A more elagant SB-220 keying interface?

Discussion in 'Amateur Radio Amplifiers' started by W1BR, Jan 6, 2011.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    Hi Tom

    I understand that the back EMF potential voltage is theoretically unlimited if there is no losses or load across the inductor.

    In practice, how high does the voltage actually go? For an amp relay being keyed by an external relay in a rig, what limits the counter EMF voltage? Is some of that energy dissipated by contact arcing as they open?

    What does the series diode on the keying transistor collector in the Mirage amplifier protect?

    Last edited: Jan 9, 2011
  2. W8JI

    W8JI Ham Member QRZ Page

    I've measured as high as 150 volts in a 12 volt relay.

    Arcing in the outside relay contacts or dissipation in the device that activates the relay coil. Also losses and things that extend the time the field collapses in the relay coil. There is a certain stored energy in the magnetic flux that must be dissipated somehow.

    In the external contacts that control the relay coil, yes.

    It was years ago when I looked at Mirage stuff. I did not design it, so I do not know what was in the mind of the fellow who did.

    A series diode can not reduce back-pulse. :)

    A good friend of mine wrote an article about modifying amplifiers to work with solid state rigs, and he suggested a series diode, so many people have the misconception that a series or external shunt diode (like ICOM suggests) will reduce the backpulse.

    There are three ways to limit backpulse:

    Clamp the line that opens on the relay coil side to ground with a zener or a back-biased diode.

    Put a diode across the relay coil.

    Use an R/C snubber to dissipate the energy over time so voltage only goes a little high but over a long time.

    73 Tom
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    Hi Tom

    I wasn't trying to reduce the back EMF voltage, and I never claimed that a series diode would reduce the level of the back emf voltage.
    Obviously the relay has to be clamped to do that; but my concern was protecting the keying transistor from the reverse voltage spike.

    150 volts doesn't seem terribly high (I understand we are talking 12 volts) but I must be stupid... I would have thought a 1kV PIV diode would not pass a 150 volt pulse in the reverse direction and would have protected the keying transistor.

    I don't get it. But thanks for trying.

    Last edited: Jan 10, 2011
  4. K7JEM

    K7JEM Ham Member QRZ Page

    You are obviously mistaken in how you think the voltage spike occurs. A series diode can do nothing to reduce that spike, other than its .7v forward voltage drop.

    You see, a series diode is never reversed biased in this sort of operation. It can't be, since the only voltage source is on the anode of the diode. The anode will always be more positive (or the same) voltage as the cathode, at least until that flyback voltage hits its peak, which could be a couple hundred volts in a 12v circuit.

    A 150v spike could easily destroy a 2N3904 type transistor, even if it only lasted for a few microseconds. The snubber diode limits that flyback voltage to .7v (or so) above whatever nominal voltage is applied, say 12v. This could lengthen the time the relay takes to drop out, but probably only a few mS additional, certainly nothing noticeable.

    There is really no downside to using this sort of protection diode, in most applications. It has been done this way for decades, where a relay is being driven by a transistor.

  5. W1BR

    W1BR Premium Subscriber QRZ Page

    I'm going to have to draw this out and sleep on it. I am missing something obvious.

  6. K7JEM

    K7JEM Ham Member QRZ Page

    I hooked up a relay to a scope to see what was happening with a relay without the protection diode. I have drawn up a picture of what I was seeing on the scope.

    The voltage would rise to about 350-400 volts on removal of the ground lead to the coil, then immediately (about .1 mS) drop back to 12v.

    The addition of a series diode would do nothing to reduce this high spike to the collector of the driving transistor. The "kickback" voltage is not negative, it is positive.

  7. KM1H

    KM1H Ham Member QRZ Page

    When running digital modes that few extra mS may be intolerable. On some amps, especially VHF and above with remote preamps the complete relay system must be very carefully sequenced in both directions.

    With older amps such as the Heath SB family the stock slow open frame relay is often hot switched by a SS rig resulting in switch and capacitor arcing and burnt relay contacts. (Some very misinformed folks start claiming parasitics) It has its own shunt diode across the coil. Any method to use those xcvr/amp combinations needs to be looked at as a complete package.

  8. W1BR

    W1BR Premium Subscriber QRZ Page


    Did the same test a few days ago using a large inductor. I saw a voltage reversal when the DC potential was removed. The problem is I had the scope probe at the same pole I was appliying and removing the DC voltage from. I reached the wrong conclusion based on flawed premise and flawed bench setup. The EMF voltage reverses, but the voltage on the keyed end of the relay is indeed driven positive. The diode never sees a voltage reversal. Indeed, it looks the voltage would be even a tad higher than the back EMF since the supply voltage in series would be additive. Damned brain fart on my end.

    Sometimes it's better to know the answer before doing a lab demostration to know what to expect.

    Thanks for the detailed explanation; now the world makes sense again.

    My appologies to Bryan.

    Last edited: Jan 10, 2011
  9. K7JEM

    K7JEM Ham Member QRZ Page

    The protection diode will have no effect on how fast the relay picks up, which is the big problem. The only effect will be on the dropout timing.

    If the relay takes a few extra mS to drop out, it shouldn't bother anything. It will only serve to ensure that there will be no RF present on that relay when it drops.

    I'm not sure of any digital modes that would be affected by a few mS of delay in antenna switching. Modes that require ultra fast switching will not use a relay, since the mechanics are just too slow. Normal modes like packet, PSK, RTTY, etc would never see the difference.

  10. KM1H

    KM1H Ham Member QRZ Page

    I never said it would, my comment specifically addressed your comment about release delay.

    Thats very subjective.

    Those old tech slow modes werent mentioned. The effect of any delay all depends upon the rig and amp. Many have had good results with a sequenced reed relay for the input and a vacuum relay at the output of the amp (or 2 vacuum relays) using relay speed up circuits. The necessary delays are programmed into the xcvr.
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