Modifying a BB700 for variable output

[Mednanu]

So I ordered a BB750, but only received a BB700 - not a big deal, as I figure they are the same boards and only have a slightly different resistor which is telling the chip to put out 700 ma instead of 750ma. I see two resistors right next to each other on the board - does anyone know which of these resistors is telling the chip what ma value to output ?

I want to hook up a pot for variable output and need to know which of these resistors to remove and replace with connections to my pot. Normally I would just start tinkering until I got the right mix, but BB boards are very particular about inputs, outputs, and load. They tend to fry themselves if you get one of these variables too far out of balance (hence no tinkering having been done yet).

Also, given stellar heatsinking on both the BB board and my 5Watter, how far can a BB700 be pushed over spec ? I was wondering if I might be able to push it as high as 900 ma with a silver/diamond heatsink setup. For those modders who may have already tried this, please clue me in on whether or not this has worked for you.

 

[dat2zip]

The resistors are very small in resistance and I would doubt you will find any standard trim pot that is precision control and under 0.5 ohms or resistance. On top of that you will need to take into the wiring distance as each inch of wire will affect the resistance.

There is no way to adjust this except by replacing the SMT resistors with others similar to that.


Just FYI as you probably already know this. But, the BB is a boost only, so for a 5W, the max Vin needs to be less than Vf which should be less than 6.5V or so.

The reason I mention this is the BB can only put out a limited amount of power and as the Battery voltage goes down the current goes up.

Assume with the BB700 the min working voltage is around 4.5V. If you cranked it up to 1A and made a BB1000 for example the min working voltage would probably be 5.5V or so. With fresh 123s, you'd get a short run time before the regulator fell out of regulation.

Wayne

 

[Jonathan]

Wayne,

I presume that the 'proper' way to make this boost converter adjustable is to apply a variable 0 to 1.5V to the 'adj' pin on the boost converter chip. How easy is it to get at the pin on the BB700?

-Jon

 

[dat2zip]

The Adj pin is easy to get to if you can work fine pitch SMT and lift the IC leg. You would then need to attach a small wire like 30G wire wrap wire and hook that up to your pot.

I'd say without a good microscope it's would be rather difficult for the average user.

It would be much better if I offered the adjustable eval board that I had at one time.

Maybe I should have another batch of these boards built.

 

[Mednanu]

Thanks so much for all of the good info Wayne ! I might try fiddling with the setup on the adjust pin as mentioned by Jonathan. While I've got a thousandth of an inch solder tip on my iron, I'll definitely need to get a much better magnifying glass before I could pull the job off. I also really liked the idea of running dual BB500's in parallel listed within this post . I had thought of the notion, but hadn't believed that the two boards would be able to coexist with each other without encountering functional difficulties. I might just try out a few different power module setups and see which one I like the best (ie - dual BB500's or a single variable BB700). Either way - many thanks for the info and your response. /ubbthreads/images/graemlins/smile.gif

 

[paulr]

Wayne, can I use a BB700 or 750 with Vin=2.5 volts or so? The idea is to run a Luxeon III (in a 1x123 McLux) when the Luxeon III becomes available, so Vout would presumably be in the 3.5 to 4 volt range. I like this better than trying to run a BB400 to the rails with a 5 watt Luxeon.

 

[AilSnail]

[ QUOTE ]
The Adj pin is easy to get to if you can work fine pitch SMT and lift the IC leg. You would then need to attach a small wire like 30G wire wrap wire and hook that up to your pot.

I'd say without a good microscope it's would be rather difficult for the average user.

It would be much better if I offered the adjustable eval board that I had at one time.

Maybe I should have another batch of these boards built.


[/ QUOTE ]

I don't know what these were like, but perhaps you could modify your non-adjustable BBs so that the ADJ pin is accessible without microscope? Then we could use external dimming by whatever means we want (twist-head, magnet switches etc.)

 

[AilSnail]

Is that Iadj pin soldered to ground on the current boards? It doesn't look like it is.

Sig-