Cheapest and "Smallest" way to cut 6.5v to 5v

Using Arduino General Electronics
1

Hi,
I have been designing a very simple circuit for a project which works with 6.5v, it can work with any voltage from 4 to 12v but I'm using 6.5v just because I had a 220v to 6.5v adapter laying around.
In the circuit, there is a relay, which works with the circuit main voltage (6.5v now) but I couldn't find a 6.5v relay so I'm gonna use a 5v relay instead, but since this relay will be turned on for long periods of time, I don't want to over-drive it and shorten its life span.
now I'm looking for a simple way to cut down the 6.5v to 5v for that relay, any advice?

p.s: I don't want to use a buck converter module or a 5v regulator like 7805, first because I should keep the PCB as small as possible and second because I should keep it as simple as possible.

p.s2: I don't want to make the 5v for the whole circuit, it will be used only for the relay

2

If it's just a relay, which will draw constant current, put a resistor of appropriate value in series with it.

The total resistance is (6.5 / 5) * the resistance of the relay.
Subtract the resistance of the relay from the result above to get the resistor value.

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3

Yes, or two silicon diodes in series... that drops about 1.4V.

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4

so if the relay is 420 ohm (measuring a 12v relay just for an example),
a 546 ohm (the nearest one obviously) in series with the coil will do the job?

5

That's also a great idea! thanks
1n4007 is fine right?

6

Err, no.
6.5 / 5 = 1.3
1.3 * 420 = 546
546 - 420 = 126

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7

oh yes sorry I missed the subtraction part, Thanks a lot! I'll try it out.

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9

It's a 1A diode IIRC, should be enough for most relays.

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10

If you make your project for 5V, then you will benefit from it later.
Suppose you need to replace that adapter some day, then a 5V adapter is easier to find.
Did you know that a USB "charger" is just power supply of 5V ?

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11

Not a meaningful concept. :upside_down_face:

The only consequence of the excess voltage, is increased operating temperature.

The standard "Songle" relay operating at 90 mA has a coil resistance of 55 Ohms. At 5 V it will dissipate 450 mW; at 6.5 V it will dissipate 770 mW. It will get a little warm if actuated for long periods.

To drop 1.5 V at 90 mA, you need a resistor of about 17 Ohms. The nearest common value is - well either 15 or 18 Ohms would be just perfect! :sunglasses:

Not a good example though. :grin:

470 or 560 would be the nearest common values.

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12

It should be connected directly to a plug,
I can make it 5v I just need to buy a 220-5v adapter but I just have a 6.5v so you know, doesn't really matter that much.
Also, it may be a school project for an A.H :poop: professor and I really don't feel like ordering anything else for him lmao :joy:

13

I know yeah it's not a big deal, that's why I said I don't wanna use a regulator or a buck converter. but if just a single resistor can fix it, then why not?

14

Which is the most reliable way to shorten a component's life-span. Runs hotter, expands and contracts more, stresses the solder joints with the PCB fatiguing them. Also the contacts oxidize more and become less reliable.

15

Ha ha. How many times have I seen the area underneath a through hole resistor on the PCB, brown or black from the heat? :slight_smile: Because someone only looked at the optimistic numbers.

16

Well the usual problem is not knowing that a resistor run close to its max power should not be mounted flush to the PCB, but spaced up above it for increased air-flow and to avoid scorching the board - power resistors can run at pretty high temperatures.

17

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