charging capacitors help.

[Joepage2008x2]

Hi, i want to charge some filter capacitors rated at 4500uF, I will be charging them by rectified 230V mains (325Vdc).

Im just wondering how to charge them, needs to be under 30 seconds.

I thought about using resistors but Im looking at values of 100R which means about 500W peak, would a 100W or 300W resistor handle this for a few seconds. I also though of using a 500W halogen lamp to charge them as a space saving alternative.

I will also be discharging the capacitors through these resistors.

My problem is that power resistors are bulky but the halogen bulb may/will blow if I discharge 325V through one.

Ideas anyone?

 

[bychon]

I think nobody is answering because this is the kind of power that can kill you or blow a chunk out of your finger.

 

[JimB]

The idea of using a lamp is fine.
You dont need a 500w lamp, certainly not for charging, a 60w will be fine.

To discharge the capacitors again, if you are worried that the the 325v DC will kill the lamp, use two lamps in series.
Dont forget that as soon as you start to discharge the caps, the 325v will start to fall so the lamp will not be overvolted for too long.

JimB

 

[MrAl]

Hi there,

A 300 ohm resistor rated at 300 watts will definitely handle this. Might even get away with a 300 ohm 100 watt device. This will charge the caps up in under 10 seconds.

A 600 ohm resistor rated at 200 watts will also definitely handle this. Again maybe 100 watt resistor might work too. The charge time in this case would be under 20 seconds.

A 900 ohm resistor rated at 100 watts will be another choice that will definitely work for this application. Possibly even a 50 watter. Charge time under 30 seconds.

If you use light bulbs you cant use too low of a wattage bulb or it wont charge as fast as you need it to.
A 500 watt bulb is roughly equivalent to 100 ohms, 500 watts, but then again the initial surge may damage it. It will also be much less reliable than a power resistor which would probably outlast all of us here. If the filament blows, it doesnt work anymore, and vibration is a problem with filament bulbs anyway. For the longest lasting solution, use a power resistor or something like that.

 

[Hero999]

How about using a motor start/run capacitor or a magnetic fluorescent a ballast?

A lower value, non-polarised capacitor can be connected in series with the rectifier to act as a ballast and limit the current. A 10µF capacitor will have an impedance of around 320R and 4.7µF will have an impedance of about 680R.

An old magnetic fluorescent ballast will also work and is probably a better idea as using a smaller capacitor will still create a large inrush current, if it's connected when the mains voltage is at its peak voltage.

 

[Warpspeed]

A resistor will certainly work, and a ballast inductor would be even better.

But how about just a series capacitor on the ac side of a full wave bridge rectifier ?
A 30uF (mains rated) motor run capacitor, or power factor correction capacitor would be well up to the job.
It would be small and have no heat dissipation issues.

 

[tcmtech]

On most modern inverter based welders and plasma cutter power supplies they have far more capacitance than that and many just use a standard direct connect charge up system without any current limiting pre charge systems in place.

However some of the smaller portable plasma cutters will dump the line power through a 25 - 50 ohm 10 watt resistor for 3 - 5 seconds before the main contactor activates and puts the full power to the system.

 

[Joepage2008x2]

I thought about using the lamps and I know the filaments are sensitive. two 500W ones in series will give me about 200R - this is the last resort I think but the easiest alternative.

I have about 150 x 130 x 50mm of space to spare for this. I think a ballast may be a bit too heavy and large at this rating - I'm trying to save a bit more weight.

I could use a capacitor to charge them but how about discharging.

Power resistors are huge at these ratings, could I get away without using a heatsink as the power drawn will quickly decrease, I have a temperature circuit to protect this.

Quote from tcmtech: However some of the smaller portable plasma cutters will dump the line power through a 25 - 50 ohm 10 watt resistor for 3 - 5 seconds before the main contactor activates and puts the full power to the system.

This is exactly what Im doing, I have MCB's to protect from a surge. What voltage will I need in the filter capacitors before I switch on the relay to stop the mcbs from tripping.

