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gionag 14th May 2008 09:50 AM

Wrong Voltage / current on BoZ... or Wrong Measuring ?
2 Attachment(s)
Hi guys,

i have a problem with my new born Boz.

I've use the irf520 instead of the irf610 and when i'm goind to measure voltages and currents there are somethings that not convince me:

1 ) the 1K 3W resistor (i've used a 1k 4W non-inductive type) ran very hot... is it normal ?
2) the current between Drain and GND is 55mA (instead of the 40mA...)
3) The Pot P102/202 is to adjust the gate voltage, isn't it ? i've adjusted voltage for 7.5Vdc.
4) Between Source and the Top of R108 100Ohm (i used a 110 one) there are only 4Vdc, instead of 14V.

FROM PSU i've 59.8V and, for tests, i don't use the Output POT of 5k, the output is direct connect to the exit RCA.

I double checked each wiring and they aren't wrong.
What i have to do ?
i attached the schematics and the PCB used.

gionag 14th May 2008 09:53 AM

2 Attachment(s)
The PCB :D

thanks for the supports, you are fundamental for me :angel:

AndrewT 14th May 2008 09:57 AM

the load resistor has 40V across it. It will run hot. Do the arithmetic.

The output offset voltage is specified as 20Vdc.
Use the offset adjustment pot to achieve the specified output offset voltage.

Look again at the voltage on the 100r resistor. It really is ~4V. (~10% of the voltage across the 1k load resistor.) Again do the arithmetic.

gionag 14th May 2008 10:07 AM

excuse my n00bing :cannotbe:

i've already thinked about the voltage dropping on the 1k resistor but i can't imagine that it can be ran so hot !

about trimmer... i've understand that is for the input voltage... not for the output.

where i have to measure the output voltage ?

gionag 14th May 2008 10:08 AM

ops... excuse me...
obviously i have to measure output offset on the Drain pin.

gionag 14th May 2008 10:38 AM

and ... for the fact of the current ? is it normal 50 ma ?

if i do (bad) arithmetics for a 40mA of Drain current i have to put a 1.5K resistor instead of 1K :

R = V/I : R = 60/0.04 = 1500ohm

with a 1K res : I = V/R = 60 / 1000 = 0.06

but i'm sure that i'm wrong. :D

AND WHAT ABOUT MOSFET MATCHING ? is suggested to do ? for which current ?

thanks a lot

Magura 14th May 2008 10:59 AM


Originally posted by gionag

AND WHAT ABOUT MOSFET MATCHING ? is suggested to do ? for which current ?

thanks a lot

No need for matching.

See my mail for further explanation.

Magurs :)

AndrewT 14th May 2008 11:11 AM

If you use the offset adjusting pot to set the output offset voltage to 20Vdc then the voltage drop across the 1k0 load resistor is 40Vdc. This ensures the correct 40mA through the output stage. Job done.

If you change to output device to another type/make then the optimal operating current may be different.

gionag 14th May 2008 11:24 AM

thanks a lot... in fact the current now is fixed at 40mA.

thanks for the hint... i misunderstood the job of P201.


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