Any lamp has about an eight times resistance increase from the cold temperature to the resistance at normal operating voltage.  This can be useful in protect circuits.  Devices like PTTC  solid state fuses also have a vary low resistance that increase in resistance with a rapid knee when they heat up.  I have some 4.5A PTTC that can take a 40A surge.  When they start to react, they will limit current to 1/4A till current is removed.

If you are contemplating a MOV you don't want a lamp.  The proper  use of a MOV requires that there is 'some' resistance in the supply.

Keep in mind that when a MOV is triggered it becomes a short.  If it is the only resistance in the circuit it will fail instantly.  If instead there is some other impedance in the circuit with it then that resistance will take some of the power dissipation.  In fact the proportion is to the ratio of the MOV's resistance and that "other" resistance.  Often that other resistance is better able to dissipate the resulting power spike.  An example would be the power cord going to the system in question.  If there is some other resistance, better yet.

In, say, a small dc instrument you would want to put a resistor in series with the local supply for the MOV to work with.  Of course you need to watch the power dissipation and hence loss across this element during normal operation as it represents wasted energy.

Keith Cress
kcress - http://www.flaminsystems.com

Hi Keith;
How about incandescent lamps in series with the supply?  

Bill
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"Why not the best?"
Jimmy Carter

My problem is that my system can take from 0.3A to about 2.0A. This means that I have a 2V drop for each ohm in series with my local supply. As my source is automotive battery (from 12 to about 32V) I weren't able to find a resistor to attend my especification. That is, a resistor with less than 4 ohms (4ohms x 2A = 8V droped) that stand about 10kW (V²/R = 200²/4 = 10.000, being 200V the maximum surge current). Furthermore, a 4 ohm resistor will make a 200/4 = 50A through my MOV during surge voltages, requiring a robust MOV.

Facing this problem, a PTC could be a good solution, as it increases its resistance with the increase of current. The problem is that if I specify a 2A Hold Current PTC, it will take to long to increase its resistance, once I have surge voltages of about 400ms of duration.

The ideal would be to have a resistor that varies its resistance with the current. That's why I thought that the lamps OperaHouse said would fit my application.

My questions are:
the lamps' resistance increase with current or with temperature?
Wont they heat and limit my current in normal operation coditions?
Do they increase they resistance fast enough?
Do you think they could be used in my application?


Thans for the patience of reading it all, :)

Eng. Filipe Carneiro

The voltage and current of the normal load was not posted.  Lamps can be great protectors in some applications because they provide resistance and some inductance. May not seem like much, but MOV performance is rated with less than 1/2 inch leads because of inductance.  Lamps are unconventional because of cost and reliability.  I use 300W 120V lamps in test applications that have a 4 ohm resistance when cold.

Consider using inductors to limit power dissipated in the MOV.

A simple three-leg voltage regulator seems to be the obvious and simplest solution. It will keep your voltage at the precise level you want without the response time problems you will have with a lamp and also without the rather unsharp characteristic of a MOV.

MOVs are seldom used to limit voltage in such a narrow range as your application needs. They are better suited for transient protection where accepted voltage is around 50 % higher than normal working voltage.

Also, they are not good for long overvoltage periods because they overheat easily. A few milliseconds are typical maximum surge lengths.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Light bulbs are fine for one time test applications but not as reliable permanent devices in a mobile situation.  Their filaments can break at any time.  Also their resistance can  take a long time "electronics wise" to change since it is a thermal response of a piece of metal.

filipespc; You should really describe what your load is and why you are interested in the voltage limiting before we can help you with the 'correct' solution.

Keith Cress
kcress - http://www.flaminsystems.com

Positive Temperature Coefficient (PTC)...  If current gets too high, the resistance goes way up.  Unfortuatnely they are "big".  (My size reference is 0201 SMD components though ;))

If you can stay in the milliamp range you can use a JFET with the gate wired to the drain.  At 0V (Vds=Vgs=0) the JFET is on (low resistance), so as the voltage from drain to source (and now gate to source) increases the JFET turns off (increases resistance).  

There are two terminal devices out there that are wired internally just for this purpose.  

I've never seen a JFET that works at 2.0A, but maybe you can use this to control something that can handle that current.  

Z