Transformer hot

[RMIM]

HI,

I made a circuit that worked very well on a 9V battery. The only problem was it only lasted 2days before the battery went flat and the circuit would not operate properly.

The circuit uses 6mA at rest and about 100mA when the relay energises which remains like this for 2hrs. (How many mAH is a typical 9v battery?)

Parts in my circuit
1A Regulator – 7805
14-stage ripple-carry binary counter/divider and oscillator - 4060
5vDC relay
Various resistors/capacitor/a transistor etc.
[My circuit is a 2hr timer. It keeps a relay energised for 2hrs when a push to make is hit]

Anyway I had the bright idea of running the circuit off the mains (240v ac), using a transformer and rectifier circuit.

Im new to electronics, I have never used a transformer before and I’m not sure if its working correctly. I believe the transformer is getting too hot!

I say this because it seems real hot to the touch (but maybe that’s normal). But it smells bad too. Also the brownish lacquer on the outer metal parts seemed to have melted. Plus I can here a buzzing sound too.

On the box of my transformer it says Primary 0-230v. Secondary 9-0-9v 250mA

I wired the primary winding to the mains. Brown to Brown (live). Blue to Blue (Neutral).
The secondary I used the black wire and ONLY 1 white 1 (and just cut the other white wire off) to a bridge rectifier (which was a W08).

Please can someone help – why is the transformer so hot? Is this normal? Do they need Ventilation? The whole thing is in a sealed biscuit tin.

(by the way my circuit works fine off the transformer - its just so damn hot. Im very worried about it. Its unplugged now.)
I have justed worked out the buzzing was from the lid of the box I made touching the transformer. It does not buzz with lid off.

Thanks.

 

[audioguru]

Why use a relay with such a high current coil?
Why use a relay? Why not use a low current transistor or triac?

You didn't attach a schematic of your circuit so we can't see what is shorting the transformer. The transformer is ruined from overheating, is dangerous now and should be replaced.

A 9V alkaline battery is made for supplying a few milli-amps, not 100mA. A carbon-zinc or "heavy-duty" 9V battery is made for almost no current.
Here is a graph of a 9V alkaline battery with a 42mA load, yours will last less than half as long:

 

[RMIM]

Why use a relay with such a high current coil?
Why use a relay? Why not use a low current transistor or triac?

You didn't attach a schematic of your circuit so we can't see what is shorting the transformer. The transformer is ruined from overheating, is dangerous now and should be replaced.

Thanks for the reply audioguru.

>>Why use a relay with such a high current coil?<<

New to electronics I guess. I just bought it from the shop only looking at the 5v coil as my circuit runs on 5v (but I know now the 4060 chip I bought does not have to have 5v – I thought it did. Used a 5v regulator could not be bothered to change it).

Actually (please don’t laugh) the 5v relay causes a 12v relay to energise (using yet another 9v batt)!
**broken link removed**
**broken link removed**

I originally had the 12v relay only. I used a transistor to try to get it to energise. It refused. As the circuit was 5v I guess. I even tried to connect the emitter to the 9v +ve of the battery and not the 5v regulated line. It still refused to energise. (I think I was working with a flat battery that day anyway).

Why the 12v relay you say? The 12v relay contacts are connected to a washing machine. The relay said 10A. But then I read something about inductive loads. This relay only does 3A inductive loads. I believe washing machines are inductive loads? How many amps will a washing machine draw?

Why do I want my washing machine on a 2hr timer you say? I was making a crude pay washing machine for a house that I let. You drop in £1 – the £1 hits a micro switch with lever – this then starts the 2hr timer, which gives about 1 wash cycle which is 2 hrs. (It all works like a dream by the way. I have only tested it with a fan not washing machine as Im not sure if my relay is adequate. It is only triggered by £1 as 5p 20p 1p are too light to trip the switch)


>>Why use a relay? Why not use a low current transistor or triac?<<
I have not read that chapter yet ;-) Are there Triacs that can handle washing machine currents?

As for the circuit diagram, I modelled it on this
**broken link removed**

Timer 2 is the circuit I used.

My rectified 9v (W08 bridge rectifier) supplies my 7805 which supplies my circuit and 5v relay. My 5v relay contacts then supply my 12v relay coil with power from the unregulated but rectified 9v supply.

So I am shorting my transformer with the 12v relay which I thought I could do (is that what you mean by what is shorting your transformer).

(I wanted to test the ac current but found out my VM only does DC current. And I have not tested the dc current with the transformer in place. I have only done that with the 9v batt.)

