current limit and controlling variable d.c power supply - Page 1
Forum section: electronics / testgear

author: R_G_B_
author_rank: Frequent Contributor
timestamp: February 03, 2014, 02:48:53 pm
content: Hi, I have a 30v 3amp power supply. With no current limiting or control. Would it be possible to do this by putting a multimeter in series with a variable resistor just a thought?Your help or suggestions are much appreciated.Thanks in advance

author: papo
author_rank: Regular Contributor
timestamp: February 03, 2014, 05:42:30 pm
content: I don't think either of this would work. If you put a multimeter in series, presumably to measure the current, you would still not have any means of limiting the current. Save from the fuse in your meter but that's probably not what you had in mind...?As for the variable resistor for voltage control: You want to hook the potentiometer across the 30 V and grab a voltage somewhere between 0 V and 30 V at the wiper? That's not a good idea for two reasons: First, typical potentiometers are not rated for high currents. So you can't get any serious current in the first place. Second, whatever you hook up to that potentiometer, i.e. your circuit, acts as a load resistor. That load is effectively in parallel with the lower part of the voltage divider inside the potentiometer. Assuming you set the potentiometer to some output voltage and hook up your circuit, that voltage actually changes. Same if your circuit changes its resistance / current consumption.You run into the same problem if you use the potentiometer as rheostat: That resistor then forms a divider with your circuit.Assuming you don't want to tamper with the existing device, the proper way of doing this would be to design a switcher between the 30 V supply and your circuit. You can also add a linear regulator in series to reduce noise, and/or incorporate current limiting there.RegardsMatt

author: papo
author_rank: Regular Contributor
timestamp: February 03, 2014, 05:48:46 pm
content: Re-reading your post, you might also have a variable supply up to 30 V and are just interested in current limiting using both the meter and the variable resistor. I still don't see how you would achieve that since you would essentially run into the same load problem I discribed above. Maybe a schematic would help.In any case, this is typically done by a low-resistance shunt resistor measuring the drop across that shunt, calculating the current out of that drop and the known shunt resistance and feeding the result into a controller that turns off the supply or regulates it in case of an over-current condition.Using this strategy, you could cut off your device rather easily when the over-current condition occurs, but it takes more than just a potentiometer and a shunt. Regulating the voltage to essentially get constant current control is a bit more difficult.Matt

author: con-f-use
author_rank: Unknown
timestamp: February 03, 2014, 07:15:16 pm
content: You can limit the current by hand with a variable resistor and a multi-meter. Your variable resistor needs to be able to handle a fair bit of wattage, i.e. dissipate all the heat without burning up. You'd go about it by cranking up the resistance as high as possible, then turn on you power supply. After that you successively lower the resistance of your varistor until your meter reads the desired current. That however manual, slow and there for a hassle and potentially dangerous.Constant current limiting in almost every commercial power-supply relies on analogue feedback loops of some sort. You seem to be a novice at electronics in general, so I can only recommend you to take a look at thisseries of tutorials:This will teach you basic transistor-based current limiting. Dave has also described how to build a constant current load, which can be used to limit the current of a power supply:

author: R_G_B_
author_rank: Frequent Contributor
timestamp: February 03, 2014, 09:23:18 pm
content: Seeing if its possible after seeing Dave's videos on constant current and dumby loads I thought along that ideas a possibility.