I've salvaging varous parts from broken electronics problems for modifying my DS-1 and SD-1, resistors, capacitors, opamps (found four JRC4558DDs) etc, since it's cheaper than going to radio shack and faster than ordering online. I've been collecting various diodes to try to see what kinds of flavors of clipping I can get, and which ones I like. I've collected some 1N4148s, various rectifier diodes (1N4004s and 7s) some fast recovery rectifier diodes (FR107s), some zenners (somewhere...) and some 3mm green and red LEDs. My digital multimeter has a diode test function. I put the leads across the diode the right way and get forward voltage reading in mV. I put them across the other way and get an out-of-range reading. Which, if I'm understanding things correctly, is how things are supposed to go.
I've tested all the various diodes and get consistent readings on all of the kind of one type or another, but when I try testing the 3mm LEDs, I can't get consistent readings. On one of my red LEDs and my two green ones, when I connect the multimeter leads to the LED, the LED lights up and my meter breifly reads 1800 or 1900 some odd value then goes out-of-range. Two of the red ones show a forward voltage of somewhere between 1600 and 1700 mV but only when I connect the leads backwards (connecting them forwards shows out-of-range as usual), but they don't light up. Finally I have two more red ones that show some super low number (like 16) when I connect the leads in either direction and don't light up. The latter four are from disposable camera flash circuits, and light up just fine when I put a battery in the circuit and start it up (don't touch the capacitor! ouch!)
So, what am I doing wrong here? All I want to know is the forward voltage of the LEDs is so I can try to adjust the clipping flavor in my pedals. How can I get an accurate forward voltage reading from these LEDs?
Testing LEDs and other diodes
It's normal to read different foreward V in LEDs. It varies with the colour, the type (ultrabright, standard etc.), and the size too I think. For a blue ultrabright led is normal to have a Vfw > 2V.
I think that your DMM's diode test has a limit of 2V and then it goes out of reading.
It would be better to build a very simple circuit to test leds: a 9V battery, a 10k resistor. Hook up the led in series with the resistor and then read the Vfw measuring the voltage drop of the led with your DMM.
I think that your DMM's diode test has a limit of 2V and then it goes out of reading.
It would be better to build a very simple circuit to test leds: a 9V battery, a 10k resistor. Hook up the led in series with the resistor and then read the Vfw measuring the voltage drop of the led with your DMM.
I figured that different LEDs would have different forward voltages. That's why I was wanting to test them, so I could knowingly tune the clipping levels in my pedals, instead of just blindly experimenting. I also figured that my multimeter has a limit of 2V on the diode test, and that's why they were testing out of range. But I thought a diode had to have a certain forward voltage across its junction, otherwise it wouldn't conduct. So if the LED illuminates when put on the meter, and the meter says out of range, what's going on? if the meter can only go up to 2V, but the threshhold forward voltage is beyond that, how is it possible for the LED to light up?
What about the LED that reads a forward voltage on the meter, but doesn't light up? If the forward voltage has been reached, shouldn't the LED light up? Is there something that the photo flash circuit it was salvaged from is doing differently that my meter isn't?
Finally, what the LEDs that show an extremely low (<100mV) forward and reverse voltage. Are those damaged, broken, or leaking somehow? If so, why do they still light up when their original circuit is energized?
Looks like it's time to pick up some breadboard.
What about the LED that reads a forward voltage on the meter, but doesn't light up? If the forward voltage has been reached, shouldn't the LED light up? Is there something that the photo flash circuit it was salvaged from is doing differently that my meter isn't?
Finally, what the LEDs that show an extremely low (<100mV) forward and reverse voltage. Are those damaged, broken, or leaking somehow? If so, why do they still light up when their original circuit is energized?
Looks like it's time to pick up some breadboard.
It would take advanced knowledge in semiconductor physics to give you a comprehensive answer, I don't have it so I'll do what I can.
The first thing that I tell you is to quit using the diode test mode of your DMM for testing leds. It is designed to test normal rectifier or signal diodes in common DMMs. A simple circuit involving a resistor, a battery and the DMM to read voltages is a far better way to test leds.
The leds don't light up on the DMM 'cause the current flowing in them is too low to be able to produce visible light. Maybe in the dark you could see a little dot of light. The photo flash unit 's circuit just provide more current.
For the odd "bipolar" leds I just don't know...
The first thing that I tell you is to quit using the diode test mode of your DMM for testing leds. It is designed to test normal rectifier or signal diodes in common DMMs. A simple circuit involving a resistor, a battery and the DMM to read voltages is a far better way to test leds.
The leds don't light up on the DMM 'cause the current flowing in them is too low to be able to produce visible light. Maybe in the dark you could see a little dot of light. The photo flash unit 's circuit just provide more current.
For the odd "bipolar" leds I just don't know...