Wednesday, 27 May 2015

PCB made

Well, I etched my first PCB in several years. I'm pleased to say that it was a very clean etch in spite of some reservations I had about the old sensitized Photoboard 2 (6 years old!) I had used and the two year old developer. That developer has now been ditched and a new batch made up. I' e also bought new sensitized board.

Tomorrow I intend making up the 4m 30W PA. my plan is to finish the transverter by this weekend as I want to use it in the present good ES conditions.

I had a disappointment with the Magnetic Devices 951 relay I had intended to use in the transverter. It turned out to be a 24V rather than a 12v coil............the 700R coil should have told me that!

To overcome this problem I started to look for a suitable circuit to drive the 24v relay coil from the transverter 12v supply. I did not want to use a voltage converter as there is a potential for audio frequency spikes causing interference. 
An alternative solution is to use a circuit like the K04BB all-DC circuit. This circuit charges a rather large value electrolytic capacitor to 12v. When you want the relay to operate, the charged capacitor is placed in series with the 12v supply to give 24v. Although the capacitor will discharge rather quickly as the relay is operated, the remining 12v is sufficient to hold the relay operated. 

The disadvantage of this circuit, as shown, is that it is not possible to operate the relay with a press to talk ground connection. Usually an isolated relay contact is used to apply a positive voltage to the 'trigger' input.
I didn't want to use a relay so I thought it might be possible to use an Infineon high side switch MOSFET to do the same thing, but with a ground PTT input.
I am pleased to say that this worked extremely well. I was wary that under certain conditions it might be possible to defeat the circuit by rapidly switching between on and off. I'm pleased to say that with the relay and circuit component  values that I used it was not possible to defeat the circuit!
I'll publish the circuit on my web page.
I will be making a printed circuit board for the relay circuit as my first new PCB with the resurrected PCB facility. I see this could be a very useful board for all sorts of uses!


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