From: Watson A.Name
Subject: Re: Free "TENS" Circuits
X-Newsreader: MicroPlanet Gravity v2.60
NNTP-Posting-Date: Sun, 15 Sep 2002 12:15:48 EDT
Organization: Prodigy Internet http://www.prodigy.com
Date: Sun, 15 Sep 2002 16:15:48 GMT
In article , "Tony (remove
\"_\" from email address)" talked about...
> The NPN version of the circuit satisfies the "no power rule" anyway, so there's
> no problem. I've used similar circuits before, and find they work quite well.
> The only slight gotcha (as mentioned on the site) is that the common base input
> impedance of the amplifier is way too high to damp the speaker electricall. So
> you need lots of mechanical damping, eg place the speaker at the end of a tube
> tightly packed with fibre filling.
> One other suggestion - it's messy and not very stable to have to select the 3M3
> resistor to suit the gain of the transistor. It's better to delete it, and
> instead put a 1M resistor from C-B. Then you can use any transistor.
With the 1M resistor from B to C, there is still some dependence on
transistor beta, altho it's less than with the 3.3M. Also the negative
feedback from C to B reduces the gain somewhat. But it's worth the
sacrifice to gain the stability.
If you put a speaker at the end of a tube, even with damping material, the
tube will reduce the bandwidth because it's a half wavelength at some
frequency. That'll give a peak at that freq.
Also a comment about the following:
> Also, usually this
> >circuit is run from a small battery, totally independant of the
> >amplifier it is driving, as this also eliminates a possible source of
> >60 hz./120 hz. hum from being introduced through the supply. Therefore
> >the possibility of high current going through the defective speaker
> >you are talking about is just about impossible.
A short across the contacts of a 9V alkalkine batery will cause it to get
real hot and it can explode. Also, the short circuit current of a 9V
aklaline can be upwards of an amp, and can cause the very small wire of a
speaker voice coil to "let the smoke out."
> >If you have a better design for this type of application, Why don't
> >you show it?
You can use a resistor, say 100 ohms, from emitter to ground or battery.
Then couple the speaker to the emitter thru a 100 uF or more DC blocking
capacitor. Voila! No more DC thru the speaker. This method adds two
other possible benefits. One is that the coupling cap can be made small
enough to roll off the lower freqs, which will help reduce the boominess
that is typical when a speaker is used as a microphone. Also the 100 ohm
load across the speaker will also dampen the boominess a bit, however if
the cap is small, then this won't help.
> >I am just trying to give people who love electronics and expermenting,
> >possible ideas and ways of doing some things.
> >Besides, There aren't any circuits which don't have some drawbacks.
But you don't expect to stay in business by selling circuits that are a
potential fire hazard. I didn't see any disclaimer on your site.
> >Take care......Gary