Current limiting circuit / short circuit protector - Page 1
Forum section: electronics / beginners

author: ALonda
author_rank: Contributor
timestamp: January 07, 2017, 02:50:39 pm
content: Hello there.Im just starting to pick up electronics as a hobby again. I did some as teenager but never got far, Im heavy into computers, low level programming, linux, etc, but got interested in raspberry pi and arduino lately. I started with a raspberry pi 2 and in the same order chucked in an arduino uno because they were so damn cheap. Then bought a 37 sensor kit, and now recently got another order delivered with a rbpi3, an (second) arduino uno, two arduino nano (v3), sd card readers, gsm module, 8x8 led matrix, etc.My new favorite is the arduino nanos, so much versatility in such a cute little package. Unfortunatly I quickly messed up and short circuited one (shorted the ICSP vcc-gnd pins). That was a sad moment not because of money loss but because getting new ones from china takes so damn long. Yes Im cheapo ordering from Aliexpress.Anyways this got me interested in this current limiter circuit which I think could save me from some future losses. Im especially worried about burning power supplies because my next expected shipment contains two 5V 5F supercaps. At first I was trying to understand the article on wikipedia (https://en.wikipedia.org/wiki/Current_limiting) but didnt have much luck since I still dont have components to experiment with. So I found the evaluation copy of Multisim which allowed me to come up with this circuit that seems to work.The conditions for the circuit above is;C1 is a small placeholder for the supercap,R4 is a placeholder load for a 32x8 (4x 8x8) LED matrix (or up to 6 of them), I measured a single element to draw ~60mA with "normal text" scrolling on it.R5 is shorted out (zero ohms) to test the circuit. At 100 ohms this circuit passes about 110 mA through U1.Now I would be grateful if someone with more insight and experience than me could comment on this circuit. About how safe, efficient, reliable, etc, that this circuit is or could be? Could it be improved?Id like to add an indicator LED to light up for overcurrent/short circuit warning but Im still too dumb to figure out where and how to place it without breaking this circuit completely./A

author: cheetahkid
author_rank: Newbie
timestamp: January 07, 2017, 03:55:38 pm
content: It looks complication for a beginners but it is not. I see you use R5 as for load or output.I go straight to the problem, Q2 seem wrong the C - E is reversed. I use resistor about 390 ohms(on Q1 base instead of a link. Because the resistor is in place, you can use a LED and resistor (low current as possible) to suit in parallel with R6 and R1.

author: ALonda
author_rank: Contributor
timestamp: January 07, 2017, 04:54:17 pm
content: Thanks, I turned the transistor right and according to the simulator it works pretty much the same. Im surprised <-- as you can see I dont understand enough yet.I thought the way to go with the indicator led would be to hook it up with a resistor over where U3 is hooked up since the voltage over those connections only goes up when there is a short but I havent been brave enough to try it in the simulator yet because Im so happy it works and Im worried about "breaking" it. Afraid of failure is incompatible with hobby electronics?The reason for R6 is its just a complement to R1 so if in my testing I drop R1 to zero, it doesnt collapse the whole circuit.The plenty Wattmeters is to test how big the components needs to be without burning up (the circuit started as two 2N2222, but then I saw the 2W load on Q1...)./A

author: ray-san
author_rank: Contributor
timestamp: January 07, 2017, 06:12:43 pm
content: /offtopic: ALonda, what software are you using? It looks pretty nice

author: ALonda
author_rank: Contributor
timestamp: January 07, 2017, 06:31:44 pm
content: Multisim from national instruments, 7 day trial but after that 400€+ for a proper license/A

author: Wilksey
author_rank: Super Contributor
timestamp: January 07, 2017, 06:38:31 pm
content: Do Mouser still do the Multsim Blue "free" version? I believe it is just a repackaged cut down version of Multisim.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2017, 08:32:53 pm
content: That circuit will work but it does have its down sides: The current regulation is far from perfect. It will change both with the supply voltage, load and temperature. The voltage drop will be high at full load: on the order of 800mV, which is quite high, compared to the supply voltage.An LED with a suitable series resistor across Q1 would indicate a short circuit but, you'd have to add the LED current to the load current.

author: cheetahkid
author_rank: Newbie
timestamp: January 08, 2017, 08:40:27 am
content: Reading up on BD243, the Hfe is minimum of gain is 20, so you want to draw 100mA as example. R3 at max 0.6v drop added to Q1 another 0.6v minus the voltage 5.16 equal to 3.96v that be across R6+R1 to get current drawn 100mA/gain 20 = 5mA. R1+R6 should be a total of 3.96v/5mA = 792 ohms, it would be quite happy with less than that perhaps 470R or even 270R as it need to draw more because of Q2. Hfe varies depend on the transistors. This is only an Example.