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lyzrdstomp
Joined: 13 Apr 2007
Posts: 20
Location: Midwest US
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 10 May 2007 14:03 Zener Diode Resistor Value? |
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I want to use a 12Volt zener diode and a resistor to bring an unregulated 36V input down to 12 volts to run a 12V/110mA fan. I used the calculator here:
http://www.opamplabs.com/zenerreg.htm
Entering the values 36 for input, 12 for Zener voltage, 110 for mA, it says I need a 200ohm resistor at roughly 3 watts.
For a bit of safety headroom, would a 5 watt resistor be sufficient for this or should I go for an even higher wattage one?
Just making sure before I order parts. 
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IanP
Joined: 05 Oct 2004
Posts: 6306
Helped: 1474
Location: West Coast
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| 10 May 2007 14:41 Re: Zener Diode Resistor Value? |
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Apply the Ohm's law and for your own safety add at least 200% margin ..
My guess is that an 5W will be still to hot ..
Regards,
IanP
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mcoster
Joined: 27 Jan 2004
Posts: 57
Helped: 6
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| 10 May 2007 16:38 Re: Zener Diode Resistor Value? |
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Hello lyzrdstomp,
I would strongly advise you not to build a power suplly using a resistor and a zenner diode. Do this ONLY if you have to do it for a school project or something like.
It is ok to use this topology to a very low current power supply. The main drawback in this circuit is that a Zener diode isn't just a voltage source. It also has a series resistance.
Its voltage depends on the current that is passing trough and if you drive a high current device it might not work properly
I would suggest you to use a LM317. This is a integrated circuit suitable for power supllies.
Here is the data sheet link:
http://www.fairchildsemi.com/ds/LM/LM317.pdf
In the data sheet you can find everything that is needed to make the cicruit.
hope i could help,
mcoster
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lyzrdstomp
Joined: 13 Apr 2007
Posts: 20
Location: Midwest US
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| 10 May 2007 21:54 Re: Zener Diode Resistor Value? |
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mcoster-
Thanks for the heads up-but I've already made the power supply with the 317t.
I'm just using a resistor/zener to regulate enough voltage to power a 12 volt fan prior to hitting the 317t, straight out of the filter cap.
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Kral
Joined: 28 Mar 2005
Posts: 978
Helped: 157
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| 10 May 2007 23:51 Re: Zener Diode Resistor Value? |
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lyzrdstomp,
I agree with mcoster that this is not the optimal approach. However, if you want to procede with this design, here are the steps to follow:
1 Determine the maximum worst case load current (lets assume that it's Ilmax=120 mA)
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2 Determine how much current you need to flow thru the Zener to achieve the desired operating point. Remember that the higher the Zener current, the lower the effective series resistance of the Zener. In general, the test current from the data sheet is a good value to use. Lets assume it's Iz = 10 mA.
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3 The total current thru the resistor = Ilmax + Iz = 120 + 10 = 130mA.
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4. Determine the minimum unregulated voltage. Let's assume it's Vinmin=35V.
5. Determine the maximum Zener voltage at the test current. Let's assume its Vxmax = 12.2V
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6. Calculate the resistor value that will provide the required Zener current under worst case (minimum) input voltage and worst case (maximum) Zener voltage.
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7 R = (Vinmin-Vzmax)/0.13A = (35-12.2)/0.13A = 175 Ohms.
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8 Power = i^2R = (.13^2) X 175 = 2.96W.
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9 A 5W resistor woud be suitable. However, as Ianp stated, it will run hot.
Regards,
Kral
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pmx
Joined: 11 May 2007
Posts: 3
Helped: 2
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| 11 May 2007 13:24 Re: Zener Diode Resistor Value? |
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Hi,
I would suggest to consider the case where the fan is unplugged (or dead).
In this case, as the voltage across the zener and the resistance don't change (that exactly what a zener regulator is used for), the voltage across the resistance don't change, and then, the current is the same. With the values of the previous post by Kral, it's 130mA.. As no current is derived anymore to the fan, the current in the zener diode increase drastically from 10mA to 130mA, and the power from 12V×0.01A=120mW to 12V×0.12A=1.44 W.
To be safe in any case, the zener should be able to dissipate at the very least 1.5W at the maximum temperature of operation into the project's case.
And as the zener begins to be very hot only when the fan is missing or malfunctioning, you can't count on it to cool the zener.... Too bad!
It's an IMPORTANT point with a zener regulator that the diode absorbs all the variations of the load's current . I this case the variation is extreme : from 0 to 120 mA. But in many case, a circuit can have an important variation.
For exemple, consider a load's current variation from 15mA to 100mA at 12 V, and a minimum zener polarization current of 10mA. The current in the series resistance will have to be set to 110mA, and the power dissipated in the zener will vary between 12V×(110mA-100mA)=120mW to 12×(110mA-15mA)=1.15W : a 1:10 ratio, and a very hot zener.
A little suggestion, now:
As you zener power supply is only an auxiliary supply for a fan that don't really needs to be regulated, and as I suppose that 120mA is the fan current at 12V, I suggest that you drop the zener. (12V fans are rarely zener regulated....)
