# 30VAC to 5VDC

Discussion in 'Electronic Basics' started by Ryan Kremser, Oct 8, 2003.

1. ### Ryan KremserGuest

Hello, hopefully someone can help me. i'm not thinking straight so this
question is probably quite simple. I have a 30vac momentary switch
mechanically triggered and need to be able to close another circut through a
relay while also using the 30 volts to drive a solonoid. the relay I wanted
to use is rated at 5 volt dc for the coil and i wasn't sure how to drop the
30vac down to 5dc. Thanks in advance

2. ### Rod SpeedGuest

It is indeed. You will kick yourself in a moment |-)
Just think of it as a need for a 5VDC power supply from 30VAC.

In other words just rectify the 30VAC and use
a voltage regulator to get it down to 5VDC.

Dont forget that you will be throwing away quite a few
volts getting down from the rectified 30VAC to the 5V
and you need to allow for the current the 5VDC coil
draws multiplied by the voltage dropped, in the regulator.

3. ### Ken TaylorGuest

With the power applied only momentarily to the relay, heat dissipation in
the regulator circuit probably won't be a problem - a resistor, diode,
capacitor and zener would do. However, why use a 5VDC relay in the first
place? What's wrong with a 24VAC relay? You're just making your life
unnecessarily difficult.

Ken

4. ### John FieldsGuest

---
___
30VAC>----O O--+-------+
| |
| | +---[<DIODE]---+
| | +-----+ | |
| +--|~ +|--[R]-+-[RELAY COIL]-+
[SOLENOID] | | |
| +--|~ -|---------------------+
| | +-----+
| |
| |
30VAC>---------+-------+

The value of R will be equal to the supply voltage minus the relay coil
voltage multiplied by the relay coil current. For example, if the relay
coil currenr is 100mA,

E 30V - 5V 25V
R = --- = -------- = ----- = 250 ohms
I 0.1A 0.1A

The power the resistor will dissipate will be

P = IE = 0.1A * 25V = 2.5W

However, if the switch closure is only going to be momentary, the
resistor will only dissipate power while the switch is closed, so the
power rating of the resistor could probably be made much smaller. I'd
use 270 ohms at 1 watt, which you can easily make out of 270 ohm 1/4
watt resistors, like this:

<---+------+
| |
[270] [270]
| |
[270] [270]
| |
<---+------+

5. ### Ryan KremserGuest

its only a matter that i have a bunch of 5v relays handy

6. ### Rod SpeedGuest

Sure, it wasnt completely clear that that relay
would only be powered momentarily tho.
Lot easier to produce the 5VDC than get another
mechanically suitable relay in some circumstances.

7. ### Roger JohanssonGuest

A quick solution may be to use a resistor in series with the relay to
get the proper current.

We usually think of relays as voltage driven, because they are
specified for a specific voltage, but it is actually the current which
is important to achieve the magnetism, so we can just as well control
the current, if the voltage is not the specified voltage.
Use a resistor which gives enough current to achieve the result
needed. Think about the power and choose a resistor which can take the
power.

8. ### Lizard BlizzardGuest

I don't see any need for a regulator. The relay is a fixed resistance
and current draw, so a resistor would work just as well. So it's a
rectifier, filter cap and the relay and resistor in series, across the
filter cap. 5V should across the relay. The resistor depends on the
relay resistance. Make sure the resistor can handle the power. The
capacitor should be 50VDC rating.

--
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9. ### Lizard BlizzardGuest

I would say that without a filter cap after the bridge, the relay will
chatter with every pulse of the DC.

___

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----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

(Problem) Evidence of leak on right main landing gear
(Solution) Evidence removed

F
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r

f
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10. ### John PopelishGuest

If you only intend to drop voltage, you might just as well put the
rectifier directly across the relay coil and use a dropping capacitor
to the 30 VAC source. Choose a capacitor that provides about 5 volts
across the relay. It will cost more than the resistor but will not
produce any heat. Come to think of it, the capacitor will cost more
than a 24 volt relay.

11. ### John FieldsGuest

---
Depends on how hard you bang it.

I tried an old 6V 52 ohm coil Allied Control T154-CC-CC with a 24VA
control transformer, a bridge made from 1N4004's, and a 150 ohm resistor
in series with the coil and it draws 113mA and works perfectly. No
chatter, no nothing.

Note that 6V/52 ohms = 115mA, so with 113mA through the coil, it's
coasting!

What's happening is that once the armature mates with the
electromagnet's pole piece it doesn't take much of a magnetic field to
keep it there, and at 120PPM (DC) it just doesn't have time to get away.

Also, since it's DC it's not like the magnetic field has to take the
time to completely reverse polarity and allow the armature to escape.

12. ### Rod SpeedGuest

Sure, its just a quick and easy way to do it.
Sure, but its only easier if he understands how to size it.

13. ### Lizard BlizzardGuest

Yeah, I thought about that. But then I thought, how many uFs is it
going to take for 25V at 100 mA? Almost a dozen? Lots more than a
typical 1 uF poly capacitor usually has. So yeah, the cap will be large
and expensive.

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

(Problem) Evidence of leak on right main landing gear
(Solution) Evidence removed

F
o
d
d
e
r

f
o
r

s
t
u
p
i
d
"
n
o
t

e
n
o
u
g
h

i
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c
l
d
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14. ### AnonymousGuest

what about 20 diodes in series?