 

[audioguru]

You are a nooby so stay away from high current triacs. But the contacts on your relay won't last very long when driving an inductive washing machine.

I have never seen a transformer that gets hot with only a little more than 100mA as its load. So your rectifier's wiring or something else is shorting and overloading it.

 

[RMIM]

Do you think my 2 relays and circuit are drawing more that the stated 250mA for the transformer? My circuit should not draw more that 100mA. I will try and check tomorrow.

What do you mean by the transformer is now dangerous? Is that because some of where the brown coating has melted away it is now live? And/or it is now internally damaged. Circuit still seems to be working fine with the transformer.

 

[audioguru]

Do you think my 2 relays and circuit are drawing more that the stated 250mA for the transformer? My circuit should not draw more that 100mA. I will try and check tomorrow.

I don't think your 5V relay has more than a honking huge 150mA coil, does it? Maybe your 12V relay's coil draws much more than 100mA.

What do you mean by the transformer is now dangerous? Is that because some of where the brown coating has melted away it is now live? And/or it is now internally damaged. Circuit still seems to be working fine with the transformer.

The insulation has melted and now the transformer is a shock hazzard and a fire hazzard.

 

[RMIM]

Im new to this transformer business but there are only a few wires and I can explain what I have done – Im sure you will pick up on any mistake if I have made any.

Bought a transformer that says Primary 0-230v. Secondary 9-0-9v 250mA on the box.

It has a total of 5 wires colour coded.

The primary wires are the same colour as we have here in the UK in our plugs. Brown for live and Blue for neutral. So I connected them to live and neutral.

Now comes the bit im not too sure about. The transformer is centre tapped – secondary has 3 wires. 1 black 2 white. I cut 1 white off and just used the other white and black. Measured Ac voltage between black and white and got about 12v.

I bought something called a W08 it has 1 long leg with + stamped above it on the case. It has the symbol ~ stamped on the case above 2 legs. I connected the black and white wire to the legs with the ~ symbol (did not bother with which way round).

The other 2 legs I treated as +ve and –ve.

As I have said everything works fine in the circuit – only transformer seems hot.

 

[RMIM]

I don't think your 5V relay has more than a honking huge 150mA coil, does it? Maybe your 12V relay's coil draws much more than 100mA.


The insulation has melted and now the transformer is a shock hazzard and a fire hazzard.

So you saying the only reason for a transformer to heat up like that is if it’s drawing more than what was intended? mine says 250mA - and it must be drawing more than 250mA to get hot like this?

my 12v relay also has a diode on it. When I first started I hooked the power to it the wrong way and there was a lot of relay buzzing. Do you think this diode has been damaged and it is this causing the excessive current draw?.

 

[audioguru]

Yeah, the transformer is wired correctly and is overheating because something draws too much current.
The diode that was connected backwards might be drawing a lot of current. Replace it.

 

[RMIM]

Yeah, the transformer is wired correctly and is overheating because something draws too much current.
The diode that was connected backwards might be drawing a lot of current. Replace it.

Is it not possible just to bin that diode and not have it. As that relay is not connected to a transistor, it is only connected to the contacts of the other relay.

I thought the diodes were there to protect things like transistors.

 

[audioguru]

Without the diode across the coil of the 12V relay, there will be hundreds of volts on the contacts of the 5V relay that is close to the sensitive low voltage IC that will be damaged if the high voltage arcs.

 

[RMIM]

well I ran a test with the diode still in place - the whole circuit never draws more that 100mA. (both relays on)

I only ran it for about 2mins. Very warm to the touch (Only touched it after I unplugged).

I tested it with a neon screwdriver - I touched the metal casing where the brown coating had melted off, nothing. Tested it again later and the neon faintly came on. What does this mean?

Could I have some intermittent problem with the diode? Or was the transformer faulty in the first place? (it was new).

 

[audioguru]

When the diode was backwards then it and the transformer could have been damaged. Replace both.

 

[RMIM]

When the diode was backwards then it and the transformer could have been damaged. Replace both.

I see. Thanks for the help audioguru.

Also you said triacs are dangerous for the beginner (AC) - how are they more dangerous than using relays? (with ac)

Coming into contact with a 240v ac live wire is not going to be fun. But will you experience similar discomfort coming into contact with the 9v ac? (not that im ever planning to touch anything like that).

 

[audioguru]

You won't feel 9VAC. 240VAC can kill you.