>Try using PNP transistors in the "top" legs of the H, and NPN in the "bottom" legs.
>Also use biasing resistors to switch the transistors off (base to GND on the NPN
>and base to + on the PNP)
>You have to invert the drive signals (transistor 1 = PNP drive base low, and
transistor 4
>= NPN driven high)

True, very true,
If possible and I reccomed that you change over to mosfet transistors.  Much lower on
resistance and heat buildup especially if this is a production prototype.  Also
protect your transistors and/ or mosfets with snubbers or at least protection diodes.

There are a few H bridge IC chips out there that do all of this for you and may
reduce your overall cost.

>
>I think your problem is that both transistors in a leg are being switched on
together, thus
>shorting the supply and cooking transistors.

Yes, I agree, or in a worst case senerio the transistors were zapped by inductive
kick when the switches were turned off thus possible massive short.

You may have seen this as arcs/sparks if your relays had a clear case.

Please, put the emitters on the supply rail side, not the "load" side.  Much better
for switching applacations, otherwise the transistors act as (at best) a voltage
drop.

I think...

Carl Bright
Communications Mfg. Co.
carl@mmcable.com
http://home.mmcable.com/cuttler/