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.
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.
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.
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.
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?
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?
The PTC on PCB is hot but PTC fitted outside PCB is cool.
Yes.
It is in original circuit & getting tripped after becoming hot.
When installed out of pcb, no problem. Amplifier working ok.
This is PA amplifier.
pacificblue wrote
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
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
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.
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.
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).
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).
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.
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.
It is total load of 3 LM317 on powersupply. So it is not much. otherwise 1N400x diode bridge must hv been blown.
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.
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.jayam000 said:
it is very difficult to provide any further help until you can identify the PTC that you are using. But I'll try.
Check "PTC (positive temperature coefficient) resistor" on Google - it will help you.
Look at the manufacturer's datasheet for the PTC.
It might help to understand how the PTC works. So here goes ...
It is a resistor whose resistance increases as the temperature rises. So it can be used to measure temperature and/or provide a means to trip a circuit at a particular temperature. Used to measure temperature, it is very simple in operation.
But it can also be used in an indirect way to limit current. As the current through the PTC increases, the temperature of the PTC increases, and, as before, the resistance increases. The PTC can be designed so that at a particular combination of temperature and current, the resistance increases rapidly. This increased resistacnce is then used to protect the circuit.
But note that it is the temperature that ulitmately determines the 'trip' point. So if you put the PTC in a hot location, it will trip at a lower current then if you put in a cold location. This is exactly what you are observing.
You must decide what you are trying to achieve. You only thing that you have stated so far is that you do not want the PTC to trip - in this case the easy answer is to remove it!
Check "PTC (positive temperature coefficient) resistor" on Google - it will help you.
Look at the manufacturer's datasheet for the PTC.
It might help to understand how the PTC works. So here goes ...
It is a resistor whose resistance increases as the temperature rises. So it can be used to measure temperature and/or provide a means to trip a circuit at a particular temperature. Used to measure temperature, it is very simple in operation.
But it can also be used in an indirect way to limit current. As the current through the PTC increases, the temperature of the PTC increases, and, as before, the resistance increases. The PTC can be designed so that at a particular combination of temperature and current, the resistance increases rapidly. This increased resistacnce is then used to protect the circuit.
But note that it is the temperature that ulitmately determines the 'trip' point. So if you put the PTC in a hot location, it will trip at a lower current then if you put in a cold location. This is exactly what you are observing.
You must decide what you are trying to achieve. You only thing that you have stated so far is that you do not want the PTC to trip - in this case the easy answer is to remove it!
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