Need help with RV7/7880 driver

[geek4christ]

I have an itch to get into modding and thought I would start out easy. I am attempting to do the popular RC-G2 + Q5 + 7880 driver mod. I have all of the necessary parts and have begun the process, but somehow I unintentionally got the positive wire to break off the board.

I looked at the pictures of that board on DX and it's not clear exactly where it goes. It seems like it may be soldered to one of the IC legs, but the inductor covers it up too much in the picture for me to be sure. Besides, I'd really like to know how to find this kind of information out for myself.

So how can I use my DMM to figure out where to re-solder that red wire to the board? I initially tried to use the continuity setting. I touched the neg. multimeter lead to the neg. contact on the board and then touched various other contacts on the board, hoping for a reading but got none. My next thought was to hook it up to a AA battery and put the neg. lead of the multimeter onto the neg. contact on the board, then probe around with the pos. lead until I get a voltage reading. But the more I think about it the more I fear that I could cause a short and completely destroy the board.

I know it's a cheap board and I could just buy another, but this is also something I'm using as a learning experience. So I'd like to know how to find it out for myself for future mods I may attempt. Any advice is much appreciated.

 

[Al Combs]

If you go onto any onto any big name IC mfg's site, do a search for switching regulators. AFAIK (read not expert), only switching regulators use inductors to smooth the output. The point of that would be to look at a few recommended schematics to see where in the circuit the inductor goes. National had a good site on LED constant current regulators that was very instructive to me.

Anyway the inductor usually gets wired into the chip output either directly between the chip output and the load, or to the sense resistor used to create the voltage reference for the constant current output. Most cheap regulators don't have protected outputs so you can't just apply power to see where it comes out. That would probably smoke it.

Do you have a good loop? Can you see a dimple in one of the solder joints that looks like it might be the remains of the original wire site?

Good luck

 

[geek4christ]

Al Combs,

Thanks for taking the time to respond so thoroughly. So a multimeter isn't much help in my case and my best resource is a good knowlege of circuit design and preferrably a schematic as well?

I am completely green on electronics terms and concepts, and your post has given me a lot of things to read up on. I have a trusty copy of the ARRL Handbook for Radio Communications on loan from my library that I will now have a good set of keywords to base my jumping off point from.

In the meantime, your question about the dimple prompted me to grab a magnifying glass and look over the driver more closely. Lo and behold, I did find the tiniest hint of a wire sticking up through one of the solder joints. I soldered the wire back to that joint, and rigged up a crude test circuit with battery, driver, and Cree star. It turned out to be the right connection!

I've now got the driver and star mounted and potted with thermal epoxy and I'm 30 minutes into the initial run to ensure there is a good thermal pathway to the body of the light. Draw from the battery is about 2 amps and brightness is on par (maybe even brighter) than my D10. I'm so excited that I got this pulled off! Thanks again for your post, it was extremely helpful.

 

[StefanFS]

That should do it I think. Just lift the inductor and solder it to the blob between the little capacitor and the other components behind the microcontroller.

 

[Al Combs]

Lo and behold, I did find the tiniest hint of a wire sticking up through one of the solder joints. I soldered the wire back to that joint, and rigged up a crude test circuit with battery, driver, and Cree star. It turned out to be the right connection!

Glad to hear you got it going.

This is all pretty much new to me as well. After you see enough terms mentioned in different ways, some of the pieces start to come together. I found that National link I was thinking of. About half way down the page, there are a series of four articles called, "LED Driver Articles."

 

[geek4christ]

That should do it I think. Just lift the inductor and solder it to the blob between the little capacitor and the other components behind the microcontroller.

Stefan, thanks for taking that shot. I happened to snap a picture of mine before I got it fixed and potted into the light. It seems the physical layout of my board is different than yours.

My pos. wire connection wasn't located under the coil like yours is. Also, it seems the inductor coils on yours are heavier. The pictures of this board on DX are very similar to the one you have, though. Maybe newer versions are laid out differently? Either that or I connected mine up wrong (but it does seem to be working fine) :shrug:

EDIT to add: thanks for the link, Al.