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# Limiting Mains current.

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Hi,

I'm making a circuit that in part will charge a capacitor from the mains. When the cap is empty the current would be very large. I want to limit the current to about 1.5A so that the rectifier is not damaged.

Should I just use a resistor on the rectifier output, or should I use a capacitor in series with the AC input to limit the current reactively?

Cheers!

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It's up to you but bear in mind that for 1.5A you'll need a pretty large capacitor which will also require something to limit the current as well as a bleeder resistor.

You could use a resistor but you have to live with the power loss.

Also note that, in this case, the rectifier's peak current rating is more important than its continuous rating.

What size is the capacitor and how quickly do you want to charge it.

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Hi Riccardo,

You must use a resistor. The series capacitor would also be discharged at the beginning thus acting like a 'short circuit'. Although the capacitor would present capacitive reactance as you say, at switch-ON the waveform is a 'step' signal going from 0 to whatever the mains voltage is at that time. A step signal is made up from infinite sinewaves at higher frequencies. As you might be aware, capacitive reactance is inversely proportional to frequency so like I summarise above, the capacitor will behave like a short circuit due to the higher frequency components. That said, if you choose your capacitor values wisely, you should be able to distribute the charging current across several mains cycles by managing the voltage each capacitor sees across it at each given time. A resistor is easier to calculate in this respect.

So, use a resistor. I have seen people use ligh-bulbs to charge capacitor banks for rail and coil-guns if thats what you are after.

What capacitance do you want to charge?

P.S. I just saw Her'os reply. An idea would be to use a relay to exclude the resistor from the circuit once the capacitors have been charged not to waste energy.

Regards,
Alex

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The capacitor will be about 1000uF. I'd discharge it before it reached full voltage. Hoping this could be done every second or so.

It's UK voltage so the rectified voltage could be over 300V. I'm wanting to limit the supply to 500W.

If I use a 200 ohm resistor the power dissipated would be 450W right? That is a lot!

Is there a transformer I could use for this?

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The power dissipated would be for a fraction of a second and it will repeat every time you want to recharge.

The rectified UK mains voltage should be about 340 Volts peak.

To limit the maximum input current to 1.5 A you could use a 226 Ohm resistor or the closest E series equivalent.

Very crude calculation ignoring diode drops and capacitor ESR but the tolerance of the resistor isn't any better.

What do you mean by transformer? I thought you wanted to charge a capacitor off mains? Maybe you mean an isolation transformer? - Edit - I see why you though of transformer. No, you dont need a transformer. A much much smaller wirewound resistor would be able to handle the power for this short duration.

Regards,
Alex

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Yes, I looked at some isolation transformers but I wasn't sure if they would actually limit the current. What do I need to look for in the specs? Maybe I can just use a 100W transformer as the peak load would be brief enough not to overheat it  :-\

Since the high currents would only be brief, I suppose the resistor wont need to be rated for 500W. How might I calculate what rating would be ok? I get the feeling that may be a more complex answer than it seems.

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Yes an isolating transformer will limit the current for several reasons. However you should consider that only if you require electrical isolation in your system. It is an overkill otherwise. 100W is a very big transformer to use. you can power your TV at the same time as charging the capacitor  ;D

To calculate the resistor's rating you need to decide on how fast you want to recharge the capacitor and to what extent. The RMS values of voltage and current will need to be calculated from the voltage and current waveforms and multiplied to give you a power rating.
The resistor used will still need to be able to cope with the current surge that is why I suggested a wirewound resistor.

I bet a 10W 220 Ohm resistor:

http://uk.farnell.com/welwyn/wh5-220r-ji/resistor-10w-5-220r/dp/9507817

would be up to the task with a lot of margin.

Regards,
Alex

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Thanks.

Yes, I saw the isolation transformers would be about 5kg ! I will avoid that if possible

The attached circuit shows the power electronics that I have in mind.

Another question. Should I make all my GND common with mains earth?

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See attached diagram with notes.

This is definitely a coil gun.  ;)

You have already connected the ground to mains and broken the isolation the 12V transformer provides by grounding the 1000uF capacitor. If I were you I would use an isolated control section powered from the 12V trafo and leave the high voltage side floating.

I have serious fears that your power section in the red circle will blow up. What is the NMOS you are using, which one? It is connected wrong. Anyway, I doubt it would be able to withstand such voltage and current. In this sort of application you can use a thyristor. To trigger it you can use an optocoupler or relay to maintain isolation between the control circuit that you have greater chances of touching and the power circuit.

What is the inductance and ESR of your coil? The free-wheeling diode D7 might not be able to withstand the back-EMF when the capacitor is drained. My point is that you should get mathematical with C1, D7, Q2 and L1 maybe run a simulation too.

Alex

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Oh, I don't know why I put the MOSFET (actually it will be an IGBT) like that, I should have noticed before I posted! I'd also forgot to add a resistor. Revision attached.

Its not a coil gun, but it is for pulsing a coil. I cant use a Thyristor because it would not switch off until the cap is discharged. I want to control the pulse time precisely.

