PTC Behaviour

[jayam000]

In the attached circuit there is PTC.
when connected in the PCB it becomes hot after some time & trips.
But if we put it outside, it works well for very long hours.
we connect PTC in series with one transformer wire but just outside same as required in the circuit.

Unable to understand why PTC becomes hot?
The current is also checked. It is nearly 1.1 amp with full load.
The diodes after PTC are 1N 400x type.
It is bridge rectifier, means will work for 1 amp current in normal use.
The PTC is of 2A but still gets hot while on PCB.
The bridge diodes r not hot.

[pacificblue]

The PTC belongs on the primary side of the transformer to limit the transformer inrush current at power on.

As current rises the PTC heats up and its impedance rises. As impedance rises, so does the voltage drop across the PTC, heating it up further.

On the primary side you only have a short high inrush current. Once that has passed, the PTC cools down again, its impedance sinks as does the voltage drop across it.

On the secondary side the current is much higher. If you don't take losses into account a 230/24 V transformer with 230 VA would have a nominal primary current of 1 A and a secondary current of 9,6 A. P = I² / R, so the PTC has to dissipate much more power on the secondary side, as it would have to dissipate on the primary side.

[AndrewT]

Hi,
before and after the PTC is either hot or much cooler.

Is it in the secondary in both cases?

Why have you fitted the PTC?

[jayam000]

before and after the PTC is either hot or much cooler.

The PTC on PCB is hot but PTC fitted outside PCB is cool.

Is it in the secondary in both cases?

Yes.

Why have you fitted the PTC?

It is in original circuit & getting tripped after becoming hot.
When installed out of pcb, no problem. Amplifier working ok.

This is PA amplifier.

[VivaVee]

pacificblue wrote

The PTC belongs on the primary side of the transformer to limit the transformer inrush current at power on.

This is incorrect An NTC is used for inrush current limiting and an appropriately rated NTC would work in this circuit position.

The PTC is used to protect against an over-current situation. If the steady state current is 1.1A then the usual design rule is to selct a PTC with a trip current of twice this current, i.e. 2.2A or higher at 25 deg Celsius. This needs to be derated if the ambient temperature is siginificantly different. Since this is a PA amp and you have fans I think it is reasonable to guess that it gets hot in your box. So derating will be necessary. This means a higher trip current spec. You need to measure the temperature inside your box.

The simple answer is that your PTC needs a higher trip current rating. When you select an appropriately rated PTC you will understand why they are required by UL to prevent fires but not so useful when it comes to protecting electronics

[jayam000]

Thanks VivaVee,
My question is very simple.
The PTC connected in the PCB after connector of ac supply on PCB. I just removed it from there, & connected it in series of ac wire outside of pcb. Due to this the in circuit PTC connection remains same. Just location changed. But in PCB PTC becomes hot & after changing location it is not that hot. The PTC works normal after the change of just location.

[VivaVee]

My guess, since you have not supplied a datasheet for the PTC or measured the temperature at the two PTC locations, is that the components on the PCB are running hot enough to raise the local temperature enough that the PTC trips. You did say that the load current is 1.1A - the LM317s will be getting very toasty.

So again, the PTC that you are using is too small. Increase the rated trip current, if you want to place it on the PCB.

But check/read the PTC datasheet to understand at what temperature/current it will trip (go high resistance).

[AndrewT]

Hi,
how close is the PCB mounted PTC to any other heat producing components?

It's maybe designed as an excess temperature trip, not as an excess current trip.

[jayam000]

You did say that the load current is 1.1A - the LM317s will be getting very toasty.

It is total load of 3 LM317 on powersupply. So it is not much. otherwise 1N400x diode bridge must hv been blown.

how close is the PCB mounted PTC to any other heat producing components?

Near the pcb are big rectifiers. They are more suspected heat generating component.
Yesterday i tested amplifier with PTC on pcb & PTC got tripped in say 10 minutes.
Then i tested the amplifier with PTC relocated & tested for more than 2 hours.
It was normal hot & no tripping.
Hence I am thinking to change the location of PTC in amplifer.

[AndrewT]

Originally posted by jayam000
Yesterday i tested amplifier with PTC on pcb & PTC got tripped in say 10 minutes.
Then i tested the amplifier with PTC relocated & tested for more than 2 hours.
It was normal hot & no tripping.
Hence I am thinking to change the location of PTC in amplifer.

if the PTC is tripping in 10mins, in it's normal location and it is intended as an over temperature trip, then running it for 2hours in this overheated condition could be disastrous. You will completely defeat the over temperature protection by relocating the PTC.