Dodgy Chinese flip flop...or my circuit?

[rockrockmcrock]

Hi Guys,

So I *think* I've got this circuit right, at least I can't see anything wrong with it. The idea is that a push button driven flip flop triggers the relay which switches the 20A current I need for a solenoid. If power is lost from the circuit the relay should stay open. Likewise for starting conditions.

I decided to buy a cheap flip flop from ebay supposedly rated at 1500mA at Vo and 3-18v. I tested it before putting it in the circuit below (it worked). Then I stuck it in the circuit for a test. I reckon this circuit should draw way way less than the stated 1500mA from my calculations - but it blew the flip flop. So what's wrong? My circuit or dodgy Chinese promises?




Here's the Chinese flip flop ICB that I bought and some specs:



With the help of you guys (thanks) I've been slowly teaching myself electronics - all going well then I hit this bump in the road....ah well! Thanks in advance for any help RRMR

 

[duke37]

As I see it, the flip flop produces a switched voltage at a maximum current of 1500mA. I do not know where 1500mAh comes into it.

The led should have a controlled current of 15mA. Putting 14V across it will kill it, possibly making a short across the output and so killing the flip flop. The led should have a series resistance to limit the current

The relay should have a diode across the coil (the correct way round) to limit the inductive voltage when the relay is turned off..

 

[rockrockmcrock]

As I see it, the flip flop produces a switched voltage at a maximum current of 1500mA. I do not know where 1500mAh comes into it.

The led should have a controlled current of 15mA. Putting 14V across it will kill it, possibly making a short across the output and so killing the flip flop. The led should have a series resistance to limit the current

The relay should have a diode across the coil (the correct way round) to limit the inductive voltage when the relay is turned off..

Oops - that's a cut and paste typo in the diagram, sorry. All instances of mAh should actually be mA - teach me to be working on Lipo battery diagrams at the same time.

On the LED - that's again sloppiness on my part, it's actually a LED unit which incorporates a resistor and draws 15mA.

According to the specs of the flip-flop it has a diode built in to protect its use with relays so if I believe them then an additional one would be nugatory. It's one of the reasons I bought it.

Other than that does the circuit look correct?

 

[Bluejets]

I'd go with Dukes reply except it does mention there is an on-board flyback diode for the relay......oops....same time zone???

 

[Bluejets]

Show us a photo of what you have connected.

 

[rockrockmcrock]

Time zoning for sure!

Not sure a photo will help much as it's such a simple circuit I didn't breadboard it. But here goes - I've removed the power supply to help make it clearer.

Solid Orange wire with voltmeter clip on it is the switched output from the relay.
Yellow/black wire the supply voltage
Momentary button is on orange/white and black wires.
Led unit is on pink and grey (-ve) wires. Grey wire connects to black wires at relay.
Vo from flip flop is pink. Vcc yellow/black. Tg orange/white.

 

[Bluejets]

O.k........ some of these relays (automotive) have an internal flyback diode.
Anode to 85 and cathode to 86.
If your wiring would have been connected this way (same as your drawing) then everything would have been ok.
However, according to your photo, you have the switch line going to 85 and ground to 86 which is forward bias on the flyback diode and a short to the flip flop unit.
As I say, it depends on whether or not your relay has an internal flyback diode.
If it did it probably blew the **** out of it anyway by now along with your board.

This is why a photo helps

 

[Bluejets]

Edit: Flyback should be "freewheeling"

 

[rockrockmcrock]

Thanks, possible I suppose as I went by the circuit diagram which is printed on the relay and shows no freewheeling diode but didn't check manually. Hence didn't bother to much with coil polarity. I'll check it manually with a voltmeter.

If it does have a diode I'll never go by diagrams printed on these relays again! Man, you've got good eyes to spot that

-------------EDIT----------
Just checked the relay - slightly more than the 330 Ohm quoted, but at 332 Ohm coil resistance each way I guess the circuit diagram was right and it does not have a freewheeling diode built in....at least right at present...

...because if the diode blew I guess it would read that now. So I opened the relay up and....no diode.

Thought you had the answer there Blue...this is a puzzler! I'm starting to think that the flip flop may not be rated to the promised 1500mA output...

 

[rockrockmcrock]

Ok so I've been thinking about this and I might have an answer - I'd be grateful if I could have some views on this as I have only one Chinese flip flop left (my spare).

