I know the usual reasons, increase wattage, decrease resistance, but I came across a schematic that insists on using two 10R 3W (metal films) in parallel and NOT a single 5R 6W.

The resistors go between a bridge rectifier and the input of a 7805.

Any ideas?

 

how is it "insisting" ?

I have never had a problem changing a cct

Could be to keep inductance low (power resistor inductance start getting significant), could be to spread the heat over an area

 

Could availability be the reason?

 

It says to use two 10R and not to use a single 5R. It could be for improved heat dissipation, the resistors have visible signs of over-heating, but that dosent seem like a good idea to me since there is always going to me a minor difference in resistance between the two and one will heat up more than the other, and sure enough, one is more "baked" that the other.

I don't think availability is a problem, I have plenty in my private stock, and I looked at a few distributers sites and they all had them too.

 

justy out of interest what is the cct?

There really isnt any extra benefit to using two instead of one (unless the power dissipation was a problem - but that can still be sorted).

Maybe failure modes. If using one 5Ohn it blows - NO resistor - cct prolly stops working

If one of the 10Ohms blows then the other will be there, less power disipated - cct could continue to operate for a bit or reduced performance.

We have FMEA's in the Aerospace and they like failure mode stuff, even increasing the time-to-failure from 1sec to 10sec is highly praised.


Sure with two 10Ohm there will be impedance miss-match and one will draw more current than the other

 

It may be for the failure rating of the resistors. The designer may have found that it was more likely for one of the 10 ohms to fail quickly at a lower over-rating, than wait for a single 5 ohm to fail. Seems kinda fishy to me though. Often in a critical application a "fusistor" or PTC device is used that will open/go large in value fairly quickly on over load.

You will see fusistors on things like telephone line inputs on modems or answering machines, where a surge could do alot of damage.

<edit> arrggghh ... Styx types faster than I do....LOL :lol:

 

Sorry Styx, I dont know what cct stands for.

 

sorry, it cause I type fast

cct = circuit

 

It's in a power supply for a Harman Kardon receiver. The power supply is a very ugly design, maybe I should ignore the service manual.

 

If the two 3W resistors are getting very hot anyway, I wouldn't replace them with a single 6W one - I would like a much higher wattage so it would run much cooler.

The original resistors were probably used solely on cost grounds, it's a cost cutting exercise which gives rise to poor reliability!.

As a service engineer I've always found that combinations of resistors (either in series or parallel) are EXTREMELY prone to failure, over the years I've informed various manufacturers of this (and they know it from warranty claim data). But, although the designers may well use better spec resistors in the design, they often get changed during actual production for cost reasons!.

Nice simple tip! - ALWAYS check series and parallel resistors when looking for faults!.

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current dividing, if there is nothing else running off these resistors then I not sure either, the only reason I parallel 2 like resistors is that, increase wattage halve resistance, and current divide.

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I might temper Nigel's advice about replacing an over-dissipating resistor or resistor pair with a higher wattage assembly. Assuming the circuit was designed properly and worked fine originally, an overdissipating resistor is usually an indicator of another problem in the circuit causing excess current to be drawn. Replacing the part(s) with a higher-wattage would be similar to replacing a 3 amp fuse with a 10 fuse to keep it from blowing so often.

Another reason for using two 10's in parallel vs. a single 5 is that "5" is not a standard value. In some circuits, 5.1 is not close enough or 5.1 isn't available in that high of a power rating and the 4.7 that is available is too far off the design value. Two tens in parallel gives the required 5. That may not be the reason here ... it just answers the general question of "is that another reason to use parallel resistors".

I've often seen parallel resistors, as many as four, used in the emitter circuits of power amps. Finding resistors of low values can be a problem that paralleling answers. Also, the lower the value, the more the solder connection and leads contribute to the overall resistance. It may be that several larger resistors in parallel will take care of that problem.

A careful calculation may find that two resistors in parallel have more surface area than that of a single resistor of double the power rating. Or two resistors give better air flow for cooling.

When power resistors are replaced, you should mount them on a PCB with a bit of clearance, 1/8" is usually good, for good air circulation and to keep hot resistors from browning the PCB.

Dean

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I replaced the resistors today with a single cement type wire wound, I left plenty of clearance and put a bead on the long lead, it seems the heat problem is solved. There was also a "Johnson noise" effect (noise caused while a components temperature is rising) that is mostly gone now, I'll run the unit for a while and hopefully its cured. What made them think that 21v at the input of a 5v regulator was a good idea? :lol:

 

I guess this is nitpicking, but I don't believe the Johnson noise of a 10 ohm resistor on the input of a 7805 would be seen at the output. Johnson noise is independent of the resistor's material, and of current. It is dependent on temperature, but the total wideband Johnson noise of a 10 ohm resistor at 100 deg C, with a 100pF load, is only about 7uV rms. For a technical discussion, see "Thermal Noise" from the Physics department of the University of Toronto.

 

Also, any small AC signal on the input of the 7805 will be attenuated by at least by 60dB at the low end and about 40dB at 100kHz

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