power supply problem pls help :(

hi guys,

i have an issue powering my project - cant work out whats wrong.

the project has an arduino, serial 192 x 64 lcd from ebay and a 8 channel relay also from ebay.

basically if i turn on all 8 relays at the same time the lcd goes faint.

in the machine is a 24v psu that i have tried adding this circuit - http://www.eleccircuit.com/power-supply-5v-2a-by-ic-78s05/ which claims to be able to supply 5v 2a. i find with the screen and 3 relays on the voltage regulator gets really hot and everything turns off.

each relay switched on adds around 80ma to the load.

i have tryed 5v from usb 500ma and the screen dims but dosnt turn off, even tryed a ipad charger 5v 2a same problem with screen.

i have noticed that the voltage drops to about 4.4v when its over loaded.

does anyone know where im going wrong, desperate for advise on this!

ali

Sounds like there might be a short circuit somewhere. You say; "each relay switched on adds around 80ma to the load." Is that measured or calculated?

500 mA power supplies will not be sufficient anyway. (8 * 80 ma = 640 mA)

What happens when you try turning on one relay at a time? Try measuring current for each relay. You should notice if one draws more current than the others.

yes even without the arduino just the lcd and the relay plugged into my breadboard each relay switch adds just under 80ma. using a multi meter to measure. which is what the data sheet says.

as i switch more and more relays on the screen gets dimmer and dimmer.

they are both plugged into the same 5v source - i am a beginner but would assume the voltage to stay at 5v and the 2a 5v ipad plug to be sufficient.

Are you powering the relays directly from the Arduino or are there transistors between?

tested each one they draw between 70 and 73ma its a relay board designed for arduino -> http://www.ebay.co.uk/itm/New-5V-8-Channel-Relay-Module-Board-for-Arduino-PIC-AVR-MCU-DSP-ARM-Electronic-/170745298865?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item27c1367fb1

when none are on the voltage from my usb port is 5.03v when i switch on 5 relays it drops to 4.7v

sorry for the newbie question but cant understand whats going on

aliboss: tested each one they draw between 70 and 73ma its a relay board designed for arduino -> http://www.ebay.co.uk/itm/New-5V-8-Channel-Relay-Module-Board-for-Arduino-PIC-AVR-MCU-DSP-ARM-Electronic-/170745298865?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item27c1367fb1

when none are on the voltage from my usb port is 5.03v when i switch on 5 relays it drops to 4.7v

sorry for the newbie question but cant understand whats going on

What is going on is that with each relay you turn on you are increasing the current draw on the arduino's 5V pin and the board cannot reliable handle having all 8 relays on at the same time. You need to use an independent +5vdc power supply to power the relay board if you are going to ever have more then just a certain number of relays on at the same time, which is not a good method to rely on.

Lefty

The description says: "8-Channel Relay interface board. Each one needs 15-20mA Driver Current."

So, without proper schematic, all I can say is that your 70 mA per channel is more than Arduino can safely handle on 8 outputs.

thanks for your replys!

ive taken the arduino out of the circuit and the screen is still dimming and the voltage is dropping.

looking closer you can isolate the power so its not drawing all from the arduino.

you ground the pins to switch on, it draws more from the separate 5v power to switch the relay. to turn off the relay you give the pin 5v 20ma for example.

my issue is why that lcd is dimming and the voltage on the 5v line drops.

i was expecting to power the arduino with 5v to the vin pin, but going under 5v will make it unstable.

should i be looking to separate the 5v on each of the arduino, lcd and relay board, is there something i can research to split 5v into separate lines? is there something i can add to the circuit to stop the voltage dropping? to convert 24v to 5v is there something that can handle more power than the circuit i made - it got very hot very quick but says 5v 2a

desperate to work out the solution - on a deadline for the project :(

Why don't you post some schematic on how you wire all this, what is this circuit you made etc.

the 24v to 5v circuit is on this page http://www.eleccircuit.com/power-supply-5v-2a-by-ic-78s05/

but dosnt handle very much, about 2-3 relays on + the lcd

i am simply plugging the lcd in + and - to a bread board plugging the + and - from a usb cable to the bread board relay - the vcc and gnd to breadboard and the in0 to 7 of the relay to the gnd on the breadboard to turn it on.

seems very basic thats why i can understand where im going wrong

must be something simple im doing wrong

Did you try measuring the current when all the relays are powered?