Limiting the current in ATX suply - Page 1
Forum section: electronics / beginners

author: deshmukhmalhar
author_rank: Contributor
timestamp: January 26, 2016, 10:08:08 am
content: I use a ATX supply for all of my projects. However, last night I let the magic smoke escape from two voltage regulators,burnt few wires and managed to save the arduino. So I want to limit the current. It will be nice if a little modification in current limit circuit of the supply itself.( There must be some current limiting circuit in each supply.)Thanks.-Malhar.

author: bktemp
author_rank: Super Contributor
timestamp: January 26, 2016, 10:31:56 am
content: Most ATX supplies do not have any current limiting, only a short circuit protection for the current at the primary side.It will not be accurate, but you can limit the output current by injecting a voltage into the voltage regulation loop:You need to sense the current in the GND wire using a small current shunt (to avoid a high voltage drop) and amplifiy the signal. Then connect the voltage over a diode to the feedback pin of the control ic. This can be a TL431 or the often used TL494(=KA7500). Here it is pin 1 or 16 depending on the actual circuit. Whenever the amplified current signal goes above 2.5V, the output voltages will be reduced.The current regulation can be improved with a proper current control loop, but it will be much more difficult than the crude modification I described.Or simply buy a proper power supply with adjustable current limit. 1A is probably more than enough for most arduino projects.

author: Paul Price
author_rank: Super Contributor
timestamp: January 26, 2016, 11:24:55 am
content: I always use a locally mounted 3-term regulator (7805, LM317) on a heatsink as required). This gives better regulation and complete protection from fire alarms waking the children at night.For all my solderless breadboards, I mount a heatsink (free in cost, plucked from a discarded PC MB north or south bridge chip) and install a LM317 or LM7808 on it to provide current-limited and clean power for my projects. There is usually just the right amount of space on a solderless breadboard near the banana jack area to mount the heatsink-regulator with one or two screws.If you want it is easy enough to make a small perfboard or PCB to be externally surface mounted (with spacers) on the ATX case to provide current limited and especially clean power for your projects. These small satellite boards exploit the convenience of "current-limited aux outputs" feature option of your ATX precision lab power supply. A small trimpot on the LM7805 or LM317 can easily set each custom output voltage to a fine accuracy and optional banana jacks provide easy output connections. In the case of adding banana jacks, a glass-expoxy FR3 PCB is usually best to to quickly make to provide robust support for banana jacks.

author: deshmukhmalhar
author_rank: Contributor
timestamp: January 26, 2016, 11:40:48 am
content: Quote from: bktemp on January 26, 2016, 10:31:56 amNope,I need up to 5 A depending on what I am doing. Still, I'll try your solution.

author: Paul Price
author_rank: Super Contributor
timestamp: January 26, 2016, 11:52:27 am
content: It is easy to boost the output of a LM317 or adj'able 7805 regulator with a power transistor to give you all the current you need.There are also 5-Amp versions of linear regulators cheap enough to buy on ebay, but the large heatsink/fan is a bit of a bother.See datasheets for each for schematic. It is also easy enough to make a very simple one op-amp and darlington pass transistor regulator.I use low cost PICC MSOP MCU's to control output voltage and current using a national/TI switchmode regulator with comparators(LM317) in the feedback loop to set both output voltage and current and have a 2-4 digit 7-seg display(at least on one of them to pimp up the precision lab P/S look.) The output current and voltage are set using filtered PWM outputs of the MCU connected to the comparators to set current and voltage. Same scheme works as well with an op-amp darlington pass transistor circuit using a tiny (.01-ohm) current sense resistor (op-amp amplifies sense voltage x100) to feed into A2D of the MCU.) Easy enough to get precise voltage and current with software dithering and setting is done with two small knob-pots feeding A2D inputs.The idea in this case is to have the MCU monitor and control a 5-amp switching reg to set(pre-regulate) the voltage input to a linear stage to get just the right amount of voltage to limit power dissipation in an op-amp-darlington linear output stage.Sometimes a small fan may be required. A thermistor can easily control the speed of a 12-V fan using a 555 timer PWM circuit.

author: deshmukhmalhar
author_rank: Contributor
timestamp: January 26, 2016, 11:55:55 am
content: Quote from: Paul Price on January 26, 2016, 11:24:55 amI had a 7805 with a huge heatsink , which I smoked.After that I realised that I can use 5V from ATX itself.I am building a tiny board with LM317 for breadboard.I'll take your suggestion about banana jacks.

author: deshmukhmalhar
author_rank: Contributor
timestamp: January 26, 2016, 12:03:15 pm
content: Quote from: Paul Price on January 26, 2016, 11:52:27 amLike the uSuply?I want to do it. But in vacetions, If I take this big project in works, my parents will stop my E stuff being 250km away.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2016, 02:04:22 pm
content: Have you considered a Polyfuse (PTC thermistor)?Or a circuit breaker? A self-reset automotive circuit breaker might be just what you need, and you should be able to find them at an auto parts shop anywhere in the world.