Resistor Value

Item 1 - yes, Guv. Of course it exists. As to replacing it with something else, well, only you can see your circuit and what it does!

Item 2 - yes, though you will ordinarily find one as two bits of wire that aren't touching each other.

You can obviously tell which one it is using your trusty Avo, set to ohms.

.8 ohms using Fluke 87 multimeter.

When you short multimeter leads do you get a 0.0? If not, subtract the value you get from resistor measured value. Do try leads both ways to minimize possibility of forward biasing a semiconductor junction and getting erroneous readings (this is not fool-proof, you must disconnect one side of the resistor to be certain) S.M.

"I have with me a pcb with a resistor having the following colour..."

Have you removed the component from the board to measure the value?

[edit] Never mind, SM has it covered.

Measuring anything with less than 1 ohm of resistance is a tricky thing because existing circuit wiring, cable leads, and contact resistances will directly add to the resistance measured by any hand held meter. Additionally inductive pickup from power distribution magnetic fields can add significant error currents. Fluke's instrumentation circuitry does a better than average job of handling these concerns but nobody is perfect.

As for your actual question about a 0.47 ohm resistor existing, Newark lists 102 resistors with this value being in stock.

One clue is that resistors rarely (if ever) short. So, the idea that the resistor was 4.7 gΩ and suddenly changed value to less than one Ohm is probably not realistic just on that accord.

Second, a resistor of 4.7 gΩ is not normally commercially available anyway.

Third, your empirical reading is probably off due to a variety of reasons. If the resistor has failed they generally do so by opening up completely and leaving a burn mark.

Does the resistor look discolored? Why are you suspecting this resistor anyway?

FYI .8 Ω is also a standard value, but not often used. .47 is very common.

The pcb be is a variable regulator with RFI suppression-QVR/RFI-TB. I have traced out and drawn its schematic only I dont know how to link it up with this message. The Input voltage is 240 and the output is supposed to varry from 0 to almost input voltage. Istead off-load the out put is varying from 178 to 220V and 0 to 4V only on-load. The circuit consists of 11 components only:Values using my Fluke ; R1= 5.58k; R2= 2.70k; R3= 1.50meg; R4= .8ohm; R5=1.0meg; C1= 0.148microF; C2=0.216microF; C3=2.2mF; VR1=250k; TR1=BTA25600B and DB3= no reading.This gadget was working perfectly before but now R1 connected in series with DB3 to the triac gate heats up and changes colour when varying the voltage using the variable resistor. I have replaced the triac, R1 and DB3. Does not solve the problem.

the triac gate heats up and changes colour

You mean it got red hot?

Is there a short circuit on it's output?

Have you lifted a leg on R4 to confirm its resistance yet?

You need an image of your sketch or a URL to it to post it here. Which browser are you using?

I read it as "R1 ... heats up and changes colour"

Your color code is sound, if you zeroed your Fluke and it read 0.8 ohms, then the resistor is out of tolerance and should be replaced. I think your resistor, given the tolerance, is probably an emitter resistor for a power transistor, low ohm values between 0.1 and 1.0 ohm are common. How your resistor looks can give you a clue about which type it is, short of a description from you, it would be a shot in the dark from us.

As to the 4.7GΩ value, it is available as a stocked, high voltage resistor, at least here in the USA, it may not be so common elsewhere. These resistors are generally long with a small diameter compared to a general type resistor, although some are available in a small rectangular case for low power applications, such as Caddock makes (they make both types and Ohmite also makes such high value resistors). I really doubt you have one of these in your circuit.

When you find resistors of 1 ohm or less, what should spring to mind is "shunt" so a 0ohm link will most probably not work (sight unseen).

Measuring resistance in circuit should return a value that is less than expected not more (resistors in parallel...), so it may indicate that this resistor is in fact o/c and that the 0.8 ohms you are meauring is a circuit artefact.

Lift a leg of the resistor and measure again to be sure.

If it is o/c (or high ohms) with a lifted leg then you want to have a scrute of the circuit to determine how this may have happened.

Do you still have the instructions for the fluke meter?

Dear Max, to my mind you have a resistor of value of 0.47 Ohms ( I think that grey band is silver, eg. move decimal point two places left, and it is a 5% accuracy resistor.

If the resistor is still soldered in at both ends, you need to lift at least one end before measuring. Measuring "in-circuit" is prone to serious errors....particularly resistors.

Most meters are not that accurate under 1 Ohm. Also, you need to measure the resistance of the meter test leads first and take that away from the value you measure on the component.....

Others have mentioned that here, but you haven't come back to them yet.....

GA Andy,

Most meter will start with a 0.1 to 0.5 ohms when shorting the leads together.

R(Ω) = ρ(Ω·m) × L(m) / S(m²) = .48(Ω) =
= 1.68×10^-8·[11(cm)/100(cm/m)](m)/{π·[(Ø0.07/2)(mm)/1000(mm/m)]²}(m²)

so it's 11cm of 0.07 mm wire -- it's not totally neglectible

The value of his resistor is of 0,47 ohms. The measurement of 0,8 ohms with the fluke owes to the mistake of putting to zero and tolerance of measurement

Hi Max,

Reading through all the answers to your resistor question and saw a few good ones.

My take on your question is:

a) the circuit was working correctly and then stopped working.

b) for some reason you suspect that the resistor in question is at fault although it is not dis-coloured (you can see the colour code clearly) or open circuit (you were getting a reading across it). If you can read the colour code then just convert this to a value http://myfreewares.weebly.com/resistor-colour-code-solver.html and then you can compare this with the reading of the suspect resistor (measured properly as in item c).

I doubt that if the unit was working correctly that the colour codes on the resistor would have suddenly changed and therefore if it tests out properly then you should carry out proper circuit testing to determine the origin of the problem.

c) you believe that the make and model of your multimeter influences the reading of the resistors value - incorrect use of any multimeter (even the 'Rolls Royce of meters) is not going to improve your measurements. As other posters have indicated you must first 'zero' the meter or subtract the resistance of your meter leads from the reading obtained. Secondly the other posters are 100% correct in advising you to lift one 'leg' of the resistor from the circuit before measurement (in circuit measurements of resistors are rarely accurate).

d) replacing resistors (even very low value ones) with links may solve your problem - serious damage may be done to the circuit which would then relieve you of the effort of fault-finding and take you off to the shop to purchase a new unit.

The above may seem a little bit sarcastic, but in view of the fact that you are dealing with potentially lethal voltages (230vac can hurt you), it was intended to be so that you will take note of the advice.

I'm sure you will find the link to the 'Resistor Colour Code Solver' useful - it will assist you in correctly identifying resistor values.

Wish you success with your repair efforts!

Regards, Keith