Thanks everyone for your input

 

[unclejed613]

you could also us NTC thermistors as a "soft-start" circuit, which will limit the inrush current, but rapidly decrease resistance within a few seconds. after the cap is charged, little or no current is flowing, so their resistance goes back to the high cold value. NTC (Negative Temperature Coefficient) thermistors are commonly used for inrush current limiting on switching power supplies that have a bridge rectifier and large capacitors as their input.

the other option would be to charge the cap using a constant current source using a transistor, a couple of diodes and a resistor. in this case the current would remain at a constant value that is preset, until the cap is fully charged. this actually charges the cap faster, since the charging current remains constant instead of exponentially decaying. this has the added advantage that the resistor required can be a relatively inexpensive 5 or 10 watt resistor. the transistor will need to be a power transistor with the voltage and current limits well beyond what the transistor will encounter in this application. since it is difficult to find transistors that can handle much more than 300V, you might be better off using a pair of cascaded transistors (not darlington, and not cascode) so that each transistor sees only 1/2 of the voltage. i've seen it done, but not in a current source, so give me a while to work it out on paper.
te reason the current setting resistor can be so small, even though the current being sourced is going through it, is that there's a constant voltage across it, usually 0.7V, but a zener reference can also be used for higher currents, but the voltage is still relatively low (a 12V zener could be used with a 1 ohm 15 watt resistor to set the source at 12 amps, but i doubt it needs to be set that high when 1 or 2 amps will most likely be plenty)

 

[MrAl]

I thought about using the lamps and I know the filaments are sensitive. two 500W ones in series will give me about 200R - this is the last resort I think but the easiest alternative.

I have about 150 x 130 x 50mm of space to spare for this. I think a ballast may be a bit too heavy and large at this rating - I'm trying to save a bit more weight.

I could use a capacitor to charge them but how about discharging.

Power resistors are huge at these ratings, could I get away without using a heatsink as the power drawn will quickly decrease, I have a temperature circuit to protect this.

Quote from tcmtech: However some of the smaller portable plasma cutters will dump the line power through a 25 - 50 ohm 10 watt resistor for 3 - 5 seconds before the main contactor activates and puts the full power to the system.

This is exactly what Im doing, I have MCB's to protect from a surge. What voltage will I need in the filter capacitors before I switch on the relay to stop the mcbs from tripping.

Thanks everyone for your input

Hi again,


As my previous post pointed out, you could use a 900 ohm resistor rated at 100 watts and that would definitely work. You could round to 1k ohm, which could be ten 10k, 10 watt
resistors in parallel or ten 100 ohm 10 watt resistors in series...not all that big really.
The point is, ten 10 watt resistors are not all that large really, and using the series string they could be run around the inside of the case or something like that, and no heatsink required.

Yes, to charge you might even get away with a 5uf cap on the AC side if you have a full 30 seconds to charge.

 

[Warpspeed]

I suppose it depends on how frequently the capacitors have to charge up from zero.
If it is once every twenty minutes, then a fairly low power charging resistor can be massively loaded for a very few seconds.

If you are charging it more frequently, for some type of energy discharge system, electric radiator bars make very good low cost charging resistors, rated in the kilowatt, or multi kilowatt region.

 

[Joepage2008x2]

Im making a 5kw inverter, once the capacitors are charged then they only need to be discharged if I turn the unit off or an emergency stop. I have shutdown circuitry so there is no overcurrent or temperature does not get too high in the H-bridge, bridge rectifier and this charging circuit. Would it be possible to load a power resistor rated at 100W with 300W for a few seconds, the current drawn decreases exponentialy so 300W may only be for a few seconds.

@MrAl , that would be my choice if I couldnt get a single power resistor - Im just concerned about the cost. I dont want to spend much over the £10 region, I can get a 100ohm 300W resistor for £10 - its very large and would have to withstand 500W peak. If I could get 10 1K power resistors in bulk then this would be my first option, my same question again - would this 250W 100ohm bank handle 500W peak (each resistor 25W). I want to keep charging time to a minimum.

@unclejed613 , the NTC thermistors is a great idea - I thought they were only in a few watts max though. This is the idea where I got the halogen bulbs from, I measured my 500W ones at 20ohms - the hotter they get the more resistance until at about 125ohms where they cannot get any hotter.

Your other option is a great Idea but every circuit I make goes onto a PCB - I hate making PCB's. Thanks anyway

 

[tcmtech]

If you are pulling 5 Kw at 240 volts then your running current is around 21 amps. So your inrush current limiter only needs to limit it to around 21 amps as well.

