开云体育

ctrl + shift + ? for shortcuts
© 2025 开云体育
  1. QRPLabs
  2. Topics

QMX and QDX - Not enough 5V PTT oomph

 

Good evening from chilly Southwest Florida
?
I've run into an issue with a relay I wired into an adjustable attenuator board.
The PTT signals coming from both my QMX HB Rev2 and my QDX LB rev 5 aren't sufficient to keep the relay pulled.
?
Completely my bad.
I tested my chosen relays on a breadboard and used 5V from an Arduino Uno to confirm I had the correct pinouts and where to tie into the attenuator board (QRPKits - 41dB selectable). Nice solid pull-ins and signals attenuated when expected.
I didn't test with the actual PTT Voltage and current available from the Q's until I had everything tied in.
?
I vaguely remember see somewhere that it may be possible to make more +5v current available on these units.?
Hoping someone can confirm that and give me an idea where the find messages/information. And what pitfalls I might run into if there are mods for use with QPR 50W amps that I have.?
?
The relay is an HK 19F-DC5V-SHG.? It needs about 0.2W to feed the approximately 125 Ohm coil.?
On the Q's there is no solid, clearly audible relay click-in with PTT but there is faint click when dropping PTT.
I see about 1.8V across the coil when PTT is asserted.
?
Thanks in advance,
?
?
GREG KI4NVX
?
?
 

Greg, a better solution would be to add a FET switch to turn on your relay - the gate of a FET is what the output of QMX was designed for, not to directly drive a relay.
A simple FET switch and a 470nF capacitor across the relay terminals, with a 9V transistor battery (or 5V arduino power source) would work well to switch your relay.
Stan KC7XE
?
?
 

Thank you Stan?

Great suggestion!??

GREG


On Tue, Jan 7, 2025 at 7:17 PM, Stan Dye via groups.io
<standye@...> wrote:
Greg, a better solution would be to add a FET switch to turn on your relay - the gate of a FET is what the output of QMX was designed for, not to directly drive a relay.
A simple FET switch and a 470nF capacitor across the relay terminals, with a 9V transistor battery (or 5V arduino power source) would work well to switch your relay.
Stan KC7XE
?
?
 

On Tue, Jan 7, 2025 at 07:17 PM, Stan Dye wrote:
a better solution would be to add a FET switch to turn on your relay
Don't forget the flywheel diode across the relay coil so you don't blow out the driver.
There is also a current-limiting resistor behind the PTT output which could be reduced somewhat for more current, but a driver would be my choice.
73, Don N2VGU
 

Donald, most MOSFETs have an internal diode, so the flywheel diode is not absolutely necessary for a small low-voltage relay.? If the relay has a large coil or runs off higher voltage, yes, I would definitely include the diode.
The very popular ATU100 tuner (7x7) has 14 relays with 14 FETs and just capacitors across the relay terminals, and I have not heard of any of those drivers blowing out (including in the one I have).
Stan
 

Thank you again Stan.

Ordered some 2N7000's and other bits and bobs.
Got everything bread boarded and verified and am now waiting on an order of 1/4 size proto boards from AdaFruit. Will build out some dedicated, relay switched PTT attenuators of various values.?

Decided on powering the relay side with a simple USB-A cable for the 5V relay source.? ?I have had good results using PD power banks on my Q's and have leftover output ports and plenty of capacity for low amperage add on's like my mini cooling fans.?

Gosh ... This QRP'ing is great fun!?
So very glad I caught the QRP-Labs bug.?


Greg


On Tue, Jan 7, 2025 at 7:17 PM, Stan Dye via groups.io
<standye@...> wrote:
Greg, a better solution would be to add a FET switch to turn on your relay - the gate of a FET is what the output of QMX was designed for, not to directly drive a relay.
A simple FET switch and a 470nF capacitor across the relay terminals, with a 9V transistor battery (or 5V arduino power source) would work well to switch your relay.
Stan KC7XE
?
?
 

Sounds like a great plan, let us know how it turns out.
Be sure to measure your PD outputs if you try to use multiple voltages from the same power bank.? My power banks wont switch one port to 9V/12V with a trigger cable, and simultaneously output 5V on the other port: as soon as I plug something into the USB-A port, the 12V triggered output switches down to 5V also.? I understand some power banks do support independent voltage output, but you need to find it in the specs.? Or just use your second power bank like I do.
Stan

1 - 7 of 7