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Author Topic: AC vs. DC relays  (Read 38422 times)
K3STX
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Posts: 1644




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« on: September 06, 2013, 04:59:34 AM »

I want to use a relay to switch the HV off my DX-60B function. I plan to do this by changing the function switch to instead activate a 6 volt relay (which, when closed, will allow the HV to flow). I want to use the 6.3volt filament AC for the relay, but Mouser has few "6V AC" relays (starting at about $15 each with 10A current contact rating) but LOTS of "6V" and "6V DC" relays (with 20A rating and they are CHEAP).

What is the difference between an AC relay and a DC relay and will the relays marked "6V" ( no designation of AC or DC) do the job?

paul
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W2WDX
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Posts: 218




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« Reply #1 on: September 06, 2013, 06:17:45 AM »

Very little difference actually. A DC coil can be used on an AC voltage like an AC filament circuit. However it may hum or chatter. With DC the current is constant. However with AC the current crosses zero 120 times per second (in a 60 cycle system). So a DC coil may hum at 120Hz or chatter when engaged.

An easy solution is to put a diode across the coil terminals this will rectify the AC somewhat and lessen the hum and chatter. You can build a little half wave rectifier with two diodes and a small filter capacitor if you like which would work even better. However, with the filter cap the coil will disengage slower due to the time constant (discharge rate) of the cap.

John, W2WDX
« Last Edit: September 06, 2013, 06:22:09 AM by W2WDX » Logged

K1DA
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Posts: 744




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« Reply #2 on: September 06, 2013, 06:45:25 AM »

   The diode ought to be in series with the coil not across it. 
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AA4PB
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« Reply #3 on: September 06, 2013, 06:47:17 AM »

An easy solution is to put a diode across the coil terminals John, W2WDX

No! Not across the coil terminals because that would short the filament supply on every half cycle. Put a diode such as a 1N4007 in **series** with one of the relay coil terminals. In addition put an electrolytic capacitor across the terminals if you need to further reduce the hum sound that the relay makes.
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Bob  AA4PB
Garrisonville, VA
N3QE
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« Reply #4 on: September 06, 2013, 08:15:41 AM »

A DC relay coil, hooked to an AC voltage, will tend to buzz/vibrate at 120Hz. This can be somewhere between annoying and actually deleterious to life of relay and contacts.

A relay with AC coil has a "shading ring" as part of its structure that helps maintains the pull in force through zero crossing, greatly reducing the amount of buzz/hum.

Commonly available AC coil voltages are 120VAC and 24VAC. 24VAC coil is very very common for relays and contactors used with thermostats as part of both home and industrial HVAC systems.

As you already discovered, 6VAC coil relays exist but are not that commonly available. You might just want to build a small DC power supply with diode and electrolytic capacitor to power a 6VDC coil relay. The electrolytic can likely be on the "small" end, a lot of ripple in the DC is just fine, typically you would choose the electrolytic to put an average 6VDC (actually pulsating between maybe 3 and 9V) out into the coil's rated resistance.
« Last Edit: September 06, 2013, 08:19:35 AM by N3QE » Logged
W2WDX
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« Reply #5 on: September 06, 2013, 08:29:48 AM »


No! Not across the coil terminals because that would short the filament supply on every half cycle. Put a diode such as a 1N4007 in **series** with one of the relay coil terminals. In addition put an electrolytic capacitor across the terminals if you need to further reduce the hum sound that the relay makes.


Hmm ... So I guess that's a problem huh? Hmm... What about what happens when the series diode is in place and the voltages spike up to hundreds of volts from the back EMF of the coil when it's disengaged. Is that harmful too? I'd say ... much!

BTW, most tube filaments are better served by lower duty cycle. Lookup "Flyback diode on relays", you'll get it.

Now wait a minute is the flyback diode only in DC circuits? Now you have me confused!!! haha.

Generally I build up a little PS, with transformer and all, for relays. Most filaments are supplied from a tap on the main transformer, so adding extra current (on transformers that are usually at the limits anyway, like in the DX-60) is something I avoid. Here's a picture of a supply I added for T/R PTT function on one boatanchor. This happens to be a 12V DC relay coil. The switching is done by grounding the center-tap of this transformers secondary (primary is 120V AC), and the relay also grounds the center tap of the radio's HV secondary, this way no HV goes through the relay nor the microphone. Only ground in both cases. BTW, the relay itself already has the "flyback diode" across the coil built-in internally.

(Mind you, the DX-60 does not use a center tapped HV transformer tap. The HV actually goes through a switch {fs-2} which is a weak point in the radio and that switch commonly burns up). Because of this what I did was find a new main transformer with the correct voltage taps (240, 110, 6.3) and slightly higher current, but got one with a HV that is center tapped and changed the rectifier circuit accordingly. This way, I could  make a safer PTT system that doesn't out the HV through a relay. (However that is not what is pictured below. This is a Gates Radio transmitter).

« Last Edit: September 06, 2013, 09:34:14 AM by W2WDX » Logged

K3STX
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Posts: 1644




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« Reply #6 on: September 06, 2013, 12:57:10 PM »

As you already discovered, 6VAC coil relays exist but are not that commonly available. You might just want to build a small DC power supply with diode and electrolytic capacitor to power a 6VDC coil relay.

