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ron944 28th January 2013 12:12 AM

Function and Value for Resistor after Rectifier
Hi, in a tube preamps high voltage power supply there is a resistor going to ground after the full wave rectifier before the first power supply cap. I imagine this is to discharge the cap. Is this correct and what value resistor would you recommend. There is 445 VDC going to this resistor. Also will it have any effect on the voltage going to the first cap.



DUG 28th January 2013 12:50 AM

All loading will affect the supply to the filter caps. The rectifier(s) and transformer have a source impedance. Any loading will affect this.

How much? It depends.

As to what value the discharge should be...I have seen suggested 10% of max load.

Someone else with more experience will be able to give you a better answer on that.

Sometimes this resistor discharge is disguised as a voltage divider for other purposes.

Two thing to consider when selecting the value are power dissipation and time to discharge.

Well, I guess voltage rating of the resistor(s) is important as well.

5 time constants (ohms * farads) will discharge the cap to less than 1% of initial charge.

Hope that helps a little.

nigelwright7557 28th January 2013 01:02 AM

I usually use a 470K 0w5 on mains equipment.
I usually use 2, one on the input to discharge the capacitor on the plug side (to stop shocks off the plug) and one on the smoothing capacitor (post bridge) to discharge the equipment.

It will have minimal effect on voltage going to first capacitor.

Bear in mind it will still take a while to discharge the smoothing capacitors after power off. I usually give it a few minutes then short out the smoothing capo wit ha resistor to make sure before touching circuit.

ron944 28th January 2013 01:37 AM

Thanks for the info. Other than for safety is this resistor really necessary at all.


nigelwright7557 28th January 2013 01:43 AM

Its for safety only.

I heard of a famous company who made vacuum cleaners and they missed out the resistor on the plug side and customers who touched the plugs got a shock !

They had to take all the vacuum cleaners back and fix them !

ron944 28th January 2013 01:51 AM

Thanks again

DF96 28th January 2013 10:13 AM

Because it is for safety this resistor is really necessary. You could argue that it is the most necessary resistor in the whole circut. Omitting other resistors may stop the amp from working. Omitting this resistor may kill you. Which is the most serious outcome?

AndrewT 28th January 2013 10:49 AM

IEC power input sockets can be bought with a filter built in.
This filter always has this resistor added in, to protect user from shock across the plug pins.

Mooly 28th January 2013 11:43 AM

Remember resistors have a maximum voltage rating and that high voltage resistors must be specified for reliability in these kind of applications despite the power dissipation being well within spec.

(To anyone thats ever worked on TV/VCR and Satellite gear you'll know just how many SMPS start up problems were caused by open circuit start up resistors and these faults were simply because they were not high voltage rated)

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