximum current as your fan consumes then it will probably work.
just out of curiosity did anyone read my reply. i gave a schematic that shows how to hook up an TIP121 and low wattage pot to allow it to controll a higher wattage fan.
the TIP121 is rated at a max of 5 amps. and the pot used to controll the TIP121 can be less than 1 Watt.
the TIP121 is rated at a max of 5 amps. and the pot used to controll the TIP121 can be less than 1 Watt.
Do any of you people know ohms law and how it might apply in this instance and in series circuits in particular. It really hacks me off to see some of you amateurs slinging around terms you really don't understand. Go take a degree in electrical engineering and work in the electronic instrumentation industry for twenty years and then yhou'll know what you're talking about!
Yeah Black_Ops I looked at your circuit too, how much heatsinking do you think it might need to prevent thermal runaway caused by reverse leakage or recombination currents?
Yeah Black_Ops I looked at your circuit too, how much heatsinking do you think it might need to prevent thermal runaway caused by reverse leakage or recombination currents?
lrlucas said:
and i even bother to reply to messages why?
i mean look i gave a good circuit, i have been doing electonics for years. and then i make a post right. and you think its all funny to insult me. well guess what. i ****ing quit, thats right i ****ing quit trying to help people... you can all have your god dam degrees and ****. cuz this is my last post.
DeathOnator: Don't place +12 on one side of Rheo and GND on the other! Hook it up in series, I'll try to illistrate it.
+12
|
|------|
Rheo<-|
|
FAN +
FAN -
|
GND
The rheo prolly has 3 terminals, between two of the terminals is the whole maximum resistance of rheo, ok? 1 terminal is the variable out put, ok? You should connect the variable(wiper) output to either side of the rheo.
That way, you get voltage to the fan even if the wiper(moving mechanical) part should lose contact. If the rheo has low enough resistance the fan will keep in spinning at lowest speed in the event of wiper failure.
+12
|
|------|
Rheo<-|
|
FAN +
FAN -
|
GND
The rheo prolly has 3 terminals, between two of the terminals is the whole maximum resistance of rheo, ok? 1 terminal is the variable out put, ok? You should connect the variable(wiper) output to either side of the rheo.
That way, you get voltage to the fan even if the wiper(moving mechanical) part should lose contact. If the rheo has low enough resistance the fan will keep in spinning at lowest speed in the event of wiper failure.
Irlucas: Don't you think you are overreacting a bit?
Black_Ops: Don't you think your overreacting a bit?
Anyway I'd like both to continue contributing to the forums. It's easy to point fingers at others creation, but doing so you should be able and ready to show your solution to the supposedly "problem". It's easy to get hung out to dry here on the forums, so I feel for you Black_Ops, but you have to be able to reason.
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I suck so much at ASCII paintings so I made one slightly better in paint. Wohoo.... some improvement, huh?
Connect rheo this way:
Black_Ops: Don't you think your overreacting a bit?
Anyway I'd like both to continue contributing to the forums. It's easy to point fingers at others creation, but doing so you should be able and ready to show your solution to the supposedly "problem". It's easy to get hung out to dry here on the forums, so I feel for you Black_Ops, but you have to be able to reason.
-----------------------------
I suck so much at ASCII paintings so I made one slightly better in paint. Wohoo.... some improvement, huh?
Connect rheo this way:
