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Maker Pro

help with designing a circuit

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Mar 4, 2013
6
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Mar 4, 2013
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6
It may help if you simplified. It some time helps to brake a circut down to modules example would be. Mod1 timer circuit. Mod 2 isolation circuit. Mod 3 output driver circuit.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
For the current source, at 15V supply, somewhere between 3A and 10A is the dividing line between "not too difficult" and "very big with significant obstacles".

Of course, you first need a regulated DC power supply that can provide the current. A laptop power supply could be a good choice here; they tend to be around 18~20V DC and are available in currents up to 6A, maybe more. You can get cheap Chinese laptop power supplies from eBay. Just make sure your fire insurance is up-to-date and avoid the ones with mains cables that break when you bend them!

Then you'll need one of the transistors that I listed in post #17, and not the first one. The fourth one is in a TO-3 package; that's the big metal thing that you can attach to a big heatsink with two screws.

You can derive your control circuit voltage from the laptop supply using a three-terminal regulator such as a 7812.

If you prefer the idea of a two-transistor astable multivibrator over a 555, that's fine. I would recommend using a separate buffer transistor for each relay coil though. A BC337 will do, because all the relays I listed in post #13 draw less than 200 mA coil current. Build the astable multivibrator as normal, then for each relay, add a transistor with its emitter grounded, its base connected through a resistor of say 10K to the collector of the multivibrator transistor, and its collector connected through the relay coil to the positive supply. Always put a diode across the relay coil.
 
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