The AM Forum

I am using a 110 vac Dow Key relay to switch between my FT-950 and my AM boatanchor setup.
When the relay is energized for a long period of time, it gets very hot to the touch.
Is this normal?
My line voltage is 121 vac.
Is that a little too much for the relay?
Thanks,
Don W9BHI

OK, you can call yourself an Old Buzzard!  :)

Current through the wire.
Wire heats up.
Wire radiates heat.
Shorten energized time.
 :D

Had the same issue here. Also with my CE 20A I was leery about switching 120vAC with that relay in the 20A. If there was a fault, the relay and 20A wiring would fry.

I used a 1:1 ratio 12 VA Signal isolation transformer with a split bobbin:

241-5-120

http://www.signaltransformer.com/sites/all/pdf/241.pdf

So now I had isolation, and with a split bobbin (poor coupling), I could short out the transformer and not burn anything up. The poor coupling resulted in about 90V to the Dow Key, and although that worked, on occasion the Dow Key would key and buzz. So I added a full wave bridge between the transformer and the Dow Key to key the relay on lumpy unfiltered DC.

So now the Dow Key keys perfect every time, and the reduced voltage does not cause excessive heating of the relay coil.

In general, anytime you fire off an AC rated relay coil with DC, you must drop the DC voltage quite a bit from the AC voltage rating. If the DC is filtered, this is even more true.

Jim
WD5JKO

 

Try just adding a series diode. This will drop the power. Just make sure it pulls in properly. Another option is add a series power resistor to reduce the voltage.

I use a 24vdc "contactor" relay with 30A contacts to switch my ladder line. At 24vdc with a fairly long run of wire, it was sometimes a bit reluctant to pull in.
So with a little head scratching, I came up with the idea of a power supply with a pre-determined voltage "sag" built in.

I used a 24vac transformer into a single diode (half wave rectifier) with a large filter cap. Then I put a resistor between the transformer and the diode to allow the voltage to sag under load. (I don't remember the value of the resistor.)

Now when I hit the switch it hits the relay with around 40v and really wacks the relay in. Then it sags down to around 15-18v to hold it in. The relay now never gets warm and it stays pulled in for hours on end with no problems.

DC on that relay with just enough voltage for pull in could make it play better.

Alternately, baseball relays always seem to be for sale on ebay. Put a cheap a dummy load on the unused port for a reference and a way to avoid keying that expensive plastic radio into thin air. 

I tried putting a diode in series with the coil and it dropped the voltage down to 82 volts.
I powered it up for 45 minutes and it still got so hot you couldn't pick it up.
I tried this with a second relay and got the same results.
Maybe a power resistor in series as well or maybe these relays can't take being powered up for long periods of time?

Don W9BHI

Bridge rectify 24 volts and filter the devil out of that.  It should drive that relay.

It could also mean that some wires are shorted in the coil causing it to heat up. Measure the current through the thing. See if it's in spec. I use a 110V AC coil with my Apache and it only gets warm to the touch after several minutes of operation.

I bet these relays were not designed to run for 45 minutes at a time
Franks solution is a good one. Once the relay pulls in it takes less power to hold it.
A shorted turn would burn up quickly

Dowkey wont work for long periods of time like that.  I use Contactors. They log for days without heat.

C

Reading your original post, it sounds to me as if you are using the
Dow Key to switch between the new rig and the Boatanchor station...

This is not what I'd use to do this task. The Dow Key was designed
for ICAS duty, not continuous duty. Suggestions offered to decrease
the voltage, current or otherwise reduce the operating temp of the
relay coil might also include putting the Dow Key in the freezer to cool it
down.

Doing this game for 40 years, I'd recommend getting a manual
rotary antenna switch to select between the two stations.

If you want to operate this remotely, many rotary solenoids provide
that function too!
good luck es 73
Steve
w8tow

I also agree with Bacon's solution. Remember, the pull-in and the holding force are proportional to the current flowing through the coil... not the voltage across the coil. The coil has lot's of inductance... and at 60Hz... the inductive reactance is much more than the DC resistance. So... if you run the coil on DC, you need a lot less voltage to produce the correct pull-in current, and the correct holding current.

I've used Bacon's solution on some relays in my shack. You will need to measure the DC resistance of the relay... and use that, along with the required pull-in current to determine the DC power supply voltage required to work with the relay. The combination of the large capaictor (e.g. 1000uF) and the series resistor will give you the higher initial current required to pull the relay in, and the lower holding current required to keep the relay coil from getting too hot.