The only thing I can think of is that the flip flop ICB implements induction protection (assumed to be diode across Vo/GND) that is rated lower for forward current than required for my coil so it blows leaving the ICB circuit unprotected which then fries.

So I'm thinking of sticking an extra freewheeling diode across the relay and retesting the circuit with my last flip flop. Bad idea?

The only other option I can think of is the flip flop Vo is rated below what it says it is. Which would make these useless to this application anyway.

 

[duke37]

To protect your last circuit, run it with a small bulb in series with the supply. When all looks well, take the bulb out.
Try it with a 330Ω resistor instead of a relay with no led.
Try the led directly from the 14V to check that works correctly.
Add the led if it is OK.

 

[Bluejets]

I'd be making sure the LED you say draws 15mA actually does.Put say 180R in series with the LED and measure current with just the LED on the battery.

 

[rockrockmcrock]

Thanks - without the relay all checks out with a new LED unit in place and works as it should.

I'll try the relay next once I've got a suitable bulb

You'll see I mention a *New* led unit....the last one blew and I think I'm going to have to modify the circuit as today in testing everything I found out something interesting...

The LED unit is not what it was described as (another Chinese purchase). It was described as a LED in series with a resistor, but having tested it it clearly has diodes in there as it can be powered both ways round. Of course when I first tested it for polarity I didn't check it with the contacts then reversed so missed this! In effect it took a huge load when the coil was depowered and blew.

 

[Bluejets]

Sounds like it did not have a series resistor. Good you have working unit and perhaps learned that you need to check everything ( preferably before blow-up) no matter how simple the circuit seems. Can you clarify what you mean about powers the LED both ways around? Something not right there that needs your attention.

 

[rockrockmcrock]

I usually do, so it's frustrating in this case that I skipped fully checking the LED and believed the description.

By both ways round I mean that I suspect the LED has a bridge rectifier in there. It must be a seriously tiny one to fit the package.

Assuming I can get this to work with the relay without blowing the flip flop,I'm wondering if I should stick a 1.5 amp quick blow fuse in the Vo circuit in before the diode/relay. Might protect the ICB if the diode fails.

 

[duke37]

You could use a cheap (2N2222) transistor with a base resistor (1k) to drive the relay. That would protect the circuit whatever is being driven.

 

[rockrockmcrock]

Thanks that's a cracking idea - I'll dig around in my spares box and see what I have.

I'm presuming pretty much any NPN should cover this application as the current will be so low through the coil (40mA)?

The only trick will be finding a resistor with the right rating for the transistor in my box of bits (i have a bad record here!)

 

[rockrockmcrock]

Ok so I finally got to the bottom of this today - Thanks to everyone who helped.

It's a dodgy Chinese promise at fault, the flip flop is not rated anywhere near the promised 1500ma output! How do I know, well....

I redesigned the circuit to put the flip flop ICB behind a transistor and whacked a flyback diode on the relay. I tested the circuit without the flip flop ICB in place and it worked perfectly - the LED shone, the transistor opened and the relay clicked to on. The transistor base was pulling very low mA as expected. I even left it running for an hour to cover every base. I then connected the flip flop with just the LED which is on the base connector of the transistor. All ok. Finally, I connected the relay and....nothing happened. I removed the LED and the relay just buzzed a bit and then even stopped buzzing. Testing relay side I had the correct voltage but obviously not enough current to open the transistor. I'll start stepping down the resistor on the base now as a last resort, but bottom line is that this chinese flip flop, like the last one is dodgy - so don't buy one for any power application as it must be kicking out nearer 15mA not the promised 1500mA. Nothing like Chinese kit, they'll stick anything on the specs - I wouldn't have bought it only there's no comparable flip flop (one that resets to NO on power loss) available elsewhere that I could find.

 

[BobK]

It is impossible for an input to be at the correct voltage but not enough current. If it cannot supply the correct current, the voltage must drop. In other words, if you supply 3V to the base of the transistor from a battery and the relay closed, then the "flip flop" supplies 3V to the base of the transistor, it will also close.

Show us a circuit and the measurements you are getting, both working and not working. If find it hard to beleve the "flip flop" cannot supply enough base current to activate the relay if you have connected it correctly.

Bob

 

[rockrockmcrock]

Ok I'll try to do that tomorrow as I'd still like to get this working. My only remaining thought at present is that the base resistor value is too high, but it works fine without the flip flop in the loop and a direct supply. Flip flop output voltage is good too and happily powers the LED and the relay coil is behind the transistor.... Very confusing!