The AM Forum

I have a nice 24 volt 60 hz AC coil relay I'm using for a the primary of my power E-Rig supply. I figgered it would run on about 15V DC.  (24VAC * .636)  This is the heating value.

Running at 15V the relay clacked in but the coil got hot and shorted some turns. Now it's bad.  I have a spare and tried a lower DC voltage but it will not key on.

It appears that AC is needed and DC obviously causes a large static current.

I've used smaller 120V AC relays on 30V DC before without problems.  I'm looking to use this relay and eliminate the buzz of AC. What if I were to use a 1/2 wave diode rectifier off an AC transformer to make it pulsating DC - would this help or do I need to stick with AC for this application?

T

Too much voltage on the relay, Tommy - divide by 4 or 5.  Use a variable DC supply to test the relay to see where it reliably kicks in, then calculate a series resistor for it if your supply is 15VDC.

Should be no problem at all using DC on an AC relay, given the proper voltage.

About a 4X reduction in DC voltage has worked for me for decades, especially in commercial amps using AC antenna relays with SS 12VDC rigs.

Carl
KM1H

Tom,
       Use a half wave rectumfryer (single diode) and a substantial filter cap. 1000Uf
or so. then put a resistor in series with the ac inpoot to the diode. This will give a high initial wack to pull the relay in, then the the resistor will let it sag down to a much lower voltage to hold it in. I've been doing this for years. Or for intermittant use just run it somewhere around half the ac rating on dc. This will be ok for INTERMITTANT use, but NOT for continuous duty.

The Slab Bacon

Tom,
I use 115VAC relays on DC all the time. 24 to 28 volts DC is plenty so I think you are running too much voltage.

I had the same problem with the 240v a.c. contactors in my Gates.  I have often run small 110 v.a.c. relays on 28 v.d.c. and they worked fine.  But when I tried to run the contactors on about 56 v.d.c., they wouldn't pull in.  I rigged up a DC supply fed with a variac to determine the minimum reliable pull-in voltage.  IIRC, it was somewhere around 90 volts DC.  But that much voltage caused the coil to heat up.  I didn't damage any of the coils, but I could smell the hot insulation.  Then I reduced the votage to see when they would drop out.  That turned out to be at about 25-30 volts IIRC.

So I wired the control  circuitry to initially wack the relays with about 150 volts, the full output of the voltage source, but retain only about 40 volts to  hold them in.  On the main control relays, which are in a latching  configuration, it was simple.  I wired a 1000 ohm wirewound resistor in series with the set of contacts that latch the relay in the "on" position.  The initial wack, with the filament and plate "start" switches, pulls in the relays with the full DC voltage from the source.  But once the relays are pulled in, and the start switch is released, they are held in by the latching contacts, and the 1K resistor reduces the voltage/current to a safe value.

But the high/low power switching relay was another matter.  It is not in a latching circuit, but fed with a simple toggle switch.  So I used a 1000 ohm resistor, just like the ones with the other two, in series with the coil, to limit the current to a safe value.  I put a "computer grade" electrolytic (don't recall the value, but 1000-1500 mfd sounds about right) in parallel with the 1K resistor.  When the transmitter is switched to low power, the capacitor (long ago completely discharged) effectively shorts out the resistor, so the coil is wacked with the full voltage.  As the capacitor charges up, the voltage across the coil is reduced until it eventually settles to the normal value with the resistor in series, since the charged capacitor no longer acts as a short circuit across the resistor.

This is nearly identical to Frank's circuit, but allows the use of a better regulated DC power supply in  case you wish to use the same DC source for more than one independent control relay or for some additional purpose for which you would want the voltage source to remain steady.

Hmmmm.... this is what I need to try.

So far, the relay will NOT pull in with less than 14 DC volts. Less than 14 volts and it just vibrates slightly. I have four relays like this. They all have a 5 ohm DC winding.  When working at 15v, I see 3 amps going thru. However, after 30 seconds the relay coil heats up and the resistance goes to 6 ohms. I lost one relay already.

Bottom line is it seems the relay does not have the scrote to pull in at less than 15vdc, but once pulled in, it will fry at that voltage.

Yes, Frank, I use 120ac relays here too at 28VDC and they work well. The problem is I need  real brute of set of contacts to handle the keying on of the AC inductive load from the HV transformer primary. Smaller relays stick from pitting after only one closure.

T

AC relay is all about inductance to limit current. Two ways to get there 1. lots of turns 2. few turns with a big core.
Looks like you have option 2 and there is no way to apply pull in DC voltage without cooking the coil.
I guess you need a better part or a way to reduce voltage after it has been pulled in.

Howzabout a pair of 12V lamps in series with the relay coil?

They'll start out as a low resistance, then the resistance will trend upwards proportional to filament temperature, keeping your voltage in check.

On top of that, it's another "it's not broken" light you can stick on the panel.

Just a thought.

actually I have seen a series resistor just high enough to hold the relay in. This resistor charges a big cap with enough energy to pull in the relay. Once the relay is in the cap and coil sit at a lower voltage so the coil doesn't fry.
The resistor and cap sit on the positive side of the coil and the low side of the coil is switched to ground.
I think a lamp will heat up too quickly but WTF worth a try.

I have a relay with mercury wetted contacts if you'd like to put it to use.

"I have a relay with mercury wetted contacts if you'd like to put it to use."

Steve,

Maybe that would work.  I've tried all combinations of getting these relays to work, but NG.   I need to go with a 12V DC coil to fit the situation.... or 24VDC will do too.

Let me look on eBay first and if not, we'll look at the mercury type. What is the coil requirement and what are the contacts rating?  Can it handle AC inductive loads?  I have a few of these nice open frame relays with big contacts, but they stick after the first AC spark.  Those big oil burner type plungers are fine, but running them on 24V AC makes a huge buzz. Running on DC seems to magnatize the coils so that after a few clicks they stop working.... they get hot too on DC.

T

Screw it. I just bought two of these FB relays with 12VDC coils on eBay. They are exact replacements for what I have in there now that were 24VAC.

Done deal.


http://cgi.ebay.com/Arrow-Hart-ACC330UDCAC-40-Amp-3-Pole-Relay-12-Volt-DC_W0QQitemZ350281574917QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item518e68aa05


T

Interesting... I just went through this with a bunch of relays I wanted to use.  Bacon and GFZ are right. It's all about high current, low inductance to pull in and high inductance once that armature closes the magnetic path.  Can't really do that with DC. The bigger the relay to worse the difference too!   
I like Frank's (Bacon) idea with the higher voltage rectified DC and dropping resistor. Might use that in a pinch.

I'll check the specs when I'm back at HUZ Central.

Tyco has an application note that may prove helpful:  http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3250.pdf

Excellent!

I think this is the circuit I used to use to pull in AC relays and hold them without excessive current.  Think this is right.  Yes, used to use use 1/4 the voltage for AC relays when using them with DC

Al