0

28 Volt supply hot hot hot
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

28 Volt supply hot hot hot

by dawsrobe on Tue Jul 29, 2008 11:15 am

Hey:
I've tried to sort this one out... but I could use some input.
The LT1173 is putting out 52 V with an input of about 3.8 V at pin 2. Obviously getting toasty. As soon as I read the values, I shut it down.
I've tried to trouble shoot; continuity test is fine on pins and I can't find any shorts.
Any ideas?
By the way, kudos to inh... his updated parts list made life a lot easier.

dawsrobe
 
Posts: 7
Joined: Sun Apr 06, 2008 11:14 am
Location: London, Ontario

52v

by inh on Tue Jul 29, 2008 1:17 pm

Hi there - thanks!

I think that you should check what resistances the LT1173 is seeing across its pins - output voltage is set by the ratio of the 2 resistors, so if one resistor is "out" it will try to regulate to an incorrect voltage.

The actual places to check are the resistances between pin 8 and GND and pin 8 and the cathode (stripe end) of D3. Shld be 50k and 1M - though could be lower due to other components on 24v rail... the main thing is that they are not huge resistances, indicating a dry joint or bust component.

I just checked the datasheet - around 50v is the max this device can do - which implies one resistor is not connected...
hope this helps.
thanks
Ian
inh
 
Posts: 102
Joined: Mon Aug 27, 2007 6:49 am
Location: london

It's a help

by dawsrobe on Tue Jul 29, 2008 1:28 pm

Hi Ian:
I've made up two boards so far, both with identically the same problem. I used 50k and 1M on the one board (as per Ladyada) and 39k and 820k (what rys used) on the other.
Thanks for the trouble shooting ideas, especially that resistors aren't connected. I'll look at that now. Your thoughts are welcome and appreciated.
Dawson

dawsrobe
 
Posts: 7
Joined: Sun Apr 06, 2008 11:14 am
Location: London, Ontario

Checked the voltage divider

by dawsrobe on Tue Jul 29, 2008 2:15 pm

Well, I took Ian's advice and checked the voltage divider:
R6 to ground is no problem, but I can't get a reading on R7. As soon as I put a probe on it, it either climbs/falls, but settles in around 130k. Funny thing is it's happening on both boards (one with 1M and the other with 820k).[/img]

dawsrobe
 
Posts: 7
Joined: Sun Apr 06, 2008 11:14 am
Location: London, Ontario

pic

by inh on Fri Aug 01, 2008 4:29 am

try posting a picture of top and bottom...
thanks
Ian
inh
 
Posts: 102
Joined: Mon Aug 27, 2007 6:49 am
Location: london

Oopsy

by dawsrobe on Fri Aug 08, 2008 8:58 am

I am such an idiot!
I'm certain that I ordered LT1173's from Linear as samples. It would appear that I received LT1173-12's instead. I never thought to check (I'm usually thorough on the ordering end). The variation of this chip has an internal voltage divider, so I guess I gave it a bit of a nervous breakdown when I connected another one to it.
I've got more on order... just waiting for them to arrive.
I still have to give Ian credit for this one; he gave me the idea when he said the chip was putting out the maximum output, and it was like the divider couldn't connect to ground. I checked the flowcharts and then saw the variations in chip design.
Dawson

dawsrobe
 
Posts: 7
Joined: Sun Apr 06, 2008 11:14 am
Location: London, Ontario

Re: 28 Volt supply hot hot hot

by banditknight on Sun Feb 07, 2010 11:04 pm

is it possible to use LT1173-5 and modify the circuit to get 24v output?
banditknight
 
Posts: 2
Joined: Sat Jul 04, 2009 3:03 am

Re: 28 Volt supply hot hot hot

by c.b. on Mon Feb 08, 2010 3:57 pm

I had exactly the same problem :-) Didn't care about the last few digits of the part number and got a LT1173-12, too. Changed R6 to 120k.
c.b.
 
Posts: 15
Joined: Thu Oct 15, 2009 3:39 pm

Re: 28 Volt supply hot hot hot

by banditknight on Tue Feb 16, 2010 7:29 am

@c.b., I've tried your suggestion (change R6 to 120k) and using LT1173-5, but it's still get hot.

With LT1173-5 for IC4, 4.7M for R7 and remove R6, I can get 26v .
and it's still get hot sometime.
did you experience the same? did you use LT1173-12 for IC4?
banditknight
 
Posts: 2
Joined: Sat Jul 04, 2009 3:03 am

Re: 28 Volt supply hot hot hot

by c.b. on Wed Feb 17, 2010 3:26 am

Hi Banditknight,

the LT1173-5 regulates its output in such a way, that pin 8 (feedback/sense) is 5V. Try R7=180k, R6=47k. That would cut down your 24V output to 47 / ( 180 + 47 ) * 24V = 5V.
This also applies to LT1173-12, only that the feedback voltage is regulated to 12V. I use a LT1173-12 and my resistors are R6 = R7 = 120k: 120 / ( 120 + 120 ) * 24V = 12V.

I didn't mention R7 in my previous post. Sorry for that, hope nobody fried something. I had adjusted R7 with a 1M resistor on top to make the output exactly 24V. So I didn't notice the modified value.
c.b.
 
Posts: 15
Joined: Thu Oct 15, 2009 3:39 pm

Please be positive and constructive with your questions and comments.