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CJC504
08-13-2008, 10:15 AM
Hi,
I am working on a hobby project where I am trying to create a small scale VAV (variable air volume) HVAC system with electric reheat in the VAV boxes. If you do not know what that is it does not really matter for the question I have. My question is in regards to controlling current. I know that if you have a motor then the motor will draw whatever current it is rated to draw. My problem is that I trying to use Nichrome wire, which is a resistive heating wire, to heat air as it passes thru the duct heater I am trying to make. The duct heater is supposed to have three stages of heating and my problem occurs when only one stage is energized. When all three stages of heat are energized, then the wire is fine because the current distributes itself evenly throughout the circuit, but if only one stage is energized then all 3 amps go through the wire and melt it. I would like a device that limits the current going thru each stage of heat to 0.9A to 1.0A since the reed relay I am using is only rated for 1A and because the wire cannot handle 3A flowing thru it without melting. Do Current Limiting Diodes work in this fashion or do they act like a high current shutoff switch? I would like the device to let current thru whenever the Relay is energized, but I only want 0.9A to 1A to pass thru the nichrome wire. Does anyone know of a device that can do this or have any suggestions on how to accomplish what I want. Any help would be appreciated. See attached.

-I drew the sysmbol for a CLD (Current Limiting Diode), but I am not sure if I did it right or even if they work as I think they work.

Chris

Zoot
08-13-2008, 10:33 AM
Is there a reason not to use an adjustable regulator configured as·constant-current? That would let you tune the actual current out for each wire and not even be beholden to ratings on a diode. See datasheets for regs. like LM317 and LM350 which include basic circuitry for constant-current output.

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CJC504
08-13-2008, 11:26 AM
No, there is no reason not use any option with exception of unreasonable cost. I have a couple LM317s at my dispose so I will explore this option. Thanks for the input Zoot.

sam_sam_sam
08-13-2008, 07:58 PM
CJC504 (http://forums.parallax.com/member.php?u=51983)

No, there is no reason not use any option with exception of unreasonable cost. I have a couple LM317s at my dispose so I will explore this option. Thanks for the input Zoot.

I will add one thing to this I have use the LM317s·constant-current output mode

Which work·very well but if you are going to use it at 1 amp I would get them in a T03 ·can type the heat sink are easyer to get or make

Here is a Link where you buy them from

http://www.goldmine-elec-products.com/prodinfo.asp?number=A10800

http://www.goldmine-elec-products.com/prodinfo.asp?number=G15314

http://www.goldmine-elec-products.com/prodinfo.asp?number=G2017

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··Thanks for any·http://forums.parallax.com/images/smilies/idea.gif·that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 8/13/2008 12:09:57 PM GMT

stamptrol
08-13-2008, 08:01 PM
Chris,

There's something just not quite right here.

If each heater element is 6.27 ohm per foot and each heater is 2.5 ft long, that gives about 15.6 ohms total. With a 18 volt supply, the heater will only draw a little more than 1 amp, not 3 amps. The only way to draw 3 amps is if the power supply voltage is rising for some reason.....or if the resistance isn't right.

If the heaters don't burn out when they're all on, it has to be because your input voltage is being pulled down. Check with a meter.

There are two simple ways to fix this. Reduce the voltage from the regulator to 15 or 12 v. Or, make each heater element a bit longer to increase the resistance.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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sam_sam_sam
08-13-2008, 08:20 PM
stamptrol (http://forums.parallax.com/member.php?u=41420)

You have the right ·http://forums.parallax.com/images/smilies/idea.gif


CJC504 (http://forums.parallax.com/member.php?u=51983)

·Just have each heater element four foot long then you can·run it on 24 volt

that way you have 1 amp to deal with

I am working on a hobby project where I am trying to create a small scale VAV (variable air volume) HVAC system with electric reheat in the VAV boxes.

I do HAVC work for a living

Yes I do know what these are that why it would easer to use the 24 volt that is already there· If you do not want to have··four foot·long wire then use what stamptrol (http://forums.parallax.com/member.php?u=41420)·had in in his post and use a 12 volt transformer

If you are making your own variable air volume control box then use a 12 volts

If you need any help with this variable air volume control box··you can PM me and·I will see what I can do·

You take this wire and roll it around a pin and make a coil then remove the pin

just an http://forums.parallax.com/images/smilies/idea.gif

Can you Post your Project when your done I would like to see what you come up with

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··Thanks for any·http://forums.parallax.com/images/smilies/idea.gif·that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 8/13/2008 12:49:15 PM GMT

Franklin
08-14-2008, 08:39 AM
Since no one has responded to your other post you can go delete it by clicking the red X in the right upper corner.

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- Stephen

CJC504
08-14-2008, 10:58 AM
All,
First of all, thank you for all of your responses.

The voltage supply/regulator that I am using can be anything from 0-30VDC and 0-6A. My problem is that (I believe) I can take any length of wire and put as much voltage and put whatever current through it that I want. For example, I took 1 ft of wire and connected it to the voltage regulator as shown in the attached picture. I then measured the current and voltage as I gradually increased the current of the voltage (whichever one was the limiting factor at the time):

Wire Length = 1ft so R=6.27

Volts(V) I(amps) W(calculated)
1 0.15 0.15
2 0.34 0.68
3 0.5 1.5
4 0.68 2.72
5 0.86 4.3
6 1.01 6.06
7 1.17 8.19
8 1.34 10.72
9 1.49 13.41
10 1.65 16.5
11 1.8 19.8
12 1.98 23.76
13 2.15 27.95
14 2.33 32.62
15 2.47 37.05
16 2.69 43.04
17 2.81 47.77
18 2.96 53.28
19 3.15 59.85
20 3.27 65.4
21 3.44 72.24
22 3.59 78.98

With this experiment in mind, doesn't it seem that if I had three other heating coils in parallel the current would be reduced by a third. This is why I believe I need to use a current limiting device. Any thoughts?


-I have also posted a picture of the make-shift VAV box with the Nichrome wire in front of it since stamptrol was interested.

stamptrol
08-14-2008, 07:56 PM
I think you're complicating your life by fiddling with the current limit on your power supply. Crank it up to maximum and then choose some reasonable voltage to work with.

Say its 18 volts as you had yesterday. With one heater connected, the resistance of the heater will limit current to a bit over 1 amp. Two heaters will draw twice as much current from the supply. Three heaters will draw a bit over 3 amps. This is an elementary parallel circuit.

By arbitrarily adjusting the current limit, you're trying to adjust two elements of Ohm's law at the same time.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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allanlane5
08-14-2008, 09:09 PM
+1 Stamptrol. In real life, usually you set the voltage -- common values are 3.3 V, 5 V, 7.5 V, 9 V, 12 V, and 24 V. The resistance of the circuit then causes the circuit to draw as much current as it needs. A supply that CAN supply 6 Amps has no problem supplying 3 Amps, and it's not like the supply is going to "force" the extra 3 amps into the circuit, that's not how it works.

And if you DO want to reduce the current draw of your heater units -- because they get too hot -- typically you add some resistance to the circuit to bring down the current.

V == I * R.· I == V / R.· Increase the resistance, and you reduce I.

Chris Savage
08-14-2008, 10:01 PM
Duplicate post removed by moderator.

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Parallax Tech Support