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I'm trying to repair an electrophysiology amplifier (AXOPATCH 200A). It has this power resistor which I desoldered because I suspected it was broken. The color code (red, green, black, gold) gives 25 Ohms / 5%, but it measures 16.6 Ohms. It is positioned directly between +5V and GND, so at 25 Ohms it is disipating 1W. My questions are:

  • Is it really a power resistor? Is it in a bad shape or am I confused and it is another type of component?
  • Which is it approximate power rating? (there are some components on the image that can give an idea of its size)
  • Which could be the purpose of a resistor not doing anything other than disipating power??

enter image description here

El Diego Efe
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    Sometimes people put a power-dissipating resistor on the output of switching regulators, because they often have a minimal load to work properly.. – Eugene Sh. May 24 '16 at 19:06
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    Actually, that's brown-green-black, which is just 15 ohms, and a standard value. (Many manufacturers make their browns rather reddish, in order to make them more distinguishable from the body color.) However, it only appears to be a 1W unit, based on the scale of the devices around it ... MAYBE 2W, but even still, it's going to be uncomfortably hot. – Dave Tweed May 24 '16 at 19:09
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    Using the 0.1" lead-spacing on the dips for scale, it appears to be 2W based on some resistors we have in the lab. This makes sense for 15 ohms across 5V (1.67W). – Tut May 24 '16 at 19:40
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    thanks eugene, dave and tut for your comments, really useful! – El Diego Efe May 24 '16 at 20:07
  • Just as a suggestion: Please add smaller sized version of the image. To indicate details of the component you're indicating, You could add a separate small image of it . Thanks :) – Always Confused Jul 01 '16 at 15:38
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    If you do happen to replace a resistor like that, take note that the leads are left long enough to allow air circulation around it rather than being mounted against the PCB. – Andrew Morton Jul 01 '16 at 19:43
  • To read further: Patch Clamp Technique: https://en.wikipedia.org/wiki/Patch_clamp – Always Confused Jul 08 '16 at 05:24

5 Answers5

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I am pretty sure that is a 2-Watt carbon resistor (about 18 mm long), based on comparing it to the ICs. Here is a picture of 1/2, 1 and 2-Watt resistors:

enter image description here

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To answer your questions :

1-The resistor you show is not a power resistor, however it drains more current than the small ones on your picture.

2-Here is a link that might help you to determine the dissipation power : http://learn.mikroe.com/ebooks/componentsofelectronicdevices/chapter/introduction-to-resistors/

3-There are various reasons to use a resistor in a circuit, if it seems useless it could be simply a load to make sure the circuit as a minimum current to operate. A good example is for relays, they need a minimal current to make sure the contact do not oxide.

If you need more help, you should provide the schematics.

supermario
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  • Thanks @supermario, I wish I had the schematics, the board is really complex. What I meant by "power resistor" was just "big resistor". I think your point 3 is a good argument, the resistor feeds from a switching power supply that gives +15/-15/+5 (brand and model: Cosel PMC30E-2) – El Diego Efe May 24 '16 at 20:03
  • " **The physical size of a resistor is indicative of the power it can dissipate, not of its resistance**."-Wikipedia. https://en.wikipedia.org/wiki/Electronic_color_code#Examples. So it is a fact that resistors with more power-dissipation capacity tend to be fatter – Always Confused Jul 01 '16 at 15:57
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Yes, you can consider that a "power" resistor relative to common resistors of today and other resistors on the board. Since it's clearly bigger than other resistors, it was chosen in part for its power rating. From the size I'm guessing it's in the 1-2 W range. The much smaller resistors in the upper right section are probably 1/4 W. The value seems to be 15 Ω (BRN, GRN, BLK).

Keep in mind that "power resistor" isn't a definative spec. All resistors have a power rating. "Power resistor" just means that this is a large point of that resistor, which varies by usage. Nowadays, anything over 1/4 for thru hole is probably a power resistor because if the extra power dissipation wasn't required, a 1/4 W resistor would have been used.

Olin Lathrop
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    Thanks! i didn't doubt about the color of the first band, now I see it absolutely brown! And yes, the other resistors are 1/4 W. – El Diego Efe May 24 '16 at 19:58
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Are you sure it is "RED(2) green(5) black(10^0=1) gold(+-5%)"? Color quality of your photograph seems very good, and to my vision, the first band is BROWN, and NOT RED.. So it seems the resistor is "BROWN(1) green(5) black(0) black(10^0=1)" i.e. 15 ohms. that is close to 16.6 ohms.

Indeed color codes are often troublesome. Read more about it on Why do resistors still use color coding?

Such banded Color coding is common in 3 parts: Resistors, Non-electrolytic color-codes (some old-type), and some coils (maybe rare one , I've never seen one, just read @Wikipedia).

If you've check Ohms in proper-method with a multimeter; Then it is surely a resistor; and if it was a non-electrolytic capacitor, it would show infinite resistance (non-conductor) (in case of digital-multimeter, 1 (High Value) or OL(Over Limit or Open Line)). If it was a small coil, it would show Zero-ohms or hardly 1 ohms (just like conductive wire).

From an intermediate resistance value very close to color-coded printed-value, it seems to be Resistor, nothing else.

Community
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Always Confused
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No, it's not a power resistor as most colloquially know a power resistor to be, a fairly arbitrary term really. Those would be large white ceramic resistors

Of course, 2 Watt resistors are fairly large, and not typically used for signaling or other purposes. It's clearly there to use as a power resistor.

As to why it is there, if it actually connects directly between 5V and GND may be one of two reasons.

  1. Heating the board or enclosure to make sure it is within operating temperature range. Not likely here.

  2. Providing a significant load to help ensure a stable voltage rail. Especially for an Amp that likely has a transformer based power supply, not having a sufficient load may result in noise or higher than safe voltages.

Passerby
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