Hey,
so I am making an adjustable voltage power supply and it would be nice to have an overcurrent protection so it doesn't blow up...
the output will reach up to 36v and I can't have the current exceed 10A - the maximum the supply can deliver.
Is there a relatively simple circuit I can create to keep the current limited below 8A (not full 10A to be safe) but that can handle 36 volts?
*I don't want to have to deal with fuses and having to replace them every time the current goes over.
thanks in advance
Hello there!
Fuses are great for overcurrent applications, but you are correct in that they can often be more trouble than they are worth if they need replacing.
There are devices that are fuses, but do not break or fail when exposed to an overcurrent condition. These can be called resettable fuses, polyfuses, or even PTCs. The basic operation is that the resistance of the fuse is dependent on the current flowing through it.
There are two currents associated with a resettable fuse. Hold current and trip current. When the current through the fuse is lower than the hold current, the fuse will have a low resistance, and will act similar to a short circuit. When the current goes over the trip current, the resistance of the fuse will quickly and drastically increase, to where the device acts as an open circuit, which can protect the circuitry after the fuse. When the fuse trips, it will stay as an open circuit until the input current drops below the hold current again, and the device will act like a short again.
If the current is in between the hold and trip currents, the device will behave strangely and unpredictably.
I looked at Digikey and found this. Look through the datasheet for it and let me know what you think.
Sometimes current limiting can be built-into the voltage regulator circuit. There is already a feedback/control circuit so it's not too hard to turn the voltage down to zero. (Over-current and thermal protection are standard features on linear regulator chips, but at 10A you're not using a linear regulator chip.)
You can sense (measure) the current with a Hall effect current sensor or by measuring/monitoring the (small) voltage drop across a (low-resistance) series resistor. If you use a resistor, it should go on the input-side of the regulator so the regulator can still do it's job and compensate for that drop.
And once you detect over-current you need the circuitry to shut off the power.
This sounds like a none question:-
the output will reach up to 36v and I can't have the current exceed 10A - the maximum the supply can deliver.
So if the power supply has a limit of 10A then what imposes that limit? Chances are that this is a current limit circuit in the power supply itself and so adding a second limiter, while it could be done would be unnecessary. The power supply would be still safe.
Adding an adjustable current limit to a bench supply however is a good thing to do and that is not too hard. Simply throttle back the final pass transistor in response to a voltage generated in a current sensing resistor. Their are chips that can do most of this for you if you like.
I would not put a poly fuse in a bench supply, those sorts of things while resetable degrade each time they are triggered and only have a finite number of trips before they stop working. They are great for situations where you want to add protection on installed equipment that will be left, but not much use in a power supply, where they could be triggered many times. The other problem is that they are solw, depending on how much over current they get depends the speed. At about 1% over the trip current they can take up to a minute to trip.
I met someone a few weeks back at an exhibition who had his exhibit ( a fun fair crane grab machine ) fail. He blew the H-bridge from the start up ( stall current ) of the motors exceeding the current rating. He proposed to solve it with a poly fuse but I told him that was useless. However, knowing what sort of person he is, he will not believe me until at least his second H-bridge blows up.
Grumpy_Mike:
Adding an adjustable current limit to a bench supply however is a good thing to do and that is not too hard. Simply throttle back the final pass transistor in response to a voltage generated in a current sensing resistor. Their are chips that can do most of this for you if you like.
could I do this so that current is adjustable but still never exceeds 8A
where could i find an IC for this that could handle 36V and 8A?
could I do this so that current is adjustable but still never exceeds 8A
Yes, just make sure the pot that provides the adjustment of the limit can't be set to more than 8A. Do this by adding a resistor in the top or bottom of the circuit.
This looks like a good place to start to see what you need:-
Make sure the op amp you choose is happy at 38V.
This just came in, it looks like the product you are looking for, works up to 60V max.
PSU protection
Check out the data sheets.
thanks, so much!
that's exactly what I was looking for.
now my power supply won't blow up