I'm wanting to use it with various coils really, but generally it will be very low inductance, maybe just 2 loops of cable around a diameter of 0.5 - 1m in air.

For Q2 I was planning to use a SEMIX603GAR066HDS. It is pretty large and it's internal diode is too.

Not sure about leaving the power side floating. Wont that change the gate voltage requirements?

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I see. Much better now.

Yes that will complicate driving the IGBT but not too much. It comes down to whether you want electrical isolation of the control circuit or not. For example, if your control circuit is powered by some lab bench power supply that has earthed  negative terminals then you will trip the safety relay. If it is a self-contained unit then no problem.

One more point, why do you include the transistor in the current loop that is created when you switch off the coil? Why not have the diode's anode connected to the collector of your IGBT?

Regards,
Alex

PS: I happen to have some used Mitsubishi IGBTs rated at 800A if I remember correctly and some SIEMENS half-bridges again IGBT rated at 300A I think. Let me know if you are interested.

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Since the high currents would only be brief, I suppose the resistor wont need to be rated for 500W. How might I calculate what rating would be ok? I get the feeling that may be a more complex answer than it seems.

That's the point I was trying to make with regards to the rectifier.

What's the continuous current rating of the rectifier? 1.5A? The chances are it can easilly withstand a 15A surge or more.

I have a 100VA isolation transformer, it isn't that heavy, the DC resistance of its secondary winding is about 22R, assuming the primary has a similar resistance, the short circuit current will be about 230/44 = 5.2A.

One thing you should be aware of with isolation transformers is that the secondary voltage can be higher off-load, from memory I get about 250V when the primary voltage is 230V with my 100VA transformer but it's less of an issue with larger transformers.

I'd recommend using an isolation transformer for safety reasons but that does make triggering a bit tricky. I was going to suggest an SCR but then I realised you needed to control the pulse width.

How wide does the pulse need to be?

What's the peak current?

You might be able to use a pulse transformer, as long as the pulse doesn't last for too long.
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that has earthed  negative terminals...

The control electronics will be powered form that 12V regulator and another 5V regulator attached to the same supply.

One more point, why do you include the transistor in the current loop that is created when you switch off the coil? Why not have the diode's anode connected to the collector of your IGBT?

Good point, Do you think I'll need a diode rated for more current?

What's the continuous current rating of the rectifier? How wide does the pulse need to be?
What's the peak current?

I haven't picked one yet, but yes probably a few amps to be on the safe side. I see your point. So maybe I could reduce the value of R1 since the diodes could take a surge. The relay would have to have higher ratings though right, so the contacts don't get fused. (although maybe I could use some sort of ZVS arrangement)
I'm trying to make the pulse width very narrow, as narrow as the IGBT will let me. I want the pulse to be short, but it must be long enough to allow significant current to start flowing in the coil. Ideally though I also want to be able adjust it, but if this is impractical, the pulse transformer method may be best.
Peak current would be limited to about 1750A by R4. The IGBT is rated for a 10ms pulse current of 1800A.

If I use an isolation transformer, it is safer (because of limited current) , but heavier, more tricky to trigger, and more expensive?

If not, I can keep the grounding as it is in the diagram, and the triggering is easier?

In what sort of condition are the parts you guys are offering? Can you send me pics?

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Hi,

Taking the worst case, where you have 2 turns 0.5m dia. , your coil will have an inductance of about 4uH. I have assumed an ESR of 50mOhms including connectors. Once you trigger the IGBT, assuming no voltage drop there, you will get 340 Volts across your inductor. Current will start rising but not linearly due to the discharging capacitor and resistor in the path. The peak current when it occurs will be when the inductor voltage is 0 volts and it will be about 800A. This is the worst time to switch off the IGBT. If this happens, whatever current is flowing tin the coil will appear across your diode, coil and resistor and it will be a bit less than 800A due to the diode drop which will be significantly more than 0.6 Volts at that current. So do you need a bigger diode? Probably. The issue here could be thermal runaway of the diode once you reverse bias it again as it could be too hot by the end of the coil discharge period  (starting from when you switch off the IGBT) when it is in forward bias. This could destroy the diode be excessive leakage current , in the region of uA for this diode.

One thing you can do, is add a resistor, say about 0.6Ohms in the path of the diode. That will limit the current to something that the diode can cope with.

This question really has no easy answer, certainly beyond Ohm's law. At these currents even the ESR of the capacitor and the resistance of your cables will have a dramatic effect. Very low level circuit analysis is needed and good knowledge of IGBTS and Shottky diodes.

The narrower the ON time for the IGBT, the easier it will be for the diode. You can control the peak current by varying the ON time.

I tend to use an isolation transformer everywhere that I can as it saves lives. Now, you can use one during development and then use a pulse transformer or an optocoupler to trigger the IGBT.

I will take a picture of the IGBTs and half-bridges soon and post it here. They are packed away in storage atm.

