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metal film resistors

post #1 of 14
Thread Starter 
Hi,

Would a 1/2w metal film resistor use more power than a 1/4w metal film resistor? Going to be used in a portable cmoy amp

Thanks !
post #2 of 14
I believe that rating is just how much power they ca handle...not sure tho.

good to have someone else who's just as green as I am when it comes to DIY
post #3 of 14
Thread Starter 
hehe actually I think you know more than me
post #4 of 14
Watt rating IS how much the resistor can handle. Use following equations to make sure you wan't blow resistors.

V=I * R
H=(power dissipation)=I^2 * R=V * I
post #5 of 14
Go ahead with the 1/2 watt resistors. They aren't much bigger and are more versitle. The only place you have to worry about too high Wattage is when you get above 3/4-1W. Those resistors are just too big physicly to be as usefull. As long as you can get both the 1/2W and 1/4 W in the same tolerence (preferably 1%), go with the 1/2W.
post #6 of 14
Tomo, if I understand your equations correctly, at 18 volts, a 1/8 watt resistor can handle 7 milliamps? This interests me because I just made a 9V x 2 cmoy amp that uses only 1/8 watt resistors. The amp is working fine.

I tried to measure amperage in the amp at one point, but milliamps are at the limit of my meter's capabilities. It seemed to fluctuate around 1 milliamp, but I wonder if I'm hitting a precision error with my meter.

A typical 9V alkaline seems to be around 600 mAh -- with a range of 350-1200. So at one milliamp, I should be getting 1200 hours of use out of my 2-battery amp before swapping the batteries? That doesn't seem likely. At 8 hours a day for each weekday (I use it at work) that would be 7.5 months between battery change-outs!
post #7 of 14
Thread Starter 
Woo hoo !!!

Guess I'll be going with 1/2w resistors

Just checking, coz I didnt wanted to have a extra power drain on my 9v batts.

Thanks !
post #8 of 14
Just FWIW to anyone who read my last post, I re-measured the completed amp, and discovered two things: 1) my meter doesn't go down to 1 mA to begin with, only to 10 mA. 2) when it's showing 10 mA, that means "10 mA, plus or minus 10 mA". Therefore, I could be drawing as much as 20 mA, which would give me 60 hours of listening, assuming the opamp will operate through the whole 1200 mAh lifetime of my batteries. That's much closer to what I expected.
post #9 of 14
You don't need 1/2 watt resistors for a portable amp. Another thing to consider in ratings is the fact that some metal film resistors are Milspec types like the Dale Vishay I use. A resistor like the CMF 60 is rated at 1/4 watt Milspec, but will easily handle 1 watt. So I often use CMF 55 rated at 1/8 watt but will handle far more. I even use CMF-50 1/16 watt in small amps as they are far better than many surface mount types. If the little 1/4 watt Radio Shack resistors were rated at Milspec they would be rated at 1/8 watt and possibly lower.
Dan
post #10 of 14
Okay, I found someone that's got a more sensitive DC ammeter. I measured my TL082-based straight-cmoy amp, and it draws 6.4 mA at idle with no headphones plugged in, 6.5 mA when my HD570s are in, and ranges from 8-12 mA while playing music at a good rockin' volume.

I just got some OPA2132s in, and popped one in to test. The amp draw actually went _up_, which I found odd. At idle, it's more like 6.8 mA, and while playing it's 11-15 mA. It's too bad that these 2132s are less efficient, since most people run them with only one battery -- you halve your mAh, then get less play time out of each battery to boot! I may be wrong on that, though -- since they'll tolerate low voltages better than the TL082, they may actually run longer, despite their higher current draw. I'll have to test that....

Getting back to the TL082 version, the 6.4 mA at-idle draw means that if you left the amp on, it would last 8 days before completely killing the batteries, and will stop making sound well before that. It doesn't really say on the data sheets how much voltage it needs, but it hints that it will barely function at +/- 4.5 VDC. I did test with only one 9V battery, but I seem to recall being dissatisfied with its performance.
post #11 of 14

Resistor theory

Hello guys,

I was talking to someone with lots of electrical experience.

He told me of a resistor value theory.

When a resistor gets hot, its resistance changes. Yes.

If you need a 1/4 watt resistor but you can only find a 5% tolerance one rather go for a half what because, it can take more heat and thus the resistance will not change as easily as a 1/4 watt one. (when it gets hot)

Tell me if you guys agree.

He said that some people use large wattage resistors because they can take more heat before they change their resistance so they are more stable even if they have 2% or 5% tolerances.


TLE2426 help
How many TLE2426 do you use when you require + 4.5 V and - 4.5 V from a 9V PPP.

The device only has three pins : in / common / out .

Gavin
post #12 of 14
A cmoy doesn't (or at least shouldn't) have issues with heat. The cuurent going through the resistors is very small, so the power they dissipate as heat is very small too. the wattage rating tells how much a resistor can dissipate without overheating and therefore changing its resistance.
post #13 of 14

Heat Issues

Hello,

Resistors change value when temperature changes. This is one of the fundamental result of thermodynamics. (I don't think there is something called the resistor value theory.)

I have done some experiments on how resistance change over temperature variations. Metal film resistors did not change much over temperature while Carbon film resistors changed much more. However, the change is resistance is only so significant over very wide temperature range; -270 degrees Celcius to 200 degrees Celcius.

Generally, this does NOT happen in normal operations. This only happens when components reach high temperature. For CMOY amp, the current going thru resistors will be insignificant producing very very little heat. Heat production will be much smaller than 1/4W or even 1/8W. On the other hand, Szkeres' Mosfet amp, some of the parts will be very hot. Thus, for this amp, you must use metal film resistors. (With exception of power source resistor.)

It is incorrect to assume higher rating resistors are more stable. The wattage rating shows when the resistor will fail. The stability has more to do with the subtrate of the resistors themselves.

The choice of resistors are depended on the use of them. Metal film for precision and temperature constancy. Carbon for signals**. etc etc. If you don't really care, you can use metal film resistors for all the resistors in the amp.

Tomo

** Carbons may sound loose. When I say Carbon, I mean high quality carbon resistors. Not RadioShack or super cheapo no brand resistors.
post #14 of 14
Quote:
if I understand your equations correctly, at 18 volts, a 1/8 watt resistor can handle 7 milliamps?
ok, but - you are leaving out the resistance for us.

10 ohm @ 18V?
18/10=1.8*18=1800mA*18V=32,400 mW = Smoked.

10k @ 18V?
18/10,000=1.8mA*18=32.4 mW = Fine.

And you gotta consider, that's the power of the resistor's rated capability... In your amp no resistor would see 18V unless you're using it to drop power (dissipate for no reason than you need a lower voltage) it'll never see all 18V unless something went terribly wrong in your amp.
Your current-limit resistor on your LED probably sees the most work. Hey, let's find out!

A resistor divider across the rails to make your virtual ground. So if you went 2x4.7k that'll be 9.4k... while this is about the same as the 10k LED resistor, in a voltage divider (series) each resistor drops the V, in this case half of it, so really each resistor is only dissipating 18/9400=1.91mA*18=34.4mW/2=17.2mW.

So yea, the LED resistor is dissipating more. Sneaky, ain't it?
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