I am working on a project which uses a PTC heating element connected to a 24VDC power supply. I want to current limit the circuit without using a fuse or circuit breaker. What I would like to do is set a maximum current of 2A, but using a fixed resistor creates too much heat within the control box. I have the capability to use my PIC to add some control, but I am not sure the safest, and most reliable way to do this. The heater element stops working if the voltage drops below 12V, and the power supply shuts down if my current exceeds 3 Amps. I know I could get a bigger power supply, or heat sink a fixed resistor in series with the PTC element, but even though that may be the easiest fix, I am looking for something a little more sophisticated. I am a bit of a novice, but I have had some success with my projects in the past while experimenting with new components, so I am open to any and all suggestions. Please help, thanks.

 

Check this site http://en.wikipedia.org/wiki/Current_source

See the first circuit where it says 'Simple transistor current source'. This an adequate circuit for you.

 

I don't think a simple constant current sink is going to be adequate for this job, because I think you require variable current, up to 2 amps.

Is this correct?

Further if you use a PNP transistor, not an NPN one as shown you will obtain a ground referenced current source.

Give us more detail abour your requirements and we can help further.

 

I wish I could provide you with full detail, but due to my limited electronic abilities, and my "trial and error" methodology, I can only give you this information.

My PTC heating element draws 100mA in ambient temperatures at 24VDC, therefore, I am assuming a max resistance of 240 ohms. This resistance drastically changes if the element is not allowed to self heat, for instance if it is partially submerged in ice water, the current draw is only limited in practicality by the supply. The inrush current is always high, but is not catastrophic to the application, however sustained currents of near 3A will cause my switching power supply to begin reducing the voltage and will effectively shut down the system. I only need a current "limiter" for high end regulation, and not a constant current through the element. I almost think an NTC element such as a thermistor wired in series with the PTC element, yet not exposed to the same heat dissipative atmosphere, would do the job. Any thoughts on what that might cause, or how effective that might be? I would like to keep any working current levels to 2A or under. Thanks for the help.

 

I wasn't after full details, just enough, which by and large you have given.

Your idea of an NTC device could work, depending. It rather depends on whether you can accept the constriction they will also place on normal operation i.e the final current will always depend on the balance between the PTC and the NTC . If you are controlling the PTC device in some way this will also alter the NTC resistance, making the overall response difficult to predict and control.

If you simply want to limit the initial inrush current and don't care once the system has settled down to a steady state an NTC would be OK.

If , however you want full control form say 20 milliamps up to 2 amps then you need something more sophisticated.

I will look out something tomorrow, or perhaps another can provide a circuit sooner. You should be aware that whatever method you choose you will loose some of the 24 volts in the limit device.

Meanwhile please let us know what sort of component inventory you have accces to.

 

Thanks again for all of the advice and support this forum offers. I can live with a voltage less than 24, as long as it is greater than 12v. The biggest problem I have in the system is that if it is inactive for a long period of time, the heating element is subjected to sub-freezing temperatures. If you initiate startup while the element is significantly below freezing, the initial current draw is quite large. I thought of a resettable fuse, but I don't think the element will recover in a timely fashion if it is constantly shutting off and then turning back on. I really just need to limit the maximum amount of current, and not control the low end. If I put the NTC in series and locate it remotely from the PTC unit, then the temperature change induced by the system won't have great effects on the NTC, correct?

Thanks for the help, and your patience.

 

Go through this app note. You can use a dedicated IC, or construct your own using a RR opamp with low offset. Most major semiconductor manufacturers have current monitors for low or high side monitoring. Search digikey for "Current Sense"

http://www.zetex.com/3.0/appnotes/apps/an39.pdf

 

A simple light bulb would come close, I would think.

 

A simple light bulb would come close, I would think.

I am not sure I understand how that would work, sorry, can you elaborate a little.

