hi, i was need opinions from u all with the subj. i.e. i use TIP122 to switch the lamp on. which line should i use for? afaik, in theory, there is no diff between that two ways, since the current will go through both line. any?

Posted on 2003-09-09 07:27:14 by dion
You should always switch the "hot" side. In some cases it's for safety, that is, you always want a ground available should something go wrong. Switching the ground side also changes the plane impedance but that's getting technical.
Posted on 2003-09-09 09:44:39 by drhowarddrfine

Switch the ground is preferable when you must use a NPN (or N Channel) device. If you have to use a NPN to switch the high side, you'll need a boot strap voltage to do so.

If you switcht the high side with a PNP, you'll have to sink the base instead of sourcing it.

At least with MOSFET's, N usually has much lower on resistance than the equivilent P device.


How would switching on the ground affect the impedance of the ground plane? Wouldn't the switching affect both power and ground? Does a (resistive) load connected to the planes have an affect of the impedance of those planes (I'm refering to what the supply sees)?
Posted on 2003-09-10 17:15:40 by eet_1024
The TIP122 is an NPN Darlington.

Probably best to put the lamp between the collector and the supply voltage.

The two sides of the output are not equal. The emitter current is the sum of the collector and base current. You cannot raise the emitter voltage above the base voltage. You can get higher collector voltages by using a power supply different from the one that supplies the base current.

I believe drhowarddrfinedrhoward's advice is for switching the mains.
Posted on 2003-09-10 19:46:38 by tenkey
uhm... so who is right here, i am confused :(

fyi, the tip122 i saw was driving gnd side.
Posted on 2003-09-11 05:51:57 by dion
Hi, dion,

Whenever possible I prefer switching the "ground", that is placing the load (lamp) between the power supply and the collector/ drain of the transistor. That is because it makes driving the transistor easier, especially when the load power is different from the +5V logic power supply.

If the load needs to be powered from a voltage higher than 5V and you must switch the high side you will generally need some extra circuitry (an extra transistor, resistors...)

As for the confusion, the TIP122 was driving the GND side of the load, since the + side of the load must be connected to the + of the power supply and its GND side to ground, which is what the transistor does.
Posted on 2003-09-11 11:29:41 by VVV
Sorry. I've been so busy lately I try to help but obviously confused myself about the question. The other guys are right.
Posted on 2003-09-12 09:42:34 by drhowarddrfine
I think VVV was more or less getting to this, but for clarity I think i should add that to use a NPN on the VCC would not be ideal since its switching threshold will then varry.

I think i heard its a darlington pair, so for argument sake the BASE must be 0.7 + 0.7 = 1.4 volts above the EMITTOR to switch on current through the COLLECTOR -> EMMITTOR.

By placing the EMITTOR at GROUND you simplify things a bit because ground is ground, its not going to change it valve from 0 v. If you place the EMITTOR on the lamp, and the lamp to ground in series, the switching voltage will 'float' to 1.4v above the voltage drop of the lamp. As time wears on the lamp's properties (and consiquentially its voltage drop) may drift. This add more uncertainty to your design. You avoid all this by loading on the collector side, since the switching threshold is not a function of the collector.

Also, remember that 1.4v is approximatly constant across the BASE->EMITTOR. It will not vary, so you should choose a resistor in series with the base that will provide enough current to drive the transistor pair:

( V(on) - 1.4 )/Resistor > I(min on).

(This is assuming your grounding the EMMITTOR)

Posted on 2003-09-13 14:13:07 by NaN
Hello all,

It's been a few years since I sat at a bench but ....
this debate seems similar to the old one of 'electron flow' vs 'hole flow'.

I believe NAN has described the situation pretty good. Switching the vcc side can cause instability in your circuit. You should always switch ground.

Also, it's the safest.
Posted on 2003-09-14 09:17:36 by djinn
I should also add that this logic is reversed entirely if you have a PNP type device, however the names (BASE, EMITTOR, COLLECTOR) remain the same.

This is, the Emittor is on the VCC rail, the collector loads the Bulb to ground, and the Base is used in series with a chosen resistor to safely switch the device on and off. Switching on would be providing a voltage on the BASE->Emittor junction less than (VCC-1.4). Ground would fit this requirement. Switching off would be done by raising the voltage above (VCC-1.4). VCC would fit this requirement.

If you draw the two scenarios out on paper you will see in both cases the EMITTOR diode is pointing in the direction of conventional current flow each time. And the COLLECTOR and BASE pins either source current in PNP devices, or sink current in NPN devices.

Posted on 2003-09-14 09:36:32 by NaN
thanks for further infos ;)
Posted on 2003-09-16 21:05:41 by dion