equation for woking out voltage drop over resistor

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skal1

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Jun 4, 2008
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OK  i bet this ? has been answered a thousand times , but i can not find the answer in the meta anyway.



i have a rectified voltage of 414vdc  , i need 290 ish  how do i work out what resistor is needed to get the 290 volts.

thanks


skal1
 
skal1 said:
... answered a thousand times ...

Well it's ohms law really. R=V/I (or V=IR, I=V/R)

So you need to know, or guess, or measure, how much current will be drawn across the resistor too. So the answer depends on the load that you're drawing.

Imagine you have one milliamp. Your voltage drop is 414V-290V = 124V.

So R = 124 V / 0.001A = 124K.

But if it's 10 mA you'll get a different value  ;)

That's a significant voltage drop too, so don't forget to calculate how much power the resistor needs to dissipate.

(Also, you may find that under load your power supply doesn't deliver the full 414V, and will droop slightly.)
 
zebra50  do you what  the current drawn of a single pm660 board ?


this psu is doing my head in , could i build my own psu for the poorman  and get rid of this bull..... transisors.

thanks

skal1

 
thats cool.

so how would i go about building a power supply that can deliver 250v and a 136v. could this work..http://www.electronixandmore.com/articles/vdivider.gif

thanks


skal1
 
That's a pretty big supply.  Do go to the PM660 support thread and see what's being used. In case you decide to build from scratch, be mindful of whether regulation is required - it often is not in many tube circuits - this one may be more sensitive.

You can take care of a single rail of regulated 250V @ 100ma with an International PS  - or use two if needed.

http://internationalpower.com/cgi-bin/htmlos.cgi/003.1.071303921610612261
 
lassoharp , i brought the tx at the pm group buy , but for some reason now it is kicking out more dc after regulation then it should do.

all i am trying do is cut down the dc before q1 so the psu  can work , I  did some fault finding and i have worked out that Q3 keeps on blowing  because it can only take  300v max , so my 414vdc is killing that Q3 mspa 42 every time.



i have to change R1 to drop at least  120vdc ,  so now i need to know what is the current draw of a single amp board.


thanks

skal1
 
been as know one know the current draw of a single amp board, maybe i should be asking how do you work out the current drawn by a pm660 single amp board,

do i look up the current drawn by the tubes  plate , cathode  and resistors.

skal1
 
skal1 said:
been as know one know the current draw of a single amp board, maybe i should be asking how do you work out the current drawn by a pm660 single amp board,

do i look up the current drawn by the tubes  plate , cathode  and resistors.

skal1


You can do it that way, or you could measure it by putting say a big (10W should be plenty safe - if you only have a 3W or 5W use it) 10ohm resistor in the circuit at the PS out and measure the voltage drop and calculate current -  This has to be done with the full load hooked up.  414VDC sounds more like a no load voltage - is Q3 blowing with full load?  If it is, you could install a standby by switch which will keep the start up voltage from soaring so high for such a long period of time.

As for calculating the total current from individual tubes,  all you need to know is the voltage drop on the cathode resistors - this should be imperative on any DIY (high voltage) project schematic.  If its not given you'll have to measure it.

Remember that even if you install a resistor to lower the final B+, the start up voltage can still go to 400+ unless the tube heaters have been adequately warmed up prior to applying the HT - hence the suggestion for a standyby switch.

Frankly I'm surprised that the total current draw figure wasn't posted somewhere in the 660 help thread - given the size of that project.  Did you check the schematic for it?
 
lassoharp  ,

thanks for your help on this ,but i think the board has  had its day , hopefully volker has some spare , i will get a new board and caps and see what happens from their .


Are yeah forgot ,q3 blows out without  any load so that rule out the amp cards.

thanks again

skal1
 
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