So close (but still no -12v!)

**phil.45** · 08-02-2002, 08:09 AM

I only need -12v to run my serial ports and my system is finished. I built the Maxim 775 circuit, here, and when I tested it only gives -2.9v output with absolutly no current available at all. As soon as a load is placed on it it drops to around -0.8v.

So I built it again with all new components, still the same identical voltages, and built it a third time, again same voltages!!! Does anyone have any idea what is going on here? Anyone successfully built this and had it working? I have spent the last 10 hours going over the circuit, its absolutly as in the schematic, I can only assume that the pinouts for the 775 are wrong.

Help! I only want -12v!! Is there another way I can get this quickly?

**MikeHunt79** · 08-02-2002, 08:50 AM

check the 9435 chip.... the http://www.jarcom.com/inmotion/power.htm page has more details on ohw to wire it up. I have had the circuit working, btw, but i then went on to blow up the 9435 chip by using a 7805 cihp to -5v aswell.... I`m sure it would work again if I replaced the little 9435.

**phil.45** · 08-02-2002, 08:56 AM

Hmm, I am suspicous of the 9435, but I have replaced it 3 times. Is that URL you listed working, I get page cannot be displayed, dont want to move from my desk to reboot the DNS server unnessasarily.

**phil.45** · 08-02-2002, 08:58 AM

Also, is there a better (read: bigger) version of that chip. I can only find surface mount versions, and there a pain to solder.

**mp3z24** · 08-02-2002, 09:24 AM

what are you using for your input voltage?...
are you using feedback?

i would be using the circuit shown in figure 2 of page 10. the bootstrapped version of the circuit. if that is the one you are using, then you need to take input off the 5V line.

also, the chip can be obtained in a DIP size.
try part numbers
max775cpa
max775epa
max775mja

they are all DIP size, which basically means they are sized like "normal" circuit board chips with the pins extending through the circuit board. the first 2 are plastic body (different operating temperature range) and the 3rd one is a ceramic body if i were you, i would get the epa or the mja models, they have huge temperature ranges...

and, no i havent built this circuit before....
~Mike

**phil.45** · 08-02-2002, 09:36 AM

I am/was using the bootstrapped schematic.

Found this MAX765 which has a lot less potential to go wrong. I will give that a go if I can get one.

**MikeHunt79** · 08-02-2002, 09:43 AM

here, try this link . I pulled the page from my bookmarks, and i didn`t try it (it`s down now, btw).

There is an equivelent called the Si9435, but the only difference is that it`s made by a different company

.

**mp3z24** · 08-02-2002, 10:20 AM

what are you using for the input voltage... skimming quickly through the datasheet i saw somehting that said there was a limit to the difference between input and output voltages.

try using 5V input, if you havent been doing that.

~Mike

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