Rising DC Offset

[pmchoong]

There's only one earth wire at the IEC and there are two connections to it, ie. the chassis and the PSU ground. Somewhere along the way, if the the PSU ground is connected to the thermistor, the chassis connection is bound to come into contact with the thermistor. I don't think there's a way to avoid this.

 

[pmchoong]

i measure 4.17v across R27 and I have 0.5 3w source resistors instead of 0.47 3w resistors. My question is : why are these source resistors measure at 0.6xx V?

 

[finwbu]

Because off your Multimeter measuring leads!

 

[finwbu]

check here ..

https://www.youtube.com/watch?v=m9YBOwVMos4

https://www.youtube.com/watch?v=InJhgwmj2So

Br Walter

 

[pmchoong]

According to the Aleph J build guide, the R27 should be 0.4mV but mine is measured at 4.17V!

 

[AndrewT]

post53 is wrong.
It shows the PE wire connected to a thermistor.

Connect the PE wire to Chassis.

 

[pmchoong]

Hi Andrew, Thanks but if I connect the Mains PE to the chassis then where do I connect the PSU ground to? Could you be kind enough to sketch out the diagram?

 

[pmchoong]

Does Q3 and Q4 needs to be matched?

 

[Zen Mod]

Hi Andrew, Thanks but if I connect the Mains PE to the chassis then where do I connect the PSU ground to? Could you be kind enough to sketch out the diagram?

safety gnd from IEC socket directly to chassis
audio ground (central point) connected to chassis via 10R to 15R NTC
say CL60

Does Q3 and Q4 needs to be matched?

no ; they're having completely different role

upper one is for programming current in Aleph CCS , lower one is peak/safety current limiter

 

[pmchoong]

Thanks ZM and AndrewT. I appreciate both of your help very much. Does the attached 'diagram5' looks right for the PE connection? Also, what is the likely problem for the big difference of voltage across the source resistor if the mosfets are matched (aj measure2)?

 

[Zen Mod]

Thanks ZM and AndrewT. I appreciate both of your help very much. Does the attached 'diagram5' looks right for the PE connection? Also, what is the likely problem for the big difference of voltage across the source resistor if the mosfets are matched (aj measure2)?

look (red !!) where is difference , comparing to your drawing

if source resistors are close enough in value (5% usual tolerance) , then mosfets aren't matched

you need to have two matched pairs - one pair going in upper half , second pair going in lower half

edit : it seems Q8 is bad neighbor

 

[pmchoong]

Thanks ZM. Now, if i add in an additional pair of mosfets, do these needs to be matched? Thanks.

 

[Zen Mod]

then you need to have matched triplets - one up , one down

 

[pmchoong]

Thanks again, Zen Mod.

 

[pmchoong]

During testing after swapping the mosfets, I accidentally touch an output resistor (between a source resistor and an output resistor) and damn. Now I have a source resistor giving a 3.0v reading (this is the extra mosfet that i add in, which is the R19A). The Z1 remains at 8.9 and R8 remains at 8.4v. R27 remains at 4.1v. The readings between Gate and Source of the mosfets are between 0.4 to 0.45v. I have checked the mosfets and they remain good and I have even replaced Q3 and Q4. How now?

 

[pmchoong]

look (red !!) where is difference , comparing to your drawing

if source resistors are close enough in value (5% usual tolerance) , then mosfets aren't matched

you need to have two matched pairs - one pair going in upper half , second pair going in lower half

edit : it seems Q8 is bad neighbor

When you said 'upper and lower half' you mean, Q5 and Q6 to be matched and then Q7 and Q8 to be matched, right? Q5 and Q6 are the current source while Q7 and Q8 are the output mosfets, correct?

 

[Zen Mod]

During testing after swapping the mosfets, I accidentally touch an output resistor (between a source resistor and an output resistor) and damn. Now I have a source resistor giving a 3.0v reading (this is the extra mosfet that i add in, which is the R19A). The Z1 remains at 8.9 and R8 remains at 8.4v. R27 remains at 4.1v. The readings between Gate and Source of the mosfets are between 0.4 to 0.45v. I have checked the mosfets and they remain good and I have even replaced Q3 and Q4. How now?

if in doubt - replace ....... then think

price of learning process

When you said 'upper and lower half' you mean, Q5 and Q6 to be matched and then Q7 and Q8 to be matched, right? Q5 and Q6 are the current source while Q7 and Q8 are the output mosfets, correct?

exactly

 

[pmchoong]

I have changed all components to new ones except for all the source resistors but the problem persists. I don't think the problem is with the board. I suspect it's the power supply. Now, the power supply is at 54V. Previously it was at 56v, I wonder where did the 2v go? and the problem is exactly the same as before, the dc offset went up to 80mv and now it is at 100mV. The additional 19A source resistor goes up even higher at 0.900v but the R19 remains at 0.534v no matter how i swapped them.

 

[Zen Mod]

please , +/- supply ......

time for pictures ...

 

[pmchoong]

mains power at 240v.