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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

    This circuit provided by Icom seems to be the most simplest and direct approach to interfacing a transceiver to the SB-220 keying circuit that I've seen to date. It doesn't seem to be very popular, however... and that concerns me.

    Anyone see any drawbacks to this approach? I certainly have enough PNP high voltage power transistors and darlingtons to try it.

  2. KM1H

    KM1H Ham Member QRZ Page

    Icom uses SS switching in most products that can only sink about 10ma.

    A drawback with that circuit above is that a shorted transistor can blow the switching circuit in the radio. A simple DIP or reed relay is more forgiving and keys about the same speed as the relay in most other rigs.

  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Given reasonably rated components there isn't any drawbacks to the circuit. A shielded cable should be used and the circuit should be built in a metal box (small) with the shield and the box connected together.

    It sure makes it possible to keep some older amplifiers stock.
  4. W0BTU

    W0BTU Ham Member QRZ Page

    Good call, Carl! :) I wouldn't have thought of that.

    Any ideas whether or not the Harbach mod (for the same purpose; no relay) has this drawback? I doubt it, but that's what I use in my SB-200.
  5. W1BR

    W1BR Premium Subscriber QRZ Page

    Personally, I would never own an Icom... :)

    It would be used on a JRC NRD-515 transmitter, that has relay keying.
    I'm just for a simple interface that will limit the wear on the JRC relay.

    Yeah, I know a resistor in series would help there.

    Most of the other interfaces require external voltage sources and use
    more than one active component.

  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    W8JI won't like that interface. He'll suggest the transistors need resistors between the B-E junctions to ensure they turn off when desired. 10K to 100K would probably do it.

    AG6K prefers no back-EMF diode across the relay coil to ensure it drops out as fast as possible. This requires a transistor with a very high voltage rating. I used a NTE228A 350V/500mA NPN, driven by a NAND gate. It's been working fine for 20 years.

    The NRD-515 is a receiver. You probably mean the NSD-515 or NSD-505 transmitter. AG6K says the problem with driving the SB-220 is that there's a 0.02uF bypass capacitor (C52) directly across the ANT RELAY jack on the SB-220. This cap is charged to ≈ +120V during receive. When your rig's itty-bitty relay contacts close, the capacitor discharges through the rig's relay contacts. At best, this eventually burns the plating off the contacts. At worst, it welds the contacts closed. The fix (other than adding a relay driver) is to add a 100 to 200Ω ½W resistance between the 0.02uF capacitor and the jack to limit the discharge current: This might be enough, depending on how much current your rig can switch.

    While my solution was by far not the simplest, the rig has to switch only 5V at a few mA. ANY rig can drive it. [​IMG]
  7. KM1H

    KM1H Ham Member QRZ Page

    The Harbach uses an opto isolator Mike, no problem when a tube decides to let loose and take other parts with it.

    There was also a simple but much better SS design than the ICOM version for the SB-200 and 220 in Hints and Kinks years ago. The Heaths used -125VDC and +125VDC respectively in the relay circuit. Lots of ICOMs got smoked bad decades ago!

    Ive been using the same 12VDC DIP relay to key a 6M NCL-2000 for about 25 years using VOX CW/SSB. The NCL relay is 12VAC and incompatible with lots of radios. It will also work fine at about 3.3VDC.

  8. W1BR

    W1BR Premium Subscriber QRZ Page

    Hi Bryan

    I pretty much agree with your observations.

    I'd also add a pull down resistor between the base and emitter, since the base would be essentially floating when driving by the NSD relay contacts. I was wondering why Icom left the base float, but the earlier explanation of how their keying circuit works made sense of that. RF bypass caps could be added if needed. Instead of a diode across the coil, I was thinking of adding a diode in series with the relay coil, to limit the back emf seen by the transistor. I have quite a few high voltage devices sitting in drawers looking for a useful purpose.

    I liked the Icom approach because it didn't require an external source of power.

    I've scanned through AG6K's webpages a few times in the past week, and caught some of the references you mentioned. Maybe I should reconsider using the series resistor and keep it even simpler :) I'll have to check the JRC manuals and see if the relay specs are shown.

    Thanks for the input.


    Last edited: Jan 7, 2011
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    The circuit shown didn't show what was driving it. If it's something like an open collector or N.O. relay contacts, it could be trouble. If it's a totem-pole driver, it's OK. A B-E resistor makes it work with ANY rig.
    You'll still have back-EMF with a diode in series with the relay coil.
    I like my approach bestest. It's built into the amplifier so, active or passive is not an issue.
    Knowing the rig's capability is half of what you need to know. You also need to know the peak current at the ANT RELAY jack. This can be measured with a 'scope & current shunt.
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    For anyone else following this thread...

    Yes, you'll always have a counter EMF as the field collapse. but the collapsing magnetic field generates a voltage of opposite polarity. A diode across the coil shunts the back EMF voltage, and the stored energy is dissipated over a longer period of time.

    A diode in series with the coil it is reverse biased (conducting) when keyed, and forward biased when the counter EMF is generated and has no effect on the decay time. Having adequate PIV, it would allow using a keying transistor with a lower breakdown voltage. that's why I suggested. Mirage and several others have used this approach. A diode in series with the coil is just an extra precaution.

    Last edited: Jan 7, 2011
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