Need help calculating value of balancing resistors

[data]

Hi again guys, i hope someone can help me with this one

I need to add a second pair of Mundorf M-litc HV 200 +200 500v caps connected in series in my power supply to handle the inrush at power on ect'

So each cap will be essentially as 400uf/500v, and with two of these connected in series for 200uf/1000v I need to add balancing resistors but I suck at working out the value needed using equations provided on internet sites (should have paid more attention at school)

I was looking at this site as a reference Capacitors - Multiple Capacitors

My secondary voltage is 435vdc or 440vdc

Anyone good at math here that can help me work out the value I need?

Thanks in advance

[SY]

You can use the equations on the site you linked, but it's really quite a bit simpler than that for this application. Basically, they need to be identical and draw a current that is large compared to the capacitors' leakage currents, but not so much that you'll cause significant heating or drag down the supply.

Your power supply voltage is likely to be something like 580V- let's call it 600 to make the math easy. If we pick a current of 2mA (arbitrary, but it meets the requirements), the total resistor value is R = V/I = 600/2 = 300k. So each resistor needs to be 150k. At 2mA, the resistors are each dissipating 300V*2mA = 600mW. So a 1W resistor would be suitable, 2W even better.

The resistors also act as bleeders. The time constant is 0.3M*200uF = about a minute. If you let the amp sit for 10 minutes or so after power down, the innards will be safe.

Again, none of this is critical- if they were both 470k, it would make very little difference. The equations on the linked site would suggest something like 1M.

[data]

*wipes brow*

Thanks SY

Been sitting here with the calc up and the only conclusive result I had was that my brain was hurting

Thanks again.

Why can't these manufacturers make some higher rated caps for us, say 800v probably some good reason I'm unaware of

*orders two more Mundorf caps*

Hmm...I wonder if I could have been OK with 550vdc Jensen's without a series connection, they are rated for 595vdc surge?

[boywonder]

Originally Posted by data

Why can't these manufacturers make some higher rated caps for us, say 800v probably some good reason I'm unaware of

Dielectric strength..........when the voltage gets high enough, it arcs over and blows holes through the dielectric insulator between the conductive plates/foils/films.

[data]

That's reason enough

[Rod Coleman]

When you say "My secondary voltage is 435vdc or 440vdc" do you mean dc voltage across the caps? or the transformer secondary's rms ac?

If you mean dc, then it is unusual to see much above 500V at power-ON. In fact, provided the amp warms up to 440Vdc within 3 minutes, then up to 550V surge is permisible when using standard industrial 500V caps like the Panasonic TSUP or TSHA/TSHB (which still has a 500V range, last time I looked).

Still, you may actually get better performance with a stacked pair. Sy's 150K is fine. There are a couple of other precautions:

- make sure the resistor has a voltage rating! many lower power resistors are only good for 250-350V (regardless of dissipation), and I strongly recommend 2W film or WW in order to get 500V durability: even the cheap 2W CFs achieve 500V.

- watch out for the aluminium can of the 'upper' cap: this may be connected to the negative pole - which is raised 220V from chassis! Use a nylon cap-clamp for insulation, and don't rely on the sleeve for this purpose.

- Don't obstruct the vent. With large caps (20mm & up) the plastic disc at the end opposite the terminals is the safety vent. Allow 6mm or more clearance for it. Accidents (like can to chassis) are easier to do with stacked caps, and the vent prevents injurious explosions in some of these events.

[data]

Thanks Rod.

440vdc after SS rectification, but no stand-by or soft start circuits, just a thermistor or varistor on the AC input...not sure which one. The M-litc HV has a max surge of 550v.

Was going to use 5w Kiwame resistors, rated at 700v I think. Just checked hificolective's page "2W and 5W type, 750 volts DC max"

Case is MDF base, so no issue with the mounting.

The whole top and sides, plus a portion of the front & rear of the enclosure is open mesh, so excellent ventilation

Thanks for the tip on clearance at the bottom of the cap.