Simple Voltage Regulator and Wiring Question - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: July 12, 2014, 03:30:35 am
content: I've never used a voltage regulator before and am somewhat new to circuits.I'm curious about an LM78xx voltage regulator, if you connect the input and ground pins to the voltage source, without connecting the output pin will the regulator heat up?The reason I'm asking is I'm curious whether the ground pin is necessary on it if the circuit will be close (if the output pin was connected of course) with just the input and output voltage?Also an unrelated question about wiring a switch. Do you usually wire it to the positive (red) wire or to the black wire and why if it will work both ways?

author: Stonent
author_rank: Super Contributor
timestamp: July 12, 2014, 04:23:46 am
content: I don't think it will heat up because most wall-warts have a regulator and they don't die without a load, but even if they did overheat they will cut out and reset.The ground pin is required for them to work. Afrotechmods has a good video on 78xx regulators.

author: IanB
author_rank: Super Contributor
timestamp: July 12, 2014, 04:27:40 am
content: Three terminal voltage regulators like the 78xx have three terminals to be connected. The input terminal is connected to the supply voltage, the common terminal is connected to the common ground or reference rail, and the output terminal with the regulated voltage is connected to the load. Three terminals is the minimum number to achieve voltage regulation and the device will not work unless all terminals are connected as per the data sheet.On switches: As far as interrupting the current is concerned a switch will work just as well if placed either in the out or the return wire, however it will do a better job of isolating active voltages if it is placed in the live wire. If the switch is placed in the return wire then the source voltage will still reach the load and this could in some circumstances be a source of danger to components or humans. It is safer to place the switch where it both isolates the voltage and interrupt the current.

author: theatrus
author_rank: Frequent Contributor
timestamp: July 12, 2014, 04:29:32 am
content: Quote from: geratheg on July 12, 2014, 03:30:35 amMany regulators do not have a minimum load, but you should always check. You should however keep the output capacitor in place.Quote from: geratheg on July 12, 2014, 03:30:35 amVoltage is measured between two points. Without a ground there is nothing to reference and the regulator behavior will not regulateQuote from: geratheg on July 12, 2014, 03:30:35 amEither will work - convention is to disconnect the positive lead and maintain a ground in the circuit

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2014, 05:07:12 am
content: Quote from: geratheg on July 12, 2014, 03:30:35 amFirst hit from GoogleLM78xx datahttp://www.fairchildsemi.com/ds/LM/LM7805.pdfThen look in datasheet forTypical ApplicationsThe internet is more important than any test gear...use it to your advantage.

author: geratheg
author_rank: Regular Contributor
timestamp: July 12, 2014, 07:40:47 am
content: I appreciate the replies.I was just curious whether the ground pin needs to be connected or does the ground pin need to be disconnected while the regulator is not in use (meaning the output pin is not connected)?Let's assume I'm using a regulator with a 5V output.If I wanted to open the circuit where the output (5V) connects, would I just need to open that? Would it be okay to keep the input and ground connected and the 5V disconnected?If so, would it be more efficient if ground was disconnected since the voltage regulator wouldn't draw any extra "small" current while it is not in use?

author: rolycat
author_rank: Super Contributor
timestamp: July 12, 2014, 08:32:37 am
content: Quote from: geratheg on July 12, 2014, 07:40:47 amYes, it would be fine. The only disadvantage is that the 7805 regulator will consume around 5 mA without a load.QuoteYes again, but as others have said it's more usual to disconnect the positive supply connection if you want to switch off your circuit.

author: geratheg
author_rank: Regular Contributor
timestamp: July 12, 2014, 07:06:02 pm
content: Thank you for all the replies.