For example, if the DC resistance is 300 ohms, and the required pull-in current is 80mA... then you want the 1000uF capacitor to be charged to somewhat more than 24VDC before the relay is activated. A (nominally) 24VAC transfromer, driving a 1N4007 diode and a series resistor will charge the unloaded capacitor to around 34VDC (24VAC rms x 1.414)... which is okay, because the voltage across the capacitor will quickly drop to a lower value once the relay is activated. The value it drops to will be determined by the value of the series resistor. If the holding current (in this example) is 45mA, then the required series resistor's value will be in the ballpark of 100 ohms.

See the attached LTSpice simulation of the example above. The graph shows the 45mA steady state holding current (after the relay has pulled in). The larger pull-in current will last for around 0.3 seconds (i.e. 1000uF x 300 ohms) as shown in the last attachment

Stu

45 minutes continuous closure is beyond any reasonable expectation.  A manual switch such as Alpha Delta sells is more appropriate.  If you need something truly remote, MFJ and DX Engineering sell remote 'antenna switches".   

   I just measured my 110vac Dow Key coil, got 291 ohms DCR, and .995 Henries using my Beckman DM27XT meter. When I key the Dow Key as hooked up in my earlier post, using the same meter I read 57 volts DC.
I don't have a heating problem, but I've never left it keyed for 45 minutes either.  ???

Jim
WD5JKO

Have you contacted Dowkey yet to ask THEM ?
http://www.dowkey.com/technical_support.php

I have used a DOW KEY relay not a advance for years even on long transmissions the relay only gets warm (never hot).

John W9BFO

My rf relays are surplus TV stuff designed for continuous duty.

One has been energized for years here.

They are 50 bux on ebay on a bad day.

I'm just going to build a new one with a 26 volt HC1 vacuum relay.
I'll run it on 24 volts and it can take being on for long periods of time without getting hot.
Thanks for all of the input.
Don W9BHI

I guess you've never been on AM and listened to some of the "old buzzard" transmissions......................   :o   ;D  ;D

I wonder how long he would have listened to Bob/W5PYT? 

I am a bit curious, did anyone find a data point for the duty cycly of a relay?  The only spec. I have ever seen is number of cycles.  Never have I seen one on the time closed.  Someone mentioned the remote antenna switches.  They have relays in them and no mention of time of activation current applied. 

Mike/W8BAC did a nice article in ER a few years back as I recall, about Dow Key relays and their failures. Fine for an Apache or 32V, but much beyond that is rolling the dice.

So am I missing something or is the Dow Key actually just being used as a station switch and not a change over relay? If it's a matter of just switching the aerial to a different station to use, it seems like a DPDT switch would be a far better choice as others have mentioned? Heavy duty contactors would be more suited as well.

I did a search in the ER Index and couldn't find any article on "Dow Key".

On the Dow-Key DK77 series:
Life Expectancy: Over 1,000,000 cycles at rated load when operated at a rate less than 2 cycles per second.
Operating time: Less than 15 ms.
Coil rating:, 6, 12, 24, 28, 32, 48, and 110V DC @ less than 2.5 watts.

On the Dow-Key DK60 Series (this is the one with round coil housing):
Life Expectancy: Over 1,000,000 cycles at rated load when operated at a rate less than 2 cycles per second.
Operating time: Less than 15 ms.
Coil rating:, 6, 12, 24, 28, 32, 48, 110V, and 220 DC @ 2.0 watts.
Coil rating: 6, 12, 24,110, and 220 AC @ 6 volt-amps. 50-60 cycles.

I have the relay connected to the primary of the power transformer of my Hammarlund HQ-140X.
When I power it up, it switches the tuner and antenna over from my FT-950
and amp to my Johnson Valiant II.
I guess I am getting way too lazy in my old age. ;D

Don W9BHI

Actually, some models of Dow-Key are not good for that application, as they have shorting contacts in them to ground the RX antenna lead on transmit. It is a small spring loaded fadiddlin in the NC side of relay. You can tell these type as they have a longer barrel on the RX antenner connector. They can be a little troublesome and would prolly wipe out the finals in a solid state rig if they didn't make (or break) properly.

As far as transmitter power that they will handle, I have used one with my 4X1 rig for many years.

Some dow keys smell like burnt band aids when overheated.

The HA-6 here had the original Dow Key inside when I bought it about 25 years ago and its still doing fine even with 2-4 hour VOX CW pileups.

The 1500W 2M amp uses a Dow Key transfer relay and has been abused as well in contests for over 25 years, same coil as the others.

Never had a failure in the AM days at the KW level either with long winded transmissions.

Ive read of several coil failures which I suspect is a manufacturing defect or RF got in.

Carl

lasts for ever as long as you don't wet switch the contacts.
I actually go for the mil relay with potted coil and N connectors. They are cheaper and built better. They also work well at 28VDC