Current sink with delay

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mrd

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Hi,

I am trying to get a current limiter with a delay working. (feeding a dc dc **broken link removed**)

At the moment it works without the delay section in. Limits to around 130mA. However with the delay part in, it delays the start correctly, but then doesn't start the DC-DC correctly. It kind of clicks on, clicks off, clicks on, clicks off. Almost like the MOSFET isn't closing enough when current gets drawn.

Can anyone see anything that would make this circuit work correctly?

I have also tried a 2N7000 but that does not work either.

http://i47.tinypic.com/142yc6h.jpg

Thanks for any help
 
What feeds the +16V? Any chance it is not a well regulated voltage source?
 
MikeMl said:
What feeds the +16V? Any chance it is not a well regulated voltage source?
Click to expand...

Hi, thanks,

It's a well regulated linear +-16V supply feeding it.

From the sound of it, it sounds as though it starts ok, but just before it reaches a steady state it stops trying. (usually takes 4seconds, it cuts out about 2sec) Then tries again.

Could the MOSFET be switching off when a lot of current gets drawn?

Any ideas on anything I can try?

Thanks again
 
What is the resistance of the filter choke?

Is it a +16V/-16V supply (i.e. a 32V supply with a center tap), or just a single 16V supply?
 
Last edited:
Hi, it's 3.9ohms.

Yes. Dual supply with centre tap. +16V and -16V
 
Last edited:
mrd said:
Hi, it's 3.9ohms.

Yes. Dual supply with centre tap. +16V and -16V
Click to expand...

That's your problem. What is that pi-section filter supposed to do? Try remodeling your circuit in LTSpice with the series resistance of the choke included. Post the .asc file, and I can try it, too.
 
Last edited:
Centre, Eh?

Canuck, Aussie, or Brit?
 
Last edited:
MikeMl said:
Centre, Eh?

Canuck, Aussie, or Brit?
Click to expand...

Brit.. and you must be ... Poirot??

The PI filter is like that mainly for space reasons (I guess ideally I'd want one on each rail with caps going to 0V?) Am I doing more harm than good with it?
 

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One problem I noticed right away, look at the power dissipation in the 2n4401 current regulator! It needs to be a power transistor, probably on a heat sink.

Why the slow start up on the +-16V supplies?

I'm married to a Canuck.
 
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The DC-DC converters you showed in your link? None of them will work on a 130mA constant-current input?
 
Slow power start doesn't mean anything. Just something I was 'trying'

The 2N4401 in real life is a KSD1273, but haven't got a model for spice for it.

Had to google what a Canuck is! Never heard that before

The DC-DC I'm using is the 6W 24-12 dfe. I only need to limit the current for the inrush.
After that, my circuit only draws around 100mA.

It works with the constant current when I try without the delay. Just doesn't start with the delay in there.
 
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What is connected to -Vin? For sim, you can use LTSpice's built-in 2N3055 for a high power NPN, although it doesn't have the high hfe that the KSD1273 has.

Canucks spell "center" centre, too
 
MikeMl said:
What is connected to -Vin? For sim, you can use LTSpice's built-in 2N3055 for a high power NPN, although it doesn't have the high hfe that the KSD1273 has.

Canucks spell "center" centre, too
Click to expand...

-Vin is connected to the DC-DC converter. When RC is joined to -Vin the converter turns on. Datasheet says when "RC terminal open, output is OFF; RC terminal to -Vin (0-0.4V), output is ON" Could the MOSFET be dropping more than 0.4V across it?

center certainly makes more sense!
 
I think you need a "snap-action" hysteresis on the RC input. As the capacitor connected to the gate of the FET charges, the RC input to the converter begins turning it on, causing it to draw current, which drops the supply voltage to the gate circuit, which then turns RC off, etc.

Add a second inverting stage to the Fet gate so you can create some positive feedback to get a bistable delay circuit. (Or use a 555).
 

Hi,
Thanks. Yes, snap-action is definately what I need I think. I've been thinking maybe to use the fet to switch a low power (2880ohms) relay connected to the RC and -Vin, but I think your suggestion might be easier to implement and take less space.

I'm not familiar with exactly what you suggest in regards to a 2nd inverting stage. Are there any links you know of that I can update my knowledge of this?

Thanks again
 
Last edited:
Hi,

Also, I've just been 'real-life' simulating. with a switch wired to the RC and -Vin, which I'm switching after a second or so.

One thing I'm noticing is when I turn on the first time. I read 130ish mA current. All good.
However, when I cut power, wait 5 sec or so and then retry, the current reading goes up.
3rd time, I get around 150mA.

What would be the reason for this? How could I rectify it to keep current low each time?
 
Sorry for all the posts, was just thinking maybe I should just use some kind of relay circuit instead of the mosfet to switch the RC pin.

Maybe like this? Any reason why this wouldn't work well?

**broken link removed**
 
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