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Re: [Amps] Grid current protection HELP

Subject: Re: [Amps] Grid current protection HELP
From: Paul Whatton <>
Date: Tue, 14 Feb 2012 14:47:55 +0000
List-post: <">>
Hi Rick

Being a lover of the KISS principle, in several amps I've used a very 
simple grid trip circuit where a transistor drives a small latching 
relay. I claim no originality for it except I've never seen anyone use a 
latching relay in this application before? Ducks under the desk to await 
a barrage of messages that say it has all been done before.

In my case it does require a manual reset. But I've found that's good 
for training the operators who, in true Pavlov's dogs style, soon get 
fed up with the effort of leaning over and pressing the button when the 
amp trips. A high brightness LED alerts the op and an audible warning is 
also an option if a sounder is used (useful for waking up an op who has 
fallen asleep with the auto CQ running). So they sort out the problem 
and retune the amp and/or back off the drive. Maybe the button should 
also dispense a candy?

The relay I used is sold by the Maplin chain here in the UK:

73 Paul G4DCV

On 14/02/2012 00:59, rick darwicki wrote:
> Hi all,
> A while back K6XX published an article that can be found at  
> It is a simple opticoupler that triggers a 555 timer base on the voltage drop 
> across a resistor.
> I built the circuit but only connected pin 2 to the 100k resistor from the 
> opticoupler.
> It has different levels of working with different connections I have tried. 
> Grounding the relay jack will key Q1 if the output is low, but it starts 
> high. At one point I could vary the voltage in to the opticoupler and trigger 
> the 555. Also, the time delay worked too. Now I can't get it to do much hi hi 
> I need to start over.
> I'm a tube and voltage guy and almost nothing I've built SS except for a 160 
> meter surface mount SDR and my copy of the Alpha 78 QSK board has worked 
> first time out hi hi
> Can someone look at the schematic and let me know what the problem is, IE 
> correct it?
> I can see the trigger tied to B+ and do not think that is correct. 
> I assumed pin 4 needed to go to B+, maybe not.
> I made a PCB and it is a pain makeing changes and cutting foil.
> Please, any help from you solid state guys? My 8874s need you. K6XX does not 
> respond.
> Thanks
> Rick, N6PE
> ==================================
> Talk is cheap... except when Congress does it.
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