• Please review our updated Terms and Rules here

RODIME MFM HD won't spin up... without help.

I'd unsolder one end of that diode and test it again.

When testing a diode (or transistor junctions) you should have a low resistance in one direction and a high one in the other direction (as you foward and reverse bias the junction).

The same resistance in both directions in not normal and, if it is actually in parallel with a resistor, it should react about the same but with unusually high and low values.
 
Druid6900 said:
The same resistance in both directions in not normal and, if it is actually in parallel with a resistor, it should react about the same but with unusually high and low values.
Click to expand...

It depends upon the parallel resistance and the voltage source being used for measurement. Not at all uncommon to get the same reading both directions if you're using a low-voltage ohmmeter. A little pencil-and-paper work with Ohm's law can show you that any resistance that develops less than about 0.6V when measured will show the same resistance in both directions.
 
True, but, when you reverse bias the diode junction, the resistance should be a LOT higher and would change the resistor/diode circuit's total resistance as opposed to when you forward bias it, IIRC.
 
Thanks guys. I’ll just get hold of a descent soldering iron fine tip before checking the questionable diode. I’ll also check if the transistors are npn or pnp then have a go checking them as well. I’ve managed to find my old analogue multimeter as that has some sort of transistor testing capability – just need to find the instructions and test leads.
 
You said above that, when you tested one diode that seemed to be in parallel with a resistor, that you got the same reading in both directions. I meant to ask if the resistance reading was roughly equal to the value of the resistor that the diode was in parallel with.
 
Druid6900 said:
True, but, when you reverse bias the diode junction, the resistance should be a LOT higher and would change the resistor/diode circuit's total resistance as opposed to when you forward bias it, IIRC.
Click to expand...

Again, it all depends. Put a 10 ohm resistor in parallel with a 1N4148. Measure the resistance both ways using a 3V supply. You won't see much difference. It gets even nastier when a diode is used as a snubber across an inductor.

As I said, it depends. I've been there, bashing my head against a brick wall. It should also be observed that the chances of a diode paralleled by a low-value resistance in a low-voltage circuit is pretty small. On the other hand, a snubber diode across a good-sized inductance, you betcha!
 
Chuck(G) said:
Again, it all depends. Put a 10 ohm resistor in parallel with a 1N4148. Measure the resistance both ways using a 3V supply. You won't see much difference. It gets even nastier when a diode is used as a snubber across an inductor.

As I said, it depends. I've been there, bashing my head against a brick wall. It should also be observed that the chances of a diode paralleled by a low-value resistance in a low-voltage circuit is pretty small. On the other hand, a snubber diode across a good-sized inductance, you betcha!
Click to expand...

I agree, but, I can only go by what the OP has said in his post.

Since I don't know what kind of meter he is using and what the section of circuit actually consists of, I can only make a suggestion based on what he said and using the correct setting on the meter for the task at hand.

Unsoldering one end of the diode and testing it will be the optimum way of seeing if the diode is actually bad or not.
 
You must log in or register to reply here.
Back
Top