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# HELP - resistors and dip-switch

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Hi Guys

A million dollar question, and please I need help with this one. I know a little about ohm’s law etc. but still can’t figure this one out. Being a non electronic tech and wanting to build a fan controller and I need some expert advice.

In the circuit I am building I am using a 10 k ohm multi-turn trimmer, but would prefer to use switches to select different resistors via a dip switch to change the resistance and thus change the fan speed.  I have measured the lowest value at 2.5 K ohm and the top value at 3.5K ohm.

Without the dipswitch and resistors R5 would be valued between 2.5 K ohm and 3.5K ohm. depending of the fan speed selected.

Can I use different resistors R1 – R4 via the dip-switch to accomplish this?  and what values will they be?
What would the value of R5 need to be to start off with?

I was thinking of building a small unit and start with soldering resistors together, but I think there is infinite combination, your input will be highly appreciative.

Is their another way of doing this.

Please and thank you ;D ;D ;D

Virus

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Good evening.

Regarding your fan control project, you must first remember Ohm's law regarding resistors in parallel, which is what your diagram indicates.

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Thanks guys

Will try out the different options and let you know.

It could be that a miniature selector switch will do the trick. Still chewing on the idea.

Thanks for the feed back.

Virus

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allvol / Zeppelin

Seeing the dip switch way will not work, I looked around the web and find these little switch, do you

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Hello again, Virus

I'm afraid you haven't yet grasped a basic premise both Zepp and I have tried to tell you:

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Virus said that the switch with resistors controls the speed of a fan controller which is probably a triac circuit for an AC fan. So the current through the switch won't be high but it might have the mains voltage across it.

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Hi guys

Great gears for all the input.

Pity, that the kid_kv,

(http://www.electronicslab.com/forum/index.phptopic=6755.msg41336;topicseen#msg41336)

could not see this amount of response. This sure would make him g r e e n. Well we know this is a great forum, with some real people.

Back to my request.

I did do a search for, 555 timer Pulse Width Modulation, available diagrams use a variable resistor to control the motor speed. I need only replace the variable part with a selector switch of some kind. A rotary switch will work ideal as suggested. But do you get them in a small enough package to have them pcb mounted and with maybe 3 to 6 position, similar to the one in the above post. Using a multi turn pot the selection range is to big to adjust, using a normal pot it is to sensitive. Thus the option for a selector switch with preset values.

I do not intend to drive the fans directly via the “resistor selector”, I think it only acts as a reference to power a mosfet that drives the fans in this circuit. The circuit it self is working fine. Posting the circuit will not really help?

Sorry for the confusion, but can their be a solution?

Thanks

Virus ;D ;D ;D

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Hi guys

I found these switch on the web. Think they will do it ?

Thanks

Virus

R_1apr.pdf

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Hi Virus,
In a PWM circuit using a 555 with a speed-adjustment pot, the sensitivity of the pot is reduced by simply adding a resistor or two in series with it.
Attach the circuit for us to show you how easy it is.

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Zeppelin

The whole though was that dip switches would be the easiest, but then the problem with the parallel resistor etc. Unless one starts with a higher value resistor?

audioguru

It’s the same circuit we discussed before with the thermistor emulation, I now put the thermistor with a air gap just above a lamp, as the lamp heats up the thermistor causes the fans to pickup speed, the airflow cools the thermistor, going up and down etc. and it is working fine. Haven got the full spectrum of min and max but is doing ok.

Attached circuit as requested.

Is their no other way to pre-select the resistor values in this circuit.

Thanx

Virus

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Sure you can use a selector switch if you use one that selects one resistor at a time.
Adjust the trimmer pot then measure its resistance and use a resistor with that value for that fan turn-on temperature as controlled by the switch.

The circuit doesn't have a resistor in series with the trimmer pot. Adding a resistor would increase the range of the pot at the high temperature end. A resistor in series with the thermistor would increase the range of the pot at the low temperature end. Then the pot wouldn't be too sensitive to adjust.

The fan runs OK? The Mosfet is just a resistor in series with the fan, wasting a lot of power (it gets hot) and making it difficult for the fan to begin turning. Pulse-Width-Modulation is usually used to control the speed of motors because its pulses are full power for the motor to start running well and the pulses drive the transistor fully on and fully off so it barely gets warm.

