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Velleman K8055 blowing up ULN2803

windinmysails

Mar 12, 2012
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Hi everyone,

I'm new here, so please forgive me if I'm posting in the wrong place or I'm missing that someone's already asked this question, but I'm having a problem with a project I'm trying to do and I can't see why!

I'm using a Velleman P8055-1 (sometimes called K8055) to drive a couple of relays to make a motor drive in two directions. Basically I'm using 2 channels, each one driving one relay independently and the relays are double pole with one pole connected to ground and the other to 24V. The motor sits across the common pins of the relays so that if I have neither channel on there both relays are off and both inputs to the motor are ground. If I have relay 1 on and relay 2 off, left side of motor receives +24V, right side gets grnd therefore motor goes clockwise. Relay 1 off, Relay 2 on, motor goes counter –clockwise. Both on and motor receives 24V both sides and doesn’t move.

So that’s my theory… I’m using 24V relays (RS: 699-5954) – they come with a built in diode so no need for me to add one. I measured and they draw about 70ma each so this was inside the output of what the ULN2803APG should have been able to do on a single channel, but I wasn’t sure about what happens if both fired at once (although this wasn’t something I was planning as it had no effect on the output side). Anyway, I tried and it worked, but then the ULN chip blew up – so I replaced it with a ULN2803AN which should sink 500ma (if I’ve understood correctly). But the same thing happened…

Can anyone tell me what I’m doing wrong here? I’ve driven lots of smaller things before and they’ve always worked fine. Is it because this is 24V (I can’t see how, but I don’t know). Is it something to do with the surge of the motors starting up? Help!

Many thanks in advance for any assistance!

Neil
 

Rleo6965

Jan 22, 2012
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Relay have built-in diode. Is it possible that Relay coil w/ diode was connect in reverse. That is, +24V supply pass thru relay built-in diode ( forward bias ) and destroy output of ULN2803. When relay driver ic was turned on and sink its output. Thus output draw high current trying to sink +24V supply.
 
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windinmysails

Mar 12, 2012
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Hi Rleo6965,

Thanks for the reply. I don't think so - I checked it was the correct way around on the circuit diagram that came from the datasheet with the relay. It says the negative end of the diode is on pin 86, so that's where I switch connect my +24V to and then I switch the ground to pin 85 (i.e. from the ULN2803). Also, if it had been the wrong way, when I measured the current the relay was drawing it would have gone pop wouldn't it?
 

Rleo6965

Jan 22, 2012
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Can you try connecting Relay pin 86 to +24V and pin85 in series with an ammeter then connect to ground. Let's see if relay will activate or draw normal current of 70ma.
 

Rleo6965

Jan 22, 2012
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It seems everything was working. How about post the circuit diagram. We might find something.:)
 

windinmysails

Mar 12, 2012
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Hi! OK - so here it is - sorry it's a bit noddy, the only tool I've got is Powerpoint...
 

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windinmysails

Mar 12, 2012
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Sorry, the ULN2803 is inside that USB board. The DO-1 and 2 Lines are connected to pins 11 and 12 of the ULN2803. It's on the Velleman board. I'm also going to post on the Velleman site as suggested by the other poster here. Do you need me to modify the diagram to clarify?
Thanks Rleo6965,
Neil
 

Rleo6965

Jan 22, 2012
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Here's a weird possible cause of damage to ULN2803. This is my theory only and I'm maybe wrong.

I'm thinking that it's possible that "dynamic braking" of DC motor was cause of damage to ULN2803 output pin. DC motor generate dc voltage when rotor was rotating mechanically.

A situation maybe that previously 1st driver was turned on and drive the motor at high rpm. Then 2nd driver was turned on. Both relay contacts 87a and 30 was closed. This therefore the path of negative voltage generated by dc motor because of centrifugal force or mechanical load. Negative voltage find it's way to output pin of ULN2803 NPN transistor and shorting the driver transistor.

Below picture of diagram that you describe. Pls note red line path of negative voltage. Blue X maybe a thin etch that act as resistor or loose connection.

ULN2803SHORT.jpg
 
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Rleo6965

Jan 22, 2012
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What is HS1 and ES1 in your diagram?

To confirm that negative was cause of damage of ULN2803. Check the resistance of pin11 and pin12 with respect to ground pin of ic. If all the IC have same defect of open pin11. That will confirm negative from dc motor cause of damage. You have to avoid turning on both 2 input of ULN2803.
 

windinmysails

Mar 12, 2012
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Thanks Rleo6965,

I was thinking it might be related to the motor temporarily acting as a generator when power was switched off. Does this mean I can put a couple of diodes somewhere? I'm trying to figure it out!

The HS and ES are Home Switch and End Switch. This motor is running an application that turns a threaded bar to move a nut along it. I want it to go back and forward, therefore when the nut hits the ES1 the PC software registers the switch depression, and changes motor direction - that is normally when it goes pop! The same happens when it hits the HS1 but in this case relay 2 is dropped and relay 1 is fired making the threaded bar go positive direction again.

What is the best way to protect my circuit here do you think?

Thanks again so much for your help,

Neil
 

Rleo6965

Jan 22, 2012
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I was thinking it might be related to the motor temporarily acting as a generator when power was switched off. Does this mean I can put a couple of diodes somewhere? I'm trying to figure it out!

Yes, your right . You need to placed diode as shown in diagram. 1N4001 will do.
Diode will serve as one way switch. ULN2603 output can pull down end of relay but will not pass negative voltage.

ULN2803protectiondiode.jpg
 
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windinmysails

Mar 12, 2012
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Hi Rleo6965,

Well - great news! It works!

So I modified my circuit to include the diodes, but in addition I had a look at the new 24V motors I've been trying. The 12V motors I was using had a small capacitor across it's input pins, but the 24V has one between the input pins and two others, and one between each pin and the shell of the motor.

Additionally, the two input switches I was using were in screened cable to stop any stray interferance from getting in and triggering the switches, but I'd grounded the screening on the structure and I do wonder if this had caused a problem with the motor change - it could be nothing, I don't know, but you know it does bug me that it was working and then stopped working when I went for these 24V motors!

Anyway, thank you very much for your help. I'm really pleased to be running again!

Neil
 

Rleo6965

Jan 22, 2012
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Your welcome neil,

I'm glad it works.

Those small capacitors in the terminals of dc motors were noise filter for the carbon brush. For the 24V motor. Try not to connect any wire to motor body and observed. Just use the 2 terminals.

Roger
 

john monks

Mar 9, 2012
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I have two suggestions. 1. Try placing a faster protection diode in parallel with the relay coil. 2. Slow down the speed of the driver, maybe by placing a capacitor from the input to ground. Sounds like either the coil has too much inductive kick or the is too capacitive. It is unusual to drive a relay with a darlington pair. Maybe the darlington pair is too fragile for a relay coil.
 
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