Help with testing HV PSU for maximum current

[Ice-9]

I am wanting to test a HV psu to see how much current it can deliver before the voltage begins to drop off, as it's HV i would like to ask if i will be doing this the correct way. My first thoughts are to hook up a resistor across the power output and measure the voltage across the resistor, by changing the value of the resistor each time until the voltage begins to fall significantly. Ideally i would like to use a potentiometer but don't think i can get one with a high enough wattage.

Here is what i want to check with someone with a bit more knowledge than me.

Its a SMPS that puts out 230v DC
the maths i think is correct is   
POWER = V*I
R = V/I


so 230v * 50ma means i should be ok with a resistor of 11.5 watts
and 230/.05 = 4k6

If i'm correct in this example,  to test at 50ma i would need a 4k6 11.5watt resistor. Is there a better way than using multiple resistors like this as they are not cheap.

[George Giblet]

The voltage drop doesn't determine the power supplies maximum current *rating*.   You might want 10% regulation at full load but that could be more or less than the rating.  In other words load regulation and rating are different things.   If the circuit with poor load regulation then you might find the voltage drops well before the rating is reached.

Your calcs look fine.

The cheapest dummy load is often to combine 5W resistors of near equal value.  Equal values ensure the power is distributed even over the resistors.

For example for an 11.5W load the next higher multiple of 5W is 15W, so you will need three resistors.  These can be in series or parallel. 

For the series case:

3 * 1.5k in series gives 4.5k and you can add 100ohm 0.5W resistor in series to get 4.6k.

If you want to play around with different loads close to 50mA use say 3 x 1.2k so you can pull more then 50mA then place 1W resistors to up the resistance value to the precise current you want.

[Ice-9]

Thanks George, I went and bought 2* 4k7 10w and 2* 1k 10w and tried different combinations, results with the 2 4k7s in series showed no voltage drop at all so i left it running for 30mins and voltage was stable so from this i take it with this load i must have a stable supply with a current draw of about 25ma. I think this is more than enough to drive the plates on a 12ax7 at the B+ voltage of 230v. I now have to work out why my heater regulator (7812 TO220 package) has dropped to only 10.8v

[brett]

Hi

with a current draw of about 25ma. I think this is more than enough to drive the plates on a 12ax7 at the B+ voltage of 230v

Yes.  If you are using 100k or more plate resistors, you only need a few mA per 12AX7.

I now have to work out why my heater regulator (7812 TO220 package) has dropped to only 10.8v

First, I suggest boosting that 12V supply to 12.6V by putting a forward biassed diode (any Si diode) on the com leg of the LM7812.
Second - what is the voltage, VA rating and filtering cap size?  You'll need 12V AC (16V DC) to regulate down to 12.6V.  It'll need to be rated at about 2.5VA to run 12.7V at 150mA and have some to spare for the heater.  I always suggest having 2200uF per amp of filtering, so for 150mA we are talking 330uF (make it 2 x 220uF to be sure).  Two filter caps gives better filtering than one cap double the size (for reasons I won't explain here).

cheers