"per.zapro" <per.zapro@...> wrote:
Yep. I breadborded the circuit to death before
going with Hans' schematic all the way. I simply could
not get it to work! - I had problems with ghosting,
strange noise issues and so on...
Wiring up his schematic works wonderfull, and gives me
stable readout, and no flickering.
You can see the outcome here:
I just need to make a board like this for the Amp section
and then a power supply.
At the moment i tried making the exact power supply with
the MC34063A switcher, but i have problems with noise. ...
// Per.
I have a couple of those CA3162 chips on order. I'll play with them, to verify my version.
The switcher Hans uses is the MKI version of the MC34063. Its of limited use. Here's a schematic of my MK1.5 Version:
There are several improvements. (1) C1 in the feedback path adds stability to the circuit. Its gets rid of what you call noise.
The FET drive is improved, so it now both actively pulls Up, and Down, the signal applied to the power FETs gate. The old version only pulls up, and has a passive pull down (a 330 ohm resistor). This greatly improves efficiency. The old circuit could only deliver ~10mA. The new circuit, is the one I sell on eBay, and can easily deliver 45mA.
Finally, he probably used the IRF830, just because he had them in his junk box. Its a crappy FET. Use the ones mention in the schematic. Any of those is much better than the IRF830. The IRF740A has been the standard used in this group. As important as voltage and ON resistance, an important spec is the gate charge. Like the ON resistance, it should be as low as possible. Its the Power FETs 'dirty secret', since it means the impedance of gate goes down, as frequency goes up. That means you need a circuit to drive the FET, that can move current, exactly what they advertise a FET doesn't need. High impedance at DC (zero hertz) means nothing other than the marketing people just stroking themselves.