I've got 12 volts DC all over the shack, I was just hoping to avoid yet another wire to my power supply and was hoping to do it "easy" using a 6V AC relay. It is easy enough to use the switch on the rig to now switch in 12V DC from my power supply which would then in turn activate the relay providing HV to my rig.

Thanks for all the help, I don't think I want relay chatter or hum.

paul
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W8JX
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Posts: 13268




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« Reply #7 on: September 06, 2013, 01:32:32 PM »

As you already discovered, 6VAC coil relays exist but are not that commonly available. You might just want to build a small DC power supply with diode and electrolytic capacitor to power a 6VDC coil relay.

I've got 12 volts DC all over the shack, I was just hoping to avoid yet another wire to my power supply and was hoping to do it "easy" using a 6V AC relay. It is easy enough to use the switch on the rig to now switch in 12V DC from my power supply which would then in turn activate the relay providing HV to my rig.

Thanks for all the help, I don't think I want relay chatter or hum.

paul

As stated a DC coil will work on AC but you actually need a higher voltage for it to latch firmly on AC than DC because cycle and polarity changes. With a 6 volt DC coil try about 8 to 9 volts AC
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Ham since 1969....  Old School 20wpm REAL Extra Class..
AA4PB
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Posts: 15044




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« Reply #8 on: September 06, 2013, 02:04:03 PM »

If you are worried about the spike when the relay de-energizes they you could add a parallel diode in ADDITION to the series diode. However, a 6VDC relay is not likely to generate a large enough spike to damage a 1N4007 diode which is rated for 1000 PIV. The parallel diode can ONLY be applied where the voltage applied to the relay is DC or pulsating DC.

If you use a parallel diode without a series diode then your diode is essentially across the 6VAC filament supply and will represent a short circuit on half of each cycle. In addition to placing a heavy load on the filament circuit it will overheat the diode which will probably fail shorted which will damage the power transformer if it doesn't take out the primary fuse first.
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Bob  AA4PB
Garrisonville, VA
KH6AQ
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Posts: 7884




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« Reply #9 on: September 06, 2013, 06:28:06 PM »

I say do it right and buy an AC relay.
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G3RZP
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Posts: 1271




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« Reply #10 on: September 07, 2013, 01:14:11 AM »

If you have an electrolytic across the coil, there wont be much of a spike anyway. Plus di/dt on a sine wave isn't as great as doing an interruption with a switch.
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KG8LB
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Posts: 408




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« Reply #11 on: September 09, 2013, 03:47:28 AM »

  Bare in mind that with an electrolytic across the relay coil you may find yourself with a very slow release or "hold over" that could cause problems of it's own .  A good design engineer would avoid that by simply using a AC relay in the first place . Otherwise provide a proper DC source and only use an electrolytic across the coil when a timed release feature is actually desireable .
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AA4PB
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Posts: 15044




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« Reply #12 on: September 09, 2013, 05:03:54 AM »

By selecting the right capacitance value you can easily prevent any chatter without introducing excessive delay (assuming you are not trying to use it for full break in CW).
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Bob  AA4PB
Garrisonville, VA
WB6BYU
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Posts: 18459




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« Reply #13 on: September 11, 2013, 12:03:53 PM »

My suggestion:  make a small rectifier/filter assembly to provide a DC output from your
filament supply.  Might want to use a voltage doubler, depending on the relay voltage.

Once you have the DC, then pass that through the switch to control the relay - that
way the filter capacitor is on the common side rather than the switched side, so it
won't affect the relay response time.  A series resistor can be used to adjust the coil
voltage if the output from the power supply is too high.

Add a reversed shunt diode across the coil to dissipate the voltage spike across the
winding when the switch is opened.

That approach should work for any DC relay rated 5 to 12V, and is pretty simple to
implement.
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W2WDX
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Posts: 218




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« Reply #14 on: September 11, 2013, 01:01:46 PM »

My whole point is, why use the AC filament supply? It's a poor (albeit commonly used) choice.

What seems "easier" will always lead to unplanned complications. Like more load on the mains transformer, changes in filament supply voltage which in turn can lead to cathode stripping or premature failure of the filaments, etc.

Building up a tiny supply with its own 120V AC primary is simple. You could even rip apart a wall wart for a small transformer and building up a little rectifier bridge and filter is easy, design it for whatever relay you have. It would all be very small and easily fit inside a DX-60. It would cost you next to nothing.

As you can tell by all the comments on this topic, using the filament supply has implications. Using a totally separate supply, removed from any other circuitry in the DX-60 other then the AC mains jack, would be a better and no more complicated approach.

It really is a shame the DX-60 doesn't use a center-tapped HV tap. Hammond does make a transformer with appropriate  secondary taps with a CT on the HV, that fits in the DX-60. It also has about an additional .125mA available overall. It requires only a slight change in the rectifier topology. This grounding the center tap can "turn off-turn on" the HV without running HV through a relay (or a switch).
« Last Edit: September 11, 2013, 01:17:41 PM by W2WDX » Logged

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