Current amplification - Page 1
Forum section: electronics / beginners

author: elec_gra
author_rank: Newbie
timestamp: March 06, 2018, 11:16:24 pm
content: Hi, I am trying to amplify a 2mA, 3.3V supply to 3.3V 50mA(pulse) as I need to drive a switching circuit with that specific current in order to work in saturation. I have tried a few circuits such as another switching circuit to have a higher current but this results in a larger voltage. Have also used an optocoupler but didn't quite work with that. Someone I know has suggested using a darlington transistor but I haven't got a good idea how to work with a darlington transistor. Any suggestions please? I would greatly appreciate!I have also attached a switching circuit that I tried but didn't work(obviously as I need current not voltage).

author: tecman
author_rank: Frequent Contributor
timestamp: March 06, 2018, 11:23:20 pm
content: Your circuit will work, but the big question is the required (desired) rise/fall times. Transistor selection, drive current and base capacitance and transistor saturation call all effect responce times.paul

author: elec_gra
author_rank: Newbie
timestamp: March 06, 2018, 11:25:02 pm
content: But I will be having a 12V output and 50mA, can I adjust to have 3.3V output?

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2018, 11:36:58 am
content: Quote from: elec_gra on March 06, 2018, 11:25:02 pmWhat do you mean?We need to see the full picture. Pleas post schematics of the surrounding circuitry.

author: elec_gra
author_rank: Newbie
timestamp: March 07, 2018, 01:01:32 pm
content: This is the switching circuit that I am using. In order to be able to operate the transistor(BC337) in saturation region for 500mA collector current I need to supply 3.3V 50mA at the base of this transistor.However my 3.3V supply is a microcontroller output which can only output a maximum of 2mA and I a trying to amplify this current to 50mA.But i'm not sure what kind of circuitry I can use.

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2018, 02:24:29 pm
content: You can probably get away with less than 50mA.The data sheets specifies, a typical VCEof 1V, when IC= 500mA and IB= 6mA.https://www.onsemi.com/pub/Collateral/BC337-D.PDFI suspect there's little point taking the base current any higher than 25mA.What's with all the resistors, capacitors and the inductor? If the load is inductive, a simple free-wheeling diode is all that's required.All that's needed is an emitter follower, on the output, see attached. As drawn, the base current for Q1 will be around 33mA. If you want it to be higher than that, reduce the value of R1.

author: elec_gra
author_rank: Newbie
timestamp: March 07, 2018, 03:00:28 pm
content: I tried with a lower current than 50mA but I didnt get the 500mA I wanted.That's why I'm aiming for 50mA. I didn't see the Ib of 6mA when Ic =500mA.The other resistors are part of the switching of the inductor.The inductor a solenoid actuator actually I'm trying to switch that actuator. The other other diodes are protection. What can I use for the input for Q2 ?

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2018, 07:34:33 pm
content: Quote from: elec_gra on March 07, 2018, 03:00:28 pm50mA is the current required to guarantee full saturation, with a collector current of 500mA, but it's overkill. Lower currents will ensure a low enough voltage drop, for most applications. See page 3 of the data sheet. Figure 3 states an Hfe of 90, when VCE= 1V and IC= 500mA, which would mean an IBof 500/60 = 5.56mA. This agrees with figure 4, which shows VC= 1V, when IB= 6mA and IC= 500mA.https://www.onsemi.com/pub/Collateral/BC337-D.PDFIf you make IBmore than 6mA, but less, than 50mA, VCE, should still be acceptable, hence why I suggested 25mA.QuoteThe input to Q2 is the 3.3V logic signal. Q2's base current will be tiny, on the order of 100µA and is self-limiting: no additional base resistor required.

author: elec_gra
author_rank: Newbie
timestamp: March 07, 2018, 07:42:10 pm
content: Ok, I will try to work like that.Thanks!

author: elec_gra
author_rank: Newbie
timestamp: March 08, 2018, 10:39:25 am
content: BTW you were right I worked with much less than 50mA.Thanks a lot for your help