Unexpected current cap from transistor - Page 1
Forum section: electronics / beginners

author: puzzledtinkerer
author_rank: Newbie
timestamp: February 15, 2025, 06:32:40 pm
content: I'm probably missing something really basic due to never having needed to pay much attention to exactly how much current goes through tranistors in past projects.I'm trying to switch a lamp (actually an array of LEDs) using a PNP transistor and a really simple circuit. What I'm finding is that despite thinking I've calculated the base resistor correctly, current through the load is limited. In fact, it doesn't seem to matter which value of base resistor I use.With a direct connection to 12V via an ammeter, the lamp consumes 0.08A. My S9012 transistor apparently has an hFE of 120 and an Ic of 0.5A so should be fine with this. My calculations for the base resistor go:Ib = 0.08/120 = 0.0006666666 AVb = 12V-0.7V = 11.3VRb = Vb/Ib=16.95 kOhmWith the resistors I've got to hand, I can do 10K + 5.6K + 1.5K = 17.1K, which is close enough.Effectively, I have the attached circuit and it matters not which switch (actually a croc clip lead) I ground the base with; powering the lamp via the transistor I always see 0.06A rather than 0.08 on the ammeter.Any suggestions what I've mucked up this time? The lamp is a navigation light, so I really don't want it being dim.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 15, 2025, 09:20:25 pm
content: Measure the voltage on the lamp first. If it is close to 12V no matter which resistor you connect, the limiting component would be the lamp and not the transistor.CheersAlex

author: ifrythings
author_rank: Contributor
timestamp: February 15, 2025, 09:39:17 pm
content: Your base current of 0.6666mA * hfe 120 = 80mA of collector current which is what your getting, you base resistor is way to large.Ic 500mA / hfe 120 = Ib 4.166mA11.3v / 4.166mA = Rb ~2.7k ohms

author: wasedadoc
author_rank: Super Contributor
timestamp: February 15, 2025, 10:18:39 pm
content: Quote from: puzzledtinkerer on February 15, 2025, 06:32:40 pmYour brain is faulty. Since when is current measured in Ohms?

author: puzzledtinkerer
author_rank: Newbie
timestamp: February 15, 2025, 10:49:40 pm
content: This is what happens one when one transcribes a spreadsheet in a hurry with someone else yabbering on a different subject over the shoulder...I've edited my post, in shame.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 16, 2025, 12:30:59 am
content: Quote from: ifrythings on February 15, 2025, 09:39:17 pmI calculated the base current differently. Assuming the 1.5k resistor is selected..... There is 12V on the emitter, so when base current is being drawn there should be 11.3V on the base.That 11.3V is applied across the 1.5k resistor between the base and ground. Therefore the current in the 1.5k resistor will be 7.53mA.The 10k resistor has 0.7V across it, so the current in it will be 0.07mA.The current in the 1.5k resistor will be the sum of the base current and the current in the 10k resistor. Therefore the base current will be 7.53mA - 0.07mA = 7.46mA.Assuming an hfe of 100, that would allow the transistor to pass 746mA. That should be more than enough to let the load LED glow brightly.Can anyone tell me where I've gone wrong?My calculation suggests@puzzledtinkerer's circuit should work just fine: 12V minus the emitter-collector voltage drop should deliver perhaps 11.6, 11.7V to the load, yes?

author: PGPG
author_rank: Frequent Contributor
timestamp: February 16, 2025, 12:55:54 am
content: Quote from: puzzledtinkerer on February 15, 2025, 06:32:40 pmWhen you use transistor as a switch you give into its base 10..20 times higher current than you calculated.First hFE varies from part to part.Second hFE varies with temperature.Third hFE varies with collector current.Fourth hFE is parameter for transistor working in active region (not saturated). When transistor is not saturated its Vce is higher than its Vbe and Vbe is araound 0.7V.To get Vce smaller you need saturate transistor (make its Vce as small as possible) and to saturate you give it that 10..20 higher base current.Nowadays more practical as a switch are MOSFETs.

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 16, 2025, 02:03:22 am
content: Quote from: wasedadoc on February 15, 2025, 10:18:39 pmDidn't they change that last week?

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 02:47:33 am
content: Quote from: puzzledtinkerer on February 15, 2025, 06:32:40 pmYour transistor has a typical hFEof 120 but a minimum of 64. And that is at IC=50mA and VCE=-1.0V.https://www.mouser.com/datasheet/2/149/SS9012-117759.pdf?srsltid=AfmBOoofFDh31JQ-pAsjoLwrYRyB-UjFE122NqfjnWoOvv1oc-CY1hqHWhat current will your LED array draw at 11V? What voltage is actually present across the LED array when you are operating at at the 60mA reduced current? LEDs in series will be highly non linear. If you aren't trying to regulate the current with the transistor--and you apparently aren't since you want the LED lamp to draw as much current as a direct connection--then you'll need to saturate the transistor. It's possible it is saturated or close to it with the 1.5k resistor, but you'll have to measure the voltage across your load to find out.

author: puzzledtinkerer
author_rank: Newbie
timestamp: February 16, 2025, 02:55:40 pm
content: Many thanks to all, and especially those who suggested checking voltage across the load.I had tested output voltage with no load before assembly, and confirmed the transistor appeared to be saturating. It turned out that post-assembly this was still the case, but once the load was attached Vout (voltage across the load) dropped to 10.1V. That started my alarm bells ringing, and I replaced the transistor - immediately everything started working as expected. So, it looks like a(nother) bad component was at the root of this.We no longer have Maplin on the high street in the UK, nor the independent retailers it replaced, so in lieu of having to order single components online when I need them, some time ago I bought an assortment box of transistors off Amazon. This isn't the first occasion on which I've ended up wasting my own and other people's time due to a bad one, and methinks the purchase was a mistake. Does anyone in the UK have suggestions of better places to get such things, either in small quantities when required or in the form of another assortment?Nonetheless, reading through the responses on this thread has been educational. Thank you all again.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 03:03:21 pm
content: Quote from: puzzledtinkerer on February 16, 2025, 02:55:40 pmCould you tell us what currents and load voltage you got with each of the three resistors? Was 17k enough to fully turn the transistor on?

author: puzzledtinkerer
author_rank: Newbie
timestamp: February 18, 2025, 03:10:06 pm
content: Quote from: bdunham7 on February 16, 2025, 03:03:21 pmI (mA)VBaseline (direct psu to load)84.611.54Transistor with 1.5K base83.811.53Transistor with 7.2K base83.411.51Transistor with 17.1K base82.711.48

author: PGPG
author_rank: Frequent Contributor
timestamp: February 18, 2025, 07:22:18 pm
content: There are some surprisings in results (rising voltage 0.02V - rising current 0.4mA, and rising voltage 0.01V - rising current 0.8mA) but can be simply measurement inaccuracies.I understand that with direct connection (transistor shorted) you get 11.54 at output and with transistor you get 11.53.Loosing only 0.01V at transistor as a key is very good result in my opinion. What do you want more?