conduction angle, form factor etc.

Thank you, everyone, for your help.

In next couple of days I will try to contact the author about it though I don't think it's really that important because we have already figured out the problem.

Could you please help me with this query? This **broken link removed** talks about tab terminal under the heading "SCR (Thyristor) Rectifiers" just below the circuit diagram. You can view the full datasheet here. Thanks a lot.

Regards
PG

You could if you wished, use either of the two anode terminals.
It can sometimes simplify the construction of the equipment containing the thyristor.

It is not that the device was designed with two anode terminals, more that the device was designed with the anode connected to the tab.

The tab is there to make a thermal connection to a heatsink which keeps the device cool.
It just so happens that when the device is made, the anode is bonded onto the piece of metal which forms the tab.

Another type of thyristor may have the cathode connected to the tab.
(I stand to be corrected on that, over the years I have not used many thyristors).

JimB

There is an interesting development about how this type of tab is used with surface mount transistors. Look up the DPAK and D2PAK package styles. You will see the middle terminal is cut and the tab, which no longer has a hole, is soldered to the board.

Traditionally, the old package PG is asking about has the middle terminal soldered into the board and the tab is insulated when mounted to a free-standing or chassis-mount heat sink. But, there are always exceptions to rules and many times I've soldered a connection wire to a tab while bread boarding.

PG1995 said:

Thank you, everyone, for your help.

In next couple of days I will try to contact the author about it though I don't think it's really that important because we have already figured out the problem.

Could you please help me with this query? This **broken link removed** talks about tab terminal under the heading "SCR (Thyristor) Rectifiers" just below the circuit diagram. You can view the full datasheet here. Thanks a lot.

Regards
PG

Click to expand...

Hi PG,


A couple points about device metal tabs and metal cases...

Some of the packages available today have insulated tabs so that the tab is no longer one of the connections to the device. That means you have to use the lead itself, but the advantage is that the metal of the package that mounts to a heatsink is already isolated from the device electrical terminals so you dont have to use an insulating pad. Without that internal isolation, you usually have to use an insulating pad so it makes good thermal contact to the heatsink but no electrical contact to the heatsink. The built in isolation is especially useful when two more more devices have to be mounted to the same heatsink that have terminals (tabs) that must be isolated electrically because they are different nodes in a circuit.

Non insulated tabs also have their advantages however. If the center leg is not connected but only the tab is used for electrical contact in the circuit, that means the electrical contact depends on the bolt through the tab hole being tight, holding it fast to the heatsink. Now if the center lead was connected to the circuit if the bolt vibrates loose the connection would still be solid so the device would still work the same way, except it would overheat because the tab no longer made good contact to the heatsink. If the tab alone was used however and not the center lead, when the bolt vibrates loose we loose thermal contact the same way, but then we also loose electrical contact. In this way the product still malfunctions but the device does not burn up.
There is another more profound use of this technique too with three terminal voltage regulators. With some regulators if the lead that connects to the case is broken the output voltage jumps up to a much higher value and burns out the following circuit. But if the ground connection is made THROUGH the package (TO3 package for example) if one lead becomes disconnected (loose bolt) the voltage drops to zero because the load no longer gets the ground (0v) side of the supply. With the TO220 packages if we use the tab as the incoming supply and the center leg as the outgoing supply lead we get the same built in protection...if the tab becomes disconnected we loose all power rather than burn up or destroy the following circuit.

Hi PG,
If you look at the BT151 series data sheet, you will see that there are 3 devices in the BT 151 range; a 500 volt, 650 volt and 800 volt rated device. The type numbers are; BT151/500R, BT151/650R, and BT151/800R.
The R represents "Reverse polarity".
Traditionally, in Philips speak, a diode had the 'cathode' connected to the case. However, for these BT 151 types, the 'ANODE' is connected to the case (TAB)
If you consider a full wave rectifier, there are two 'cathodes' connected together at the +side, and two 'anodes' connected together at the - side. In order to get rid of the heat, as Mr AL says, it is good if the case can be directly mounted on to the heat sink WITHOUT an insulator. With the BT151/500R, the two anodes can be bolted directly to the heatsink; but for the other two diodes, they have to be insulated from the heats ink OR bolted directly to two independent heat sinks. HOWEVER, IF there was a device BT151/500, this would have the 'CATHODES' connected to the tab and then they could be bolted directly to a heatsink. I'm not sure I've seen 'normal polarity' SCR's but it was the case with diodes. See BYX52 series for example.
If you look at the data sheet for the BTW23, you will see the description says ;" ... reverse polarity types". Philips have been using this nomenclature for over 50 years.
With modern devices soldered directly into circuit, many of the previous mounting and heat dissipation problems have changed.
In the early days of high powered SCR's, the only available heat sink mounting insulator was mica, but today there are some excellent high thermal conductivity and high voltage, ceramic materials available.