[Amps] RF making it's way into a digital control box.

[Joe Subich, W4TV]

On 1/7/2025 11:17 AM, Lukasz wrote:

> This will be difficult . I can pull at most 4mA at 3.3V on the
> control lines out of the uC board. That is not a lot of power....
> I'm already at over half of that, so realistically ... I can't pull
> more than 3mA with a bit of safety factor.
Then don't drive the base of the transistors directly.  Use a
buffer/line driver between the controller and the transistors.

Alternatively, you may want to look at a voltage drive switching
device (FET) instead of the current controlled NPN transistor.

73,

    ... Joe, W4TV

On 1/7/2025 11:17 AM, Lukasz wrote:
> On Tue, 7 Jan 2025, 15:42 Ron W4BIN, <ka4inm at gmail.com> wrote:
> 
>> On 1/7/25 3:28 AM, Lukasz wrote:
>>> I'm thinking about adding these capacitors as 100nF or 10nF (depending
>>> which one will fit) smd parts right between the base and emitter the
>> pads.
>>> Do you think that will work? If not, what would you do?
>>>
>>> 73,
>>> Łukasz - SP4IT
>>
>>     The trick is to make the input to the problem relays to require POWER to
>> activate them.  Feed a high current to those channels and drop the input
>> resistor divider down to the hundreds of Ωs.
>>
>>    Ron W4BIN
>>
> 
> This will be difficult . I can pull at most 4mA at 3.3V on the control
> lines out of the uC board. That is not a lot of power.... I'm already at
> over half of that, so realistically ... I can't pull more than 3mA with a
> bit of safety factor.
> 
> I think my problem may be a variation of the famous "pin 1" problem.
> 
> In my attempts to avoid ground loops I've not connected the shielding braid
> on both ends to the case.
> 
> I just read you do that to avoid uV level hum on audio , not when a kilowat
> of RF is flying past. So that's the first thing to change.
> 
> Then the coax that delivers +12V. The braid of that too is not terminated
> on the chassis but goes inside. Despite having RF shorting cap to RF ground
> right at case wall I think this may have to be replaced by a second coax.
> 
> Finally, if nothing helps at all I'll have to get rid of transistors and
> replace them with transoptors. I wanted to avoid that, but it may be
> required .
> 
> If anyone has one ideas please share.
> 
> 73,
> Łukasz - SP4IT (also SQ4KW)