tip42 base current

[Newz2000]

So, in the exploding 556 circuit, not only did the 556 blow up, but the two transistors went too. We've rebuilt the board and found the transistors don't work. A test showed that collector to emitter has continuity (that's bad, right?).

In hopes of avoiding this in the future, can you suggest a solution? I've put a 220 ohm resistor in line with the wiper on the pot so that there will never be zero resistence on the discharge pin.

I'm wondering if we had too much base current. I've got a 1k ohm resistor in series with the base. The circuit will be running at about 13.5v, but it's possible that there will be little dips and surges (auto environment).

Also, I've put in a 12v zener diode along with a 220 ohm resistor. I'm not used to using these, so tell me if this is right:
I put the zener in backwards going from ground to +12v, then a 220 ohm series with the 12v supply. I used this as my guide: http://www.kpsec.freeuk.com/components/diode.htm#zener

[dyarker]

Yes, emitter-collector short is bad.

If you didn't electrically isolate the TIP42s from the heatsink, and the heatsink is grounded, then the collectors were shorted to ground; no motor impedance to limit current. First turn-on ... poof.

The page you linked to correct, a 10uF parallel to zener would help.

<small>[ December 01, 2005, 10:21 PM: Message edited by: Dale Y ]</small>

[philba]

there are a lot of things going on here.

A 1K resistor limits 13.5V to 13.5mA, seems pretty low to me. The TIP42 has a pretty low Hfe - could be as low as 15 up to 75 or so. This says that the current you could deliver ranges between 200mA up to about 1A. More likely is 400mA. what do your motors pull? You'll need to deliver that much current. your base_current * Hfe should equal the motor's current. You might want to lower the resistor - the 556 can easily source 100mA or more. You want to turn on the TIP pretty hard. the TIP42 has a base current max of -2A so I don't think the 556 could hurt it. If your motor needs a lot more current, I'd use an N-mosfet as a low side driver instead.

Also, I wonder how much heat the TIPs are generating.

I hope you are only using the zener for the 556 and not the motors. You should make sure your current limiting resistor is of the correct wattage.

I'm still think that your Back EMF diodes (1n4148) are way too small. I wouldn't be surprised if they blew rather quickly and then the TIPs were exposed. If you have the non-A TIP42, you might be seeing spikes close to the 40V limit. I don't think that would take out the 556, though.

[Newz2000]

Ah...

One motor is rated at 3 amps, the other is unknown but in the ball park/slightly larger then the 3 amp.

I don't understand the -2A thing... how can you have negative amps?

Also, the zener is definately in line with only the 556. We have a dpdt relay controlling power to the motors and circuitry separately.

Regarding the 1n4148's... the only other diodes I have with a higher power are 1n4007 regulator diodes... can they substitute for a 1n4148 switching diode?

BTW, the heat sinks were not connected to ground. For a while, they were connected to each other but we quickly discovered this didn't do what we wanted. They are not isolated from the tip42 though.

[Robert Reed]

A footnote to Philbas excellent suggestions--More than just attaining enough base drive to satisfy the motors demands, you want enough drive to quickly and thoroughly drive your TIPs into saturation. This will ensure minimum heat dissapation. With an Hfe of 50, you would need 60 ma of base drive minimum (assuming a 3 amp load).This would require a base resistor of 200 ohms. With lower Hfe, you may not be able to do what you want efficiently.

[philba]

PNPs and P channel mosfets are rated this way. It's about polarity - don't get hung up on the minus sign.

3 amps. yikes! your 1n4148's probably blew the first time you turned off the motor. the 4007 will work better and may survive the BEMF.

What I don't like about the TIP42s is the crappy gain (Hfe) and wide range in the datasheet. Makes it a bit hard to know if they will really work for this application. Note also that the Hfe falls with the higher current. At 4V and 3A, the Hfe is 15. I don't know what effect the higher voltage has, though. An Hfe of 15 means you need to have an Ib of 200mA to get to 3A. The spec seems to imply that you need to have an Ic/Ib ratio of 10 to get to saturation. That says you have to have an Ib for 300mA. I don't know if the 556 can safely deliver that so you'd be better off with an intermediate driver transistor.

Given that, I suggest you scrap the TIP42s and go with a mosfet. the IRF540 should work quite well as a low side driver (motor connects to 13.5 and drain, source to gnd). Probably any n-mosfet that can handle >3A continuous should work. as a bonus, you will have a lot less heat production. The IRF540 is commonly available on the surplus market but even from mouser, it's fairly cheap (ST cross ref STP24NF10 is $.88). When fully turned on, it has an Rds of .06 ohms max. At 3 A, that means a dissipation of about 1/2 watt. You might not need a heatsink though I'd leave it in for safety's aske.

[Newz2000]

Yeah, I've got a few IRF540s. What changes to do you need to make when substituting a mosfet for transistor? I think I'm correct when I say that FETs aren't turned on by current like transistors, so does that mean I need a current limiting transistor in there?

I was thinking about using the suggestion of an nchannel transistor set up as a darlington pair to turn the TIP on, but I'm happy to try the FET if it will work better.

[Newz2000]

Ooops. I have IRL540N... this is a logic level transistor... it problably won't work will it?

[philba]

actually Matt, the IRL540 should work just fine. The main difference is that it needs about 1/2 the gate voltage to turn the mosfet on and the max Vgs is 20. You could trigger it off of a PIC if you wanted. In fact, it's Rds is a bit lower than the IRF540 and thus will generate a touch less heat. I'd use it this way:
<img src="http://www.geocities.com/pcbs4less/nmosfet.gif" alt=" - " />

R1 is optional and can be reasonably high.

[Enzo]

Or use a darlington. Stick a TIP 1xx in place of the TIP42.

TIP125,126,127 are choices in a 5 amp and TIP105,106,107 in an 8 amp. That assumes you want PNP. They come in NPN flavors too. Running between +12 and ground in the circuit like Philba's, you would want NPN. That is low side switching. Your high side switching uses PNP appropriately.

With a bipolar darlington you wouldn't need to worry about drive voltage, and current demands would be very low from the device driving the TIP.

[k7elp60]

I think the 220 ohm resistor on pin 7 is still to low in value. Pin 7 is an open collector, so the resistor from pin 7 to Vcc will determine collector current. According to may data book typical current for pin 7 at Vcc of 12 volts is about 15mA. This relates to about 800 ohms. I have always used about 1K and had not problems with that value.