Transistor TIP102 as a switch - HELP!

Dear Folks,

i have Problem and i am running out of ideas: I am using a TIP102 as a switch to drive a valve and i have the following problem: when i turn on the Transistor i can measure the 12V current but the valve is not moving. I checked if it has a common ground, which is the case.

Any other ideas? What should i check?

thanks guys!

it´s connected exactly like this: http://www.arduino.cc/playground/uploads/Learning/relays.pdf

when i turn on the Transistor i can measure the 12V current but the valve is not moving.

What are your measurement points? Put your volt meter right across the two coil terminals. Off should read 0vdc, on should read around 12vdc. Tell us what you measure there.

Lefty

Hello Lefty,

thanks for your quick response.

If i connect only the Voltmeter to the screwing terminal i measure 11.7V (On) and 0V off (as it is suposed to be).

If i also connect the valve it reads 0V in every case (which would explain, that its not working) but why if i get the 12v without the valve?

By those measurements, connecting the coil has the effect of stopping the circuit working. Is that right? So, look for places where there's a bad connection (high resistance), particularly near the transistor's emitter and collector. Also, can you tell us what resistor you've used in the base circuit?

It would also help if you posted a picture so we could see if you have missed anything.
Have you tried connecting the valve directly across the 12V supply, does it move then?

Hello,

i will try to get a picture soon - yes, directly connecting the valve to the 12V works..

Thanks

The picuture attached shows the board i designed - the pins on the right map to the pins of the arduino mega 1:1 and get connected by a ribbon cable. As you can see, there are 9 transistors - and none of them is working.

i hope you have some more ideas..

Thanks, have you got a schematic that was used to make that board. You referenced a circuit earlier from the playground but that only had one value of resistor. That board appears to have two values. Also they appear to be 10K and 100M unless I am being fooled by the colour fidelity.

They look like 10M Ohm resistors to me, Mike. Last band is blue.

Yes that's right I can't count. However a 10M resistor is not worth having.

Yes you are right, the ones for the transistors are 1k, for my “sensor” are 10M. I attached the eagle stuff.

batchlab connection shield board.pdf (102 KB)

batchlab connection shield.pdf (58.2 KB)

Thanks for posting that. From what I can see then you have everything correct on this board. I checked the transistor's pinout and the diode polarity, both of which could stop it working. So I can think we can eliminate and errors on that count. From experience I have found that often a problem is not where you are looking which is why you don't find it.

Having re read your post I think the salient points are:-

If i connect only the Voltmeter to the screwing terminal i measure 11.7V (On) and 0V off (as it is suposed to be). If i also connect the valve it reads 0V in every case

That sounds like there might be something wrong with the load or connections to it.

First thing to try is a resistor and LED in place of the valve. Does it light up? Do the voltage measurements still look like they are supposed to be? If that works then it could suggest that your 12V power supply is shutting down under load, perhaps because it can't supply the current.

Hello again,

the power supply seems not to be the problem BUT - i measured the current that actually reaches the transistor - it is after the 1k resistor less than 1V, more like 600mV.

I have no idea why it is like that, but CAN that be the problem?

thnkx

One thought - you are connecting the coil between the transistor output (collector) and the +12V (not ground) aren't you?

Also are those base resistors really are 1k? (they do look orange rather than red in the photos). TIP102 darlingtons have base-pulldowns so the base resistor needs to be low enough to overcome them - 10k would be marginal, 1k should be fine. for a darlington the base should be more like 1.2V than 600mV.

For a normal transistor, 0.6 to 0.7 volt from the base to emitter is normal when the transistor is passing current, BUT the TIP102 is a darlington pair so the voltage should be about twice this.

According to the data sheet there should be 2.8V on the base when it is on. What is the voltage the other side of the resistor.

they do look orange rather than red in the photos

That's what I thought.

Hi roadar

Does not look like you have resolved the problem as of yet.

I was wondering if the solenoid requires the full 12V to activate?

Also what is the actual current when the solenoid is activated?

A simple way to confirm would be to test without the transistor and put two diodes in series with the solenoid to see if it still activates when directly connected across 12 V. The 2 diodes would simulate the voltage loss through the transistor in operation.

Regards Bill

A simple way to confirm would be to test without the transistor and put two diodes in series with the solenoid to see if it still activates when directly connected across 12 V. The 2 diodes would simulate the voltage loss through the transistor in operation.

Not exactly. A darlington transistor has a two diode voltage drop across it's base/emitter leads, but the 'saturation' voltage drop across the emitter/collector leads should be .2 volts or so depending on collector current. Still it's a good idea to power the solenoid directly with +12vdc to see if it works at all and what current draw it consumes.

Lefty

retrolefty:

A simple way to confirm would be to test without the transistor and put two diodes in series with the solenoid to see if it still activates when directly connected across 12 V. The 2 diodes would simulate the voltage loss through the transistor in operation.

Not exactly. A darlington transistor has a two diode voltage drop across it's base/emitter leads, but the 'saturation' voltage drop across the emitter/collector leads should be .2 volts or so depending on collector current. Still it's a good idea to power the solenoid directly with +12vdc to see if it works at all and what current draw it consumes.

Lefty

Hi Lefty

Without knowing the actual solenoid current, I played it save. ;^)

At 3A the TIP102 Vcesat is actually 2V max from the "On Characteristics" spec sheet which is why I approximated two diode drops (1.4V) which is still less then 2V. http://www.onsemi.com/pub_link/Collateral/TIP100-D.PDF

If I am reading Figure 10 correctly, minimum is Vcesat 0.7V at 1A, 1.0V at 2A, 1.06V at 3A, 2V at 8A

Since it is a valve that is being driven, my assumption is there will be a large initial current with the solenoid?

For roador's benefit, whatever the Vcesat (voltage across the collector - emitter) ends up being, this is subtracted from the voltage that is available across the solenoid. To minimize Vcesat, you want to overdirve the current on the base. From Figure 10, the base current should be minimum of 1/250 of the collector current.

Which still brings us to the question, can the solenoid drive the valve with less then 12V? If not, one solution would be to increase the solenoid power supply voltage to compensate.

Regards Bill

Dear Everybody,

thank you for your thoughts, i need your brains once again:

i now tried an old transistor, that was still laying around, and now it works (WTF?).

Could you please compare the datasheets and actually tell me, what i missed and how they differ?

this one works: http://de.rs-online.com/web/p/products/545-0494/ this one does not: http://de.rs-online.com/web/p/products/485-9610/

and i thought TIP102 is TIP102..