Can I split current across multiple BJTs - Page 1
Forum section: electronics / beginners

author: kicken
author_rank: Contributor
timestamp: October 28, 2016, 09:59:29 pm
content: I made a few simple halloween decorations with some 555-timers and LEDs that run off a 9v battery. For now I've just be disconnecting the battery to turn them off but after seeing Dave's World's Simplest Soft Latching Power Switch Circuit I got curious about the idea of trying to implement a power switch following that design.The only issue i have I think is that I do not currently have any high-power mosfet to use like he did in the circuit. All I have available is a bunch of 2N3904 transistors. Most of the decorations are well below the 200mA rating for these so I am assuming I can just swap out the mosfet for a single transistor and it would work. One however has many LEDs and draw around 550mA peak which exceeds the ratings. For this decoration, would it be possible to split the current across 3 separate transistors so each one would draw about 183mA peak?I attempted to adapt the schematic from the video and this is what I came up with.I've not yet tried this out on a breadboard, so I'm not sure if it works at all yet. I'm just wondering right now whether this idea of current splitting would actually work? If it would, did I correctly adapt the circuit or did I goof it up somewhere?

author: IconicPCB
author_rank: Super Contributor
timestamp: October 28, 2016, 10:16:22 pm
content: To ensure reasonable current sharing include three additional resistors into the circuit.Each of the emitters of Q2, Q3 and Q5 should have a resistors.The resistors should drop about a volt at desired current.In this case try 5.6 or 6.8 ohm resistor.Having said that...I am not quite sure what You hope to achieve ( the purpose of the circuit as it stands )

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2016, 10:29:58 pm
content: No. To implement a latching power switch with only bipolar transistors, and zero off-state current consumption, you need at least one complimentary transistor. e.g for your NPN emitter followers you need a PNP transistor to switch their base drive.For even current sharing you need individual emitter resistors which should be chosen to drop around 0.2V at the intended load current.

author: kicken
author_rank: Contributor
timestamp: October 29, 2016, 02:27:02 am
content: Thanks. I do have a some PNP transistors but only a few compared to many NPN's. I'll re-visit this a bit later, don't want to mess up my decorations just before halloween. I've downloaded LTSpice in the mean time, maybe it'll help me figure these things out.

author: Audioguru
author_rank: Super Contributor
timestamp: October 29, 2016, 04:01:52 am
content: Most little 9V batteries will quickly drop dead when they try to drive the very high current that your LEDs will draw.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 29, 2016, 03:21:52 pm
content: With 9 V supply you can likely use 2 or 3 LEDs (red) in series. Try to get a way with less than 20 mA, so that the overall current will not be that high.The usual way for current sharing between BJTs are emitter resistors. Alternatively one could use resistors at the base - though the drop should be larger (e.g. 500 mV). It might be still attractive if the transistor is used in emitter circuit and thus plenty of voltage on the drive side.