Different between 78xx regulator and zener diode ? - Page 1
Forum section: electronics / beginners

author: Nhan95
author_rank: Contributor
timestamp: April 17, 2015, 02:22:33 pm
content: What's the differences between 78xx and zener diode for regulator ? Which should we use ?

author: DmitryL
author_rank: Regular Contributor
timestamp: April 17, 2015, 02:29:04 pm
content: 78xx is a series voltage regulator and has 3 pins. Zener diode is a shunt regulator and has 2 pins

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 17, 2015, 02:31:34 pm
content: My understanding is that 78XX regulators are more stable and which means they are more tolerant of fluctuations in the input power. Also, a zener shares the current with the load whereas a voltage regulator pulls a constant for itself. I posted a source below that gives a good explanation of the differences/applications for each.Source:http://www.edaboard.com/thread37825.html

author: smjcuk
author_rank: Frequent Contributor
timestamp: April 17, 2015, 02:38:54 pm
content: Simple zener shunt regulator has more power dissipation than a fancy transistor linear regulator meaning they are less efficient. Also linear regulators are more stable as the zener isn't loaded heavily. The zener voltage changes with load.78xx's are noisier when they explode though.

author: JoeN
author_rank: Frequent Contributor
timestamp: April 17, 2015, 03:08:06 pm
content: Quote from: smjcuk on April 17, 2015, 02:38:54 pmHas that happened to you? Never happened to me. I read that 7805's have thermal shut-down built in. I've had caps explode, that was my fault though.

author: smjcuk
author_rank: Frequent Contributor
timestamp: April 17, 2015, 03:23:45 pm
content: Quote from: JoeN on April 17, 2015, 03:08:06 pmIt was indeed my fault. This was in 1992 and I was trying to build by own switch mode power supply. The output had lots of ripple on it (because my design sucked) so I thought I'd stick a linear regulator on the end (a 7812) and a big electrolytic capactor to get the desired 12v DC (sucks I know but I had no idea what I was doing then). 20 minutes of soldering later and I had mains live connected directly to the common pin due to a solder bridge. Fired it up, boom, bits of TO220 everywhere. I still have a scar on my right hand where it blew.I have a history of making things explode hence the avatar which was a recent victim (BC548, forgot load resistor, hello 30v 5A Vce!)Edit: have a larger picture of a blown up BC548:

author: rdl
author_rank: Super Contributor
timestamp: April 17, 2015, 05:40:32 pm
content: I built a device a couple of years ago and used a zener regulator for the power supply. I highly recommend you do the same so that you will understand how poorly they regulate and will know to avoid using them whenever possible in the future.

author: retrolefty
author_rank: Super Contributor
timestamp: April 17, 2015, 06:17:00 pm
content: Quote from: rdl on April 17, 2015, 05:40:32 pmToo broad a rule. A zener can be the very best solution to some circuit problem, just not as a voltage source that has to supply any appreciable current.

author: rdl
author_rank: Super Contributor
timestamp: April 17, 2015, 06:33:25 pm
content: Quote from: retrolefty on April 17, 2015, 06:17:00 pmI don't think so, not really. If you need to do something that only a zener can do, then use one. Otherwise don't.

author: edavid
author_rank: Super Contributor
timestamp: April 17, 2015, 07:03:21 pm
content: Quote from: JoeN on April 17, 2015, 03:08:06 pmThermal shutdown doesn't help if you wire them backward

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 17, 2015, 07:06:03 pm
content: Quote from: smjcuk on April 17, 2015, 03:23:45 pmHysterical. The worst I ever did was I got the TO-220 tab branded into my thumb. It smelled like meat. I don't remember why but I decided to firmly grab the case while I knew it was over 100C.

author: smjcuk
author_rank: Frequent Contributor
timestamp: April 17, 2015, 08:18:28 pm
content: Mmmm baconYou never forgot the day you first smell your own toasting flesh.

author: Falcon69
author_rank: Super Contributor
timestamp: April 18, 2015, 04:40:24 am
content: I'm glad someone brought this up.I'm working on a circuit where I have a voltage of, for example 6.7 volts. I need to get it down to a maximum of 5.5v for logic gates. My space is limited on a board, and since the logic gates require very little current, I'm thinking a 5.1v zener would be perfect for this, vs. a voltage regulator (which requires more board space and more components.)Thoughts?Someone did mention they are useless vs a voltage regulator if you need any amount of current draw.

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 18, 2015, 04:55:08 am
content: Quote from: Falcon69 on April 18, 2015, 04:40:24 amI think this would be a perfect situation for them assuming you have a constant input voltage

author: Falcon69
author_rank: Super Contributor
timestamp: April 18, 2015, 04:59:13 am
content: Yes, the voltage is supplied via an actual voltage regulator that supplies that 6+ voltage to another circuit., which is supplied by a walwart.

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 18, 2015, 05:13:19 am
content: Quote from: Falcon69 on April 18, 2015, 04:59:13 amI think this is a perfect application for a zener then!

author: Laura
author_rank: Contributor
timestamp: April 18, 2015, 07:11:59 am
content: Quote from: Falcon69 on April 18, 2015, 04:40:24 amWell, you can't usejusta zener. You'll also need a series voltage dropping resistor that can dissipate enough power (watts) to run your circuit. This may require a non surface mount resistor. I expect this combo to take upmorespace than a TO-92 or SOT-23 regulator.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: April 18, 2015, 07:46:49 am
content: Quote from: Falcon69 on April 18, 2015, 04:40:24 amHow exactly does your actual circuit look? Do you want to regulate the voltage of the supply to the logic chips, the inputs or both?If it's for the supply, and you will only use a low current, you can get away with connecting any bog standard diode (not zener) in series with the supply to the logic chips to produce the desired voltage drop. For example, if you put two 1n4148 in series with supply, that will drop the supply by 1.4 to 2 V depending on the current. Also place decoupling caps to ground on the logic chip side of the diode.If it's for the inputs, a series resistor, maybe 1k, or a series resistor plus a zener might do the trick. This depends on how sensitive the inputs of the logic chips are.A zener regulator isnotrecommended for most applications because it's wasting power. The zener is shunting current to keep its nominal voltage, which means you need a series resistor to limit the current. But the maximum power consumption of the circuit sets a maximum value for the resistor, or else the voltage drop will increase when the circuit tries to use more current, and the resulting supply voltage gets too low. On the other hand, the low resistance value that you need for the series resistance means that if the load drops, the zener will start conducting to make up for the higher voltage. In effect, a zener regulator will approximately always consume the maximum power consumption of the load, if the load is variable.

author: rdl
author_rank: Super Contributor
timestamp: April 18, 2015, 04:40:16 pm
content: Here is a real world example. This is from a project done several years ago. It is just one of many variations I tried before deciding that an LM317 was a much better choice. This circuit was to power a 7660 which in turn had to supply only about 15 mA. As you can see, over a fairly wide range of input voltages the 10 volt zener only regulates between about 10 to 11 volts. If your input voltage and output current is fairly constant a zener can work, otherwise a real voltage regulator will probably be better.