Diode advice

[Deleted member 9648]

I currently have an intermittent issue with a disabled alarm system. The pullcord has a fault monitor connected across it in the form of a resistor and diode in series which I suspect is the culprit. The diode only reads continuity one way as expected but the resistance through it the correct way is around 500 ohms. As a contracting sparks my knowlege of electronic components is limited so can someone tell me whether this is correct or whether a low resistance should be expected? The diode is marked IN4007 with F018T underneath if that helps.
Thanks for any advise....WP

 

[kenny7askew]

sounds ok to me from what i remember from my electronics days,the resistance will be dependent on the voltage applied to open the juction from your multimeter

 

[Steve1412]

Using the resistance scale on a multimeter this looks about right for a diode. Check this link. Does your multimeter have a diode test on it?
How to test diodes using a digital multimeter - http://en-us.fluke.com/training/training-library/test-tools/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter.html

 

[westward10]

Does your meter have a diode check facility.
Beaten to it

 

[westward10]

Delete.

 

[telectrix]

digi meters diode check measures the forward voltage, approx. 0.6V. an analogue meter on ohms x 1 will read in the region of 20 ohms one way and infinity the other.

 

[DPG]

A 1N4007 is a bog-standard rectifier diode, so as above volt drop will be around 0.6-07 volts if you've got a diode test function on your meter, or resistance as stated above.

 

[Lucien Nunes]

As above - just to add that if you test with a resistance range, the reading displayed isn't a genuine measurement of resistance as the meter cannot distinguish between true resistance and the non-linear characteristic of the diode. Therefore, different meters may indicate different values for the same diode, depending on the test voltage and current they apply. When the diode is conducting forwards, its dynamic resistance is actually much lower than any of the above readings, typically a fraction of an ohm, but an ordinary meter can't detect it. Hence the preference of using the much more useful forward voltage indication on the diode range.

 

[Deleted member 9648]

Thanks chaps and sorry for the late response to your replies....been a long day. Yes I have a meter with a diode test function I now discover (!) so will try it again with that tomorrow.
Once again...thanks for the help.

 

[DPG]

Call in maplin on your way to the job just in case. They're only pence.
Daz.