Driving DC relays from AC

[jrobbo]

Hi all,

I have a handful of relays with a coil rated at 24V DC. Only problem
is that the source that I want to drive them from is 24V AC....

The coil current on the relay is 15mA. I was thinking of just using a
half-wave rectifier (single diode) and a 100uF cap to drive it. Will
that work, or should I use a full-wave (4 diode) rectifier instead?

Thanks in advance for any help

Regards

John

 

[Bushy]

Depends, if you don't mind a small delay in operating the relay then use
half wave and accept up to a half wave delay in operating the relay.
Peter

 

[Phil Allison]

Hi all,

I have a handful of relays with a coil rated at 24V DC. Only problem
is that the source that I want to drive them from is 24V AC....

The coil current on the relay is 15mA. I was thinking of just using a
half-wave rectifier (single diode) and a 100uF cap to drive it. Will
that work, or should I use a full-wave (4 diode) rectifier instead?

** One diode ( half wave) rectification is fine - but use the diodes
connected alternately to the AC so the DC component in the transformer does
not build up to any large amount.



.......... Phil

 

[KLR]

Hi all,

I have a handful of relays with a coil rated at 24V DC. Only problem
is that the source that I want to drive them from is 24V AC....

The coil current on the relay is 15mA. I was thinking of just using a
half-wave rectifier (single diode) and a 100uF cap to drive it. Will
that work, or should I use a full-wave (4 diode) rectifier instead?

Thanks in advance for any help

Regards

John

experiment with both setups and see what works best. Only takes a
minute to do.

Other thing to keep in mind is that rectified 24vac is going to
produce about 34v dc (multiply the ac x 1.414 to get the DC voltage)
which might be too much voltage for your relay's coil - specially if
its left on for any length of time. An appropriate dropping resistor
would be a good idea too;.

 

[Phil Allison]

Other thing to keep in mind is that rectified 24vac is going to
produce about 34v dc (multiply the ac x 1.414 to get the DC voltage)
which might be too much voltage for your relay's coil - specially if
its left on for any length of time. An appropriate dropping resistor
would be a good idea too;.

** This sort of relay is typically rated for 150 % over the nominal DC
voltage - see Farnell cat. The average voltage can be reduced buy
selecting the filter electro value - using a 22uF will allow about 10 volts
p-p ripple at 15mA and 50 Hz hence 5 volts average drop.



............ Phil

 

[Ian Macmillan]

The best approach is to use a diode in series with the coil to the AC, and a
diode in parralel with the coil, cathode to positive. This will keep some
current in the coil during the "off" half cycle and control any voltage
transients that might destroy the series diode, especially when switching
it. A DC component in the supply may not be an issue if the relay current is
small compared with the supply capacity.

All the best

Ian Macmillan

 

[Arpit]

The best approach is to use a diode in series with the coil to the AC, and a
diode in parralel with the coil, cathode to positive. This will keep some
current in the coil during the "off" half cycle and control any voltage
transients that might destroy the series diode, especially when switching
it. A DC component in the supply may not be an issue if the relay current is
small compared with the supply capacity.

WOuldn't the capacitor do the same as the parallel diode? my
experience using relays on 50 hz half wave rectified without a cap is
that they buzz, so a cap is needed.

 

[Phil Allison]

WOuldn't the capacitor do the same as the parallel diode? my
experience using relays on 50 hz half wave rectified without a cap is
that they buzz, so a cap is needed.

** Correct - the extra diode in parallel is not enough to stop severe
relay oscillation at 50 Hz.

A 10 uF or greater cap will however do both jobs.



.............. Phil

 

[D.Castles]

** One diode ( half wave) rectification is fine - but use the diodes
connected alternately to the AC so the DC component in the transformer does
not build up to any large amount.



......... Phil

DC component in a transformer? UH?
Dave

 

[D.Castles]

** This sort of relay is typically rated for 150 % over the nominal DC
voltage - see Farnell cat. The average voltage can be reduced buy
selecting the filter electro value - using a 22uF will allow about 10 volts
p-p ripple at 15mA and 50 Hz hence 5 volts average drop.



........... Phil

Adding a Cap changes the Average Voltage...UH?
Dave

 

[Phil Allison]

Phil Allison wrote:


Hi all,

I have a handful of relays with a coil rated at 24V DC. Only problem
is that the source that I want to drive them from is 24V AC....

The coil current on the relay is 15mA. I was thinking of just using a
half-wave rectifier (single diode) and a 100uF cap to drive it. Will
that work, or should I use a full-wave (4 diode) rectifier instead?



** One diode ( half wave) rectification is fine - but use the diodes
connected alternately to the AC so the DC component in the transformer does
not build up to any large amount.



.......... Phil

DC component in a transformer? UH?


** Transformers have always been a big problem for Dave's dinosaur brain.





............ Phil

 

[D.Castles]

** This sort of relay is typically rated for 150 % over the nominal DC
voltage - see Farnell cat. The average voltage can be reduced buy
selecting the filter electro value - using a 22uF will allow about 10 volts
p-p ripple at 15mA and 50 Hz hence 5 volts average drop.



........... Phil

What about the average voltage one Phil? (see below)
I can't wait for your explanation and next insult. I love them, they
make me stronger.
Dave

 

[Michael]

** One diode ( half wave) rectification is fine - but use the diodes
connected alternately to the AC so the DC component in the transformer does
not build up to any large amount.



......... Phil

I'm not sure what you had in mind here Phil, were you thinking of a
CT with a diode to each end?

 

[Phil Allison]

I'm not sure what you had in mind here Phil, were you thinking of a
CT with a diode to each end?

** No.

My post is VERY clear - do not have all the diodes rectifying the
same. Alternate them.



......... Phil

 

[Michael]

Do you have an anger problem Phil?.
I believe there are anger management councillers that may help.
As for average voltage.... I re-ask my question because I am possibly,
as you say, a complete moron and don't understand.
If you half wave rectify a sine wave then how is it that a capacitor
lowers the average voltage of that waveform?
This is a genuine question Phil, please don't get upset.
As for posting HTML my Netscape tells me that this is the preferred way
of posting to this newsgroup. Is this not correct? Would you be happier
with plain text or that I don't post at all?
Thanks Phil for your kind attention to these matters
Dave

--

Dave, by selecting a lower capacitance, the voltage across the cap
discharges more between cycles. Hence the average voltage across the
relay coil is lower. What is your argument?

 

[Phil Allison]

"D.Castles" <[email protected]

Dave, by selecting a lower capacitance, the voltage across the cap
discharges more between cycles. Hence the average voltage across the
relay coil is lower. What is your argument?

** Dave is a highly qualified mental cripple - see:

http://www.latrobe.edu.au/ee/people/profiles/Castles.html


He gets his jollies by baiting naive Christians on their newsgroups -
and pulling the wings off flies.




............ Phil

 

[Albm&ctd]

As for posting HTML my Netscape tells me that this is the preferred way of
posting to this newsgroup. Is this not correct?

Dunno, I'm braindead but I'm considerate enough to others who have
text based readers to NOT post in html.
As for running relays from AC, I'd use a bridge rectifier with a
decent sized electro taking note of the pull in and hold in voltages,
maybe even use a variable supply to test the relays I want to power so
I could choose a suitable voltage transformer but I'm just a mechanic,
not a farkin electronic genius. Ask Phil.

Al

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