Amplifier v Oscillator

[barry wilkins]

This foxed me today ,built an amplifier during the week which is part of my radiation detector I have been working on,worked OK bit noisy it is very high gain.
Today cut up a coke can to make a little screened box to reduce the noise, it decided it didn't want to be an amplifier any more it was going to be a HF oscillator !
I had this problem before in the past ,it just disappeared on it own?
I tried a few different PP3 batteries ,each one gave me a different frequency?
So I used my bench power supply once I got the thing quiet it was OK ,went back to the batteries ,I noticed it worked with one of the batteries if I held it in my hand ,that made me think.
I have got an ESR meter I made some years ago so I did some checking ,one of the batteries didn't even register on my meter ( I dont actually have ohms on the scale) it must have been over 10 ohms.
The zinc carbon batteries do seem to have quite a bit of internal resistance whereas a Duracell Alkaline battery was more like 0.2 ohms.

Barry Wilkins

[tom66]

Re: Amplifier v Oscillator

Zinc carbon cells are terrible.
Zinc chloride cells are only slightly better.

Given the cost difference between alkaline and zinc carbon, in an application using primary cells your best bet is to use alkaline.

Rechargeable 9Vs are pretty poor, the capacity is too low. Any product I design now avoids 9V where-ever possible. You can get a lot more total energy capacity from two AA cells than a 9V (almost 3x) plus the rechargeable AA's don't suck, and are pretty cheap. The only disadvantage is the need to implement a boost converter (but such converters are so simple now, requiring only a few parts...)