Can I use darlington transistors in a power bridge?

[BrendanP]

Im controlling some LEDS with a power bridge. Im using a TIP35C and a TIP36C at the moment. Its overkill for the current I need to switch. Can I use darlington transistor such as BD681 and BD 682 ? (which have already). Also on the spec sheets for transistors I see it give a figure under freq maximum, I presume this reffers to how fast the device can switch. What spec should I look for in transistors to use with pwmout?

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[evanh]

The small base voltage of the BJT can be handy as a non-saturating emitter-follower but you might want to try MOSFETs or IGBTs for simple to drive circuits. The IGBT actually is a dalington type arrangement but turns on further and uses less drive current.


Evan

[premelec]

The main electrical differences between a darlington and single bipolar configuration are: Hfe, current gain, and input 'Vbe' voltage which is higher because of the additional transistor - i.e. about 2x Vbe for single bipolar. So, yes probably it will work fine.. with LEDs in collector circuit and a current limiting resistor to darlington base - ca. 500 ohms depending...

[BrendanP]

Thanks for replying to my post guys, I don’t have experience with power bridges so excuse me if my questions are a bit ‘dumb’.

I see there is a significant voltage loss through the power bridge. Are there any designs that avoid this? Would using MOSFETS or IGBTs be more efficient than BJTs? Or is this just the price to pay for using a power bridge? Am I right in suspecting that higher current handling transistors are less efficient that lower rated ones? Would using transistors rated at handling no more current than what I need make for a more efficient power bridge?

There appears to be voltage loss on one side of the circuit but not on the other. I.e. When the current flows in one direction I lose half a volt but on the other side its around 2.5V, is there any way around this? The LEDs are noticeabley dimmer as a result of the voltage drop.

I recently bought a program called Circuit Wizard which has been a big help in understanding how a power bridge works. It shows a voltage loss on one side of a power bridge more than another as well. Why is this?

I thought to up the supply voltage to allow for the loss, is this a soloution? (Id have to then use a voltage regulator to power the picaxe that is controlling the LEDs as supply would > that the piacxe needs 4.5 v.)


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[evanh]

You don't normally need a bridge just for dimming. Can you post a rough ascii drawing showing how you have wired the LEDs and transistors?


Evan

[BrendanP]

Thanks Evan for your interest in my problem. I apologise for not explaining the situation in more detail originally. I have a 100 LEDS wired parrallel in alternate polarity. So if apply poer to one side of the circuit all the green leds light. I apply poer to the other side all the orange leds light. Hence the need for the power bridge. I connected it all up last night. Im using a 28X as it has two pwmouts. Surprise surprise it actually works. I can infintely dim the whole the whole lot. I use a for next loop to subtract form or add to a word value which is then used in the pwmout comm.

As I mnetioned in last post Im getting a fair bit of voltage drop. THe leds light but no where as bright as they should. Theyre rated at 4 volts, 4.6 max. Fully bright you cant even look at them the glare is that stong, make them stobe and they make you feel nauseas.

I had a play around on Circuit Wizard and did some reading on Wikipedia and I think Im onto the problem with the bridge. I had thought that transistors were simply either on or off. But now my understaning is that the current flow through the transitor is dependent on the voltage applied to the base. My problem is that Ive used too higher rated resistors on the base so the transistor isnt allowing enough current through the bridge to the LED strings. So Ill try increasing the voltage to the base up to 5 volts and see what happens. Hmmm whats that smoking smell..... hopefully not!

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