PIC supply using a DC-DC converter

[deperkin]

Hello all,

I am trying to power a PIC (18f4550) using a DC-DC converter (EC4A21H from CINCON).
It is a 5W - 1A converter.

There is not alot of information in the datasheet for this DC/DC converter and I would like to ask some advice on whether or not it would be prudent to using a current reducing circuit?

If so, any recommendations?

the pic is 250 mA max.

I am using a 48V power supply to power a motor and would like to integrate this converter to also power my PIC.

I also have a few sensors, a clock, and an LCD in the circuit; all which require 5V operating voltage.

I had considered using just a resistor to limit the current but would like to avoid any voltage drop (due to having other things in the circuit).


Thanks for any advice / reading pointers.

 

[Pommie]

There is no need to limit or reduce the current. Just feed it the 48V and let it do the work it's supposed to. Your other circuitry will only take what it requires.

Mike.

 

[deperkin]

Thank you. I am concerned however, since there is no minimum current rating on the converter, that the parts i am using are summing up to current well below any required minimum. Some converters with provide this information. I will try this though and hope I do not make the PIC act crazy.

I appreciate the reply.

 

[HATHA]

data sheet for smps(EC4A21H) stats that the ripple is 100mVpp in 20MHz B.W. so i guess pic will not restart under activation of brownout reset fuse, but smps chip switch at 100kHz so doesn't this effect to the PIC

 

[deperkin]

PIC resetting

I have connected the PIC to the DC/DC as shown in the schematic:

I am having a problem with my PIC18F4550 continuously resetting.
I have disabled the brownout
I have checked the circuit with the schematic (shown in the link below) and it matches.

when i power it with the pickit 2 there are no issues.

**broken link removed**
an updated schematic...

https://www.electro-tech-online.com/custompdfs/2010/09/EC4A.pdf
the DC/DC converter...

**broken link removed**
pressure sensor...

https://www.electro-tech-online.com/custompdfs/2010/09/DS1307.pdf
RTCC....

Any thoughts why it could be resetting continuously??

 

[CafeLogic]

Pull the reset line up to VDD with a 10k (the pickit does this for you), if necessary, place a 1-10nf capacitor from the reset pin to ground. I seem to remember the 4550 being touchy on the reset. Did you measure the output voltage to make sure it is ok?

 

[deperkin]

Thank you, I will try this.
Yes I did measure the voltage and it was 5.012VDC and the current was 11 mA.
which all seems okay.
I will try the pullup... I seem to remember now that I placed this on a seperate board in the distant past.

Thank you and I hope this helps.

 

[deperkin]

I tried a 10K straight from MCLR to Vdd but alas the resetting resides. I will try to pick up a nf capacitor tommorrow and pray it works.
I appreciate the input again.

 

[CafeLogic]

It probably won't. Is that motor running yet, how long until the reset?

 

[CafeLogic]

I have some questions about your schematic.

1. What is that CCP2 line pulled up to VDD with 20 ohms and labeled brightness, what is the function?
2. You appear to have that BJT wired like an NPN, except that the TIP42 is a PNP, what's going on there?

 

[Pommie]

Have you turned off the watch dog timer (WDT) in the config?

Mike.

 

[deperkin]

I have NOMCLR, NOBROWNOUT, NOWDT...

I have added a pullup resistor but it still resets.

The motor function works fine (if i power from the pickit2), and if i completly disconnect it i still get the resetting.

(#fuses NOWDT, NOPROTECT, NOBROWNOUT, NOPUT, NOLVP, XT, NOMCLR)

updated schematic:
**broken link removed**

 

[Pommie]

I can't see any reason for it reseting. Can you try disconnecting everything except the LCD and just display a counter on the LCD to see if it still resets?

Some notes on your schematic,
The switches appear to be always open circuit.
The switches need pull down resistors or use internal pullups and wire the switches to ground.
You need a reversed diode from "speed" (motor connection) to ground to prevent back EMF.

Mike.

 

[deperkin]

I will add the resistors in the morning.

I have tried removing everything but the LCD and i still get resets. I have tried other PICs as well.

I will add a zener to the speed also. I have tried removing the motor and tansistor as well, so that is not causing it.

THanks again I really appreciate you.

 

[Pommie]

The only thing I can think of is your 48V supply maybe unstable. I think your converter needs over 30V to keep working. The fact that the chip doesn't reset when powered from the PK2 also suggest a supply problem. Try adding a large capacitor between 5V and ground after the diode. If that fixes the problem then it will confirm your 48V supply is dodgy. Where does the 48V come from?

The diode on the motor should not be a zener, just a normal diode.

Mike.

 

[deperkin]

I will try this. I will also post a datasheet for the supply. I was thinking the diode from speed to ground needed to zener.
I have a regular diode from power to ground on the motor.

how large of a capacitor? i have upto a 22000 uF :/

You think a ceramic is okay?

 

[Pommie]

The diode should go from the collector of the TIP42 (shouldn't it be a TIP41?) to 48V (where the 4.7k is). The base resistor also seems to be very high.

A 22,000uF electrolytic should be fine. A ceramic would be fine but I doubt you have one big enough.

Mike.

 

[deperkin]

you are correct it should be an NPN.

It does seem to control speed however quite well, but i will change it to an NPN non-the-less.

Thanks again...

I hope it is not my power supply, but that would be an easy answer and easy fix.

 

[CafeLogic]

If you do this sort of thing a lot. Consider an oscilloscope. Here it would enable you to eliminate or isolate he power supply as the cause in 5 minutes.

 

[Oznog]

Have you gotten this PIC to work off "normal" 5v? To pursue a difficult diagnosis with the SMPS without knowing it's the SMPS is an inefficient way to go about troubleshooting.