Choosing power resistors

[mtl777]

Hey guys, I have an application where I'm going to use a power resistor to reduce the input voltage that's going into a voltage regulator. This is for the power supply of an AD/DA converter.

Based on what I've read, wire wound resistors are not so good because they are inductive. Metal oxide or metal film resistors are preferable. OK, so I want metal oxide or metal film. Now, is there any point to being so brand conscious about power resistors (just like some people are with capacitors - e.g., Black Gate, AuriCaps, Mundorf, Roederstein, etc.)? I wonder if you guys take as much care in choosing power resistors as you would in choosing capacitors. Or are all power resistors of a given type (metal oxide or metal film in this case) gonna perform the same since this is just a voltage reduction application and not something in which the resistor is directly in the audio signal path? Should I just concern myself with choosing the resistor with the lower temperature coefficient, tighter tolerance, and, of course, adequate power rating, totally ignoring the brand or how much the resistor costs?

For a specific example, the Vishay BC PR02 at 60 cents a piece costs almost 3 times the Yageo FMP200 at 23 cents a piece for a 27-Ohm 2W value that I need, but the Yageo has better TC of 100 ppm vs 250 ppm for the Vishay. Both are 5% tolerance, though. I haven't heard of Yageo until now, but I think Vishay is well known for quality. BTW, what do you think of Yageo with regards to quality?

Thanks!

[Conrad Hoffman]

Unless you were using it as a feedback resistor in a high bandwidth circuit, the tiny amount of inductance in a wirewound resistor isn't likely to cause any problems. Wirewounds have tremendous overload capability if something gets shorted out. You can also choose other types for the express purpose of going open during an overload conditions. Be careful if you're putting the resistor in series with the regulator, as the voltage will change with load. I don't know how much you have to drop or the current level (sounds low), but take all variables into account. Other than that, it sounds like almost any type would work fine.

[KBK]

Try hard to stick to resistors with non-magnetic parts. Yes, even in that location. Digital gear is 'pulse draw' when it comes to current delivery, and thus the sonics can be notably changed by what is in series with the PS rails for the DAC and associated components.

Jitter induction via micro current modulation in the given IC sets involved. A well recognized source of jitter in digital audio gear.

I personally do take extreme care in what resistors I use. Not specifically branding issues.. but always choosing as many non magnetic parts as i possibly can.

[mtl777]

Originally Posted by Conrad Hoffman

Unless you were using it as a feedback resistor in a high bandwidth circuit, the tiny amount of inductance in a wirewound resistor isn't likely to cause any problems. Wirewounds have tremendous overload capability if something gets shorted out. You can also choose other types for the express purpose of going open during an overload conditions. Be careful if you're putting the resistor in series with the regulator, as the voltage will change with load. I don't know how much you have to drop or the current level (sounds low), but take all variables into account. Other than that, it sounds like almost any type would work fine.

Thanks for the heads up on wirewounds! So they're not bad after all for my application. I need to drop from 25V to 19V, and the current level is 175mA on the average and maybe 300mA tops.

[mtl777]

Originally Posted by KBK

Try hard to stick to resistors with non-magnetic parts. Yes, even in that location. Digital gear is 'pulse draw' when it comes to current delivery, and thus the sonics can be notably changed by what is in series with the PS rails for the DAC and associated components.

Jitter induction via micro current modulation in the given IC sets involved. A well recognized source of jitter in digital audio gear.

I personally do take extreme care in what resistors I use. Not specifically branding issues.. but always choosing as many non magnetic parts as i possibly can.

Thanks for your advice on avoiding magnetic parts! Can I use a demagnetizer to demagnetize magnetic resistors? I have one that's shaped like a toothbrush that I use for demagnetizing tape heads.

[Conrad Hoffman]

Nice thought but no cigar. If the resistor is built with steel leads, end caps or some other magnetic component (including the wire itself), it's magnetic in that it's attracted to a magnet. You can't demagnetize it. You can test to see if a strong magnet attracts the resistor. I don't consider it an issue for anything I do and happily use magnetic resistors, but to some it's a big deal and they avoid them like the plague.

[mtl777]

Originally Posted by Conrad Hoffman

Nice thought but no cigar. If the resistor is built with steel leads, end caps or some other magnetic component (including the wire itself), it's magnetic in that it's attracted to a magnet. You can't demagnetize it. You can test to see if a strong magnet attracts the resistor. I don't consider it an issue for anything I do and happily use magnetic resistors, but to some it's a big deal and they avoid them like the plague.

Yeah, as soon as I hit submit I realized it was a stupid question, duh! It's the steel lead that makes it "magnetic" (i.e., can attract to a magnet), not that the resistor is a magnet itself. Anyway, it's good to know that you don't consider it an issue, so I have one less thing to worry about. Thanks!

[mtl777]

I wonder why there is all this concern about magnetic parts. There are so many things that are magnetic in a piece of equipment that can't be avoided. For example, the screws (especially the ones that are used to secure the circuit board, transformers and voltage regulator heat sinks), the chassis, and even the power jack. Are you really gonna replace all of these?

[qusp]

well thats kinda irrelevant, the screws arent in the circuit are they? I use non-magnetic in high frequency apps, but i'm not as concerned about it as most.
I worry more about stability and matching than branding. although for power resistors I really like Caddock MP930 and the newish vishay precision Zfoil power resistors

[mtl777]

Originally Posted by qusp

well thats kinda irrelevant, the screws arent in the circuit are they? I use non-magnetic in high frequency apps, but i'm not as concerned about it as most.
I worry more about stability and matching than branding. although for power resistors I really like Caddock MP930 and the newish vishay precision Zfoil power resistors

If it's a screw that's used to secure a heatsink to a voltage regulator, for example, then it's in the circuit. Another example would be a screw that's used to bolt a power transformer to the chasis. In these examples, the screw becomes a part of the ground, therefore it is "in the circuit".