A 12 ohm 20 watt power resistor would be more than enough for the few seconds it takes for the capacitor bank to charge up before the main contactor activates and supply's full line voltage to the system.

 

[Hero999]

I think you're over thinking this and worrying too much about exceeding the maximum ratings.

It's not the power rating of the resistors that's important but their ability to withstand surges.

I suggested using a fluorescent ballast which you thought might be too big and heavy.

What's the weight requirements?

A 16W ballast should do, RS sell them.

Tridonic | Electrical Installation | Lighting | Lighting Ballasts | Switch Start Ballasts (Chokes) |EC016 C102K 240/50

https://www.electro-tech-online.com/custompdfs/2010/04/0900766b80ce799b.pdf

It will be within your dimensional requirements and weigh 300g.

The datasheet says the tube current is 195mA but it'll be more when short circuited although it'll be low enough to limit the current to a safe level, based on the tube voltage being 100V, I estimate the impedance to be 667Ω, it could be more or less.

 

[unclejed613]

i even went and found a MOSFET that you could use as the current source transistor, an STP10NK60ZFP. rated at 600V and 10A....

a 4700uf cap at 325V has a charge of 1.525 Coulomb
the current required to charge a 4700uf cap in 10 seconds is 150mA per capacitor.
HTH

 

[Joepage2008x2]

Wouldnt a 20W resistor at 12ohms explode - the surge would be huge, very fast charge time though. I do have two 22R 25W resistors though (11R at 50W). If you think this wont explode I can always try it, I have an MCB in series with the invetrer input, I can always put it in an enclosure just to be safe. I think the resistors are wire-wound which I think are good for surges - what kind of surge can a 25W wire-wound resistor take?

The ballast would be no good, perfect for charging when AC but not for discharging when its DC, I want to kill two birds with one stone.

I want to limit the weight to around 500gram

My inverter has a 2.5kW and a 5kW input with a 16A and a 25A MCB. I will mainly be running it at 2.5kW because thats what Im limited at the moment. Inrush current needs to be limited to around 15A.

@unclejed - Im not greatly experienced in electronics yet so there are a few circuits I have never come across, I starting having a look at the circuit you have proposed and it looks a very good idea. Im actually quite interested how I would do this - even if i do use the resistors I would like to know how I would build the circuit to charge and discharge a capacitor. Thanks

 

[tcmtech]

A wire wound resistor usually has at least a 10X surge capacity and most can go higher than that for a second or two so in this type of very limited duty cycle aplication a 20 watt rating is more than enough for the few seconds that it takes to charge the capacitor bank.

Try it and find out.

 

[Birdman Adam]

Please be safe.

 

[MrAl]

Wouldnt a 20W resistor at 12ohms explode - the surge would be huge, very fast charge time though. I do have two 22R 25W resistors though (11R at 50W). If you think this wont explode I can always try it, I have an MCB in series with the invetrer input, I can always put it in an enclosure just to be safe. I think the resistors are wire-wound which I think are good for surges - what kind of surge can a 25W wire-wound resistor take?

The ballast would be no good, perfect for charging when AC but not for discharging when its DC, I want to kill two birds with one stone.

I want to limit the weight to around 500gram

My inverter has a 2.5kW and a 5kW input with a 16A and a 25A MCB. I will mainly be running it at 2.5kW because thats what Im limited at the moment. Inrush current needs to be limited to around 15A.

@unclejed - Im not greatly experienced in electronics yet so there are a few circuits I have never come across, I starting having a look at the circuit you have proposed and it looks a very good idea. Im actually quite interested how I would do this - even if i do use the resistors I would like to know how I would build the circuit to charge and discharge a capacitor. Thanks

Hi again,


We can estimate the fusing current of your resistor if you measure the length and diameter of the actual body of the resistor, and that will give us the time it will take for the resistor wire to burst under extreme or semi extreme current flow conditions.
Even better, if you can measure the diameter of the wire on the core of the resistor that would give us an even more accurate estimation of the time it would take to destroy the resistor.

One thing to keep in mind however is that when you do it with a very seriously under rated resistor you have to allow ample time for it to cool off before another cycle or on the next cycle it may be destroyed anyway. A resistor that is rated near the required full power will not have as much problem handling the repeated cycles so you'll have to figure out how often you need to charge and discharge too.

 

[Joepage2008x2]

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.