Search found 1252 matches

by rshayes
Thu Dec 25, 2008 6:58 am
Forum: General Discussion
Topic: Mic Pre-Amp
Replies: 123
Views: 41363

As a further note, the output impedance is also low for negative going signals until the negative current exceeds the current through the 1K bias resistor. In this case that is about 4 milliamps. Normally a preamp is not this heavilly loaded. Using my example, if the output voltage is pulled up by t...
by rshayes
Wed Dec 24, 2008 11:36 pm
Forum: General Discussion
Topic: Mic Pre-Amp
Replies: 123
Views: 41363

Consider an emitter follower with its base connected to a 5 volt source, the collector connected to a 12 volt source, and the emitter connected to a 4 milliamp current sink. Further assume that the base-emittter voltage is .65 volts under these conditions. The output voltage is then 4.35 volts. Now ...
by rshayes
Wed Dec 24, 2008 12:01 pm
Forum: General Discussion
Topic: Mic Pre-Amp
Replies: 123
Views: 41363

The output impedance will be quite low. The dynamic resistance of the transistor output in the emiter follower connection is about 26 ohms divided by the emittter current in milliamps. Since the emitter current is about 4 milliamps, the transistor output impedance will ideally be about 6.5 ohms. The...
by rshayes
Tue Dec 16, 2008 5:36 pm
Forum: General Discussion
Topic: Mic Pre-Amp
Replies: 123
Views: 41363

The last circuit posted actually has positive feedback and will latch into one of two states. The feedback can be made negative by replacing Q2 with a PNP transistoe and interchanging the collector and emitter connections. This will make the current gain of the pair quite high (approximately the pro...
by rshayes
Sun Nov 09, 2008 3:13 am
Forum: General Discussion
Topic: Scoping the primary of a switching power supply. fuses blow
Replies: 6
Views: 3083

With few exceptions, you will need an isolation transformer to look at waveforms in a line powered switching power supply. These usually uae a bridge rectifier feeding a storage capacitor. During the positive half cycle, the rectifiers connet the hot lead of the power source to the positive terminal...
by rshayes
Wed Sep 24, 2008 10:18 pm
Forum: General Discussion
Topic: Problems with 74HC165 of ST
Replies: 2
Views: 2422

It sounds like the PISO chips are not being shifted. In one form, these chips will require two control signals. The first signal transfers the parallel data from the input terminals of the PISO chip to an internal shift register (possibly called "load", "set", "preset",...
by rshayes
Wed Sep 24, 2008 10:00 pm
Forum: General Discussion
Topic: IP Intercom?
Replies: 12
Views: 7241

One possibility is to have the telephone wiring done with Category 5 cable. This cable has four twisted pairs. A normal telephone jack would leave a maximum of three of these pairs, leaving the fourth one for other purposes. This will get you from a wiring cabinet to each of the rooms. A separate ru...
by rshayes
Tue Sep 09, 2008 11:49 am
Forum: General Discussion
Topic: Any home built thermal imager cameras?
Replies: 7
Views: 6112

Silicon photosensitive devices, such as those used in digital cameras, are sensitive to light with wavelengths shorter than .9 micron (900 nanometers or 9000 angstroms). Objects close to room temperature emit very little radiation in this region. This is considered "near infrared". For lon...
by rshayes
Thu Sep 04, 2008 1:18 am
Forum: General Discussion
Topic: thinking back......
Replies: 12
Views: 12002

Originally, a few lines during the vertical retrace interval were sometims used for a Vertical Interval Test Signal (VITS). These were usually single line analog test patterns (such as multiburst, stairstep, color bar, sine-squared, etc.). These would be observed on a waveform monitor to check for d...
by rshayes
Wed Aug 13, 2008 10:57 pm
Forum: General Discussion
Topic: Weston/Schlumberger Freq. Counter
Replies: 8
Views: 5032

There are some images of catalog pages os Heathkit counter on this site (http://www.pestingers.net/freq_counters.htm). You might find a match in physical appearance. This would give you a Heathkit model number to search for. Once you have a Heathkit model number, it might be possible to find a manua...
by rshayes
Sat Aug 09, 2008 4:09 pm
Forum: General Discussion
Topic: Help on photoresistor - how to reverse?
Replies: 9
Views: 5398

A double emitter follower could be used to raise the impedance of the voltage divider providing the feedback signal to the LM137. A PNP emitter follower could be used to drive the LM137 with a 200 ohm load resistor between the output of the LM137 and its adjustment terminal. This would require less ...
by rshayes
Sat Aug 09, 2008 3:53 pm
Forum: General Discussion
Topic: Need help identifying a semiconductor
Replies: 23
Views: 17155

This may be a programmable unijunction transistor (PUT), since it is providing a trigger pulse to the SCR's through a trigger transformer. A PUT is actually a small integrated circuit that acts like an older part called a unijunction transistor, but with more consistent characteristics. The part you...
by rshayes
Mon Jun 30, 2008 1:15 am
Forum: General Discussion
Topic: Common mode rejection...
Replies: 10
Views: 4480

My guess would be that you would want two iron bars (possibly 1 to 2 meters) with a 1 to 2 meter gap between them. A single bar passing through both coils would tend to provide a flux path that would equalize the flux in the two coils without respect to external conditions. Since the measurement is ...
by rshayes
Fri Jun 27, 2008 1:11 am
Forum: General Discussion
Topic: Very simple but odd battery charger. Need assistance.
Replies: 4
Views: 2974

This looks like a regulated supply using a constant voltage transformer. Capacitor C1 and the inductance of transformer T1 are probably resonant close to the supply frequency. Transformer T1 probably has a saturating core, which tends to change the tuning of the tuned circuit as the input voltage ch...
by rshayes
Tue Jun 17, 2008 11:46 pm
Forum: General Discussion
Topic: current sensing
Replies: 3
Views: 2589

An alternate method would be to sense the motor "back EMF". This is the voltage generated by the rotation of a DC motor. It is approximately proportional to the motor's speed. The voltage across the motor terminals is the sum of the motor current times the armature resistance and the volta...