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Topic: Having some power issues (Read 1 time) previous topic - next topic

remmy

I'm trying to put together a guitar pedal that will let me change loops as well as channels on my amplifier.  The approach I have taken is to have relays do the switching and control them with attiny chips that I have programmed with my Arduino.  Each relay (5v) has an chip (attiny85) which also controls an LED.  Four of these are connected to an attiny84 which is dealing with the switch inputs and control voltages to the 4 attiny85s.

Now the issue I am having is power, I have tried using a L7805CP 5v regulator to convert my 9v (1.2A) adapter but as soon as I connect each chip and relay module to the source the voltage on the output pin of the regulator drops.  The measure of voltage after each module is connected is 5v, 4.55v, 3.52v, 2.92v and then 2.27v.  I had the same results with another 5v regulator as well as a 5v DC adapter.  Each feed has been decoupled with a 100R resistor and 100uF cap.

Could someone explain why this might be happening as I am not sure?  I have powered multiple pedal circuits in a single enclosure before without issue so cannot work out what is happening here.

Thanks, Remmy.

pwillard

When asking for help... providing a drawing saves the most time in getting your situation understood.

Also, the behavior (symptom)  is common when current is being drawn from a power supply that cannot deliver the amount of current the circuit is asking for...   Unless you have really beefy relays and they are energized on power up... there is no obvious reason for high current draw unless your circuit has a very low resistance path between V+ and GND  (AKA a short)

Without a drawing... this is conjecture.  (Guess work)

And what exactly is "decoupling with a 100ohm resistor"?

remmy

#2
Mar 27, 2013, 07:43 pm Last Edit: Mar 27, 2013, 07:54 pm by remmy Reason: 1
Thank you for the tip, this is the schematic of how I have everything hooked up.



Each relay and attiny are on their own PCB and when I connect them to the 5v supply the output voltage from the regulator drops gradually with each module.  I tested the input voltage to the regulator and it is also dropping with each module being added.  I have looked for shorts but with no luck there, plus it does not seem to change the way the voltage drops if I change the sequence that I connect the modules to the regulator.

Regarding the decoupling I have tried this by adding a 100R series resistor and 100uF capacitor to ground to each feed 5V to see if it helped but it makes no difference so I have removed them, but the point of it was to see if it isolated each module from each other and sorted the problem.

The relays are 5v, only need around 30mA to trigger the latching coil.  Regarding the power supply I have tried a 600mA 9v one, a 1200mA 5v (bypassing the regulator) and a fresh 9v battery, all had the same behavior.

Many thanks, Remmy.

PedroDaGr8

Have you checked that your grounds are hooked up properly? This is the first thing that I am thinking of. If the ground is "rising" the apparent voltage can become less and less. Another option is the your 5V regulator does NOT like seeing the inductive load of the relay coil. If you remove the relay from the circuit (meaning just disconnect the coil) does it show the same behavior?

Something is happening that is either a)taxing your main powersupply to draw that much current (in that case I would look at the relay hooked up wrong) or b) there is a weird grounding problem wih a rising voltage floor.

1oldfox

Hello Remmy,
Let me give this a whirl. If i were going to design a circuit with the same power, input and output requirements, I would do the following:
1. We will refer to the controlling attiny85 with the 4 outputs as the *controller*.
2. I would lose the 4 channel attiny85's and replace them with a suitable transistor and associated cirtuitry to drive the base from the controller.
3. I would lose the 2 zener diodes across the coil and just use a good diode across the relay coil to absorb the inductive kick.
4. And finally, but not least, connect all 4 relay pin 1's directly to the +5 output of the 7805.

I'm not an enginner. Just my thoughts. You might have to tweak my *thoughts* just a little.
-oldfox-

ps. Ohm's law works.

remmy

Well I dug out my Diago adapter (9v 3000mA) and everything is dandy with that so I'm guessing that the adapter I was using (Maplins, I should have realised earlier :smiley-roll:) was not up to it.  I'm not sure why though as it's spec was plenty enough as far as I could tell, and when I measured the draw of the circuit when connected to the Diago adapter the draw was only 80mA.


Have you checked that your grounds are hooked up properly? This is the first thing that I am thinking of. If the ground is "rising" the apparent voltage can become less and less. Another option is the your 5V regulator does NOT like seeing the inductive load of the relay coil. If you remove the relay from the circuit (meaning just disconnect the coil) does it show the same behavior?

Something is happening that is either a)taxing your main powersupply to draw that much current (in that case I would look at the relay hooked up wrong) or b) there is a weird grounding problem wih a rising voltage floor.


Yes I checked the grounds a few times, not sure what was going on inside the adapter though...


Hello Remmy,
Let me give this a whirl. If i were going to design a circuit with the same power, input and output requirements, I would do the following:
1. We will refer to the controlling attiny85 with the 4 outputs as the *controller*.
2. I would lose the 4 channel attiny85's and replace them with a suitable transistor and associated cirtuitry to drive the base from the controller.
3. I would lose the 2 zener diodes across the coil and just use a good diode across the relay coil to absorb the inductive kick.
4. And finally, but not least, connect all 4 relay pin 1's directly to the +5 output of the 7805.

I'm not an enginner. Just my thoughts. You might have to tweak my *thoughts* just a little.
-oldfox-

ps. Ohm's law works.


I've actually put ohm's law up on my wall only two days ago  :)

Thank you for the alternative suggestions, I am using latching relays and feeding each pin 5v when needed to do the switching, I have the switching pretty quiet (in the audio signal) so I am keen to stick with the set up as is, just happy that I can power it now!

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