Search found 1252 matches
- 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...
- 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 ...
- 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...
- 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...
- 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...
- Wed Sep 24, 2008 10:18 pm
- Forum: General Discussion
- Topic: Problems with 74HC165 of ST
- Replies: 2
- Views: 2422
- 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...
- Tue Sep 09, 2008 11:49 am
- Forum: General Discussion
- Topic: Any home built thermal imager cameras?
- Replies: 7
- Views: 6112
- 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...
- 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...
- 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 ...
- 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...
- 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 ...
- 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...
- Tue Jun 17, 2008 11:46 pm
- Forum: General Discussion
- Topic: current sensing
- Replies: 3
- Views: 2589