Simplistic MosFET HV Shunt Regs

[king4joy]

Just another silly question: if I set the TP say at 50mA each....will they draw 100mA from my PSU in total??

Will that do no good to my // chokes??

 

[king4joy]

Will the board draw the current set at TP all the time....

or will it draw only 20mA + load current??

If it only draw 20mA + load current, I do not mind setting the TP at higher setting...

 

[Salas]

Yes. It will draw what TP says, all the time. Works as a current limit also. Measure what your tubes draw by VRplate/Rplate and just add 20mA each on TP not to tax your Tx and chokes. Tx ducks and chokes drop on their resistance. Factor in any parallel bleeders.

 

[king4joy]

Should I change the value of the bleeder (now at 150K) so that I can have more room to play around now with 2 boards connected??

I have been playing around with TP setting...and things...but seems PSU is max out to make any meaningful (stable) adjustment with the boards at the moment.

 

[king4joy]

""...Measure what your tubes draw by VRplate/Rplate and just add 20mA each on TP...""

So it is OK if I may end up with difference TP setting for each board (each tube)....

How do I measure VRplate and Rplate, please?? (I assume that is not the same as the voltage across the 6K load resistor).....

 

[king4joy]

thinking a bit more....I think by VRplate/Rplate...you mean the current through the 6K load resistor (V across the load resistor/ 6K)....and that should be what each tube is drawing at time....

 

[Salas]

Should I change the value of the bleeder (now at 150K) so that I can have more room to play around now with 2 boards connected??

Just 2mA bleed at say 300V, so with your value 150K, no considerations.

 

[king4joy]

So the bleeder is not a worry.......so I think tomorrow, I will try to turn back all voltage setting trimmer way back.....set TP at 30mA...and start from there...

""...Measure what your tubes draw by VRplate/Rplate and just add 20mA each on TP...""
will that change will B+??

 

[king4joy]

or the chokes are not up to spec of 40mA each...although they are Bartalucci C core chokes....??

 

[king4joy]

I have done the above, set both TP at 30mA, voltage into the boards now at 280V, I can then set boards output at 258-260V, with voltage across the //12K load resistor at about 48V...

I notice this time only, Q4 & Q5 are hot...in fact they are the only thing that is hot this time.

Everything cool, until about one hour later.....the voltage out from the new board (the one with brighter LED....suddenly went down to 158V....TP remains at 30mA.....

After switch-off, and half an hour later, upon switch on....TP remains at 30mA, volt out rise to about 200V...

What do you thing has happened??

 

[Salas]

Check Vbe for Q4,Q5 to make sure they still conduct and use another Q6. Also check its trimmer normally changes value with turning using DMM in KOhm when Q6 is out.

 

[king4joy]

I only have on hand at the moment 2 more Q4 (and/or Q5).....and 4 x Q6

What would be the best game plan that you would suggest that I should go forward B/4 I need to go shopping??

 

[king4joy]

May be you are thinkjing there are cold joints at Q4 &Q5??

I will check that first thing next.... tks...

 

[king4joy]

Pardon my poor English and/or typing....

 

[Salas]

There is no plan by intuition. Measure that the transistors Q4,Q5 show healthy Vbe (0.6-0.7V) when the circuit is operating. If not, change. Change Q6 anyway, check trimmer.

 

[king4joy]

If change...Q4 / Q5 , change which one first (according to Salas' law..../experience)...(I will change Q4 N Q5...but which one first...)

I will change Q6 first....I have only 4 left...B/4 I need go shopping...

""Change Q6 anyway, check trimmer..."" do you mean, if after I change Q6...if not working...then I should change trimmer?? what do you want me to note when I check trimmer??

It is hard to check (or not much meaning to check again)....as they all checked out for the first hour...unless it is heat related.....

Do you think I should sink all Q4 & Q5 then...

 

[king4joy]

I have not sinked Q4 Q5 before when I use only one board (set at 50mA) to load both channels....

So I now think the problem is with the new board....Q6 then (Q4 and Q%...but in what order...)

 

[Salas]

Use measurements. Normal overheat at that voltage output level is unlikely. Overbias due to a faulty transistor is more likely. Change Q4,5,6 if in doubt and the circuit is not in operational status handy to measure their Vbe. Trimmer should change value smoothly if you have a DMM on its outermost pins and turning its screw.

 

[king4joy]

Thankyou...will report back soon....Merry X'mas...

 

[roger57]

Thankyou...will report back soon....Merry X'mas...

Hi,

Not sure if you are using the small heat sinks on Q4 and Q5, if not, you might want to use them. I found it makes a huge difference. For example, even with the clip on sinks, the temperature still sits around 35 - 45C.

Merry Christmas