Motor driver problems

elMickotanko · Dec 6, 2006

Hi,

Im working on a line following robot for Uni. We've built a PCB for our motor driver which we tested and was working fine. Ive added the full schematic (and another pic with one drive circuit and internal circuit of TIP121).

We tried it again this afternoon and something went wrong:
As soon as we connect power to the board the motor starts full power, as if there is something shorting out the transistor. It is the same for all 3.

One time the transistor got extremely hot, so maybe it has been destroyed? ALthough I thought that would make it open circuit, and the motor wouldnt work at all?

It blew a couple of 5A fuses aswell.

I cant figure it out at all, I had a long hard look at the board to see if i could spot anything but couldnt, but i was pretty stressed by then cos we were supposed to demonstrate it this afternoon.

Can anyone help?

Thanks,
Michael.

PS. R1,2,3 are wire links, R7,8 are open circuit, and its the TIP121 not the tip31 like the schematic.

dknguyen · Dec 6, 2006

THe motor starts even when there is a base-pull down resistor? I'm not suprised ot starts full power when you have a pull-up resistor on the NPN's base.

I think MOSFETs can short when they fail, but isnt it really rare?

Nigel Goodwin · Dec 6, 2006

dknguyen said:
I think MOSFETs can short when they fail, but isnt it really rare?

By far the most common failure mode for power transistors (either bipolar or FET) is for them to go short circuit, going O/C would be extremely rare.

audioguru · Dec 6, 2006

D4 and D5 protect the transistors T2 and T3 from the inductive voltage spike from the motors. T1 doesn't have a protection diode so maybe it fails first and causes the others to fail.

Diodes D1, D2, D3 and the diodes in the darlingtons don't do anything in your circuit.

What is the max stalled current of the motors? It can be calculated as the voltage divided by the motor's resistance.

elMickotanko · Dec 7, 2006

Thanks for your help guys.

Nigel Goodwin said:
By far the most common failure mode for power transistors (either bipolar or FET) is for them to go short circuit, going O/C would be extremely rare.

Thanks i'll try replacing them. Someone suggested putting a 4R7 resistor for R1,2,3, to limit current. i'll try that aswell. But im thinking it should be a 5W resistor (I^2.R)? it seems wastful and will take a lot of space.

audioguru said:
D4 and D5 protect the transistors T2 and T3 from the inductive voltage spike from the motors. T1 doesn't have a protection diode so maybe it fails first and causes the others to fail.

Diodes D1, D2, D3 and the diodes in the darlingtons don't do anything in your circuit.

What is the max stalled current of the motors? It can be calculated as the voltage divided by the motor's resistance.

T1 actually drives an electromagnet to pick up discs, which already has a diode on the module.

The technician at uni suggested the diodes across collector-emitter, but it didnt seem to make a difference when we were breadboarding. Just put in anyway. Why is there one in the package if it doesnt do anything?

I'll go up just now and measure the resistance of the motor.

Cheers,

Mick.

Nigel Goodwin · Dec 7, 2006

elMickotanko said:
Thanks i'll try replacing them. Someone suggested putting a 4R7 resistor for R1,2,3, to limit current. i'll try that aswell. But im thinking it should be a 5W resistor (I^2.R)? it seems wastful and will take a lot of space.

A 4.7 ohm would limit the current a LOT, and may well give problems starting under load - and would require LARGE resistors, and get VERY hot.

T1 actually drives an electromagnet to pick up discs, which already has a diode on the module.

The technician at uni suggested the diodes across collector-emitter, but it didnt seem to make a difference when we were breadboarding. Just put in anyway. Why is there one in the package if it doesnt do anything?

It may just be a byproduct of the manufacturing process?.

I'll go up just now and measure the resistance of the motor.

How big are the motors?, stall current is likely to be something like at least 5 to 10 times running current! - so a low value resistor in series may be desirable?, but no where near as high as 4.7 ohms!.

elMickotanko · Dec 7, 2006

Nigel Goodwin said:
How big are the motors?, stall current is likely to be something like at least 5 to 10 times running current! - so a low value resistor in series may be desirable?, but no where near as high as 4.7 ohms!.

We tested the motor on its own with a proper power supply:

Initial surge current was about 3 amps. the datasheet said stall current is 2.8A.

It draws about 0.5A unloaded, and about 0.7-0.8A loaded.
This was constant from 4 to 12V.

What is the output stage for HCT logic, im trying to figure out if thats something to do with transistors blowing.

audioguru · Dec 7, 2006

The max allowed output current of 74HCxx logic is 25mA. It will try to drive 60mA or more into the transistors or darlingtons and will blow without a series current-limiting resistor.

The logic high current of an old 74LSxx IC is not much but it still might blow up when directly driving a transistor without a series current-limiting resistor.
I doubt it has enough current to work properly.

Nigel Goodwin · Dec 7, 2006

elMickotanko said:
We tested the motor on its own with a proper power supply:

Initial surge current was about 3 amps. the datasheet said stall current is 2.8A.

It draws about 0.5A unloaded, and about 0.7-0.8A loaded.
This was constant from 4 to 12V.

I take it they are very small motors then? - I don't know if you ever saw the magazine robot Cybot? - but I tested those and stall current was over 6A, and I suspect that was the batteries I was using!.