Assuming we want to obtain 12V across the fan with a current of 120mA, and a 36V supply, the serial resistance value is (36V-12V)/0.12=200Ω, and the power dissipated in the resistance is (36-12)×0.12=2.88W.
The advantage is that there is no risk to blow the zener if you unplug the fan.
Considering the opposite situation, if the fan is short circuited, the voltage across the 200Ω resistance will be 36V and the power dissipated 36*36/200 = 7W. Will blow... But a short circuited fan is often a much less likely situation than an unplugged fan (or a bad solder point), so it may not be necessary to size the resistance for this extreme situation. (Note however that 3W or 5W carbon resistors are big, with a lot of flammable (carbon) matter, and in case they burn, there may become a dangerous fire hazard - so fuses were invented ! ).
As you see, zener regulation are useful only in low current application. The serial resistance creates a large dropout voltage. Zener are seen in dedicated local regulation (regulation for a few components like an oscillator, or a polarization voltage, for example). But for general low power supply voltage regulation, they are supersede in many case by 3 pins regulators, (or even by switching regulators) that add current limiting and thermal protection at a fraction of the the cost of a power zener and a power resistance.
Hope this helps.
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nguyennam
Joined: 06 Sep 2006
Posts: 455
Helped: 27
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| 11 May 2007 14:30 Re: Zener Diode Resistor Value? |
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| lyzrdstomp wrote: |
I want to use a 12Volt zener diode and a resistor to bring an unregulated 36V input down to 12 volts to run a 12V/110mA fan. I used the calculator here:
http://www.opamplabs.com/zenerreg.htm
Entering the values 36 for input, 12 for Zener voltage, 110 for mA, it says I need a 200ohm resistor at roughly 3 watts.
For a bit of safety headroom, would a 5 watt resistor be sufficient for this or should I go for an even higher wattage one?
Just making sure before I order parts. |
5W resistor is good enough. It is better if you can get the resistor of 6W upwards.
In the past, I learnt from a book, but now I cannot remember the name, that the resistor should be at least 2 times of calculated wattage to ensure the reasonable temperature raise.
I had dealt lots with zener circuit designs and assembling, and as per my experience, the resistor wattage should be at least 1.5 times the calculated value. However, which "times" is good enough is depending how "closed" your circuit is, the "times" will be lower with better ventilation, and vice versa.
But, my opinion is the same with our guys here, you should consider the other regulator, for example, adding the series transistor into your zener regulator, you will get better circuit, and even efficiency.
nguyennam
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sbalpande
Joined: 11 May 2007
Posts: 33
Helped: 1
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| 11 May 2007 15:16 Re: Zener Diode Resistor Value? |
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want to use a 12Volt zener diode and a resistor to bring an unregulated 36V input down to 12 volts to run a 12V/110mA fan. I used the calculator here:
better to use 7812 regulator ic.
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Kral
Joined: 28 Mar 2005
Posts: 978
Helped: 157
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| 11 May 2007 16:32 Re: Zener Diode Resistor Value? |
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pmx,
Excellent point regarding the Zener power dissipation.
Regards,
Kral
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lyzrdstomp
Joined: 13 Apr 2007
Posts: 20
Location: Midwest US
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| 11 May 2007 20:19 Re: Zener Diode Resistor Value? |
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Wow, guys, thanks for the great info! Sounds like I should dump the Zener idea and go with either a 7812 or another 317?
I've been reading about using a 3055 relieving the zener of current concerns, (if I read the article correctly), but I think I'll just lose the zener and try the suggestions here.
What a great, helpful site!
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mcoster
Joined: 27 Jan 2004
Posts: 57
Helped: 6
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| 12 May 2007 0:36 Re: Zener Diode Resistor Value? |
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Hello lyzrdstomp,
One more thing:
In the absolut maximum rating of the 7812 you find that its maximum DC input voltage is 35V...
Best regards,
mcoster
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me_guitarist
Joined: 03 Oct 2006
Posts: 142
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| 05 Jul 2008 16:36 Zener Diode Resistor Value? |
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hey guys, how about a 5.1V ZD regulate from 24VDC. I only need 10mA only in my case to run a PIC.
here are my calculation:
24V-5,1V/0.01A=1890ohm
24V-5,1V*0.01A=0.189W
for the series resistor
Any comments?
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max0412
Joined: 26 Oct 2003
Posts: 221
Helped: 28
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| 06 Jul 2008 1:27 Re: Zener Diode Resistor Value? |
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Read through On semi's "TVS/Zener Theory and Design Considerations" handbook.It has everything you need to know for zener shunts and then some.
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| http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF |
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Audioguru
Joined: 19 Jan 2008
Posts: 377
Helped: 22
Location: Canada
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| 06 Jul 2008 2:15 Zener Diode Resistor Value? |
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Will the motor start running when a resistor in series limits its current?
Many motors need 10 times their running current to start.
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ronel101
Joined: 08 Jul 2008
Posts: 6
Location: red planet
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| 08 Jul 2008 9:59 Zener Diode Resistor Value? |
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hi!!!!!!!!!!!!!
i suggest that you dont have to use zener diode anymore you use 7812 regulator for it , but before you regulate it be sure to resist over current to be pass on the regulator.you can use 1k ohms for it ,cause this regulator regulates voltage. to 12 v only .
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