14 Aug
2014
14 Aug
'14
3:40 p.m.
Hmm.. but.. if both the HV (25K anode) & LV (2.5V heater) secondaries share
the same primary, then where is the HOT in the circuit? There aren't many
xistors that will take 25K, so it's got to be on the low side, right?
The HOT is the transsitor that deives the primary winding. Rememebr a
transformer cna't work on steady DC, it needs a chaning primary currnet
ot work. hThe cirucit is powered form a DC supply rai, the hOT is the
chopper trnasisotor (yes, this is essentially a specialisd SMPSU) that
interrupts the transofrmer primray current.
The idea goes back to vlave (tube) TVs. The original horizontal
defleciton stage used the fact that applying a constant voltage aross an
inductor will cause the current (and thuse the magnetic field) to rise
linearly with time. So far so good, this will sweep the beam across the
CRT. But at the end of the line, you wanto suddenly reduce that currnet
ot zero. Now, you have sotred significant energy in the magnetic field of
the yoke (and in the case of valves, matchign transformer), what do you
do with it?
Wellm, the drist ieea was just ot dame it with a reissotr, and waste said
energy. Then Alan Blumlein (I beleive) realised you could step up the
votlage produced when the circuit was suddenly opened by the output
valve (now a trnasisotr, of course) for the flyback period, by putting extra
widings on the transofermer. And thus get the EHT for the CRT. Note that
when he thoguht of this, SMPSUs were not really even thought of.
-tony