In-rush limiter

[dmgdesigns]

I am completely lost when it comes to electronic circuits. I can plug wire A into port b and c, but beyond that I am lost.

I have a 120V input 60V 30A output power supply that blows the 20A SB fuse. I need and would happily pay for a circuit that would limit the in-rush of power and bleed down the PS on shut down. I tried a high watt resistor for a while. This worked but got HOT HOT HOT.

Any and all suggestions are greatly appreciated.

BTW I did find a few circuits on CNCZONE but as I said, I'm

[Al_The_Man]

I would first confirm it is the capacitive charge that is causing a 20a SB fuse to blow!
Disconnect the bridge output and see if the fuse holds.
Do you have a minimum 2kva transformer?
What do you need rapid bleed down for?
If you have a large wattage low value resistor for bleed down, then this is also contributing to increased ripple, which is working against the object of the Capacitor bank.
Al.

[cjmerlincnc]

Hi, You could use a large contactor with normally open contacts with an addition of a coupled unit (that slides on the top of a normal contactor) with normally closed contacts.


Connect the NC contacts in series with your DC dump resistor.

Connect the NO contacts across (in parallel) a suitable resistor to the primary circuit.

The contactor's coil can be connected a veriety of different ways, used with a delay timer or connected to the main switch.

I've got mine connected to the main switch.

When you first switch-on, the current rushes upto the contactor coil and resistor in the primary circuit and slow starts the transformer. The contactor's coil has energised and there's few milliseconds before the NO contacts short across the resistor and provide full current to the primary.

At the same time, the NC contacts have open and disconnected the dump resistor from the DC supply.

This is how I've designed my servo system power supply with the exception of useing a thermistor instead of a resistor in the primary circuit.

It's 1.5kVA and works very well and as yet no tripping MCBs OR RCDs


Hope this helps

John

[dmgdesigns]

Thanks for the input.

Disconnect the bridge output and see if the fuse holds.

The fuse did not blow.

Do you have a minimum 2kva transformer?

Yes

What do you need rapid bleed down for?

I read another post here or on another forum saying this would help. I used it only temporarly thinking it would help with a problem I had with the breaker tripping. (see below)

Before I moved to a new house, we built a house with a larger garage the machine was on a 15 amp circuit. On this circuit the machine would trip the breaker once or sometimes twice and then work fine. Now I have a new circuit with a 20 amp breaker and the fuse blows before the breaker trips. I thought the new circuit would help with the problem of the breaker tripping.

Thanks for the help. Also I will look into the contactor solution.

[dmgdesigns]

UPDATE: After changing the fuse again everything work OK. I guess the capacitors had a little charge. I still have the problem though.

At this time my simple solution is a box of fuses. And run the machine everyday.

[vger]

If you can find a SPDT relay (or two in series) with about 60 volt coils (perhaps a couple of 28 VDC coils in series) and connect the comon of the contacts to the capacitor + side, then the NC contact to a sizable resistor and on to the + of the bridge, and the NO contact straight to the + of the bridge. Wire the coil across the capacitor. When power is applied the capacitor will charge more slowly with the limited current through the resistor. Then when the voltage reaches the point that the relay closes, you get a straight connection. Might save on fuses.

Steve

[irving2008]

here's an idea I've been working on for my inrush limiter... what do you think? Output is 45v @10A, 500VA toroidal transformer with two paralleled windings.

Not shown is the back-EMF catcher...

[snoopy27]

There are a numbe of slow-blow current limiting fuses that you might be able to replace your current in-rush fuse with for capacitive and inductive loads with. I would not compromise by upping the fuse rating though or changing this fuse if it is also protecting more sensitive componets directly that depend on a quicker response when excessive current draw is present.

[Mariss Freimanis]

irving2008,

That circuit has a serious problem. What happens if the load on the supply is more than 1 Amp while the capacitor is charging during startup? The supply voltage will never reach the rated value, meaning it will be unable to pick up the load attached to the supply. The output will stay stuck at a few volts.

Consider using an inrush current limiting thermistor between the bridge and the bulk capacitor. It is a resistor that has high resistance when cold (a few Ohms) and low resistance when hot (about 100 times less than the 'cold' Ohms). The inrush and load current heats it up in a few tenths of a second, way long enough to charge the cap slowly. Costs about $1.

Mariss

[irving2008]

Originally Posted by Mariss Freimanis irving2008, That circuit has a serious problem. What happens if the load on the supply is more than 1 Amp while the capacitor is charging during startup? The supply voltage will never reach the rated value, meaning it will be unable to pick up the load attached to the supply. The output will stay stuck at a few volts. Consider using an inrush current limiting thermistor between the bridge and the bulk capacitor. It is a resistor that has high resistance when cold (a few Ohms) and low resistance when hot (about 100 times less than the 'cold' Ohms). The inrush and load current heats it up in a few tenths of a second, way long enough to charge the cap slowly. Costs about $1. Mariss

Mariss, thanks for your input. I realise that possibility, but my stepper drives are disabled at switch on and draw less than 80mA until enabled. My original design for an inrush limiter was more conventional and used a thermistor in the transformer primary which was shorted by a relay once the PSU was up to volts. This worked well for a while til the thermistor gave up... three or four thermistors later I decided to look for an alternative as although higher rated products are available they are hard to find in the UK for 'normal' suppliers, so I was looking for a solution that didnt need one and also got rid of the relay (I prefer solid-state solutions). I am also looking at other solutions including using an opti-isolated TRIAC in either the primary or secondary side to control the process.... all experimental of course...

regards,
Irving...

[Mariss Freimanis]

Irving,

The primary is not the ideal location for the inrush thermistor. Here's why: The secondary of the transformer is working into an effective short circuit whilst the bulk capacitor is charging to the eventual supply voltage. During this time the primary develops significant magnetization currents because the transformer is saturated. This kills your thermistor.

The proper place for the thermistor is between the bridge and the bulk capacitor. Magnetization currents stay at sane values then and the thermistor lives.

(I have always wanted to use 'whilst' in a sentence.:-)

Mariss

[irving2008]

Originally Posted by Mariss Freimanis Irving, The primary is not the ideal location for the inrush thermistor. Here's why: The secondary of the transformer is working into an effective short circuit whilst the bulk capacitor is charging to the eventual supply voltage. During this time the primary develops significant magnetization currents because the transformer is saturated. This kills your thermistor. The proper place for the thermistor is between the bridge and the bulk capacitor. Magnetization currents stay at sane values then and the thermistor lives. (I have always wanted to use 'whilst' in a sentence.:-) Mariss

Interesting...yet all the designs I have seen do so...