How to use a RequiredFieldValidator at runtime?

I have following tag, I want to validate requirefieldvalidato runtim whenever user click on "Submit" button btnSubmit_Click event, how can I do this? Actually on my form I have 3-4 submit button for different purpose. On each buton I have set CauseValidaton=false, because some of the fields are initially hidden and depending on button click fields are visible.
<td width="26%">  <asp:DropDownList ID="ddltype" runat="server" class="ms-short"> <asp:ListItem Text="-- Select --" Value="" /> <asp:ListItem>Item1</asp:ListItem> <asp:ListItem>Item2</asp:ListItem> <asp:ListItem>Item3</asp:ListItem> <asp:ListItem>Item4</asp:ListItem> </asp:DropDownList> ...