Author Topic: MC34063 step down stability problems  (Read 1440 times)

0 Members and 1 Guest are viewing this topic.

Offline little_carlos

  • Regular Contributor
  • *
  • Posts: 129
MC34063 step down stability problems
« on: May 21, 2016, 12:05:22 am »
Hi, im using a mc34063 as a step down converter, the input is 12v and the output is 5 v at 500mA, the problem is that the regultator starts becoming very unstable, switching on and off, but it doesnt heat at all, the load is 5 relays and 5 leds.
the strange thing is, that when i put a 50k resistor between pins 3 and 4, becomes stable, but the regulator turns hot, up to 70º C.
what do you guys think is happening?
btw, when i put the resistor i mentioned before, the inductor starts making a weird noise
 

Online Kleinstein

  • Super Contributor
  • ***
  • Posts: 7048
  • Country: de
Re: MC34063 step down stability problems
« Reply #1 on: May 21, 2016, 09:15:22 am »
The extra resistor disturbs the oscillator. I don't know the details of that circuit part, but it's usually not a good idea, epsecilly without knowing what happens inside. It could even be oscillating to fast.

For the normal operation, check the circuit. There are rules / formulas / tables on how large the caps and inductors should be. Usually the MC34063 is rather tolerant, especially as a step down. There are also different possible circuits.
 

Offline batteksystem

  • Regular Contributor
  • *
  • Posts: 167
  • Country: hk
    • My ebay store
Re: MC34063 step down stability problems
« Reply #2 on: June 01, 2016, 04:07:14 am »
Hi, im using a mc34063 as a step down converter, the input is 12v and the output is 5 v at 500mA, the problem is that the regultator starts becoming very unstable, switching on and off, but it doesnt heat at all, the load is 5 relays and 5 leds.
the strange thing is, that when i put a 50k resistor between pins 3 and 4, becomes stable, but the regulator turns hot, up to 70º C.
what do you guys think is happening?
btw, when i put the resistor i mentioned before, the inductor starts making a weird noise

Can you post your circuit? Also, what is the current load for the 5 relays and 5 LEDs? We cannot help you if we don't have the necessary information. Thanks.

Offline Chris Mr

  • Regular Contributor
  • *
  • Posts: 114
  • Country: gb
  • Where there's a will there's a way
Re: MC34063 step down stability problems
« Reply #3 on: June 02, 2016, 12:14:35 pm »
Adding the resistor probably increases the operating frequency but it's not supposed to have a resistor to ground there - just a capacitor.

It's always a good idea to test these things out of the final circuit, using a dummy load, to check it will do what you want it to do before connecting up to the real load.  I have a reel of Nichrome wire I use to make up a dummy load as required, winding it round a large ceramic tile (nice white one from last tiling the bathroom which has fish swimming on it and some 'abstract' water) and a couple of bulldog clips to stop it unwinding itself.

What is the ripple current of the inductor you used (from its data sheet)?

Are you using the circuit in the data sheet for the MC34063 (which is only rated at 175mA)?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf