Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

supply through series resistor problem

Status
Not open for further replies.

brownt

Member level 3
Joined
Sep 25, 2017
Messages
63
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
597
How can this be done.

I have 5 volts coming out of an enclosure and entering another enclosure through a patch cable. The voltage needs to be current limited so that a user does not inadvertently short the supply as the patch cable and the various devices that can attach to it are designed to be plug and play and so it is quite possible that a user might plug something unexpected in, or do something to short the supply.

At times the 5v is simply across a potentiometer, with the wiper feeding back to the other enclosure, and for that application the limiting resistor is not a problem. However, the five volts is sometimes used to power an op-amp and uC circuit (about 40mA total) that doesn't like working through the series resistor.
What actually happens is that the output of the op-amp circuit , which is configured with some gain and a 2.5v swing around zero, starts to pulsate. The uC circuit seems to work ok, though I guess it is unstable.

What about a charge pump, would that make any difference, or what else can be done.
 

Hi,

You need two things:
* generate the 5V
* and current limit them

There are 5V regulators. They will "generate" clean 5V.

For current limit: (two simple ways)
* either choose regulators with current limit feature (infinite short circuit duration)
* or add a polyfuse in series with the 5V line.

There are many other possible solutions.

Klaus
 

Hi,

You need two things:
* generate the 5V
* and current limit them

There are 5V regulators. They will "generate" clean 5V.

For current limit: (two simple ways)
* either choose regulators with current limit feature (infinite short circuit duration)
* or add a polyfuse in series with the 5V line.

There are many other possible solutions.

Klaus

I see. The 5 volts is already coming from a regulator. The 'fix' needs to be within the supplying enclosure. Else a user will be able to short things.

A regulator with infinite short feature, great. But it will have to be in the future. Because the hardware is already mostly set.

A polyfuse would still allow the circuit to be shorted, wouldn't it? and so the power would go off.
 

Hi,

A polyfuse would still allow the circuit to be shorted, wouldn't it? and so the power would go off.
No circuit can prevent an output to become shorted by a user.

If you wanted the voltage NOT to go off during short circuit, then you need an infinite high supply current and zero series resistance.
Both is impossible.

*****
There are "operation" conditions:
* in your case: 5V, 40mA.....means even with a load current of 40mA the output voltage should remain 5V (with some tolerance)

And there are "fault" conditions.
* like: short circuit. Then the current is above 40mA and the voltage is accepted to drop.
During those" fault conditions the device does not operate (properly)

Decide which operation "operation" condition and what "fault" condition you have and specify the limits.

Klaus
 

If you can get inside the source enclosure, the simple solution is to fit a current limiting circuit BEFORE the 5V regulator. That way the voltage remains constant until the current limit kicks in.
What actually happens is that the output of the op-amp circuit , which is configured with some gain and a 2.5v swing around zero, starts to pulsate.
This might be a different issue, simply adding resistance in the supply rail increases it's impedance and is likely to make the amplifier unstable. You could try adding a capacitor (~100uF) across the rails AFTER the limiting resistor (on the op-amp side) to reduce the impedance.

Brian.
 

Cap.ok I’ll try that. Though I guess with a 125ohm limit resistor there is a large voltage drop. Thought that might be the problem. What does the limit circuit before the regulator look like?
 

Before the regulator it can just be a resistor but, as betwixt mentioned, the difference is that the resistive drop will have no impact on output voltage until right when it reaches the limit.

For example if its a 12->5V regulator you can put an R on the regulator input. 100 ohms would be a roughly 70mA limit (not factoring dropout). The 5V output will be perfect until the input voltage of the regulator drops to its input limit, perhaps 5.5-7V.


There are also millions of 5V current limiting IC's which are essentially solid state fuses:
https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4789.html

Note you mentioned poly-fuses earlier: be careful, they're great in theory but in practice they're pretty poor (very wide temperature variation, high trip current, low hold current etc). The above IC or one like it may be nearly as cheap/small.
 

Here's a simple limit circuit you can add before the regulator.
It uses two resistors and two transistors.

It works better than a single resistor if the voltage between the input and output of the regulator is small.

Capture.PNG
 
Last edited:

Hi,

Not sure it would work or if it did, well - using a PMOS as a pass device at the output instead of at the input as a reverse polarity protection device might be an option.
 

Before the regulator it can just be a resistor but, as betwixt mentioned, the difference is that the resistive drop will have no impact on output voltage until right when it reaches the limit.

For example if its a 12->5V regulator you can put an R on the regulator input. 100 ohms would be a roughly 70mA limit (not factoring dropout). The 5V output will be perfect until the input voltage of the regulator drops to its input limit, perhaps 5.5-7V.


There are also millions of 5V current limiting IC's which are essentially solid state fuses:
https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4789.html

Note you mentioned poly-fuses earlier: be careful, they're great in theory but in practice they're pretty poor (very wide temperature variation, high trip current, low hold current etc). The above IC or one like it may be nearly as cheap/small.

I see. That's a simple solution. though there is a lot power, correct? So I would need about a 2Watt resistor
 

I worked on an intercom system whose power supply had shunt regulation as a 5W 5V zener diode. Upstream was a safety resistor (low ohms, physically large). 16 intercoms in the building, receiving 5V. The earpieces and mouthpieces were the old-fashioned kind which ran on little current.

If a short circuit were to occur anywhere anywhere in the system (due to fault, error, soldering, testing, etc.), the safety resistor was able to withstand the entire surge. Afterward operation returned to normal, no harm done.
 

Yes although the good news is it’s taking those watts from the regulator and substantially limiting the peak total watts they’ll ever see.
 

Do you know what the input voltage is to the 5V regulator?
That determines the maximum resistor you can use at the regulator input to limit the current.
 

Do you know what the input voltage is to the 5V regulator?
That determines the maximum resistor you can use at the regulator input to limit the current.

Yes, it is 9.9 volts
 

Yes, it is 9.9 volts
Depending on the regulator dropout voltage, that means you can have only about 2-3V drop in any current limit resistor in series with the regulator input.
Thus for 40mA of current, the maximum resistor value would be 2/40mA = 50 ohms.
That means the short circuit current would be about 8V / 50 = 160mA.
The resistor will be dissipating about 1.3W so a 2W resistor should be sufficient.
 

The voltage needs to be current limited so that a user does not inadvertently short the supply

If an user accidentally shorts the power supply there is nothing much you can do. But local regulation on each unit is not a bad idea and each module should be provided with a short circuit and current limit so that other modules are not affected. If the power demand is reasonable and you have some voltage headroom, you should use the good old three legged regulators; but if you want to go with a simple resistor use also a reserve capacitor to take care of surge current demands. Both are energy inefficient but the regulators provide many useful features.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top