Wondering about this circuit - Page 1
Forum section: electronics / beginners

author: RissViss
author_rank: Regular Contributor
timestamp: May 27, 2019, 11:27:43 pm
content: Building a power supply, and wanted to power a fan off of the 19v 7amp laptop brick. Wanted to drop the voltage down to between 10 - 9 volts.In picture 1 I used a 75 ohm 3w 5% axial resister and had a voltage of 10.37.In picture 2 I used the 75 ohm resister and a Zener Diode 9.1v 5w axial and had a voltage of 10.00.Is there an advantage to using the Zener over just the resister?And any other info would help.

author: RissViss
author_rank: Regular Contributor
timestamp: May 27, 2019, 11:28:15 pm
content: extra pics

author: StillTrying
author_rank: Super Contributor
timestamp: May 27, 2019, 11:41:13 pm
content: "In picture 1 I used a 75 ohm 3w 5% axial resister and had a voltage of 10.37.In picture 2 I used the 75 ohm resister and a Zener Diode 9.1v 5w axial and had a voltage of 10.00."You must have the zener the wrong way around.Is there an advantage to using the Zener over just the resister?The voltage at the fan would be more constant, if the fan needs a high start current the volt drop across the resistor can sometimes be enough to stop it starting.

author: RissViss
author_rank: Regular Contributor
timestamp: May 27, 2019, 11:47:59 pm
content: When I flipped the zener it was reading around 5 voltsWhen I used a 100 ohm 1/4 watt resister in place of the 75 the zener was right on the 9.1 volts, but the resister got hot.

author: RissViss
author_rank: Regular Contributor
timestamp: May 27, 2019, 11:51:32 pm
content: could i put the 100 ohm resister after the zener and lower the temps on the resistor.

author: Nerull
author_rank: Frequent Contributor
timestamp: May 27, 2019, 11:59:21 pm
content: Zeners don't provide perfect regulation, its normal for their voltage to vary with current.Increasing the resistance of a shunt regulator will reduce the current in the shunt, but it will also reduce the current you can supply. Essentially, the zener is trying to keep the current through the resistor constant, and will take up any 'extra' current not being used by the load. If the load draws more current than this you will fail to regulate and output voltage will drop.You should figure out how much current your load requires and pick a resistance that produces a voltage drop with that current that is low enough for the zener to regulate. If you're supplying 19V and need 9V out than the voltage drop across the resistor should be less than 10V at max current.

author: StillTrying
author_rank: Super Contributor
timestamp: May 28, 2019, 12:07:27 am
content: From the75 ohm 3w 5% axial resister and voltage of 10.37 at the fan, the fan is drawing (19-10.37)/75 = 8.63/75 = 0.115A.So the resistance is dissipating about 8.63*0.115 = 0.992 W, so a 1/4 W resistor would getquite warm.Anyway you do it there will be 1W to dissipate, for ~10V at the fan. Use the fan to cool the dropper!@NerullI think the 9V1 zener or resistor is in series with the fan to drop the 19V to ~10V.

author: RissViss
author_rank: Regular Contributor
timestamp: May 28, 2019, 01:23:21 am
content: Thanks for the info.Will use the 75 ohm resistor and the zener.

author: digsys
author_rank: Unknown
timestamp: May 28, 2019, 01:30:14 am
content: Quote from: RissViss12V 0.13A is equivalent of a 92R resistor, total power = ~1.6W. With any series solution,somethinghas to dissipate the remaining power.First, a 12V fan can handle a reasonable range of voltage eg 6-8V low (fan speed will be affected) > 13-14V (fan will run a bit faster and maybe hotter),BUT there won't be any appreciable degradation at the slightly higher voltage. They will also handle a short spike of even higher.IF you are not concerned about the EXACT fan speed, then a resistor is well good enough. Any rating from 1W to 2W is fine.The advantage of using a resistor instead of a zener tolosethe ~6V is - when a zener fails, itusuallygoes short cct, a resistorusuallygoes open.Both will have the SAME heat to get rid of. If you need an exact voltage (=speed?), then a resistor / zener regulator, or just use a LM7812 (xx) regulator.If you also want near zero heat, then there are plenty of dc-dc regulator modules for ~3 bucks+ out there. Depends on your criteria.

author: RissViss
author_rank: Regular Contributor
timestamp: May 28, 2019, 01:34:13 am
content: The resistor puts the fan speed down low enough that it is almost silent.I made a mistake when I order the resistors they should have been higher 85 - 90 ohms.The speed with the 75 ohms is good enough and gets the job done.There is little to no heat on the 75 ohm 3 watt resistor.Thanks for the help

author: StillTrying
author_rank: Super Contributor
timestamp: May 28, 2019, 01:34:57 am
content: "Will use the 75 ohm resistor and the zener."LOLThe "Zener Diode 9.1v 5w axial" will do it just by itself, when connected to drop 9V and not just 0.7V.

author: RissViss
author_rank: Regular Contributor
timestamp: May 28, 2019, 01:38:58 am
content: I was using this as my information.https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/zener-diodes/all the circuits had resistors.this is not about be practical, for me this is about learning different things.

author: RissViss
author_rank: Regular Contributor
timestamp: May 28, 2019, 01:43:33 am
content: Just tested, using the zener diode, and the voltage is down to 8.9 volts.thanks for the help.more experimentation is better, never hurts to learn something new.

author: StillTrying
author_rank: Super Contributor
timestamp: May 28, 2019, 01:53:40 am
content: "I was using this as my information.https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/zener-diodes"They're all shunt voltage regulation, which needs a resistor, yours is in series volt dropping rather than regulation."Just tested, using the zener diode, and the voltage is down to 8.9 volts."If your 9V1 zener is really a 5W it won't mind dissipating the ~1W of heat at all.

author: RissViss
author_rank: Regular Contributor
timestamp: May 28, 2019, 02:11:55 am
content: For me this is extra information, that I can use.Much better for me to learn with building and testing then just reading.Now I have a way to get the voltage anywhere from 8.9 to 10.37Electronics is way more fun when you have a power supply.Thanks everyone for the info, if you have any other things I can do with this would be great.

author: digsys
author_rank: Unknown
timestamp: May 28, 2019, 02:34:04 am
content: For "fine tuning" the voltage, if you need to keep the supply fixed, you simply add series diodes, at 0.6V each, as many as you like. In fact, technically,you can drop the entire amount with just "forward biased" diodes - 10X at 0.6V = 6.0V approx. The heat also get "shared". Or zener + diode comboLots of options