Hero999 · Dec 7, 2006

Nigel Goodwin said:
By far the most common failure mode for power transistors (either bipolar or FET) is for them to go short circuit, going O/C would be extremely rare.

Yes, the semiconductor junction fuses together forming one big conductor, in MOSFETs the gate normally becomes conduction too thus ruining the driving circuitry.

elMickotanko · Dec 10, 2006

Nigel Goodwin said:
I take it they are very small motors then? - I don't know if you ever saw the magazine robot Cybot? - but I tested those and stall current was over 6A, and I suspect that was the batteries I was using!.

i've heard it been mentioned on here before. I'll need to look it up on your site! Our motors are quite big and I suppose they must have drawn over 5A to blow the fuse. But the datasheet and the current gauge on the power supply said about 3A. They are geared right down (128:1 i think), dont know if that has anything to do with it.

audioguru said:
The max allowed output current of 74HCxx logic is 25mA. It will try to drive 60mA or more into the transistors or darlingtons and will blow without a series current-limiting resistor.

The logic high current of an old 74LSxx IC is not much but it still might blow up when directly driving a transistor without a series current-limiting resistor.
I doubt it has enough current to work properly.

Its HCT were using. I cant find any internal schematics or anything. I think its MOS outputs? Is it that when logic '1' the o/p pin is connected straight to the supply?

Its annoying cos weve already got the board made. It was working for a week or so aswell. Not sure why our supervisor told us to use this pull-up method instead of our original way of 'O/P->series R->base' now.

audioguru · Dec 10, 2006

Your original schematic shows 74LS04 TTL inverters driving a weak output high current therefore requiring the output pullup resistors.

Then you changed the IC to a 74HCT04 which has the same compementary high current Mosfet outputs as a 74HC04 and a warning not to exceed 25Ma. They can provide 60mA at the output for a moment to quickly charge wiring capacitance for high speed. A resistor must be added to their outputs to limit the current to 25mA or less.

Maybe you can use darlington output transistors with the series base resistors built-in.

elMickotanko · Dec 12, 2006

audioguru said:
Your original schematic shows 74LS04 TTL inverters driving a weak output high current therefore requiring the output pullup resistors.

Then you changed the IC to a 74HCT04 which has the same compementary high current Mosfet outputs as a 74HC04 and a warning not to exceed 25Ma. They can provide 60mA at the output for a moment to quickly charge wiring capacitance for high speed. A resistor must be added to their outputs to limit the current to 25mA or less.

Maybe you can use darlington output transistors with the series base resistors built-in.

Thanks again for all your help.

If I just go back to an LS chip will that work without a base resistor? I think they may be hard to get though.

I might just cut the track and botch in a base resistor. Would that work with either logic chip, and with the pull-up still there?

For the base resistor:
If the gain is 1000, and my motor current is say 0.7A, i should make Ib at least 0.7mA to saturate. Is that all?

if say the output from the inverter is 5v and i want my Ib to be 5mA, then my resistor should be
R = (5-1.3)/5mA = 740.
Or if i use 470R: Ib = 7.87mA.
and if inverter output is lower, say 3.8v: Ib = (3.8-1.3)/470 = 5.32mA.
concl. : 470 ohm.

Is there anything wrong with these calculations/estimations?

audioguru · Dec 12, 2006

The output voltage of an inverter is not 5V unless it is Cmos without a load.

An old TTL output high has a low current and goes up to about +3.5V without a load.

A 74HCxx output can drive a 150 ohm resistor in series with the base of the darlington and a pullup resistor is not required.

Darlington transistors amplify well with a gain of 1000. The datasheets show that they saturate well if you allow enough base current so it is 1/250th of the load current.

elMickotanko · Dec 14, 2006

audioguru said:
Darlington transistors amplify well with a gain of 1000. The datasheets show that they saturate well if you allow enough base current so it is 1/250th of the load current.

Thanks for your help.

The store at Uni only stock HCTs.I cut the track and stuck in a base resistor to give about 5mA and removed the pull-ups. It seems to work now but it blew another inverter chip first. Hopefully it doesnt do it again.

Cheers,
Mick.

audioguru · Dec 14, 2006

The inverter chip and maybe also the darlington transistor will continue to blow if you don't add a reverse-biased diode across the motor to arrest the very high voltage created by its inductance when it is turned off.

The 74HC inverter is rated for a max output curremt of 25mA. With only 5mA into the base of a darlington transistor then it will satuate well with a max load current of only 1.25A. With a 20mA base current then it can drive 5A well.

elMickotanko · Dec 14, 2006

Yeah, I have protection diodes already.

The motors take 0.5 - 0.8A.

audioguru · Dec 14, 2006

Hello el Mick,
Please attach your schematic for us to see why the inverter ICs blow up.

elMickotanko · Dec 15, 2006

Its my first post in the thread.
The schematic show 74LS04 but Im using HCT.
R1,2,3 are wire links, R7,8 are open circuit, and its the TIP121 not the tip31 like the schematic.
(Couldnt find tip121 footprint in eagle)

R4,5,6 have been removed and i cut the track and put 470ohm resistors in series with the base.

I ran it today for 5 mins and it worked fine. Im worried it might happen again though cos yesterday it worked for a while then fried the 7404.

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