power supply

[Jimmy]

I was given a DC to DC converter a couple of years ago and finnaly got
around to trying to do something with it.When I traced out the circuit I
found that the devices driving the transformer were BIG SCRs. I had never
seen this before. I am familiaar with transistors FETS IGBTs but not with
SCRs used in this way. It kind of puzzles me as to how it would work as I i
dont readily see a way the SCRs can be turned off once they are turned on.
My best guess is that a COUNTER EMF pulse generated when one SCR is
triggered on turns the the other off my momentarily reducing its cathode to
anode current to zero/near zero.

 

[Form@C]

On Mon, 19 Apr 2004 17:36:19 +0000, Jimmy wrote:

I was given a DC to DC converter a couple of years ago and finnaly got
around to trying to do something with it.When I traced out the circuit I
found that the devices driving the transformer were BIG SCRs. I had never
seen this before. I am familiaar with transistors FETS IGBTs but not with
SCRs used in this way. It kind of puzzles me as to how it would work as I
i dont readily see a way the SCRs can be turned off once they are turned
on. My best guess is that a COUNTER EMF pulse generated when one SCR is
triggered on turns the the other off my momentarily reducing its cathode
to anode current to zero/near zero.

I don't know your circuit, but I used to work on fork lift trucks, where
we used thyristor motor drive. One thyristor had the motor as a load on
the anode and IIRC the other, much smaller, had a resistor. The cathodes
were common -ve and the anodes were connected to each other by a
capacitor. The drive produced a pulse to switch the main thy on
(switching the motor on and charging the capacitor via the resistor. This
also switched the small thy off), then a pulse to the second thy which
switched on, pulling the anode of the main one down via the capacitor -
switching it off in turn.

The secret is in using the capacitor - known as a "commutating" capacitor
here.

Memory is vague - I haven't looked at this for about 30 years...

--
Cheers...
Mick
Gave up on viruses & trojans - moved to Linux...
Nascom & Gemini info at http://www.nascom.info

 

[Jimmy]

"Form@C" <mickREMOVE-THIS@mixtel.co.uk> wrote in message
newsan.2004.04.20.17.56.51.854373@mixtel.co.uk...

On Mon, 19 Apr 2004 17:36:19 +0000, Jimmy wrote:

I was given a DC to DC converter a couple of years ago and finnaly got
around to trying to do something with it.When I traced out the circuit I
found that the devices driving the transformer were BIG SCRs. I had
never
seen this before. I am familiaar with transistors FETS IGBTs but not
with
SCRs used in this way. It kind of puzzles me as to how it would work as
I
i dont readily see a way the SCRs can be turned off once they are turned
on. My best guess is that a COUNTER EMF pulse generated when one SCR is
triggered on turns the the other off my momentarily reducing its cathode
to anode current to zero/near zero.


I don't know your circuit, but I used to work on fork lift trucks, where
we used thyristor motor drive. One thyristor had the motor as a load on
the anode and IIRC the other, much smaller, had a resistor. The cathodes
were common -ve and the anodes were connected to each other by a
capacitor. The drive produced a pulse to switch the main thy on
(switching the motor on and charging the capacitor via the resistor. This
also switched the small thy off), then a pulse to the second thy which
switched on, pulling the anode of the main one down via the capacitor -
switching it off in turn.

The secret is in using the capacitor - known as a "commutating" capacitor
here.

Memory is vague - I haven't looked at this for about 30 years...

Ive seen a flipflop circuit that works this way