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Old 04-24-2012, 04:18 PM   #1
mockingbird
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Default Giant FSP Resistors

What are these gigantic pink resistors FSP used to use in some of their PSUs?



This one comes from an AOpen branded FSP250-60GTW.

The colors are, from top to bottom:Red (Auburn), Green, Brown, Gold.

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Old 04-24-2012, 04:22 PM   #2
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Default Re: Giant FSP Resistors

From their position (secondary side) they are probably minimum load resistors, which provide a small amount of load to the power supply allowing it to start with little or no external load.
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Old 04-24-2012, 04:44 PM   #3
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Default Re: Giant FSP Resistors

I have another one where some of the thin ceramic has chipped off exposing metal underneath... Is there something of higher quality I can replace these with?
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Old 04-24-2012, 06:11 PM   #4
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Default Re: Giant FSP Resistors

That resistor seems to be a 250 ohm one based on the colors you say, probably 5-10w based on the size.

As it's already said, it's there to provide a minimum load for the power supply, it helps it stabilize the voltages when very little is attached to it, or when in standby for example, when only the 5VSB is used.

There are various packages, for example wirewound resistors in ceramic package (acting as a heatsink) :



or thick film resistors that look like this:


(but these would be bad for you as the wires are much closer in the fsp power supply)

You can find them on Digikey easily.
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Old 04-24-2012, 09:34 PM   #5
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Default Re: Giant FSP Resistors

I'm going to guess that the first color is actually brown, which would make it 150 ohms, a standard 5% value (220, 240 and 270 are standard values, but not 250). Actually, the more I think about it, I wonder if the third band (multiplier) might be black, a 15 ohm resistor (or even red-black-gold-gold, a 2 ohm part). I'm going to guess that it is metal oxide film, 5W; I could be wrong, but I don't think that technology/package goes up to 7W or 10W.

Love those vertical-mount bathtub types mariushm had in his post! Good PCB real estate efficiency, nice cooling air profile. They can be found in 2W, 3W, 5W, 7W and 10W, wire-wound for lower values, and metal oxide film for higher values. Cheaper ones are wire-wound (or film) on a fiber glass core; better ones have a ceramic core. The other ones are in the TO-220 package and need a heatsink to be able to dissipate their rated power.
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Old 04-24-2012, 10:29 PM   #6
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Default Re: Giant FSP Resistors

Quote:
(or even red-black-gold-gold, a 2 ohm part)
I think this is the closest. It hink it's supposed to be brown black gold gold the more I look at it. This would make it a 1 Ohm Resistor. Does this make any sense? Maybe it's just big because of a high watt rating?
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Old 04-25-2012, 12:41 AM   #7
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Default Re: Giant FSP Resistors

Why not lift one leg and test it?
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Old 04-25-2012, 01:22 AM   #8
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Default Re: Giant FSP Resistors

Quote:
Originally Posted by tom66
Why not lift one leg and test it?
+1

I regularly replace those minimum load resistors if they dissipate more than 0.5W of power - it's just a waste of power, hurts the overall efficiency, and cooks nearby caps if there are any. If the resistors had low values, generally I try to use 33 to 50 Ohms for 3.3V rail, 50 to 100 Ohms for 5V rail and 5vsb, and 330 Ohms or more for 12V and -12V rails, all 1W parts minimum (or at least 2x the power they will be disippating).

If a power supply has such huge load resistors, then the designer must have been too lazy to design a proper control loop. Most reputable power supplies have very small load resistors, and they are usually of high resistance too so they don't waste power.
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Old 04-25-2012, 07:30 AM   #9
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Default Re: Giant FSP Resistors

I thought of 1 ohm as a possibility, but on a 3.3V O/P that would be throwing away 10W, a big waste of power, putting a lot of heat near lytic caps, and if they are using a 10W part to dissipate 10W, that is beyond !
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