using a solid-state relay…
© 2018 by KV5R. All Rights Reserved. Rev. 2/9/2019
So you got your nifty HF transceiver and now it’s time to move up to the big leagues and add some power to your signal. You order a nice amplifier, only to discover you also need an “interface” to connect the radio and the amp.
The problem is that there are no standards for connecting the keying circuits of transceivers and amplifiers. Each manufacturer uses their own voltage and current for this simple operation. Of course, radio manufacturers sell (very expensive) amps that will plug right in to their radios, but most hams opt for less expensive options. Other companies make amps that are much lower cost-per-watt, and just as good, but the keying circuit is generic, with interfaces provided at extra cost. The problem is that they cost too much! Why? Because they are built to be compatible with a wide range of radios and amplifiers, and thus have a high parts count. They are plug-n-play, so you don’t have to look up specs for your radio and amp or do any designing. If that’s your goal, then by all means buy one, but if you’d rather roll your own, please continue reading.
You can build an interface for $10-$30; all you need is an old stereo audio patch cable and a small relay. The audio cable is the typical stereo RCA phono plug cable, 3 to 6 feet as desired. Cut the phono plugs off one end and wire them to the radio’s accessory jack and relay. One cable (with relay) for amplifier keying, and the other (directly connected) for ALC. Most amplifiers use RCA phono jacks for both keying line and ALC line. At the radio end is the accessory plug that comes with most radios. Icoms come with an 8 or 13 pin DIN plug with pigtails (short wires), so you can look up the colors and connect to the wires without having to solder to the tiny pins in the DIN connector.
The relay is selected to comply with the needs of the radio; typically the coil is 12VDC at 10-20mA. Icom radios use a pin on the accessory jack called “HSEND” that when transmitting can sink 13.8V at 200mA to ground. (Some Icom models, like the 706, are limited to 20mA.) Most third-party amp’s keying circuits will overload it and blow the switching transistor in the radio. Also, a relay coil will need a very fast power diode across it to absorb the high-voltage pulse generated by the collapsing magnetic field in the coil when it is switched off. This pulse will greatly exceed the peak inverse voltage (PIV) rating of the radio’s switching transistor and POOF! — time for an expensive repair… The diode is connected across the relay coil reverse-biased, and the pulse (which is reverse-polarity) forward-biases it, causing it to conduct and absorb the pulse.
Mechanical relays have that nasty coil spike, switch relatively slowly, can get dirty contacts, and are noisy. They also require extra parts, such as a resistor (in-rush current-limiting), a capacitor (RF bypass), and the diode. A better solution is to use a solid-state relay. These typically have an LED, photo-detector, and power MOSFET encapsulated in a very small package. No other parts are needed, not even a perf board or enclosure. The device is small enough to simply “float” in the cord assembly. Prices are a little higher than comparable mechanical relays. The one I used is about $22 (plus shipping) from Mouser.com.
Disclaimer: The author is not responsible for your mistakes. If you fry something, it’s your own fault! This article is intended for people that know how to look up specs, solder, and test simple electronic circuits.
With thanks to KK5DR, I discovered the Crydom DMO063 (Mouser p/n 558-DMO063), a solid-state relay (SSR) with an SPST-NO MOSFET switch rated at 60VDC at 3A. The input is 3-10VDC at 20mA (at 5V). It appears to work at 13.8 volts also, drawing 12mA. The switch-on time is an incredibly fast 50μsec. See the datasheet for all the details. I built the following isolated solid-state keying interface according to KK5DR’s article. It also shows how to build one using a mechanical relay.
My radio is an IC-7100 and the amp is an AL-80B. The 7100’s HSEND line can sink 200mA, and the AL-80’s RLY jack supplies 12VDC (through the coil) and when grounded draws 100mA, so it looks like they could be directly connected—but wait! Each side supplies its own voltage source and ground. A direct connection would mean that HSEND closure would be sinking the amp’s 12V to the radio’s ground, and yes, their grounds are connected, but I don’t want to risk it, so I built the isolated interface so each side can sink its own power to its own ground. It’s just better to load the radio with the SSR’s 12mA, not the amp’s 100mA. And it’s also safer that way, particularly when considering that amplifier relays can fail and arc RF to the coil of the relay, which will surely fry a directly-connected radio! Isolation is a very good thing.
