BOSOZ's PSU runs very very hot

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R

ruangrit

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Joined 2003
The PSU seems to work fine with no connection to the Pre circuit.

Once I connected the PSU to the Pre circuit (no signal fed yet), the FET Q101 and Q102 started to run very very hot (could not touch after 5-6 minutes).

On the Pre circuit everything seemed to be OK, all FETs were warm, I got voltage at G, D, S accourding to the document (approx. +31 at Drain pin and 3.X at Source pin).

I have also noticed that the PSU voltage dropped to +58.XV (did not measure the -voltage as I had to switch the circuit off).

By the way, there are a couple of changes I have made (1) to the zener pack to obtain +-60V (2) 2200uF/100V and 100uF/100V instead of 1000uF and 10uF accordingly.

The transformers are 2 x 35-0-35/600VA but I do not think this will produce any problem.

Any thoughts on this heat problem...??
 
R

ruangrit

Member
Joined 2003
You are right, I got 111V at the BR, although the PSU has to drop >40V I thought this is what each zener pack does though.

Correctme if I'm wrong.

By the way, I have tried with a bigger heatsink (considered big comparing to the size of the PSU.

Not sure if those who have posted their bosoz pictures with small heatsink would experience the PSU heat problem as such or not.

Thanks,
RR.
 
eLarson

eLarson

Member
Joined 2002
The zener diodes will establish the Gate voltage, with the Source voltage being about 4 volts less than that. The Drain, though, is attached to the unregulated voltage (about 111, I think you said.)

So, yes, you will probably want to some hefty heatsinking to prolong the life of the MOSFETs.

Erik
 
R

ruangrit

Member
Joined 2003
You are right Erik, I've really overlooked this...I better drop off the Drain voltage in stead of trying to adjust the heatsink size...

Any suggestion? if I maintain the 2 x 35-0-35 transformer.

One quick way is to replace the transformer to 2 x 24-0-24 where I can easily find "ready to use" product locally. This will supply Drain voltage less than 70V...However, I should be able to maintain the output at +-60V...

Thanks,
RR.
 
eLarson

eLarson

Member
Joined 2002
I'd probably just intersperse some resistors among
your filter caps to drop the raw supply down some.

You could throw in a choke, too, if you're so inclined.
As long as the total resistance (including that of the
choke) is more than

2* sqrt ( L / C) where L is in henries and C is in Farads,

then you wouldn't have to worry about any nasty
peaks.

Erik
 
jh6you

jh6you

Disabled Account
Joined 2002
ruangrit said:
One quick way is to replace the transformer to 2 x 24-0-24 where I can easily find "ready to use" product locally. This will supply Drain voltage less than 70V...However, I should be able to maintain the output at +-60V...
Click to expand...
With 2 x 24V transformer, I would try to get +/-50V using 6 x 9.1V zeners.

JH
 
J

JDeV

Member
Joined 2002
On my XBoSoZ, using same supply as for BoSoZ, ac onto the rectifiers are 65v and after rectifier 80vdc. My FETs was 90deg C on the tabs while fitted to heatsinks. I now removed them and fitted them onto a 6x60x230mm aluminium plate. They are now 47deg C on the tabs, nice and cool.
 
R

ruangrit

Member
Joined 2003
I've been trying to adjust R as what Erik said, so far haven't found an appropriate one yet.

Unfortunately, I have blown up few resistors alaready, yet, with my rusty electronic skill.

Anyone has any suggestion on the calculation (the transformers used in the circuit are 30-0-30V, 30VA each).

To Jadev, thanks for the sise of the heatsink, I could not find similar size yet, will keep searching.
 
eLarson

eLarson

Member
Joined 2002
ruangrit said:
I've been trying to adjust R as what Erik said, so far haven't found an appropriate one yet.

Unfortunately, I have blown up few resistors alaready, yet, with my rusty electronic skill.
Click to expand...

Uh oh. I'd hate to see you toast any more resistors
in your quest.

Here are the equations you need to bear in mind.
In all of these I is the total bias current you are
drawing in amps, R is the value of the resistor,
V is the number of volts you are trying to drop
from your current value, and P is the power that
the resistor will need to dissipate in watts.

(If memory serves, your initial voltage was around
110 volts and you wanted to go down to about 80
or so. So V would be 30 in that case.)

Since Ohm's Law states V = I * R, you can get your
R value from rearrangement:

(1) R = V / I

Pick the closest standard resistor value you have
available. It doesn't have to be very exact.

The other thing you need to be aware of is the
power that the resistor will need to dissipate.
(Otherwise ::pOP:: as you've seen.)

(2) P = I * I * R

So the more current your trying to draw from your
supply, the lower the voltage will go and the more
power the resistor(s) will need to dissipate.

Pick a resistor that can dissipate more than what
you get in (2) and you shouldn't blow any more up
barring a short downstream drawing more current
than normal.

Erik
 
eLarson

eLarson

Member
Joined 2002
grataku said:
I seem to have read 111V at the rail with capacitors
rated at 100V... may I suggest you use a cap with
higher voltage rating, too?
Click to expand...

Whoops! A very good point. I think that part slipped
by in my quest to protect the noble transistor. :)

(Noble? Certainly! So many times have I seen a
multi dollar transistor give itself up to save the life of
a few cent fuse. A touching display of selflessness.
Once I get over all the profanity over having shorted
the output, anyway. *grin*)

Over voltage is a good way to let out the proverbial
magic smoke.

eL
 
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