Problem with a MC34063: Can't Handle Any Load - Page 1
Forum section: electronics / beginners

author: nathanpc
author_rank: Unknown
timestamp: November 28, 2013, 06:12:40 pm
content: I was testing a simple boost circuit based on theMC34063to boost a 9V battery to 15V (100mA max). Here are the values that I've used:L = 100uH, Ct = 200pF, Co = 47uFWithout any load I'm getting 15V, but as soon as I try to load it with a 1k resistor the output drops to 3V. Any ideas on why this could be happening?

author: filip_cro
author_rank: Regular Contributor
timestamp: November 28, 2013, 07:19:24 pm
content: Are you sure that you connected everything right? Are you sure that you are using 1k and on the right place?You are using breadboard?

author: nathanpc
author_rank: Unknown
timestamp: November 28, 2013, 07:28:25 pm
content: If I didn't connect everything right I wouldn't be getting 15V without load, correct? I'm putting the 1k resistor in Vout. I'm using a breadboard. Also, I'm using a brand new 9V battery to test the circuit.

author: filip_cro
author_rank: Regular Contributor
timestamp: November 28, 2013, 07:35:52 pm
content: 0.76 ohm may be 76 instead and be limiting current to much lower value. Second, breadboards are known to not work good at higher frequency, and this may cause problems.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 07:47:58 pm
content: What inductor do you use? If it is small axial one or something like this, it will saturate.

author: nathanpc
author_rank: Unknown
timestamp: November 28, 2013, 07:57:22 pm
content: Tried with 470uF, exactly the same result. I'm using 2x 1.5 ohm in parallel, I rechecked, they are 1.5. I'm using this inductor:http://www.digikey.com/product-detail/en/RLB0914-101KL/RLB0914-101KL-ND/2352772

author: AndyC_772
author_rank: Super Contributor
timestamp: November 28, 2013, 08:06:19 pm
content: It's probably a layout issue to do with the current sense resistor. Breadboards are horrible when it comes to resistance and, especially, inductance.I'd bet if you probe pin 7 with a scope, you won't see the nice, clean voltage ramp that the device requires, but instead you'll be seeing spikes and noise that will stop the closed loop feedback circuit from working properly. The more current flowing in the main loop, the worse those spikes will be, and the worse the error.There's nothing for it but to improve the layout: very short wires, solid ground plane, all the usual advice applies. Ditch the breadboard, though.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 08:09:04 pm
content: Should work. Problem is likely breadbord or wrong wiring. Make it at least on proto board with connections as short as possible. Or make something like this:

author: mariush
author_rank: Super Contributor
timestamp: November 28, 2013, 08:21:46 pm
content: Make sure the 9v battery can actually supply the peak currents. Test the circuit from a bench power supply or a 7.5v wall adapter.Keep in mind 9v batteries will be dead at about 6.5v and full at about 10-10.5v so the circuit should go as low as 7.5-8v if you really want to power something from a 9v battery.I have doubts a 9v battery can actually give you 15v 100mA ... 15v x 100mA is ideally 9v 160mA ... but you'll have probably peaks of 250-300mA pulled from battery. Most 9v batteries can probably only do about 40-60mA before seriously dropping voltageSee datasheets :http://data.energizer.com/PDFs/522.pdfhttp://users.tpg.com.au/users/p8king/files/va_9v.pdf

author: c4757p
author_rank: Super Contributor
timestamp: November 28, 2013, 08:30:46 pm
content: Quote from: wraper on November 28, 2013, 08:09:04 pmJust don't use the same inductor I used...

author: codeboy2k
author_rank: Super Contributor
timestamp: November 29, 2013, 03:11:36 am
content: Quote from: c4757p on November 28, 2013, 08:30:46 pmbut I really like how you made the IC into a convertible.. top down, silicon flapping in the breeze

author: c4757p
author_rank: Super Contributor
timestamp: November 29, 2013, 03:12:56 am
content: Well, it sure did cool down once the top came off! (Beforethe top came off, it made a damn good simulation of the Big Bang...)

author: matkar
author_rank: Regular Contributor
timestamp: November 29, 2013, 01:46:12 pm
content: I don't know if this has any influence in boost mode, but it sure did in buck mode. I had a similar problem once. Power supply was not able to deliver calculated current. The reason was the diode. I firstly mounted a 1N4007 (or something similar). After changing with a schottky diode, everything was fine. I didn't think it will make such a difference. What diode do you use?

author: nathanpc
author_rank: Unknown
timestamp: November 29, 2013, 02:18:37 pm
content: I used a 1N5817