mastech PS killed my cree

[VegasF6]

Well what happened here. I have a test led setup on a CPU heatsink, a cree q5. Finally got a Mastech hy3010D to play with.

For those of you familiar with it's operation it uses relays for switching being the linear version.

I followed the instructions for restricted current protection mode.

They state:
"For restricted current protection mode switch on the power on/off switch 7, adjust controls 3 & 4 anti-clockwise to the miniumum position, adjust controls 5 & 6 to set the desired output voltage level and then connect the load to the output terminals 8 & 10. Adjust controls 3 & 4 clockwise to set the output current at the desired level for restricted current protection."

3 & 4 are the fine and coarse current controls and 5 & 6 are the fine and coarse voltage controls.

So, that is what I did. Prior to connecting the load the power supply indicates constant voltage mode. Once you connect your load it switches to constant current. I think what happened is that at about 3.6 to 3.7 volts you hear a relay click internally and when that happened it allowed a higher voltage spike out of the unit.

Of course, it happened so fast I am not entirely sure what happened. I never did see any shift to blue of the led or anything, and I didn't even seem to get all that bright, it just stopped working.

Now that I switched to an old luxeon led for testing of course wouldn't you know it everything is working fine. Just user error? Is it normal to potentially have a spike when the relay switches? If so I am not quite sure how to avoid this in the future.

 

[jtr1962]

Ugh. Why on earth are they using relays? I made a constant voltage/constant current power supply which does everything electronically. And those instructions aren't really the best way for setting up the knobs. Best bet is to first set the current level to whatever you want by using a non-critical load like a large resistor. After doing this, set the voltage very low, well under what the LED needs to draw the desired amount of current, better yet to zero or as low as it'll go. Connect the LED, then gradually turn up the voltage until you see the current reach the value previously set. Stop there, are if you like go a bit further just in case the LED's Vf rises slightly. If the supply is in constant current mode the current shouldn't rise any more no matter how high you turn the knobs.

To me it appears that when you first connected the LED and the supply was in constant voltage mode you may have gotten a current spike from the power supply's filter capacitors as they were probably charged to a far higher voltage than the LED can take. In general the best way to test LEDs is to use a pure constant current circuit. The ones I make don't even use or need caps on the output. It doesn't matter if you connect the LED even if the output is 12V as there is no stored energy in an output cap.

 

[rantanplan]

My cheap (=chinese noname) linear bench power supply (0-30V,0-5A) uses relays too ... and it works fine with LEDs. I guess the relays are used to tap into different transformation ratios of the transformer. Maybe that better power supplys use solid state relays for switching, but I doubt that you can build a linear power supply with a wide output voltage range without it ...

I usually do the following: Dial the voltage down to max 3V, dial down the current control to zero, plug in a cable to short the PS output and adjust the current flow to e.g. 350 mA, remove the short, connect the LEDs and adjust the voltage up till the set current limit is reached. That procedure has worked fine for me so far ...

Guess I´ll have to hook up my oscilloscope and check how "smooth" the transistion between the "clicking gears" is ...

 

[VegasF6]

I was a little surprised by the switching relays myself. Perhaps if I understood how to build a power supply I wouldn't have been. I am thinking because I chose the D series, the linear version that uses transformers that is why. If I had bought the E series, which are solid state power supplies the switching is all electronics. I chose the D because supposedly it is better for testing electronics. Cleaner power with less ripple to worry about.

How critical would the resistor choice be for setting up the power supply before connecting my leds? I have a stack of 2W 22 ohm resistors here, should I just parallel 2 or 4 or so of them?

I am a bit math challenged so I have a terrible time with formulaes for figuring this stuff without an online calculator.

 

[VegasF6]

...

I usually do the following: Dial the voltage down to max 3V, dial down the current control to zero, plug in a cable to short the PS output and adjust the current flow to e.g. 350 mA, remove the short, connect the LEDs and adjust the voltage up till the set current limit is reached. That procedure has worked fine for me so far ...

