charging capacitors help.

[MrAl]

Hi again,

Is that 47 ohm resistor a 10 watt wire wound? Couldnt find that one on the RS site.

That may just take the punishment you intend to give it just barely below what will make it an open circuit
It depends partly on how the device was manufactured, as sometimes to increase production yields the device is 'adjusted' using some sort of grinding which can adversely affect it's joule capability (surge rating). That means one device may work and the other may blow out (cheaper versions would most likely be the adjusted type).

What will be harder to predict would be how long the resistor would last in this kind of application which regularly subjects it to high stress. The high dT/dt might crack the case the first time it is used for example, or later down the road when it looks like it is working just fine.

If you get a chance to pull one apart and check the wire diameter we can estimate the fusing current vs time.

 

[Joepage2008x2]

The resistor is 47R at 25W. The RS number I gave you is for a 22R 25W resistor, my 47R is the same manufacturer and therefore should accept the same conditions.

It may prove a bit awkward to get to the inside of one of these, I may be able to spare one of them.

 

[Warpspeed]

I just decided to look at the resistors I have got on RS, 157-566.
They are in fact wirewound and I had a look on their datasheet.
10 times commercial wattage rating for 1 second
5 times commercial wattage rating for 5 seconds
2 times commercial wattage rating for 3 minutes

If I use 500R resistor to charge then it will be 10X for short period, only 5x after 500ms

Do you think these resistors could handle 100x for maybe 50ms. If I use a 47R resistor it will charge to about 220V in only 200ms, fully charged in just 800ms.

Fusing current is Amps squared multiplied by time. But for MASSIVE short term peak surge currents, the heat does not have time to conduct from the wire away into the body of the resistor. The heat release is explosive, so the I^2t rating must be reduced accordingly.

5 times for five seconds is (5 x 5 x 5 = 125)
10 times for one second is (10 x 10 x 1 =100)

40 times for 50mS is (40 x 40 x .05 = 80) which is probably more realistic
or maybe ?
100 times for 5mS (100 x 100 x .005 =50) only a guess really

 

[tcmtech]

The 47 ohm 25 watt resistor should be just fine. At 240 volts it will carry around 5.1 amps that rapidly tapers off as the capacitors charge.

The means the peak wattage will be at most 1225 watts dropping to under 25 watts within a few tens of cycles. If its a heavy wire wound or potted ceramic type they can take considerable surges without damage.

 

[Joepage2008x2]

Thanks everyone, very much appreciated.

 

[Hero999]

I forgot to mention the basic safety stuff.

Use a fuse, as it's a surge 5A will probably do but go higher if it blows.

If the resistor has a metal mody then you should connect it to the earth/ground safety conductor, if it's connected to any exposted metal surfaces.

 

[Joepage2008x2]

Dont worry, I already have a fuse in place. If somehow the fuse doesnt blow I have the MCB's at the power inputs anyway.

The resistor is metal but I wont need to ground it, it will be in plastic case because this is the only thing I fear may explode. + I have no ground on my circuit anyway because the case is perspex and there are no other components that require ground,

Havnt had any time to test the circuit due to college work and some of the circuit isnt complete yet, + dont like testing high power electrolytes without them being enclosed and out of my way.

Thanks anyway

 

[unclejed613]

back in the early days of switchmode power supplies, "launching" caps was mentioned in design notes as a failure mode to be feared. i never have seen it happen, and most modern caps vent at rather low pressures. unless you make the mistake of somehow blocking the vent (like gluing a cap with the vent side down and the vent enclosed in the glue), i think most caps, if they fail will vent safely.

 

[MrAl]

Hi again,


Just to note, i have seen large electrolytics blow after an inappropriate voltage had been applied in the hundreds of volts and each one at that point works basically like a pirate cannon going off where the top blows just about as far as it possibly can and guts all over the place. It isnt pretty and is very unsafe to say the least. The sound of the explosion alone will make you jump about 10 feet or more depending on how close you are.

The best way to wire the caps is probably with heavy copper buss bar at least 1/2 inch across, with holes drilled to line up with the screw holes at the top of the bank of caps and screwed firmly into place. If the top blows off, the buss bar holds it from going too far even though the force is enough to bend the heck out of the heavy buss bar, but of course the noise of the explosion is still the same and the guts can still fly all about the place with seemingly no rhyme or reason.

 

[Joepage2008x2]

The reason I mentioned it is because I have had them blow before - only small ones though, they were old style and sounded like a handgun - they made a huge mess and stunk.

My caps have no screw terminals (solder tag) and therefore can't use a bus bar + I have already attached wires to them and dont want to de-solder. The caps are held firmly down and the tops will blow into a shelf in my inverter, they have a plug on the top of them. Hopefully they will not blow.

I blew some old-stock ones for the fun of it under a 2mm acrylic box - blew a hole straight through it. Fortunately modern ones have burst disks.

 

[tcmtech]

I have a pair of 2400 uf 450 volt electrolytic capacitors wired in reverse series on a 15 Hp phase converter unit for my big air compressor in my shop. They get the full 240 VAC at around 100 amps shot through them for about 3 - 10 seconds every time the compressor starts. They have been doing that for nearly a year now without problems.

Point is the big electrolytic capacitors can take some serious and regular abuse without damage.

 

[unclejed613]

there is a failure mode particular to capacitor banks, where the "weakest link in the chain" fails, and has the available charge from all of the caps that are still good dumping through it while it fails. a cap that would normally fail slowly after developing leakage, has the process accelerated by all of that available instantaneous current, and fails spectacularly and catastrophically (with an impressive pyrotechnic show)

 

[unclejed613]

I have a pair of 2400 uf 450 volt electrolytic capacitors wired in reverse series on a 15 Hp phase converter unit for my big air compressor in my shop. They get the full 240 VAC at around 100 amps shot through them for about 3 - 10 seconds every time the compressor starts. They have been doing that for nearly a year now without problems.

Point is the big electrolytic capacitors can take some serious and regular abuse without damage.

i would recommend checking their ESR periodically. once they begin developing ESR, the increase in internal heating will accelerate the increase in ESR quickly. the first tangible sign that the caps are developing ESR may be slower starts for the motor, or the caps might just "let the magic smoke out" depending on their internal temperature rise and their construction.