Is it possible to use TwinGig Converter Module with the 3750 series 12 port sfp slots. We would like to utilise the 10Gb feature while creating a core layer switch by stacking multiple 3750-12 switches to provide redundancy at the core. and which model 3750G or 3750 E series
The TwinGig Converter Module is used with the 3750-E (or 3560-E) 10 gig module slots. (Instead of a 10 gig connection you obtain dual gig connections for the module slot.)
If you mean, can a TwinGig connection connect to a 3750 SFP port, believe the answer is yes.
If you mean, can you have a mixed stack containing both 3750-Es and 3750G-12Ss (or other 3750s), also believe the answer is yes.
There is only one non E-series 3750's that has 10GB support - the 3750G-16TD. This has a single 10GB XENPAK transceiver slot, not X2 TwinGig like the 3750-E series. All of the 3750-E series switches have two 10GB X2 slots that each can take an X2 transceiver or a TwinGig converter. The TwinGig converter allows two SFP transceivers to be installed instead of a single 10GB X2 transceiver.
Have a read of the hardware install guides for each platform to see what ports are available:
In answer to your question, no a 3750G-12S only supports SFP modules, however there would be no need for TwinGig modules in this switch as it already supports 1GB SFP's - 12 of them...
Is there any specific command to convert the 10G into 2 1G ports on the 3750-E after you use the SFP module/converter?
We have a 3825 router with a 1 Gb uplink connected to the 3750 10Gb port but the link is not coming up.
No. When you insert the Twingig converter module in the Cat3750E, the appropriate gig1/0/x port become available in your configuration.
In other words, if you have one Cat3750E-24 then you have gig 1/0/1 - 1/0/24 for the 24 copper ports and you'll see Ten1/0/1 and Ten1/0/2 as the two 10GE port. If a TwinGig converter is inserted in the upper 10GE slot, then you'll see Gig1/0/25 and 1/0/26 in your config file and one of those is the interface you configure to match your 3825.