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Old 3rd May 2004, 09:46 AM   #1
Wagener is offline Wagener  South Africa
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Default tolerances of 1% metal film resistors

I noticed that some of my 1% metal film resistors are very far out. I calculated them and they fall under 1% but it is very close to the limits.

How accurate do I need to have them? Some of them are precisely the value they should be. I suppose if Dr. Leach specified 1% values then using a resistor which are 1% out is ok? right?
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Old 3rd May 2004, 09:49 AM   #2
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Don't forget to check your instrument too. Maybe the instrument is showing too much wrong?

My experience is that 1% resistors mostly are around +- 0.3% but under +- 1% OK.
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Old 3rd May 2004, 10:37 AM   #3
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How accurate do you think your ohm-meter is?
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Old 3rd May 2004, 11:55 AM   #4
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I've used Beyschlag 1% ones that are often 0.1% or better, measured on a 6 digit HP meter that gets calibrated by a NATA lab once a year.
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Old 3rd May 2004, 12:02 PM   #5
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I'm sure you would be OK with the 1% parts, but it might be worth doing some repeat tests on the same batch of resistors to see if your meter is consistent.
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Old 3rd May 2004, 12:20 PM   #6
SY is offline SY  United States
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Also, unless you're talking about EQ circuits or special differential amps, 1% is already tighter tolerance than you need for most uses. If the project you're doing is one of Leach's power amps, most resistors (except the feedback Rs) could be out 5% without affecting much.
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Old 3rd May 2004, 12:35 PM   #7
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Quote:
Originally posted by traderbam
How accurate do you think your ohm-meter is?
I have a 0.025% LCR bridge plus an APPA305 0.3% but calibration showed 0.06%!
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Old 3rd May 2004, 12:43 PM   #8
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Quote:
Originally posted by SY
Also, unless you're talking about EQ circuits or special differential amps, 1% is already tighter tolerance than you need for most uses. If the project you're doing is one of Leach's power amps, most resistors (except the feedback Rs) could be out 5% without affecting much.
before I address Sy's comments -- if you are truely going to measure resistance you need a meter with 4-wire capability -- the auxiliary leads compensate for the resistance in the leads and clips. this doesn't have to be very expensive if you look around for Fluke and Keithley, the Genrad RLC meters also test very acurately -- the HP meter Circlotron refers to is probably a HP3478 or HP3456 (or in that family). The HP3456 has Guildline resistors in the current source for very acurate, lab-type measurement.

1% is "OK" -- but you should be made aware that in cascaded filter designs the requirements for higher tolerances goes up, much more so for Sallen Key than MFB -- you can see this demonstrated with either the FilterPro software from TI, or Analog Devices interactive java applet -- they both allow you the option of seeing your results given varying component tolerances. Of course they assume that you are getting making a whole bunch of filters and not testing the individual components. In this case the manufacturer is supplying some statistical parameters -- either a histogram or a mean, standard deviation table -- so that the filter designer knows what tolerances they can spec.

Capacitors are going to be more problematic than resistors -- this is where a good bridge is helpful (and for $50 bucks, well that's the price of a decent Bordeaux so what the heck.)

btw, a couple decades ago TRW and GE made capacitors with 0.02% tolerance -- you can still find these in surplus shops.
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Old 4th May 2004, 11:37 PM   #9
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Default Re: tolerances of 1% metal film resistors

Quote:
Originally posted by Wagener
I noticed that some of my 1% metal film resistors are very far out. I calculated them and they fall under 1% but it is very close to the limits.

How accurate do I need to have them? Some of them are precisely the value they should be. I suppose if Dr. Leach specified 1% values then using a resistor which are 1% out is ok? right?
Yes, using a resistor within the specified tolerance is fine, everywhere. There are no resistors in this design that demand higher than 1% precision. There are a couple of cases where I might suggest matching even the 1% resistors (see below).

I suspect it is easier to just spec 1% resistors everywhere than to minimize cost by using the more typical 5% carbon film resistors. Remember this design is aimed at neophytes, so he tries to keep it simple. You probably only need 1% metal film resistors in a few places, some for precision and some for noise reduction.

First, the 300 ohm emitter resistors should be "matched", using the same meter, as close as you can get them. That is, try to find two that measure the same value, say 301.2 ohms, for each pair. The absolute accuracy does not matter, since it is swamped by the beta differences of the NPN and PNP types. This will help to minimise distortion and is significant.

It is also a good idea to match the 22k input bias resistor with the sum of the two 11K feedback resistors, to equalize input current to the pairs. I suspect this is less important, since again the differences may be swamped by beta differences between types.

I think the rest of the amp can be safely built with 5% resistors, but 1% is a safe overkill, not too expensive.
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Old 5th May 2004, 09:03 AM   #10
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Quote:
Originally posted by jackinnj
-- the HP meter Circlotron refers to is probably a HP3478 or HP3456 (or in that family).
The meter was a HP34401A. It does have 4-wire resistance measurement facility but I wasn't using it at the time. It also has a null button for zeroing out the ~0.038 ohms (YMMV) of the test leads. It not mine - it's a work one.
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