irf530 and relays

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bratt2032

New Member
Hi !

First : please excuse me for my english but i m french !

My project is to control a aquarium : lights, neons, heaters... with a picaxe system.

I ve already made little robots with picaxe and coding my project is not a problem for me (i know Delphi, Python, Basic...)

But, i've never used medium or high power, so i'd like you to help me to choose good drivers :

To drive neons and heaters (just on/off) i think this is ok ...but i'd like to know the difference between this one and this other one. If these relays aren't a good idea, please tell me. (i need 250W max, under 220VAC)

I'd like to drive these relays with a ULN2803...is it good ?

I'd like to control 12VDC lights with pwm (max 50W, 4A) : i wanted to use a IRF530. Is it ok ? do i need to cool it with a thermal spender ?

Thank you.
 
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eclectic

Moderator
@bratt
Welcome to the Picaxe forum.

Please post your question in the ACTIVE forum.

This area is for finished projects.

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BeanieBots

Moderator
Moved to Active Forum.

Those relays would do the job as far as electrical spec is concerned but you could be giving yourself problems by using them.

1.They are PCB mount.
That means having mains on a PCB. To be avoided whenever possible.
There are socket relays available which have screw terminals.

2.They are 6v. Your PICAXE will run off 5v.
It probably will switch from a 5v source, however, if you intend to use a ULN2803, remember that a darlington driver drops about 1.2v. That would mean only 3.8v to trip a 6v relay. Most unlikely.
You could of course use a 7.2v supply for the relay coil, but that adds an extra complication which can be avoided by using 12v coil relays. (you already have 12v available for your lamps?)

3.Switching mains lamps (especially flouroescent) makes a lot of electrical noise.
You could use a solid-state relay with zero-crossing.
Such a realy would be ideal for the heater control but it won't switch flouroescent lights. I don't know how it would behave with neons.

Driving an IRF530 with PWM to control 4A will require a proper FET driver.
If a proper driver is used, only a very small (if at all) heatsink will be required.
Without a proper driver it will overheat even with a large heatsink.
 
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Andrew Cowan

Senior Member
The difference between the relays is that one is DPDT, and one is SPDT.

Doube Pole Double throw = two isolated switches, with a common pin that can be be connected to a Normally open pin, or a Normally closed pin.

Single pole Double throw = one switch with a common pin that can be be connected to a Normally open pin, or a Normally closed pin.

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bratt2032

New Member
Hi all !

Thank you for your answers :

MPep : all helps are welcome ! even if i use 220VAC heaters... i like to "store" docs, it can be useful.

Andrew : thanks, now I know the between DPDT and SPDT relays.

BeanieBots : i have a few questions for you again !
I understood the problem with relays : i must take care of supply for the relay coil. If i use 12v coil relays, could i have problems with neons ? Should I use a solid-sate relay ? Why zero-crossing ? (I thought zero-crossing was used in order to syncronize pwm with main to drive a 220VAC light with a triac ?)

I'm not sure I understand your sentence about IRF530 (remember I'm french...lol) : what do you mean when you write "a proper FET driver" ?

Thank you again
 
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Andrew Cowan

Senior Member
Your english is very good.

A FET driver is needed if driving a FET at load.

While a FET has a very low resistance while it is fully on, it has capacitance, and so takes a while to turn it fully on. This 'half on half off' state is known as linear mode. It will have several ohms of resistance, and thus heat up.

When you are using it as an on/off switch, a few microseconds of heating don't matter. However, when PWMing it quickly, it all adds up, and makes lots of heat. A good driver will push in the charge quickly, and pull it out again afterwards.

An example would be the MC4427 from microchip.

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BeanieBots

Moderator
About the FET driver, what Andrew says.
About switching neons, as I mentioned earlier, I don't about neons.

Zero crossing switching is exactly what it says.
It only switches when the mains is at zero.

Yes, zero-crossing can be used to synchronise the timing of phase angle triac fire for dimming lights, but a zero-crossing switch will ONLY switch when mains is at zero.
Equally, most (if not all) solid state switches will not turn off until the triac current is at zero.
A flourescent lamp starter switches off mid cycle so that the ballast sees a large instant change in current. That in turn produces a large back-emf voltage which is used to 'strike' the initial arc. A zero crossing switch would upset that process unless the ballast is an electronic type.

Do neons have the same problem? I don't know.
Just remember that many solid state relays have zero crossing.
It could be a problem for you with the lights.
It should be OK for your heaters.
 
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westaust55

Moderator
Logic Level Gate Drive FETs

FET's are not a device I have played with in the past xx years.
But now starting to study up for some of my forthcoming experiments/projects.

I note that Rev Ed literature does reference parts such as the IRF530 which are not really suitable for direct logic driven - barely turn on at Vgs =5Vdc as I see it.
Rev Ed examples with the IRF530 include:
- PICAXE Manual 3 page 7,
- the EXEMPLAR STUDENT PROJECTS Fish Tank Heater

Would examples such as the IRL540 and IRL530 with Logic-Level Gate Drive
and Vgs threshhold of 1-2Vdc and good Id with Vgs in the range 3 to 5Vdc not be a better recommendation in general? :confused:

The CHI035 project board datasheet indicates that kit uses the IRL520 which has Logic-Level Gate Drive.
 
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BeanieBots

Moderator
Yes, if you are driving from a PICAXE output and NOT using PWM.
If using PWM, probably doesn't make much difference as you need to use driver anyway, so the driver determines the gate voltage rather than the PICAXE Vcc.
 
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Andrew Cowan

Senior Member
Just a note on the bog standard IRF530:

I did some tests recently, and with a gate voltage of 5V.
- IRF530, Rgs = 1.5 ohms
- Logic level FET = 0.02 ohms

I'm using three in my RGB floodlight, and with 100mA across them, they feel slightly warm and drop about 0.1V.

I'd say they are OK unheatsinked for up to about 700mA, which is about the limit of most educational battery powered projects. I have found that IRF530s seem to be more robust (temperature, static, voltage) then some of their logic level counterparts. Maybe why they're used in education?

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bratt2032

New Member
Thank you for your answers... i have much to learn about electronic and i must study before i can begin my project. Coding won't be the most difficult thing for me, but now i'm "stopped". I'll find... with your help !!! please !!!! lol :)

i've found projects to control a 12VDC (under 4A) light with pwm , but without pics and the frequency was low (about 400 hz) and i think the problem of heat is not the same at this frequency.
 
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