Hello everybody i'm building a project with arduino uno R3 and i want to power this 8-relay board(Working voltage:5v)
What i want to do is to :
1)power vcc from arduino 5v pin
2)power jd-vcc from external power source 6.5V-500mA
Is it ok to power 5V relay with 6.5V?Thank you in advance!
if you are asking if the relays will blow with an extra 1.5V, the answer is no, but you are over-driving them by an extra 30% or so which just means extra current thru the relay coils, not good for longevity.
I see there are 8 relays each one's coil will consume around 50mA when energized, so when all are energized there will be a current consumption in the order of 400mA (call it 1/2 amp).
Why not just put 2 diodes in series with the 6.5V supply and the J-VCC point?
Although 1N4007's will fit the bill, they may get warm if all 8 relays remain energized for extended periods of time.
Rather use beefier ones like the 1N5408.
EDIT:
And whilst you are about it, replace diodes D1 to D8 with proper ones like the SMD version of 1N4007s.
Those puny (IN4148 or similar) types are not meant to supress inductive kick back from relays, they are small signal diodes
and won't last very long in that application and once they start going, the relay drive transistors will shortly follow.
UnoDueTre:
And whilst you are about it, replace diodes D1 to D8 with proper ones like the SMD version of 1N4007s.
Those puny (IN4148 or similar) types are not meant to supress inductive kick back from relays, they are small signal diodes
and won't last very long in that application and once they start going, the relay drive transistors will shortly follow.
That's not true. When the transistor driving the relay turns off, the current through the diode is briefly equal to the current that was passing through the relay just before it was turned off, and decays more or less exponentially from there, becoming negligible in a fraction of a second. So the diode only needs to have a peak current rating equal to the relay coil current, which is 89mA for those particular relays @ 5V. The 1N4148 is rated at 450mA peak, 200mA continuous. So it is more than adequate for the job.
yes 450mA peak forward current, the kick back spikes are hardly forward.
As a general rule, 1N4148's for small reed relays, for anything bigger, 1N4001/4/7
UnoDueTre:
yes 450mA peak forward current, the kick back spikes are hardly forward.
You obviously don't understand what you are talking about here. The kickback spikes ARE forward, from the perspective of the flyback diode.
OK maybe I'm explaining wrongly then.
Not to worry, the OP can post back here in a few weeks and tell us how is relay board is faring.
Thank you all for you replies.Here are some more weird (for me)facts:
i have a panasonic ac/dc transformer in the label of which is written: output 6.5V-500mA but when i try to measure its voltage,multimeter shows 7,35V.
Next, as UnoDueTre suggested, i connected one 1N4007 diode in series with the transformer an then the multimeter shows 6.96V
then I added one more diode in series with the first one and now the multimeter shows 6.58V
so what is wrong with the whole thing?is my transformer faulty or those measurements are meant to be this way if there is no load?
vagktm:
Thank you all for you replies.Here are some more weird (for me)facts:
i have a panasonic ac/dc transformer in the label of which is written: output 6.5V-500mA but when i try to measure its voltage,multimeter shows 7,35V.
Next, as UnoDueTre suggested, i connected one 1N4007 diode in series with the transformer an then the multimeter shows 6.96V
then I added one more diode in series with the first one and now the multimeter shows 6.58V
so what is wrong with the whole thing?is my transformer faulty or those measurements are meant to be this way if there is no load?
Yes, that transformer is rated to supply 6.5vdc when the load is drawing 500mA. At less load the voltage will be higher and lower if current draw goes over 50ma. This is due to the internal impedance of the transformer.
Lefty
Ok thank you all for your answers:) the two diode in-series worked like a charm for me.Now with one relay on i have 5,5V and with all 8 relays on i have 4,9V on my relay board 
one last question if i cover those diodes tight with a plastic tape around are they going to have any problem with temperature?
First switch all the relays on.
Leave them on for a few minutes and feel the diodes.
If they are getting warm to hot then rather use the 1N5408 I suggested.
If after 10 minutes or so with all relays on, the diodes are still OK to touch then go ahead and use tape or heatshrink sleeving.
vagktm:
if i cover those diodes tight with a plastic tape around ...
Why would you do that?
There won't be any current through any of those diodes but super-briefly, resulting from coil (relay) de-energisation.
If you are referring to the two series diodes then there will be current flowing for as long as one or more relays are energized.
I'm more concerned about your power supply than the diodes. Those relay coils take 89mA each @ 5V (unless they are the low-power variant, which takes a little less). If you have all 8 of them on at once, that, that's 712mA. The 1N4007 series diodes are rated at 1A so they can take that (although they will get a little warm), but you said the power supply was rated at 500mA.
The relays in the picture are SRD-05VDC-SL-C which indicate the lower current ones (0.36W)
Therefore 8 of them consuming 71.4mA, equates to 571.2mA.
Surely the transformer will handle that.
It is a Panasonic after all, not some no-name brand from some dubious manufacturer in some far flung corner of the world.
UnoDueTre:
The relays in the picture are SRD-05VDC-SL-C which indicate the lower current ones (0.36W)
Therefore 8 of them consuming 71.4mA, equates to 571.2mA.
Surely the transformer will handle that.
That's still overloading the transformer by about 14% when all 8 relays are switched on at the same time. Designing a circuit in which components are overloaded is not good. But it may be that the OP never needs to turn on more than 7 relays at once.
Strictly speaking you are correct, but one has to take the following into consideration:
All reputable manufactures always add a certain amount of safety margins to their products, normally around 20%,
so 14% is not unreasonable.
It's not as if this arrangement is going to be mass produced and sold, it's a one off for the OP to mess about with plus he already had the transformer/adaptor to hand.
The chances of all 8 relays being on for extended periods is probably remote.
You will probably find that the transformer is capable of mote then the rated 500mA but is "limited" by what the manufacturer set as the limit of acceptable ripple due to the value of the smoothing cap/s.
Hello again! Just for your information my project is up and running for 24h+ and everything seems to be ok.I forgot to mention that i use 6 of the 8 relays in the board so with all of them working the multimeter shows 380mA, easy to handle from my transformer.The diodes heat up a little when all 6 are turned on but i dont get burned so i guess they are ok 
Thanks