Simple & cheap 'off line' power supply?

[Rolavine]

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

 

[Active8]

On 18 Nov 2004 07:58:08 GMT, Rolavine wrote:

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.

--
Best Regards,
Mike

 

[Spehro Pefhany]

On 18 Nov 2004 07:58:08 GMT, the renowned rolavine@aol.com (Rolavine)
wrote:

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
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com