Hi all, I’m wondering what the best way to maintain +5volts to my pic in a circuit that has a voltage drop caused by a low resistance load. Multiple power supplies are not an option unfortunately. I currently have a voltage regulator that drops 12v to 5 but as the load switches on the voltage drop causes the pic to reset. Could I add a boost converter after my existing voltage reg or would a buck/boost be a better option? There is already a 1400 micofarad cap before the regulator (on the 12volt side). The load is PWMd by the pic. Any help appreciated, sorry I can’t do a circuit diagram at the moment…

 

You will need to isolate the load from the microcontroller. Can the load run from a second parallel regulator? Or alternatively, if the supply line from regulator to microcontroller incorporates a diode followed by another high value capacitor, the diode drop my be an acceptable cost, as long as the voltage drop at the regulator caused by the load was transient.

 

Are you sure it's a voltage drop that is causing the problem, 12 to 5 shouldn't really have such problems unless the supply is dropping below the dropout of the regulator.

Also, if you are overloading the regulator, this will cause problems.

How much capacitance do you have on the output of the regulator?

 

You will need to isolate the load from the microcontroller. Can the load run from a second parallel regulator? Or alternatively, if the supply line from regulator to microcontroller incorporates a diode followed by another high value capacitor, the diode drop my be an acceptable cost, as long as the voltage drop at the regulator caused by the load was transient.

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Thanks very much for the suggestions I will try the diode but I haven’t scoped it so unsure re it’s transient behaviour. The parallel reg would need a more complicated circuit as the load side is quite high current.

 

Are you sure it's a voltage drop that is causing the problem, 12 to 5 shouldn't really have such problems unless the supply is dropping below the dropout of the regulator.

Also, if you are overloading the regulator, this will cause problems.

How much capacitance do you have on the output of the regulator?

Thanks. I think that maybe what is happening with the supply. I have no cap on the reg output currently…

 

Try turning off Brown Out Reset.

 

Hi again, I have attached a schematic of this very simple circuit. The resistor heats proportional to the rpm of an encoder wheel Unfortunately i don't have access to a scope at the moment so here are some notes:
The circuit works as intended when the resistive load is greater than 3 ohms. (running from single supply)
When run with less than 3 ohms the pic resets and the current never rises above a threshold.

The circuit works when i run it with dual power supplies (common ground)

I tried the addition of the diode and cap after the reg. and it definitely improved things but does not perform as when run off of 2 supplies.

Any help appreciated, thankyou.

 

I tried the addition of the diode and cap after the reg.

What value cap did you use? What are the specs on the 12 volt supply?

 

Is there any common path from the supply ground to the micro and to the load?

Bob

 

A circuit! That helps a lot.

If the load burdens the 12V supply to the point that the 5V supply post regulator is affected, then the 12V supply is inadequate for the purpose. The linearity of the heater will be affected as well.

The fix is to provide a 12V supply with adequate current for the job.

A low-dropout regulator might reduce the impact on the 5V supply. Or moving the diode/capacitor isolation to before the regulator.

 

I am betting on ground bounce.

Bob

 

Are you sure about the value of your loads?

That supply should have no problem with a 1ohm load.

 

That supply should have no problem with a 1ohm load.

I missed that - the 12V supply is plenty big enough.

Switching noise? Is the load ground taken back to the supply separately from the control supply ground?

 

Thanks guys. The cap I added was 2400 micF. The load ground and breadboard ground are separate back to the supply. I have tried 2 power supplies, the mean well one and another lower power bench supply (the same two I coupled together). When i use only the bench supply found here https://www.jaycar.co.nz/0-to-30vdc-0-to-5a-regulated-power-supply/p/MP3840
it flicks between current control and volt control when set at max current. Its frustrating as if i could use a higher resistance load it would be adequate but unfortunately i cannot...

 

That's only a 5 amp supply.
What are the specs on the Meanwell supply?

 

Yes you are right and it makes sense that it does that with 1 ohm and is fine with 3 ohm load... It just doesn't make sense that the higher power P.S doesn't work at 1 ohm... I even tried a 40 amp P.S and same result...

 

Show the actual schematic of the 317 regulator in your project, not a representation.

 

This is how i have the LM317

 

Try connecting a 10uf cap across the 680 ohm resistor.

 

Ok i will try this in the morning.
Thankyou