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Audioguru

Again a eye opener from your side ! ! !

Just one question, if I opt to go with the resistors and the dip switch switching the resistors 1 at a time, what will happen if for example 2 switches are on at the same time ?

Would a PWM circuit use much less heat. And how would I incorporate the thermistor to do the temperature control ?

Can this exciting circuit be modified to do just that ?

Sorry for all the questions but if PWM is better can you help me with it.

Thanks

Virus

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Just one question, if I opt to go with the resistors and the dip switch switching the resistors 1 at a time, what will happen if for example 2 switches are on at the same time ?

Resistors in parallel have a total resistance that is less than either of them.

Would a PWM circuit use much less heat.

Of course. The transistor that is driving the motor is switched completely on when it doesn't have much voltage across it so it doesn't get hot, then it switches completely off, when it doesn't have any current so it doesn't get hot. The power is sent to the motor in pulses at a high rate and the average current which is made by the duty cycle percentage controls the motor's speed.

And how would I incorporate the thermistor to do the temperature control ?

Then you need to make a PWM circuit.

Can this exciting circuit be modified to do just that ?

There are a few ways to make a PWM circuit to control a motor's speed. Look at PWM Motor Speed Control in Google.
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Thanks

Will have a look see

Virus

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Audioguru

I have find a circuit at a quite informative site,

http://casemods.pointofnoreturn.org/pwm/index.html

and have attached the diagram, could you please indicate where I should attache the thermistor in this circuit?  Also I have 10 K ohm termistors, could they be used?

Can their be any enhancement made to the circuit. ?

Again all the questions.

Thanx

Virus

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Hi Virus,

If you were going to lift a circuit from that site, why didn't pick the one that WILL WORK!

The attatched is from the same source, but unaltered, and is what we've been trying to accomplish lo these many days.

Try it.

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AllVol

2 Questions, please I am very greatfull for the help.

1. Where do I connect a thermistor to this circuit, to regulate the fans?
2. Why would the circuit I posted, from this site not work?

Thanks

Virus  ??? ??? ???

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Hi, Virus

The circuit you attached could possibly have some drawing defects, but the first thing I noticed was that pin 8 of the 555 is not connected to anything.

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The circuit you attached could possibly have some drawing defects, but the first thing I noticed was that pin 8 of the 555 is not connected to anything.
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Yes Sir, Audioguru,

and pin 3 is still unconnected.

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and pin 3 is still unconnected.
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Yes, I see that now.

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Guys

Sorry I did not notice the connection in circuit3 posted above where missing, my mistake.

Question still remain, where do I connect the thermistor, in the circuit 3, or in circuit forum lab 3, above ? ? ? ? ?. which ever is the better or more reliable of the two. I do have some 10k thermistors can I use them?

Re: the two resistors in parallel – this site where the circuits come from, on page http://casemods.pointofnoreturn.org/pwm/741.html

In the 2nd  line he states – “using a rotary encoder to set your resistance.”

Does any one have an idea how to use an encoder switch to select resistance, (This could be the answer to the original question)

Thanks once more.

Virus

PS: audioguru, can you please explain to me how to use the "quote" section. Do you select a section you want to use and then hit "Quote", or how?

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Question still remain, where do I connect the thermistor, in the circuit 3, or in circuit forum lab 3, above ? ? ? ? ?. which ever is the better or more reliable of the two. I do have some 10k thermistors can I use them?

In the circuit you posted with the 555 and the opamp, replace R4 or R5 with your 10k thermistor. One position will make the fan speed up when the temperature is colder, so use the thermistor for the other resistor.

audioguru, can you please explain to me how to use the "quote" section. Do you select a section you want to use and then hit "Quote", or how?

Push the Quote button, delete text that you don't want in the quote then end the quote with "
Each time a new sentence is quoted it must begin with "
" and end with "
".
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In the circuit you posted with the 555 and the opamp, replace R4 or R5 with your 10k thermistor. One position will make the fan speed up when the temperature is colder, so use the thermistor for the other resistor".

audioguru

Thanx will build one and give feed back.

Still no idea on the resistor and the selector thingy. Any ideas ?

Thank you

Virus

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