current limter

[apprentice]

hey guys im currently building a dual power supply and need to incoorporate a current limiter, i know you can place a high wattage variable resistor in series with the output but is there a better way without getting to many components involved? (space is very limited).
thanks looking forward to your reply.

[Chris Smith]

Yes
There are many ways. <p>One is to use a digital pot, hooked up to a Fet. <p>You can also use ordinary means to control the Fet. <p>The old fashioned way is to use a ten turn pot, wire wound, heavy amperage handling.

[rshayes]

One method is to put a low value resistor in series with the output. The voltage across this resistor is used to turn on a transistor when it reaches about .5 to .6 volts. When the transistor turns on, the collector current can be used to remove the base drive from a series pass transistor. This is not a particularly accurate method, but it works well for protection purposes.

[Bernius1]

Or an incandescent bulb in series. Also will act as a fuse.

[terri]

The bulb is my favorite Q&D R&D current regulator/limiter. Their resistance is low when cold, and high when hot. Excellent for charging completely dead batteries, for example.<p>(Q&D R&D = "Quick and Dirty Research and Development" --meaning basement and backyard experimentation.)<p>Incidentally, a good source of high wattage resistors (for low frequencies) is old hair dryer coils. And you can clip a lead to anywhere along their length. (More Q&D R&D stuff.)<p>[ February 04, 2005: Message edited by: terri ]</p>

[haklesup]

I cannot completely answer without seeing the regulated but unlimited circuit you are trying to modify. A series resistor is among the simplest ways to measure current so that it may be fedback to the comparitor and clamping stages. Choosing a small value resistor will reduce the power wasted in it and reduce the necessary size but the voltage drop will be that much smaller requiring more gain in the comparitor section and therefore less accuracy.<p>A longer ansswer:<p>A current limit circuit basically has three parts. One part to measure the current at the output, a second part to represent a reference current current with a way to compare it to the output current and a third part to use the output from the comparitor to divert power away from the input stage so that the output is reduced (a clamp).<p>In many DC supplies, the first part is acomplished by a series resistor. The voltage produced across the series resistor can be applied to the compartitor section directly. Other times you may not want the extra impedance of the series resistace since it acts to limit max current. In this case, you can look to some other part of the circuit that will vave a voltage proportional to the output current. <p>The comparitor section can be in fact an opamp setup as a comparitor or as simple as a voltage divider network. What is important is that when the output current signal exceeds the reference voltage the circuit applies a bias to the clamping section.<p>The clamping section often takes the form of a transistor where the base (gate) is connected to the comparitor output (with whatever bias resistors that may be needed to make the signal compatable) and the Emitter (Source) grounded and the Collector (Drain) connected to a point (in the input stage) that will reduce or turn off the output (often the base or gate of the output driver transistor)<p>To acomplish a variable curent limit you can either switch out the series resistor and compare the new voltage drop to the same reference. This gives a limit with stepped ranges. Or you can leave the current sense part alone and vary the reference voltage or gain of the comparitor stage. This results in a continuously variable limit.<p>Shorter Answer:<p>If space is limited and only one value non-variable current limit is required then choosing a primary transformer with the right max current will intrinsically limit the whole circuit without any additional components. It is not necessarily precise but adequate for many applications. This is typically where the current rating on a wall wart comes from, no complicated limit in there.<p>This all applies to linear supplies and amplifiers. Switchers are a whole different breed.<p>[ February 04, 2005: Message edited by: haklesup ]</p>

[viveguy]

You could also use "toaster wire (nichrome)", like a heat gun element. Some high current lab supplies use this method.

[apprentice]

thanks guys for all the reply, i think im a little overloaded with info but im trying to get it all in. what stephen said sounds simple do you have a circuit diagram?

[rshayes]

This web site (www.rason.org/Projects/discreg/discreg.htm) shows a fairly typical series regulator circuit. The parts involved in current limiting are R2 and Q4. When the output current reaches about 1.5 amps, the voltage drop across the .47 ohm resistor will be about .7 volts. This turns Q4, a 2N3904, on and diverts current from the base of the two darlington connected transistors Q2 and Q3.<p>The current limit will be set by the value of R2. A .68 ohm resistor would limit the output current to about 1 amp.<p>This circuit uses a 1N4001 in series with the base-emitter voltage of Q1 as a reference voltage. Normally, a zener diode in the 5.6 to 6.8 volt range would be used instead since this results in a temperature coefficient closer to zero than two series connected diodes. Other than that, this is a fairly standard circuit.