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Go through this app note. You can use a dedicated IC, or construct your own using a RR opamp with low offset. Most major semiconductor manufacturers have current monitors for low or high side monitoring. Search digikey for "Current Sense"

http://www.zetex.com/3.0/appnotes/apps/an39.pdf

Thanks for the link, how would the sense resistor not overheat? My circuit is 24VDC and I want to detect anything above 2A. I don't think I understand why it wouldn't just try to pass 2A through the small value resistor which would create a lot of heat. I am missing something, aren't I??

Thanks guys!

 

Originally Posted by Bill_Marsden
A simple light bulb would come close, I would think.

Yes how would this help,cold incandescents draw large inrush currents.

Thanks for the link, how would the sense resistor not overheat?

Using a 0.05 ohm resistor would only dissapte 0.2W.
P= EI. 0.1V * 2A = 0.2W.

You may be able to do something like this for inrush limiting.

An example of using a PFET and an OPAMP for inrush suppression.

http://www.nxp.com/acrobat_download/applicationnotes/AN93008_EIE.pdf

Accuracy is dependent on the time and money you are willing to spend,maybe you could control the gate of a PFET keeping it in it's linear region when required (cold start),and a shutdown for a sustained short circuit,using your existing up.

Using mosfet gate control for inrush current limiting is nothing new you should be able to find more examples on the web.

 

The easiest way to limit your inrush current is to use an LM150/LM350 3A adjustable positive regulator with a current selection resistor between the output and adjust terminals. This is documented in National Semiconductor's datasheet for those part numbers; see page 10 and the Electrical Specifications in the beginning of the datasheet.
Iout=Vref/R1
Conversely, R1=Vref/Iout.
Vref is typically 1.25v, with extremes from 1.2v to 1.3v. Therefore, R1 may be from:
1.2/2 = 0.6 Ohms to 1.3/2 = 0.65 Ohms. You should use a 5W resistor.
This will typically reduce your maximum voltage available to the heater by 3v (so, 21v) - but this won't make a difference. If your heater's resistance is less than 10.5 Ohms, it's current will be limited by the LM150/LM350.

If your expected inrush current will take more than a few moments, be sure to heatsink the LM150/LM350. It has thermal protection, but this will stress the regulator and cause early failure.

 

Yeah good thinking Wookie.

Alternatively my sketches show some simple alternatives.

As I said it all depends upon what components you can get your hands on.

The pass transistor TR1 is operated in saturated mode so dissipates the minimum power in normal operation.
When the current is such that the voltage across R2 plus the base emitter voltage exceeds the diode forward drop the transistor desaturates and the output current is limited to the base current times the gain.
Depending upon the components chosen it may be necessary for D1 to be two diodes in series.

 

Thanks for all of the help gentlemen.

I was looking through my meager stash of components and came upon a tde1737 which appears to be a driver chip with current limiting capability. Is anyone familiar with this device? I don't disagree with any of your previous suggestions, it's just that this is a component that is already in-hand. I am not opposed to ordering some new components, but if I don't need too, then it's one less thing to worry about.

Thanks again.

 

any help with the schematic is appreciated.

 

According to page 3 of the tde1737 spec the absolute max output current is a tad short at 1000mA. Sorry.

http://www.st.com/stonline/products/literature/ds/1515.pdf

Will suggest a few components for my NPN circuit. This can't be used for negative ground, if the negative terminal of the 24 volt supply is also grounded.

Do you prefer american,(2N..0) European (BD...) or Japanese (2S..)?

 

Thanks for reviewing the spec sheet, I passed right over it, being more concerned with the functionality rather than the actual values. But, I would assume that a driver chip like this would tend to perform the functions I am looking for, only at a lower current? American component specs would probably be best, I usually order from Allied or Digikey, but occasionally a nice trip to RadioShack yields an urgent find. Thanks.

 

Let's start wth this extract from the Mullard power suply design manual. the circuit is actually atributed to RCA.