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Testing LEDS w/ Cen-Tech 98674

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TT_Vert

New Member
I know many don't like cheap meters but I'm trying to figure out if my LED testing issue is user error. I've set the tester to diodes and ensured correct polarity of the LED across the meter leads but I do not get anything out of the LED (Known good). Looking at the manual for this meter it does not mention a forward voltage only the below info. I do not have another meter to see if there is any voltage/current out of the leads with the meter in this mode. The meter is powered by 3 AAA batteries.
3.2.13 Diode Test
Resolution 1mV
displaying approximate forward voltage of diode
Forward DC current ~1mA -
Reversed DC voltage ~1.5V
Thanks much

Dave
 

ericgibbs

Well-Known Member
Most Helpful Member
hi TT,
Most LED's have a forward voltage drop greater than the 1.5V specification of the meter.

E
 

TT_Vert

New Member
Thanks eric, but nowhere does it mention forward voltage or should I be able to deduce that from the specs I left (I'm a newbie)?

Dave
 

TT_Vert

New Member
I may have worded my question poorly there. I know what the forward voltage of my LEDS is and none are lower than 1.5 V. So is the reverse voltage in the specs I listed above what is applied to the LED during diode tests since you mentioned 1.5V? Thanks for that image though I'll definately save that.
 

ericgibbs

Well-Known Member
Most Helpful Member
OK,
Your meter Diode spec states Reversed DC voltage ~1.5V
This means the the actual voltage on the meter probes is 1.5v
So it is too low to forward bias an LED, so it cannot test an LED.
E
 

TT_Vert

New Member
Ahh ok thank you. So in this case reversed DC voltage is applied voltage to the LED via the probes? I'll have to get another meter to test this, probably one that illuminate a damn LED LOL. So if the spec was >2.0V I could at least light my yellow LEDS. It's sort of annoying that this thing cannot light/test an LED.
Thanks for the very fast replies.
Dave
 

ericgibbs

Well-Known Member
Most Helpful Member
I would use a 9V PP3 and say a 1K series resistor, use the meter on volts DC.

EDIT:
if you have 5Vdc supply use that with say a 220R
 

TT_Vert

New Member
Yeah I can certainly test the LEDS other ways, I was just wondering if I was doing something wrong w/ this meter. I do have a 30V/5A adjustable DC supply so I can just do continuous current to prevent over current to them.
 

ericgibbs

Well-Known Member
Most Helpful Member
That would be OK, if you can set the LED current to say 15mA thru 20mA [typical 3mm LED 's]
Without the limit the LED will quickly die.!
E
 

TT_Vert

New Member
Yeah I never go above 20mA intentionally. They seem to be fully lit ~10mA. Can't say i've not torched a few LEDS in my early stages of experimenting w/ this new DC supply..

Dave
 

audioguru

Well-Known Member
Most Helpful Member
The LED meter uses a 4.5V battery and says its forward current is 1mA so obviously it forward biases the LED with 1mA and displays an accurate forward voltage, maybe up to almost 4.5V. It might also test the reverse leakage current with -1.5V but the manual should say what it does.
Doesn't it light an LED fairly dimly?

EDIT: The manual says that in the Diode Test if the leads have the correct polarity then it displays the forward voltage of the diode but I think it might not have enough voltage for some LEDs.
 
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Tony Stewart

Well-Known Member
Most Helpful Member
You can make a breadboard or box with dual row header and series R's and then measure the Vf forward drop to measure diode Voltage and VR drop to measure the current by calculating I=V/R for the same Vf measurement with only 1 resistor connected at a time or the LED in one position on the header socket at a time.

Since all LEDs are rated at -5V max, this may be safer than a 9V battery. Also only a few LEDs models are ESD protected so a static dissipative bench or 1M wrist strap ought to be used.

upload_2017-1-18_22-52-42.png
 

TT_Vert

New Member
I can easily provide the correct current/voltage to the LEDS for testing w/ my adjustable power supply I was just wondering if my meter was acting weird or it just doesn't test LEDS.

Dave
 

audioguru

Well-Known Member
Most Helpful Member
You do not have an "LED Meter", instead you have a cheap multimeter. Maybe it only tests ordinary diodes.
 

TT_Vert

New Member
Fair enough thanks guys. I'll have to look into a little bit better of a meter. I only really use it for minor DC work and learning basic circuits right now.
 

audioguru

Well-Known Member
Most Helpful Member
I have an expensive accurate Fluke multimeter. It can show the forward voltage of ordinary diodes and red, some green, orange and yellow LEDs but not white, blue and some green LEDs that have a higher voltage.
I found some bags of LED Christmas lights thrown away that had some LEDs with their wires rusted away. I tested and harvested hundreds of them. Most in a string have almost the same forward voltages and therefore were connected in parallel. That is how I am wiring them in my new LED projects.
 
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