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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: REQ: reverse polarity squarewave circuit
Date: Tue, 10 Dec 2002 07:48:13 GMT
NNTP-Posting-Date: Mon, 09 Dec 2002 23:48:13 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
> I am doing some copper electroplating. I want to supply a squarewave to
> the plating electrodes. I want the squarewave to go positive and
> negative. I also want to see both the peak positive and negative
> voltages. I would like to be able to adjust the positve and negative Vp.
> I know how to get a variable duty cycle from the 555 timer. I would like
> a simple way to measure the duty cycle.
> I have a 30V 3A Elenco pwr supply. There is a lot of room in the
> chassis. I would like to put the circuit inside the chassis. This
> supply will be used in a fume hood so I would like to keep it compact and
> as sealed as possible.
> I have access to every freakin' kind of test equipment there is (this is
> an exaggeration but I have lots)
> I've drawn a simple circuit. Please see:
> I only show one peak reading meter because I was too lazy to draw the
> other one but it would be the same.
> I used NPN transistors for (drawing) convienience I can use FETs also.
> I would appreciate any suggestions you may have, even if it is a
> completely different circuit.
1) Measure duty cycle at output of 555 with an averaging meter (analog
voltmeter); 0V= all low, Vcc=all high, 0.5*Vcc=50 percent.
2) There are a number of reasons the circuit (as shown) cannot work.
There is no current path from the floating 15V supply, so there can be
no base current. Also, one cannot have current going out of one
collector and going into the other.
To partly solve #2 and also to get independently variable positive and
negative voltages, you need two power supplies, one for each voltage
(obviously each one goes to ground, thereby allowing current to flow for
base (or gate) drive).
Also, you will need to take that unipolar 555 output and convert it to
a bipolar voltage for driving anything.
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