Rantanplan, I just followed that procedure, except instead of a dead short I substitued 3 parallel resistors ( the 2W 22 ohms I mentioned above). That seemed to work great. I couldn't bring myself to short the outputs, it just seemed wrong. Is it safe to do that?

Prior to blowing the test Cree I had the voltage cranked to max but the current to zero. So, the caps must have been charged and that is what happened. Well it was only about a ~$6 dollar lesson and I will remember it.
Thanks!

 

[Mr Happy]

You can short the output safely (you need to do so in order to get the best result).

When working with any power supply, always start with the voltage at min and turn it up slowly.

So the procedure is:

1. set voltage at min and current at zero.
2. short the output with a wire
3. slowly turn up the current until the desired value is displayed
4. leave the current setting at that value and remove the wire
5. connect the device to be tested
6. slowly turn up the voltage until the device is working as desired, and no further

 

[jtr1962]

My cheap (=chinese noname) linear bench power supply (0-30V,0-5A) uses relays too ... and it works fine with LEDs. I guess the relays are used to tap into different transformation ratios of the transformer. Maybe that better power supplys use solid state relays for switching, but I doubt that you can build a linear power supply with a wide output voltage range without it ...

If it's a linear supply they probably do that to avoid using an overly large heat sink (the one on my home made supply is pretty large). But in any case it shouldn't cause voltage spikes. Anyway, the OP explained what he did (cranked the voltage to max), and that unfortunately caused the problem. And I've done the same myself on occasion (thankfully I only blew cheap 5mm LEDs, not Crees).

 

[VegasF6]

Yah, not the best instruction manual in the world, but then I guess they expect you to know what you are doing.

Procedure for constant current mode (not the restricted mode I listed earlier) states to turn the volt knobs to max, adjust the current knobs to minimum, turn on the PS, connect load then adjust current.

In my excitement I probably started this procedure, then skipped to the restricted one without connecting a load in between. I am not sure that is what I did, but it certainly could be.

I was also hoping for finer resolution with this thing to play with 5mm leds and such but this is more like trying to drive a finish nail with a 10 lb sledge hammer. Well, I still have a LM338 switch mode supply with fine resolution to play with those. I just don't like the interface on it.

All the advice I have read on here say to get the biggest power supply you can afford and then buy the next more expensive one than that

In reality, buying one power supply seems to be like buying one flashlight.... any here own one flashlight?

 

[Mr Happy]

Yes, choosing the right power supply is like choosing the right hand tools: there are small tools for doing fine detail work on jewelry and wristwatches, medium tools for crafts and hobbies, and large tools for building construction.

 

[VidPro]

my cheap china power supply 3003 came "loose" and went to a 30V constant one day while running some 3amp (its max) on it.
i still havent fixed it, but i suspect it will be fixable.
whatever was on it at the time went ballistic, i forget what all happened, but i just decommisioned it, suspecting that i will find some poor heat syncing inside it somewhere , and a de-soldered joint because of it.
i went out and bought the 3003D , because i didnt learn anything the first time and i treat it a little better, so far its still working.

they CAN punch out more total wattage at unexpected times, due to a delay, so if you had it set for some 8+v and that relay is switched and have the current controlled Way down, whatever you dump on it at that moment can take a few miliseconds to switch the relay back,, so i always watch out for the voltage AND the current, even if the current is being controlled.
you know when you got the current turned down AND something connected , the voltage doesnt move, but disconnect or burp the connections and it will snap up to the high voltage it was turned to, with the relay adding in a bit of delay to the return. which isnt much of a problem for the big stuff.

 

[Sean168]

Sorry for your loss, VegasF6. I am a technician with Mastech Power Supplies, and have written a tech notes about how to set current limit for Mastech Power Supplies, and you can see it at our website at www.mastechpowersupplies.com: http://www.mastechpowersupplies.com/howtosetculi.html

 

[VegasF6]

Thank you Sean, I just read all the tech notes, and sent request for a schematic as well.