Aleph current limiter question

Mad_K

Member
2002-09-12 11:14 am
Norway
Hi!

I am in the progress of scouting out parts for an Aleph 3 with 2 output FETS per channel (2A bias), and I think I have figured out all the new values, except for R110 (Aleph 3 Scematic). From searching the forum and studying the manuals, I think it should be about 200R (thinking of 221R). I can't figure out how this little critter works, and therefore I am unable to calculate the value.

-Is there anyone here who can help me out a little?

BTW I am going to use 0,25R source resistors and BC550C's. Does anybody know if I should change any other parts of the circuit? (R114 for example?)

Any comments/suggestions/answers would be greatly appreciated.
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Q104 starts conducting when its Base to Emitter voltage
approaches .6 volts, so you set up R109 and 110 to
divide the voltage across the .47 ohm resistor, setting
the limit. On the Aleph 3, the mimimum limit would be
about 2 amps per Mosfet, but the divider has been set
at about 6 or so, so that you are unlikely to activate
it into an ordinary load.
 

Mad_K

Member
2002-09-12 11:14 am
Norway
-Really? No one knows?

-Maybe I should try to be a little clearer (Aleph 3 schematic):

I understand that R109, 110 , 122 and Q104 creates a current limiter for the output, and that this current is set by the R109/R110 ratio. (only R110 is changed)

What I don't understand is how to calculate the max output current.

Since I am only going to use 1 FET on the output (and 1 on the ACS), I am going to give this fet double the current of the original to keep the original bias setting. Since the original setting of the current limiter is set to limit all the paralell output FETS, based on the current going through the first, I need to double this limit to keep the original limit.

I am wondering if doubling the value of R110 achieves this?

Even better, -how to calculate this?
 

Mad_K

Member
2002-09-12 11:14 am
Norway
- So It Shall Be -

Thank you Nelson.


:eek:

i.e: Original Aleph 3:

Imax=2*(0,6/(R122*(R110/(R110+R109))))
=2*(0,6/(0,47*(100/(100+221))))=8A

(also what it says in the manual)

My Aleph:

Imax=0,6/(0,25*(100/321))=7,7A

Hope this is correct, and may also help some fellow diy'ers who also has his head in the sand sometimes.