Update September 2019 — Bill, KO4NR (see comment below) found a better solid-state relay for this application, particularly the TS-850 and others that have a 10ma amplifier keying output. It is the Crydom MPDCD3 for DC output switching (up to 60VDC at 3 amps), or for old amps with line-voltage keying relays, look at the AC versions (MP120D3 MP240D3 MP240D4) in the same Crydom MP Series Datasheet. $25.80 at Mouser (watch out, other places sell it for ~$50 with a DIN rail mount).
Bill verified with tech support that the MPDCD3, which is rated to pull 23ma at 32VDC on the input, pulls 8ma at 12V. And since the minimum turn-on voltage is 3VDC, I suggest that the input might be connected with a series resistor to drop the voltage and current even further. So don't get get the DMO063 I used, rather get the MPDCD3 for amps with DC keying, or the MP240D3 for amps with line voltage AC relays.
All these devices are optically isolated and much safer to use than bipolar transistors, which if they fail to base will put amplifier relay coil power into the radio and damage its teeny SMT amp keying output. (end of update)
Crydom DMO063 solid-state DC-DC Relay, $22. Note that part number is DM "oh zero" 63. (Use the MPDCD3 instead.)
I used the 13-pin DIN accessory pigtail that came with the IC-7100.
Part of IC-7100 manual for 13-pin ACC. Pin numbers and pigtail colors are the same for the 703, 706, 718, 7000, and 7100. Other Icoms use an 8-pin DIN Acc. Refer to your manual.
Make a little sketch to plan your soldering. Yes, red is ground. Nobody knows why…
First, connect the red phono-plug cord to the SSR’s output. Obviously, the center is + and the shield is - .
Dress everything up with heat-shrink tubing.
Connect the other phono plug cord to the ACC plug’s ALC and ground lines. Heat-shrink it.
Connect ACC plug’s 13.8V line to CONTROL+, and the HSEND line to CONTROL-. heat-shrink them. NOTE: If these short, it will blow the transistor in the radio! Make sure to insulate them well. (Please ignore my nasty soldering, it wasn’t a good day to solder…)
Fold the cables back to the jacket of the ACC plug with a cable tie, to provide a strain-relief.
Plug it in. Notice how the little device is lost in the maze. We don’t need no stinkin’ little box!
Test it by connecting an Ohm meter to the red phono plug.
Key the radio. TA-DAAAA! It works.
Connect to amplifier
Everything working fine!
This SSR device (DMO063) is not suitable for old amps that use AC keying. The DMO063 is 60 volts max, DC only. Use a suitable relay for those. See your amp’s manual to verify its keying voltage and current. See 2019 Update, above, and use the MPDCD3 for DC relay amps and the MP240D3 for AC relay amps (unverified, but should work fine).
I suggest using a series resistor on the input that's low enough to trigger the SSR (3V minimum), but high enough to limit the current to 10ma if the internal LED fails shorted. Yes, the Crydom SSRs have internal resistors, but more wouldn't hurt, particularly if using a radio with a 10ma keying output.
If you get erratic behavior from either amp keying or ALC, wind the cords a few times through a FT-140-77 (low HF bands), or -31 (high HF bands) toroid from Amidon or Palomar. (Snap-on split chokes are crap).
Make sure to cable-tie (or tape) the phono cords back to the ACC plug to prevent pulling on the connections. Make sure all connections are properly soldered and well insulated.
Make sure to set a 15-30 millisecond TX DELAY in the radio. This gives the amp time to close its big open-frame relay before receiving RF drive. Failure to set TX DELAY may result in popping sounds when using VOX, and "hot-switching" may damage the amplifier. In the IC-7100, TX DELAY is in the Set → Functions menu.
Next, let’s save another $75 and build a good old-fashioned salt-water dummy load!