zener diode resistor - Page 1
Forum section: electronics / beginners

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 01:16:56 pm
content: what i have got is 30 volts want to run 13 volts using 1N4743 i believe its max current is 69ma what size of resistor would i need to limit currentim also using bd137 base connected to supply higher currentapologies in advance if this is a dumb question

author: ledtester
author_rank: Super Contributor
timestamp: January 26, 2021, 01:28:12 pm
content: Solve this equation for R: R > (30-13)/I, where I = 0.069. R will be given in ohms.

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 01:31:43 pm
content: my maths was never to great but i got 158

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 01:34:55 pm
content: just as a side note was experimenting with 1K resistor kept killing it

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 02:04:52 pm
content: was my maths right at 158 tried it zener went to 14,5 volts and very hot

author: ledtester
author_rank: Super Contributor
timestamp: January 26, 2021, 02:23:51 pm
content: Quote from: ascot on January 26, 2021, 02:04:52 pm158 ? ? ?There's no need to do the math yourself, there are plenty of online calculators:https://duckduckgo.com/?q=(30-13)%2F.069&ia=calculatorWith a 158 ohm resistor you are running around 100mA of current which is over your spec.

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 03:12:19 pm
content: i was using the calculation of 30-13 /0.069 not 30 -13 = 17 /0.069 both seem the same different outcome is that algebra or maths thanks anyway

author: Terry Bites
author_rank: Super Contributor
timestamp: January 26, 2021, 05:11:57 pm
content: Some one was taking the quack out of you I think, shame on them. There are real on line calculators for Zeners. There are many crap ones - this is a good onehttps://www.petervis.com/electronics%20guides/calculators/zener/zener.htmlIt will calculate the ratings for your components too- in your case a over 1W in the zener. A 1N4743 will get hot and die. I'd be looking for a 2 or 3W type.They can be harder to find than 5W ones like the 1N5350B.If you stick a 1/4 watt 1K resistor across 30v it will find its self struggling with 0.9W (P=V.V/R or 900/1000) it will release smoke and shrapnel! So in case your zener goes short you will need at least a 1.5W resistor. 2W are more commonly available.

author: Jwillis
author_rank: Super Contributor
timestamp: January 26, 2021, 05:26:46 pm
content: Quote from: Terry Bites on January 26, 2021, 05:11:57 pmThe calculator is correct but the formula is deceptive. ISwould read as Current Source .R=(VS-VL)/IZR = Series ResistorVs= Voltage SourceVL= Voltage LoadIZ= Current across Zener

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 05:57:59 pm
content: would the zener go boom if i connect a transistor to boost current

author: ledtester
author_rank: Super Contributor
timestamp: January 26, 2021, 08:04:02 pm
content: Not if you are using a circuit like described here:https://www.digikey.com/en/maker/blogs/zener-diode-regulator-with-transistor-current-buffersIn these cases you can get by with a much smaller ILwhich in turn means a higher R which means it will need to dissipate less power.In the MOSFET circuit no current flows through the gate, so I would try designing with IL= 1mA. In the BJT circuit you can design with IL= 1/10 of your load current, perhaps even a smaller fraction depending on which transistor you are using.I'm using the parameters defined by the above mentioned calculator page, so IS= IZ+ ILwhere a recommended value of IZis 5mA.Keep in mind that the power dissipated by the transistor is within its spec.Btw, if you only have 1/4 watt resistors but need a higher wattage, just use N copies of R/N resistors in series for some value of N. For example, if you use four 250 ohm, 1/4 watt resistors in series it's the same as a 1000 ohm, 1 watt resistor.

author: TimFox
author_rank: Super Contributor
timestamp: January 26, 2021, 08:13:33 pm
content: As mentioned above, it’s not a good idea to run a semiconductor at its maximum power rating (69 mA at 13 V). The Zener voltage will increase with temperature, so 14.5 V is believable. What is the power rating on your resistor? Do you own a calculator?

author: Jwillis
author_rank: Super Contributor
timestamp: January 26, 2021, 10:13:51 pm
content: I thought resistors needed to be in parallel to increase power rating .

author: ledtester
author_rank: Super Contributor
timestamp: January 26, 2021, 10:28:32 pm
content: Either configuration will work. Each resistor is dissipating the same power.I suppose parallel is more often mentioned because it is easier to find higher value resistors as opposed to lower value ones.

author: ascot
author_rank: Contributor
timestamp: January 26, 2021, 10:56:29 pm
content: the idea was to put the zener after C1 to run a thermostat that controlled a fan which ran about 72 mA given that R1 is 1/4 watt and didn't burn out