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Simple & cheap 'off line' power supply?

R

Rolavine

Jan 1, 1970
0
I need to make about 24 volts DC (good enough for dc relays) at about 250 ma
maximum from either 110v or 220v ac input.

I've found a bunch of circuits online, and quite a few chips.
I would like to avoid using a switcher because of EMI.
I think I need too much current for a shunt type approach.
Does anyone know of any simple SCR based, 'light dimmer' type circuit that can
be used for an off line supply. I have a PIC onboard that has a battery back
up, so it could run the SCR? I just looked up sensitive gate SCR's and it
didn't even seem impractical that I could trigger it through a capcitor?

Any other advice on a best cheapest approach would be helpful. I've done these
before for cheap control circuits but have never needed this much current
before.

Thanks

Rocky

Rocky
 
A

Active8

Jan 1, 1970
0
I need to make about 24 volts DC (good enough for dc relays) at about 250 ma
maximum from either 110v or 220v ac input.

I've found a bunch of circuits online, and quite a few chips.
I would like to avoid using a switcher because of EMI.
Er...

I think I need too much current for a shunt type approach.

If you mean "divider", 2.5 A through the divider.
Does anyone know of any simple SCR based, 'light dimmer' type circuit that can
be used for an off line supply.

A triac uses both half cycles. Both give EMI in the simplest
circuits. Hint: thyristor data book.

I don't like to recommend an offline supply that's not isolated,
something easily afforded in a switcher - and your batt backup might
power the controller. Or a PVI opto isolator driving the thyristor,
I guess.
 
S

Spehro Pefhany

Jan 1, 1970
0
I need to make about 24 volts DC (good enough for dc relays) at about 250 ma
maximum from either 110v or 220v ac input.

I've found a bunch of circuits online, and quite a few chips.
I would like to avoid using a switcher because of EMI.
I think I need too much current for a shunt type approach.
Does anyone know of any simple SCR based, 'light dimmer' type circuit that can
be used for an off line supply. I have a PIC onboard that has a battery back
up, so it could run the SCR? I just looked up sensitive gate SCR's and it
didn't even seem impractical that I could trigger it through a capcitor?

Any other advice on a best cheapest approach would be helpful. I've done these
before for cheap control circuits but have never needed this much current
before.

Thanks

Rocky

For 250mA, use a transformer.


Best regards,
Spehro Pefhany
 
F

Fred Bloggs

Jan 1, 1970
0
I need to make about 24 volts DC (good enough for dc relays) at about 250 ma
maximum from either 110v or 220v ac input.

I've found a bunch of circuits online, and quite a few chips.
I would like to avoid using a switcher because of EMI.
I think I need too much current for a shunt type approach.
Does anyone know of any simple SCR based, 'light dimmer' type circuit that can
be used for an off line supply. I have a PIC onboard that has a battery back
up, so it could run the SCR? I just looked up sensitive gate SCR's and it
didn't even seem impractical that I could trigger it through a capcitor?

You can't beat something made out of junk TV parts- this circuit should
suit you well:

View in a fixed-width font such as Courier.
 
R

Rolavine

Jan 1, 1970
0
From: Fred Bloggs [email protected]
Date: 11/18/04 6:55 AM Pacific Standard Time


You can't beat something made out of junk TV parts- this circuit should
suit you well:

View in a fixed-width font such as Courier.
.
.
.
. RG
. +-/\/\--+
. FW BRIDGE | |
. +--------+ R | SCR |
. ----|~ +|---/\/\--+--+-|>|------------+-------------->
. | | | / | |
. | -|>|- | | | | | comp/one-shot
. | | | | | | |\
. ----|~ -|-+ | +-|<|-+--------|-----|-\
. +--------+ | | | | | >-+
. | c ---/ +-----|+/ |
. | HV \| // \ 24V | |/ |
. | xstr|--+ --- | + |
. | /| | | === |
. | e | | |BFC |
. | | | | | |
. +-------+----------+--------+----------|---->
. | |
. +-------------------------+
. 100us
. +---+
. | |
. ---+ +-----
.

Great circuit, I think, where did you get it from? It was exactly the kind of
thing I was thinking about.

Rocky
 

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