Question about relay

Ages ago I posted a transformer question related to a demo box I had from an old welding machine, inside is a power board with one of the biggest heat sinks I have seen. I also have some server fans with extreme high output airflow. For a laugh when I get back Friday I will put a 220 on the heat sink and see how much current I put through it and still keep it at 25C. I am curious to see just how far you have to goto to meet the specs in the sheets.

The other point I didnt mention earlier was the bit about package limitation, I always thought that meant the figure was valid but the package stopped it reaching that. I wonder if others or the manufactures mean it would be higher if the 220 wasnt being used? Anyway time for some fun Friday , or batch testing whichever you prefer

spec said:

Hadn't thought of that LG.

spec

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The trick is too have a semi plausible excuse ready if the boss turns up. Thats why I burn the reed grass in our fields so I kill the sheep ticks . Nothing to do with liking fires

We never got asked what we were doing. Those were the good old days. We would think nothing of knocking off for lunch at noon and coming back at around 4pm. Mind you, we also thought nothing of working 84Hr weeks.

Someone was always doing something weird. One guy had a golf practice net set up in his lab and used to practice swinging all day (he was developing a synthetic system).

Another guy lived in the attic. He was working on voice recognition before the days of integrated circuits. Most of the factory had to go up and see him to test if their voices could be interpreted. He was eccentric and could only speak in grunts because he was so pre occupied with his work. He had dummy heads all over the place and racks of orange dekaton tubes spinning around in the dark. The attic was dusty, dark, and spooky with spiders webs and hundreds of spiders. He was an irritable old git too, but one of the companies top scientists.

In another lab they spent all their time blowing up large and very expensive capacitors.

There was a whole factory floor taken up with marine radar development and production. The floor was literally stewed with components which were swept up once a week and put on the scrap heap. There was dish antennas all over the place and bits of boat here and there. Most of my home projects were built with marine radar parts: output transistors and beautiful Sprague and GE reservoir capacitors especially, for HiFi amps. Sometimes they would dump complete marine radars with magnatons, and boxes of brand new components of all types.

spec

Little Ghostman said:

So..and please please correct me if I am wrong, the datasheet value is based upon theoretical limits of junction temperature and package rather than on real world application. Pass 75A @10V through that with as big a heat sink as you like and I cant see how you are going to get anywhere near the stated 25C. I could be wrong but I have a Textronic multi tester with a massive Transistor (cant remember the package name now!!! but large round type all metal), this sits on a heat sink 12 inches across and 9 inches high, its really huge with large fins. It passes 45A and yet the transistor gets hot to touch. So how is a To220 going to handle 75A 10V and stay at 25c in the real world?

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I agree with you, that it may be theoretical, and as I said earlier, I accept it, but I don't push the limit either. The main factor in determining the current the silicon can handle is the Rds(on), and thus the voltage drop & power dissipated, on your 45 amp TRANSISTOR, it probably has a higher vce than the FET has for vsd, so the heat generated is less in the FET. The heat generated at max package current for the IRF2805 is 75A X .0047Ω=26.4375W and the FDB016N04AL7 has a Rds(on) of 1.16,so it will generate about 25% of the heat of the IRF2805.
I don't know what determines max case current, maybe melting leads (as an electrician, I was taught the #12 wire melts at 212 amps), or what determines max silicon current, but it must be under the ideal condition that we don't get to have.
I have had FETs melt their solder and I have seen melted silicon after cracking the case off the tab. Ain't these things fun!!!!
Jeff, AKA Kinarfi

Little Ghostman said:

For a laugh when I get back Friday I will put a 220 on the heat sink and see how much current I put through it and still keep it at 25C.

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Afraid the answer is no current, in theory that is. The thermal budget illustrates that:

MOSFET Maximum Junction Temperature (170DegC or 150 degC) → Thermal Resistance Junction to Case (0.5 degCW) → Thermal Resistance Case to Insulating Washer & Insulating Washer to Heatsink (0.75 DegCW) →Th Resistance Heatsink to Ambient Air (2 degCW)→ Ambient Air Temperature (50 Deg C).

Taking the typical thermal resistance values in brackets, the total thermal resistance from junction to ambient is: 0.5 + 0.75 + 2 = 3.25 Degrees C W

Taking the maximum junction temperature to be 175 Deg C, the differential temperature is 175- 50= 125 Deg C

From this it follows that the maximum allowable power dissipation of the MOSFET is 125/3.25 = 38.46W

Suppose you had a 100A, 120W MOSFET and in your circuit you had 24V between its drain and source, in an amplifier for example, the maximum current you could allow is 38.46W/20V= 1.923 Amps.

On the other hand suppose you were using the MOSFET as a switch:

Say the resistance between drain and source of the MOSFET were 10mili Ohms, at 25 Deg C junction temperature. At a guess the resistance would increase to 50m Ohms at a junction temperature of 175 Deg C .

Thus, the maximum safe drain current would be found from, I * I * 50 mili Ohms = 38.4W. Thus I = 27.71A

It would make no difference how high the maximum current or power is shown on the MOSFET data sheet, it could only handle 38.4W and as a result it can only ever switch 27.71A.

