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Dead M140?

Benm

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Well, if you are building the dummy load from scratch i'd go for 1n 540x diodes as they can handle 3 amps and are still very cheap.

Some designs have been proposed with more powerful diodes as well, but they require to be mounted to a heatsink - not really a problem, but if you plan on staying at the 'around 1 amp' range probably overkill.

There is also the resistor. If you want it at 1 ohm, you could just put 10 standard metal film 10 ohm resistors in paralel. This would be fine up to 2 amps continous operation. You could also use 20 pcs of 20 ohm resistors in there, making it reliable up to (almost) 3 amps, same limit as the diode.

Actually putting a load of resistors in paralel has advantages: they are often cheaper than high power resistors even if you need 10 or 20 of them, and they have low inductance. Cheap power resistors are often wire-wound which work fine for DC, but present a pretty inductive load to the driver under test.

There are low-inductance high-power resistors too, with heatsink mounting, but those only make sense when you want a dummy load that handles currents of 5 amps or so, where the diodes would also require external heatsinking.
 

paul1598419

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Last time I bought rectifiers, I got 50, 10 amp 1000 VPRV for under $3.00. I also got my resistors in 1% tolerance at equally cheap prices. They aren't wire wound. I can only imagine the cluster f@#k look of 20, 20 ohm resistors soldered together in parallel. I suppose it would dissipate heat better, but seems like overkill.
 

WizardG

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" I can only imagine the cluster f@#k look of 20, 20 ohm resistors soldered together in parallel."


Well, that would depend on the skill of the one soldering them together. :rolleyes:
 

IVO

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Hi!

There it is...
Dummy Load.jpg

Dummy Load Test.jpg

And it was set to 280mA. Measuring in series with the multimer it reads 279mA. I think It's within the margin of error.
The resistors are x10 10R 1W resistors, so the should hold up well at the max 3A that the diodes can resist.
 
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paul1598419

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Looks good. If you have a meter that can read low resistance, it wouldn't hurt to confirm your 10, 10 ohm resistors in parallel are in fact 1 ohm. You will want to limit the time you use this dummy load for drivers above 1.5 amps as the rectifiers will overheat. Otherwise, it should work well for drivers up to 2.5 amps. Maybe a little more.
 

IVO

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Looks good. If you have a meter that can read low resistance, it wouldn't hurt to confirm your 10, 10 ohm resistors in parallel are in fact 1 ohm. You will want to limit the time you use this dummy load for drivers above 1.5 amps as the rectifiers will overheat. Otherwise, it should work well for drivers up to 2.5 amps. Maybe a little more.
My multimeter shows 0.9ohm, but i don't know how good it is for measuring low resistance.
 

paul1598419

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My multimeter shows 0.9ohm, but i don't know how good it is for measuring low resistance.

What does it read when you short the leads? If it is above 0.0 ohms you might have a problem. I have an ESR meter for capacitors that can measure down to mohms. Maybe you do too?
 
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IVO

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When I short the leads of the multimeter It reads 0.0 Ohms.
I don't have an ESR meter.
 
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paul1598419

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Hard to say. Do you have any 1 ohm resistors to measure to see what they read? Or even a 2.2 ohm. Something else to test against?
 

IVO

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I have 1.8 Ohm resistors. Ok, it now reads 1.0 Ohm, but it sometimes goes up to 1.1, or down to 0.9. I think it's just not soo good for low seristances.
 
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paul1598419

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You can only go by what you have to work with. I'd check some 1.8 ohm resistors just to see.
 

IVO

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It sometimes reads 1.8, but sometimes lower, like 1.6. I've never calibrated my multimeter, so it might be reading lower than It should.
 

Benm

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Soldering many resistors in paralel is not that uncommon really, it was a pretty common practice to create low-inductance dummy loads to test radio transmitters in (20 1k resistors make a neat little 50 ohm dummy load, soldering right onto the connector working fine up to 100s of MHz at least).

To measure the resistance: using your multimeter directly is not realiable due to resistance in the leads and all that. If you have a constant current lab supply, just set that to 1 amp and measure the voltage. Or measure the current using the multimeters 20A range and then the voltage across.

Essentially you're doing a four point measurement without the specific meter to do that - if you have one of those you'll not need to read this ;)
 

paul1598419

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It sometimes reads 1.8, but sometimes lower, like 1.6. I've never calibrated my multimeter, so it might be reading lower than It should.
It's hard to say for sure, but it could be that you aren't making a good connection every time you are measuring resistance. The leads shouldn't make that much difference as they should be stranded wire in a fairly heavy gauge. But, the connect you make with the probes can make a difference in small resistances. The quality of the meter will make a difference as well. You can use a constant current, constant voltage power supply to test the dummy load by setting it for 1 amp and figure the amount of voltage drop across your diodes and resistor. Set the output voltage for the amount you calculated. Then measure the drop across the paralleled resistors to see if you get 1000 mV.
 

IVO

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Hello, guys!

I have good news. The new laser arrived this Friday... and... It works great!
Here it is:
laser diode.jpg

To test it, I used the same driver, but I added two caps near the diode. I followed the steps: 1º set the current to 1.4A, 2º test with the dummy load, 3º Disconnect driver and short output to discharge caps, 4º connect laser diode, 5º power up the driver.

The setup:
setup.jpg

Working:
working2.jpg

A closeup:
closeup.jpg

Beam shot:
beam.jpg

I'm very happy it worked this time... Thank you very much for all the help.
Now I'll keep on making the rest of the laser.
By the way... this is the switch mode 350mA driver that I'm working on right now. It's designed for red lasers from DVD and the current is adjustable up to 350mA.
350mA driver.jpg
I still have to test it, but when I have something working I'll open a thread.

Thank you all again!
 

Benm

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Looks like a nice beam and heatsink to me :)

Putting in in a nice host with a driver will be the next challenge, but i suppose you'll get there.

As far as using multimeters on low value resistors go: give up. If i just grab one of my multimeters here at random in resistive measurement mode, shorting the leads to eachother already gives me a value of 0.5 to 1 ohm, depending on which leads i use and how hard i push their tips onto eachother.

If you want to measure resistances in the order of a few ohms or less, a simple DVM will not perform well. You get all kinds of problems with contact and lead resistances, you need a 4 point measurement at this point.

Upside is that most resistors are actually pretty close to their indicated value. Ones with 5% tolerance often are within 1% of the marked value, unless you get a real crappy batch. At the point where you get under a few ohms i'd rather go on the indicated value than on what a simpe dvm displays... or measure it the right way if critical for the application.
 




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