How to use PTC with coil

Hi to All,

I having the 24V Solenoid Coil to activate the small iron rod. Iron piece is inserted in between the coil, whenever the 24V given to it, the coil will energize and activate the iron piece for the displacement. For this action it needs 650mA current. I will give the 24v Supply for 1Sec only, after that it remains off until it gets next activate signal.

Meanwhile, coil wrongly activate for a minute or longer time, the coils tends to heat and its burnt. To avoid this, i have used the PTC along with the coil. It will sense the temperature ratio from 0 to 60'C, when the temperature exceeds the limit, it will open up the circuit and prevents the damage, this is what i have studied theoretically, when i implement in practical, what i had thought in theoretical it comes, bu my current entirely reduced, i have to work in 650mA, but it came to 20mA. This is will not enough for me to displace the iron piece. I have attached the image, how i connected the PTC.

What my question is,
1. I have to get 650mA when PTC connect with Coil.
2. PTC has to open when the temperature exceeds the limit(0 to 60)

I have selected PRG18BB330MB1RB and solenoid is SDO-0520S-24A33

anboli said:

coil wrongly activate for a minute or longer time, the coils tends to heat and its burnt.

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A gram of prevention is worth a kilogram of cure. Can you adjust or modify your control circuit so that it prevents wrong activation? If so then it should be unnecessary to install resistive devices, etc., which reduce current flow. Although you have experimented with a PTC, and it is needed in some situations, it does you no good in a place where it prevents proper operation.

Hi,

I agree with Brad. This is the better solution.

If you still insist to use the PTC...
Then you need to take care about self heating of the PTC

The current (650mA) is flowing through the PTC. It has a known (datasheet) cold resistance.
Heating power is: P = I x I x R.
Avoid self heating above some 100mW (depending on PTC size).

Klaus

It's not impossible to use a PTC for the intended purpose, but its parameters must be matched to coil current and activation time. The choosen type is simply unsuited, looking at the operation time curve in data sheet clarifies why.

You should however consider that a suitable PTC protection device will also reduce the coil current by some amount.

If anything happened in the board or some short happened means, the coil tends to activate. On that time how can i prevent the circuit to open? is there any other way to gets open?

Important thing is, if the coil gets 24V Continously means, it tends to tremendous heat. Please suggest any other idea.

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I have selected the 900mA PTC, then how it will get heat? @klaus
when i connect this with coil, the curent getting low. Why it is happening? whether i have to select very low resistance PTC?

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FvM said:

It's not impossible to use a PTC for the intended purpose, but its parameters must be matched to coil current and activation time. The choosen type is simply unsuited, looking at the operation time curve in data sheet clarifies why.

You should however consider that a suitable PTC protection device will also reduce the coil current by some amount.

Click to expand...

Please suggest what are the parameter i need to see for my application

I have selected the 900mA PTC

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No you didn't. You have seen the words "Max. Current 900 mA" and apparently stopped reading the datasheet at this point.

Getting 650 mA for 1 sec with this PTC connected is impossible both by ohms law and by the other datasheet specifications, partcicularly "Operating Time" curve. It shows that the PTC fuse trips typically in 1 sec if the current is larger than 80 mA.

As said there are probably other better suited PTC fuses.

Hi,

I have selected the 900mA PTC, then how it will get heat?

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I´v already given you the formula how to calculate power dissipation. Did you use it?

The given 900mA is the max. rating. Any current above those 900mA may kill the PTC.

The selected PTC has a hold current of 25/36mA no wonder it trips with 650mA.

I can´t find the datasheet of the solenoid. Please give a link.


Klaus

KlausST said:

Hi,



I´v already given you the formula how to calculate power dissipation. Did you use it?

The given 900mA is the max. rating. Any current above those 900mA may kill the PTC.

The selected PTC has a hold current of 25/36mA no wonder it trips with 650mA.

I can´t find the datasheet of the solenoid. Please give a link.


Klaus

Click to expand...

I have attached the 12V solenoid datasheet, it is similar to 24V solenoid, voltage only the difference.

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KlausST said:

Hi,



I´v already given you the formula how to calculate power dissipation. Did you use it?

The given 900mA is the max. rating. Any current above those 900mA may kill the PTC.

The selected PTC has a hold current of 25/36mA no wonder it trips with 650mA.

I can´t find the datasheet of the solenoid. Please give a link.


Klaus

Click to expand...

see my current flow is 650mA only, PTC has the 900mA current rating. Then how it will get heat. One thing i having the doubt, wether the PTC will work upto 900mA or it will start to work more than 900mA?

anboli said:

I have attached the 12V solenoid datasheet, it is similar to 24V solenoid, voltage only the difference.

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see my current flow is 650mA only, PTC has the 900mA current rating. Then how it will get heat. One thing i having the doubt, wether the PTC will work upto 900mA or it will start to work more than 900mA?

Click to expand...

And one more point for you, i have to place the PTC over the coil to sense its temp. If it is over heat, the circuit has to open, when its come to normal, the circuit has to close.

Apparently you have difficulties to apply ohms law to your circuit or to understand the datasheet details, parameters like trip current or the given time curve. In so far I'm helpless to proceed.

Here's the questioned PTC datasheet https://www.farnell.com/datasheets/2048985.pdf?_ga=1.115371334.1723756869.1452450976

I see this possible options:

- Use a PTC fuse with e.g. 0.2 - 0.4 A trip current that trips in about 2 to 10 sec with 650 mA
- Use a regular slow blow fuse with similar characteristics
- Use a temperature sense PTC or whatever at the coil and an electronic switch
- Use a redundant on-time limiter circuit