I seem to have a picture of the IGBTs and other power modules before cleaning them to store them(attached), I will have a look in storage and give you part numbers very soon.

Regards,
Alex

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If I use an isolation transformer, it is safer (because of limited current) , but heavier, more tricky to trigger, and more expensive?

You're right that using an isolation transformer would be more expensive and heavier but you've missed an important point: safety. An isolation transformer, disconnects the neutral from the earth, making i so you have to touch both wires to get a shock. As you currently have the circuit connected up, you can receive a shock by touching any part of it, even the low voltage DC part.

I was wrong about the triggering, see my response to the next quote.

If not, I can keep the grounding as it is in the diagram, and the triggering is easier?

I was wrong earlier, you can keep the grounding as it is with the isolation transformer.

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Riccardo,

Here are some pics of the single big IGBTS. They are these:

http://www.mitsubishichips.com/Global/content/product/power/powermod/igbtmod/hmd/cm400ha-34h_e.pdf

400A 1700V and 800A pulsed.

Regards,
Alex

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And the smaller IGBTS...I also have these in half-bridge style with two devices per package. Bear in mind, some of the smaller IGBT units have one of the two unit toast.

This is a SIEMENS BSM75GAL120DN2

http://www.infineon.com/dgdl/75gal120dn2.pdf?folderId=db3a304412b407950112b4095b0601e3&fileId=db3a304412b407950112b431af4e5520

These are 1200V 105A or 210A pulsed.

The dual units are identical in terms of package with I think a bit lower specs.

Let us know when you have seen them so we can remove the pics to save space. Also let me know if you need higher res pics.

Regards,
Alex

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• 2 weeks later...

Hi, Thanks for the info and pics. I had somehow not noticed the posts had gone on to a second page!  ::)

I'm still a bit confused about, the isolation / safety. Some points and questions below.

The unit will be self contained and in a plastic box.
The 12V transformer  is just for changing the voltage for the control electronics rather than isolation.
I envisioned the output of an isolation transformer would be connected where I currently have the Live and Neutral markers (the fuse would be moved ahead of this transformer). Would that improve the safety of the attached circuit?
Should I connect the metal parts of any controls to mains earth?
Should the Earth wire from the mains be connected to my circuit GND?

I've attached a revised schematic showing how I intended to control the IGBT.

PS: I'd be interested in the parts offered. Just pm me the prices (incl shipping to UK)

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The unit will be self contained and in a plastic box.

Then an isolation transformer is not necessary as long as all parts have class 2 insulation from the user i.e. there's no chance of the insulation failing resulting in an electrocution.

See the Wikipedia article: http://en.wikipedia.org/wiki/Double_insulation

The 12V transformer  is just for changing the voltage for the control electronics rather than isolation.
I envisioned the output of an isolation transformer would be connected where I currently have the Live and Neutral markers (the fuse would be moved ahead of this transformer). Would that improve the safety of the attached circuit?

Yes, if an isolation transformer is used the fuse should be before the transformer, to prevent the cable from overheating if a short circuit develops in the primary.

Should I connect the metal parts of any controls to mains earth?

Is depends on how well the metal parts are insulated from the live circuitry: it goes back to the question of double insulation. If you're unsure, then connect all metal parts to earth. This only applies when no isolation transformer is used.

If you're using an isolation transformer then there isn't any need to earth any of the metal parts as long as they're double insulated from the primary side of the transformer. The only thing you should ensure is that if the metal parts only have single insulation from the transformer's secondary, you should electrically bond them to each other. This is to ensure they're all at the same voltage i.e. if the insulation fails there's no chance of one plate being at 0V and the other at 230V.

Should the Earth wire from the mains be connected to my circuit GND?

If no isolation transformer is used, connecting the DC side to earth would at worst blow the rectifier, at best cause the earth leakage breaker on your building's distribution board to trip. The earth is electrically the same connection as the neutral: look at the circuit and predict what will happen if you connect the neutral to the circuit's 0V rail.

If you are using an isolation transformer then nothing bad will happen but connecting the 0V to earth would defeat the whole purpose of using an isolation transformer.
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Your schematic looks OK. I think you can get away with increasing the value of R3 to a few kOhm unless your relay's coil draws something like 6W.

What did you have in mind with D9? Is that there for protection?

Finally, the zener diode at the gate of the IGBT. Maybe you dont need that if IC2 has internal clamping diodes to +12V.

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I think I will not use an isolation transformer then as it is bulky and expensive. All controls will also have plastic covers, and there will be no exposed metal parts. I think I'll earth the metal cases of the pots too just in case a cover gets broken off.

Oh yes, R3 is a bit low. Yes D9 is supposed to be there to keep out any negative transients, probably overkill but for the price of it, I thought I might as well.

The TC4428 data sheet says the pins are ESD protected but I couldn't see if the output was clamped to 12V. Again, possibly overkill, but I'd rather play it safe.

PS: D7 would be a UFB200FA40P rather than what is listed on the diagram.

Thanks for all the help guys!

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