I meant mosfet and was talking package size but got lazy, All I was going to do was see how much cooling I could throw at it and keep it at 25C. I am thinking heat sink front and back and high air flow fans . Its just a laugh, I might try evap cooling as well or use the acetone type heat exchanger

Little Ghostman said:

I meant mosfet and was talking package size but got lazy, All I was going to do was see how much cooling I could throw at it and keep it at 25C. I am thinking heat sink front and back and high air flow fans . Its just a laugh, I might try evap cooling as well or use the acetone type heat exchanger

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If the ambient air temperature is 25 deg C, the case can only be at 25 deg C if the item housed in the case, whatever it might be, is not generating any heat. Even mounting a TO-220 case on to a heatsink directly without an insulating washer forms a thermal resistance, a low one, between the case and the heatsink, even a heatsink with a 0 degree C watt rating.

spec

spec said:

If the ambient air temperature is 25 deg C, the case can only be at 25 deg C if the item housed in the case, whatever it might be, is not generating any heat. Even mounting a TO-220 case on to a heatsink directly without an insulating washer forms a thermal resistance, a low one, between the case and the heatsink, even a heatsink with a 0 degree C watt rating.

spec

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Ambient 25C!! I am moving there!! Ambient here is 12C at the mo!! your jumping guns again wait and see .

Would a 1N4148 over coil be fine?

Hi,

You mean to clamp the inductive kickback? 1N4004 or similar is much better, more hardy, stronger leads.

MrAl said:

Hi,

You mean to clamp the inductive kickback? 1N4004 or similar is much better, more hardy, stronger leads.

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Thank you! I also have 1N5404 home. Very thick leads on them. Maybe overkill?

Don't forget that MOSFETs have a large parasitic Schottky diode connected between the source and drain that will catch any over swing voltage in certain configurations. For example a positive supply rail with a PMOSFET as a high side switch.

spec

Rorut said:

Thank you! I also have 1N5404 home. Very thick leads on them. Maybe overkill?

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Hi,

Yes maybe an overkill, but will do the trick without buying anything else

spec said:

Don't forget that MOSFETs have a large Schottky parasitic diode connected between the source and drain that will catch any over swing voltage in certain configurations. For example a positive supply rail with a PMOSFET as a high side switch.

spec

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Hi there,

I'd be careful with that. When an upper side mosfet turns off with an inductive load, the kickback goes negative not positive. the internal diode will catch positive going spikes but not negative going. With that in mind, the upper mosfet diode would catch the lower mosfet's kick back if they were connected as a half bridge, and the lower mosfet's diode would catch the upper mosfet's kick back. So it's always the opposite device's diode acting as the catch diode

MrAl said:

Hi there,

I'd be careful with that. When an upper side mosfet turns off with an inductive load, the kickback goes negative not positive. the internal diode will catch positive going spikes but not negative going. With that in mind, the upper mosfet diode would catch the lower mosfet's kick back if they were connected as a half bridge, and the lower mosfet's diode would catch the upper mosfet's kick back. So it's always the opposite device's diode acting as the catch diode

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Thanks MrAl- too many balls in the air at the same time.

spec

Rorut said:

Thank you! I also have 1N5404 home. Very thick leads on them. Maybe overkill?

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Overkill.
Look at post #19 in this old forum thread. The peak current in the snubber diode is whatever was flowing in the coil at the instant the switching device turns off. The duration of the current pulse is very short, so even a 1N4148 will snub a 1A load. The 1A is well within the "non-repetitive surge rating".

If a MosFET is used as a switch, the body (parasitic) diode does nothing to act as a snubber; a diode (or RC network) is always added...

https://www.microsemi.com/document-portal/doc_view/14695-defining-diode-data-sheet-parameters
**broken link removed** 1N4148 data sheet

Shows the meaning of IFsm. Ie testing to destruction and that is at a junction temperature of 45 deg C a figure that can easily be exceeded in a practical circuit

It is impossible to establish what the maximum up catching diode current is or its duration without knowing, in addition to the resistance, the inductance and the capacitance of the relay coil.

The IN4148 is a high speed small signal, low capacitance, low conductance diode and is totally unsuited as a catching diode for a relay. I have seen these used before across relays and they have been unreliable. Besides which, why sail so close to the wind when it is totally unnecessary. Much better to use 'overkill', especially as there is no penalty.

It has already been established that the intrinsic diode in a MOSFET does not catch over swing, but it does catch under swing.

spec

spec said:

...It is impossible to establish what the maximum up catching diode current is or its duration without knowing, in addition to the resistance, the inductance and the capacitance of the relay coil.

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Only the DC resistance of the relay coil, which is what establishes the current that is flowing in the coil just prior to interrupting the current, is relevant in picking the snubber diode.

... Much better to use 'overkill', especially as there is no penalty.

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I have used 1n4148 diodes as snubbers on dozens of small 12Vdc relays (automotive sugar cube relays whose coil current is 250mA or less) and never had one fail...

MikeMl said:

Only the DC resistance of the relay coil, which is what establishes the current that is flowing in the coil just prior to interrupting the current, is relevant in picking the snubber diode.


I have used 1n4148 diodes as snubbers on dozens of small 12Vdc relays (automotive sugar cube relays whose coil current is 250mA or less) and never had one fail...

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Hello Mike,

What is the price of the 1N4148 vs the 1N4002 or similar?
That is, the price you pay yourself.

I've seen all kinds of crazy things done electronically, some work some dont, but mechanical stability also comes into question sometimes. For example, a 1/2 watt resistor is more durable than a 1/4 watt resistor because of the stronger leads.

But other more serious mistakes i've seen include trying to clamp the two windings of a center tapped transformer to +Vcc when the center tap is grounded and the two windings are driven with the collectors of two bipolars. Very bad
This also might be tempting to do with a stepper motor, the kind with center tapped windings. Again very bad

With the higher power circuits it is also not uncommon to see external diodes added to the mosfets in parallel with the integral body diode to remove some of the heat from the mostfet package.