Short circuit protection for my voltage source

Thread Starter

xxxyyyba

Joined Aug 7, 2012
289
Can someone suggest me some short circuit protection schematic? I have 12V DC source (1.2A max. output current) and I will use it in some amateur electronics projects, but I would like to add some short circuit protection (between voltage source on one side and load on other side). I found some schematics on web involving BJTs but I would like to see opinion of professionals here :) Thanks in advance.
 

dl324

Joined Mar 30, 2015
9,150
I have 12V DC source (1.2A max. output current) and I will use it in some amateur electronics projects, but I would like to add some short circuit protection (between voltage source on one side and load on other side).
It depends on your design objectives.
  • Do you want to interrupt current above a certain amount?
  • Do you want to provide max supply current into a short?
  • Do you want to fold back current to some level when some maximum is reached?
  • Do you care if sensing current affects voltage regulation?
 

Thread Starter

xxxyyyba

Joined Aug 7, 2012
289
It depends on your design objectives.
  • Do you want to interrupt current above a certain amount?
  • Do you want to provide max supply current into a short?
  • Do you want to fold back current to some level when some maximum is reached?
  • Do you care if sensing current affects voltage regulation?
I would like to interrupt current. Source max. output current is 1.2A, When load draws about 1A I would like to interrupt current to prevent source damaging.
 

dl324

Joined Mar 30, 2015
9,150
When load draws about 1A I would like to interrupt current to prevent source damaging.
  • A fuse will interrupt current, but trip point is not precise.
  • A PTC fuse will reduce current significantly, but will not interrupt. As with a "wire" fuse, trip point is not precise.
  • You could insert a relay (mechanical or solid state) controlled by some current sense circuit. This is probably your best bet, but current sensing will affect voltage regulation.
 

MikeML

Joined Oct 2, 2009
5,444
Normally, such current limiting is installed inside the power supply, not external to it. That way, the voltage drop caused by the current-sensing "shunt" resistor is inside the voltage regulation feedback loop, so that the regulator compensates for the voltage drop....

Look up the app notes for an old LM723; they show examples of how to add simple or fold-back current limiting without degrading the voltage regulation.
 

PeterCoxSmith

Joined Feb 23, 2015
148
Can someone suggest me some short circuit protection schematic? I have 12V DC source (1.2A max. output current) and I will use it in some amateur electronics projects, but I would like to add some short circuit protection (between voltage source on one side and load on other side). I found some schematics on web involving BJTs but I would like to see opinion of professionals here :) Thanks in advance.
There is a range of current limit devices from TI that operate in constant current mode...

http://www.ti.com/lsds/ti/power-management/fixed-current-limited-switch-products.page
 

PeterCoxSmith

Joined Feb 23, 2015
148
Can someone suggest me some short circuit protection schematic? I have 12V DC source (1.2A max. output current) and I will use it in some amateur electronics projects, but I would like to add some short circuit protection (between voltage source on one side and load on other side). I found some schematics on web involving BJTs but I would like to see opinion of professionals here :) Thanks in advance.
...or this might do the trick...

http://www.ti.com/product/tps25944l
 

AnalogKid

Joined Aug 1, 2013
8,153
There are three basic ways of limiting current in an error condition. First, a fuse, but even a "fast-acting" fuse is relatively slow compared to how fast a transistor can go poof. Next up is foldback current limiting. This is essentially an electronic fuse or circuit breaker. When the output current exceeds a set point, the circuit reduces the output voltage to almost zero, preventing almost all current flow. Usually you reset things by removing power briefly. This is great at protecting parts, but not very diagnostic; all problems look the same, no output.

Last is probably what you are asking about, constant current limiting. In this case, when the output current set point is exceeded the power supply adjusts itself by turning down the output voltage until the current is below the set point again. Depending on how bad the overcurrent situation is or isn't, the output voltage might sit there are 50% or so of the intended output value, waiting for the overcurrent to correct itself. A typical situation might go like this: You have a new audio amp design that should draw about 1 amp when running, but only about 1/10 amp if there is no input signal. So you set your power supply for 12 V, set the current limit to 0.2 A, and fire it up. If everything is wired correctly, the amp sits there nice and happy and the power supply output stays at 12 V. But if there is a wiring problem, you might see the power supply output drop to 8 or 9 V, an indication that something is trying to draw more a little more than 0.2 A, or drop to 2 or 3 V, indicating that something is trying to draw much more than 0.2 A and blow up, but the supply is limiting the available current (and power).

As mentioned, there are many current limiter schematics on the web. Many of them fall into 3 or 4 basic approaches to the problem. Shop around, find one that you think might work for you, and post it here for comments.

ak
 

crutschow

Joined Mar 14, 2008
23,531
Below is the LTspice simulation of a circuit that limits the current to 1A with less than 100mV drop (50mV across the R5 shunt and the rest across the ON resistance of the MOSFET) below the current limit point. Thus it does not seriously compromise the regulation of the power supply.

This is a "hiccup" type circuit that stops the current for about a half second if the limit is exceeded and then tries to restart. It keeps doing that until the overload is removed.
(Rather like an electronic fuse with an automatic reset.)
This limits the average power dissipation in the MOSFET to less than a 100 mW for long duration shorts so it doesn't need to be on a heatsink as a linear current-limit circuit would require.

The simulation shows the current for loads of 11 to 20 ohms with a short being applied by switch S1 at about 1s for a 1s duration. As you can see the average current is limited to approximately 1A maximum.

The P-MOSFET can be any 20V or higher device that has a low ON-resistance rating (<50mΩ).

Current Limit.PNG
 

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