Any electronics experts out there?

[npf1]

If a heating thermostat is capable of switching a max of 250VAC @ 3.5A what the max amps it could handle it 12VDC? The stat will work with DC. I need to be able to switch 1.5A @ 12VDC

If the stat isn't capable of of 1.5A @ 12V, should I use a relay or a transistor for the switching? If a transistor, any idea what type?

Hopefully there's enough info there to advise. Thanks in advance.

 

[nimbusgb]

The answer is of course 'it all depends' If it is a triac based thermostat then it wont switch DC ( or it will, once ) triacs depend on AC to switch themselves off.

If its a mechanical contact type ( with a relay or other mechanical device output ) then 12v dc at 1.5 amps no problem!

The MAX value of 250v at 3.5A is based on the mechanical characteristics of the switching device. beyond about 800w the heating of the contact could be a problem. At 12v and 1.5 amps you are passing less than 20 watts through the contacts. Well inside the parameters spec'd.

If they said it would switch DC then you are well inside the specs.

 

[npf1]

It was the manufacturers tech dept who said it would switch DC and mentioend something to do with volt free switching (which I didn't understand!).

 

[LadyInBed]

It isn't a direct correlation but it is down to power and contact sparking.
Power (watts) = V x I (amps) and sparking is proportionate to voltage level.
So using a dry contact thermostat on DC should allow for more current to pass rather than less. To switch 1.5A @ 12VDC should be no problem.

 

[npf1]

Many thanks for the replies.

 

[DaveS]

[ QUOTE ]
It was the manufacturers tech dept who said it would switch DC and mentioend something to do with volt free switching (which I didn't understand!).

[/ QUOTE ]

I'm afraid I don't understand this either. Volt free switching is, in my experience, a technique normally employed by electronic "relays" (actually solid state components) in which power is applied or interrupted coincidentally with a zero crossing of the AC voltage waveform. The advantages of this, assuming a resistive load, are fairly obvious (no inrush current, no r.f. interference, etc.), but it is not a technique which is possible with DC.

 

[Hoolie]

[ QUOTE ]
... ... volt free switching (which I didn't understand!).

[/ QUOTE ]
I think it refers to a relay contact that has no connection to the mains or 12v DC and is usually single pole double throw (SPDT) with all three connections available externally.

 

[DaveS]

Well, yes, it could mean that in other circumstances, e.g. auxiliary switching contacts. But here the OP was specifically talking about ability to switch load. If the temperature sensing device is not itself switching load then it must be connected to something else (relay, SS circuitry, whatever), that does perform the actual load switching, and it is the capability of the latter which is, presumably, the issue.

 

[William_H]

The rating of a simple contact switch is quite complicated. I might disagree with Ladyinbed in that usually a contact has a higher rating for AC than DC.
You have firstly the need for a low resistance contact which will carry the current when "made" without heating. This is not usually a problem.
The real problem is when the contact opens. You have 250vAC across the contact needing a suitable amount of insulation and gap. However with DC you have the risk of an arc forming as the contacts separate this can be a real problem if switching an inductive load. The thing is that because AC goes to zero volts and current 50 times per second an inadvertent arc tends to extinguish itself quickly whereas a DC arc can continue in the ionised air to melt everything.
So npf1 is right to ask the question and assuming it is a simple mechanical contact it will probably be OK on 1.5 amps DC. But beware of inductive loads (motor or relay)

If you want to use a transistor, use one of the high current switching N channel FET transistors.(BUZ71 MTP3055 BUK456-60A are some examples) But do fit the transistor in the negative line. Fit a high resistance from the gate to ground (100K ohm) and switch the gate to 12v positive to turn on. With 12v from gate to the source (grounded) the FET will switch to a very low resistance conductivity drain to source.
good luck olewill

 

[Anonymous]

[ QUOTE ]

I'm afraid I don't understand this either. Volt free switching is, in my experience, a technique normally employed by electronic "relays" (actually solid state components) in which

[/ QUOTE ] I think that this refers to what is known (and has been for decades) in the electrical controls industry as a "volt-free contact" . It can be CO, NO or NC but you get two or three terminals available to you and isolated from the control panel's electrics. Normally you will be told something like -- Alarm output, volt-free changeover contact 250Vac 5A, 12Vdc 5A resistive. Ideally you will also be given the contact material to determine whether you can use it for very low power signals.