Question about relay

[Rorut]

Hi,
How many amps can a relay like this handle? Not over coil but load?
Thoose leads looks so tiny. Can it really handle 7 or 10 Amps or did I miss something.
My load is between 16-19v DC, 3-5A
http://www.sunshinele.com/pdf/pcb-relay-T73.pdf


Thank you!

 

[MaxHeadRoom78]

What type of load? AC? DC? Resistive? Inductive?
They typicaal use those to switch elements in stove now, and seem to be capable of switching high resistive loads.
Max.

 

[Rorut]

What type of load? AC? DC? Resistive? Inductive?
They typicaal use those to switch elements in stove now, and seem to be capable of switching high resistive loads.
Max.

Its DC. I want to use it in my boombox. Power is coming through from a battery or a DC adaptor to a class d amp

 

[MaxHeadRoom78]

Should be OK.
Max.

 

[Rorut]

Should be OK.
Max.

Thanks max! But whats the limit on it for my specification?

 

[MaxHeadRoom78]

If DC then it is shown as 10amps at 28vdc for both Chinese standard CQC and CE.
Well within 16-19v DC, 3-5A
Max.

 

[dr pepper]

I've thought that too, some 30a ones I have use tiny leads, but I spose if the length is short there isnt a lot of resistance.

 

[large_ghostman]

I have some of those I got them cheap in a batch of 50 for about £6 ages ago, I found they failed at 12V and 5.5A, mine were ebay specials so could have been fakes, the contacts on mine welded together

 

[Diver300]

I have some of those I got them cheap in a batch of 50 for about £6 ages ago, I found they failed at 12V and 5.5A, mine were ebay specials so could have been fakes, the contacts on mine welded together

If there is a diode across the coil, that can slow the relay release time, so it can result in contact arcing, and then welding.

A resistor, varistor or zener can be used to absorb the coil energy instead, which speeds up the release while keeping the voltage transient manageable.

 

[kinarfi]

If the leads of a TO220 can handle 75 amps (package limited), these relays should certainly handle at least as much. IMHO

 

[Rorut]

If there is a diode across the coil, that can slow the relay release time, so it can result in contact arcing, and then welding.

A resistor, varistor or zener can be used to absorb the coil energy instead, which speeds up the release while keeping the voltage transient manageable.

So just soldering a zener across coil leads will make some improvements? Important zener has same voltage specification as coil? Or any other requirements to take notice about? Thanks

If the leads of a TO220 can handle 75 amps (package limited), these relays should certainly handle at least as much. IMHO

75Amps!? Sounds crazy

 

[dr pepper]

Not quite as simple, for a r/c you'd calc the resistance so that its equal to the relay coil dc resistance, and the cap needs to hold the same energy as the inductance of the coil.
That way when the coils switches off the current stays the same thanks to the resistance, and that keeps up till all the energy in the coil is shoved into the cap absorbing the spike, this too can hold the relay on longer.

 

[kinarfi]

 

[large_ghostman]

Isnt that slightly misleading? Theoretical limit for To220 maybe 75A @25C BUT isnt this based on an industry theoretical infinite heat sink? I have seen designs where the traces are very thick and wide and the case soldered directly to the copper, but even then at 12 V the current was way below 75A or the solder melted.

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?

I am not arguing but I always assumed the figures were based purely on a theoretical perfect heat sink rather than what really happens. Look at a CPU and the number of pins and size of package, then consider the heat sinks used with power fans on and yet they still reach 50-80C without much effort and thats at less than 3.3V through them, no idea how many amps they are pulling but my point is I cant see you putting 10V and 75A through that package and not melting the solder no how big the heat sink. But I could be wrong


edit

Apparently I am wrong but you need one hell of a heat sink!

 

[ChrisP58]

The current capacity of a pin is all about temperature rise. If the pin is short, and the two ends are thermally connected to something that will act as a heatsink, then you can push more current through it than you could if it were a long pin of the same cross section.

So for the relay, the current rating assumes that the pins are soldered to PCB traces that will carry away whatever heat is generated by the current through the pins.

For the TO-220 package, it's current rating is based on you holding the case temperature at or below 25C, while it's generating whatever heat that the drain current produces. Which, unless you're running it outdoors in Antarctica, probably means cooling with cold water or refrigeration.

 

[large_ghostman]

If the leads of a TO220 can handle 75 amps (package limited), these relays should certainly handle at least as much. IMHO

The difference between the two is the contacts, the higher the A the more arking as they close and open, that isnt something you get inside a Mosfet package. The ones I got from ebay were likely copies anyway

 

[large_ghostman]

The current capacity of a pin is all about temperature rise. If the pin is short, and the two ends are thermally connected to something that will act as a heatsink, then you can push more current through it than you could if it were a long pin of the same cross section.

So for the relay, the current rating assumes that the pins are soldered to PCB traces that will carry away whatever heat is generated by the current through the pins.

For the TO-220 package, it's current rating is based on you holding the case temperature at or below 25C, while it's generating whatever heat that the drain current produces. Which, unless you're running it outdoors in Antarctica, probably means cooling with cold water or refrigeration.

That was my point Chris, the figure in the data sheet is a bit misleading, it give the impression you can do it no problem, but like we both have said to keep it at 25C is going to take some serious cooling

 

[spec]

You have all brought up a very important aspect about the specifications of components and how they relate to a component's application. As has been said, the ratings can be very misleading.

There is a surprising number of aspects to consider in the application of a relay and the closely associated simple switch, as has been mentioned already. In fact, contacts have caused more trouble than all the most complex integrated circuits put together, in my experience anyway. As well as restive loads there is the added complication of reactive loads, both capacitive and inductive, but on top of that there are low currents to consider. If the contacts are not of a suitable material, notably gold, too lower current will result in the relay contacts going open circuit because there is insufficient current to keep the contacts clean (the same with battery contacts on most remote controllers).

Some contacts have a high current contact with a gold flash over them. This type can be used for low currents, but once they have been used for high currents the gold is stripped off and they are no longer suitable for low currents.

The data sheets for power transistors can be especially misleading and the maximum power rating of a transistor is pretty much meaningless and in only a marketing ploy. The maximum current rating is similar. kinarfi posted a very good example, the IRF2805 NMOSFET, but some of the new breed of MOSFETs are even more extreme. For example, https://www.fairchildsemi.com/datasheets/FD/FDB016N04AL7.pdf

There are a number of factors that make these power and current limits fanciful: junction to case thermal resistance, case to heatsink thermal resistance, maximum junction temperature being one set. Secondary breakdown characteristics being the other main limitation. But then there are the basic problems of a high current which have already been mentioned: bonding wire capability, connections to the terminals, and the actual current capability of the PCB traces/wires. Even the solder melting as LG says.

In a juvenile moment we used to tack solder three wires to the pins of an inverted 2N3055 and see how long it would take to power itself off by melting the solder, or fusing and shorting out the power source.

spec

 

[large_ghostman]

In a juvenile moment we used to tack solder three wires to the pins of an inverted 2N3055 and see how long it would take to power itself off by melting the solder, or fusing and shorting out the power source.

spec

No thats called batch testing when asked what your doing, Or fun if no one asks

 

[spec]

No thats called batch testing when asked what your doing, Or fun if no one asks

Hadn't thought of that LG.

spec