Looking for a non-switching power supply...

RussB

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Oct 2003
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I was just asked to find a power supply that will limit its output to 24 VDC at 2 Amps without shutting down completely.
I am not sure that this is even possible but what they are looking for is:

PLC that runs at 2 Watts but can draw up to 15 Amps for 20 mS on power up. There is a slip ring between the PS and the PLC that is rated for 2 Amps at 24 VDC.
"As is" it does not work with a standard switching power supply.

Is there a solution other than the obvious, change the slip ring?
If so what would that be?
 
Try putting a light bulb, say, 40 - 60 watts, or an AC capacitor sized for correct reactance in series with the primary of the power supply?
 
I'm a little confused, perhaps describe better what "does not work" mean. Do you fry the slip ring? Does the power supply just shut down?

Do you really need a linear power supply or just any power supply that will limit current to 2 amps during start-up.

Is the slip ring rated above 24Vdc? Could you run 120Vac to a power supply on the other side of the slip ring?
 
Russ, I'm not sure that what you are asking for is technically possible. For example, if the power supply voltage is 24VDC and the output current is to be limited to 2A, what other choice is there when the load impedance goes below 12 ohms other than to reduce the voltage?

Or, am I misunderstanding what you are asking for?
 
Russ,

as DickDV stated, if current stays at 2 amps, the only choice is to reduce the voltage.

non switching power supplies are calles linear power supplies as far as i know. sola and automation direct are the two i am familiar with. i know there are others.

regards,
james
 
I believe that any standard "short-circuit protected" PSU does exactly that, lowers the voltage automatically.
Here is one for 2.5A (says that the "typical" current limit is 2.6A):
https://eb.automation.siemens.com/m...alogTree&activeTab=order#R&activetab=product&

With a bit of searching, I think you can find one for 2 A.

But I am worried about that maybe the PLC will not "like" to have a low voltage (low, but not zero) on its supply. That would be kind of a "brown out". Normally not acceptable.
 
Curious, what PLC is it that draws 15 A on power up ?
edit: I guess it must be an older type. New PLCs doesn't draw nearly that much.
For example a Siemens IM151-8 draws 1.8 A inrush on power up. And maximum 380 mA after power up.

edit: And what PSU is it that shuts down completely if it encounters an overload ?
 
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Russ, I'm not sure that what you are asking for is technically possible. For example, if the power supply voltage is 24VDC and the output current is to be limited to 2A, what other choice is there when the load impedance goes below 12 ohms other than to reduce the voltage?

Or, am I misunderstanding what you are asking for?

That is about what I told my contact. Technology has advanced but not to the point of overcoming physics. Not yet anyway. :cool:

But I promised to "check around" and the Forum here is a virtual pool of knowledge and experience.

As to Timbert's questions, I do not have any more information than what is in the OP, even that I received third hand.

I did ask that they get me the details of what is being tried now and exactly what the real problem is.
 
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On a standard switching supply, as the current demand increases, the power stage increases the duty cycle of the PWM section to modulate a power transistor. That in turn feeds a PWM wave to a filtering / rectifying / smoothing section. This ultimately becomes your DC output.

The duty cycle changes to keep the output voltage constant. When the modulation is at 100%, there is no longer any PWM wave at the transformer, just a DC signal which cannot pass through the transformer. This results in your output going to 0V. It's a very abrupt change.
 
Here is what I think I understand
1. PLC is mounted on a rotating machine
2. Power fed to PLC via slip ring
3. Slip ring limited to 2 A
4. PLC and other stuff on machine draws more than 2 A on startup.

Possible solution??
Put a 24 V battery on rotating portion. It will supply the surge current. Power supply can trip on overload - in meantime battery will carry the load. After reset the power supply will recharge the battery.

Dan Bentler
 
Without knowing alot of specifics about the application and the mechanics of it all, just bypass the slip ring on power up with a bypass plug in connection. You're not expecting motion on PLC power up, so plug in the bypass, power up, unplug and go.
 
The duty cycle changes to keep the output voltage constant. When the modulation is at 100%, there is no longer any PWM wave at the transformer, just a DC signal which cannot pass through the transformer. This results in your output going to 0V. It's a very abrupt change.
Sounds convincing, but I dont think it can be right. The manufacturers of current limiting PSUs specify the value that the current is limited to (like the one I linked to), and that value is not zero. So I am thinking that the PSUs have a little bit of intelligence built in. Such as, when the voltage cannot be kept even if the PWM is near 100%, then it assumes there is an overload situation and drastically reduces the PWM duty cycle. There must be a regulation loop of some kind.
 

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