Asking on how to choose proper relay

[1MK5]

Dear All,

I have a task at hand to design a controller circuit to modulate ON/OFF a high voltage function generator (sinewave 500 Vpp, f 100 kHz) via a relay.

I have never handled a relay before, and I am positng this question here to ask for your kind help to give me some suggestions.

Some of the questions I have are:

1. Is it possible to control the relay via FPGA card?
2. What are the parameters I should look for to choose the proper relay to do the job?

Many thanks for your suggestion =)

Cheers,

1MK5

 

[mdjensen22]

What is it that you are switching exactly?
Are you trying to switch on/off the output of the function generator, power to it, or some master enable input to the function generator?

Generally speaking, there are two sets of ratings for a relay. The input side and the output side.

The input side specifications will tell you what it takes to energize the relay (could be coil, LED, etc.). There will typically be ratings for turn on as well as release.

The output side specifications will tell you the kind of load the relay can switch, life expectancy, etc.. This will vary depending on relay type (solid state, reed, traditional, etc.).

There are some types of relays that can be driven from a few mA of current (solid state relays, for example), while other types of relays can require hundreds mA of current to energize. The type of relay you can use all depends on your application.

 

[1MK5]

Hi mdjensen22,

Thanks for your your very quick response. I am trying to build an application where the output of the function generator to be switched ON/OFF automatically with a very high speed.

I know that I can turn ON/OFF the F gen directly by using a PC. However, this method is very slow and makes it unsuitable for my application.

Hence, in lieu, I am thinking of using an a relay between the F gen and my chip, and control the relay using an FPGA card (the FPGA here is used to avoid communication with the PC, and hence it is hoped to bring more speed to my application).

Hence, I guess what I would like to know is if there are any relay which can be controlled via FPGA to handle the sinewave generated from the F Gen (500 Vpp, f 100 kHz).

Thanks once again for your kind advise =)

Cheers,

1MK5

 

[sophiecentaur]

If you want "really high speed" and 'clean' switching then you may be better to use something more sophisticated than a relatively crude pair of contacts that open and close. Do you need to shape the on and off envelope of your signal (to avoid unwanted 'splats')?

 

[1MK5]

Hi Sophiecenteur,

Thanks for your response. Yes, basically what I would like to from the relay can be summed into these requirements:

1. Controllability via FPGA (TTL, etc)
2. Switching speed
3. Ability to handle high voltage sinewave from the F Gen (sinewave 500 V, f 100 kHz)

Although, I am not quite sure what you mean by clean here?

Thanks once again =)

1MK5

 

[sophiecentaur]

When you turn a cw signal on and off there are 'splats' due to the fact that you are modulating it. This means extra, out of band signals. It is normal. if you don't want this, to have a 'soft' switching, which changes the level over a few cycles. This can easily be done with electronics.

 

[1MK5]

Hi Sophiecentaur,

I see, do you mean those high frequency components which usually appears at the ON/OFF transition?

If this is what you mean by splats, I guess I am not too worry about them for my application now. =)

Cheers,

1MK5

 

[sophiecentaur]

As long as interference generation is not a problem then the cheapest / easiest solution is the one to go for.
btw, is it not possible to control your function generator directly to achieve what you want?

 

[1MK5]

Hi Sophiecenteaur,

Well...I know that I can control my F Gen directly via PC...but, this method is too slow...so that's why I am thinking of modulating the output with the help of a relay.

Hence, what I need is to find a relay that (1) can be controlled digitally (via FPGA in my case), (2) capable to switch ON/OFF quickly (is it possible to get to 1 ms or less?), and (3) capable to handle high voltage sinewave (500 V, f 100 kHz) coming from the generator...

...and my problem is that I have never handled a relay before, so I would like to know what type of relay suitable for my requirements, and what are the parameters I should consider to choose one...

Thanks for your reply =)

 

[sophiecentaur]

This is getting less and less trivial. A relay contact could do a lot of sparking if you're trying to break 500V (even AC).
What actual Power is involved - i.e. 500V at what current or into what load-?
A bit about the actual application could help. Relays are essentially for low frequency / DC switching and this is an unusual situation. You could always just try one and see how it performs. It would need to be specified for hundreds of volts switching, of course, so a reed realy, which would have been an obvious choice for lower voltage operation, may not do the job. How fast (what repetition rate) do you need to switch at?

 

[dlgoff]

Like sophiecentaur has said, "A relay contact could do a lot of sparking..."

Contact bounce (also called chatter) is a common problem with mechanical switches and relays. Switch and relay contacts are usually made of springy metals that are forced into contact by an actuator. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electric current instead of a clean transition from zero to full current. The effect is usually unimportant in power circuits, but causes problems in some analogue and logic circuits that respond fast enough to misinterpret the on-off pulses as a data stream.[5]

http://en.wikipedia.org/wiki/Switch_bounce#Contact_bounce"

 

[1MK5]

Hi Sophiecenteaur and digoff,

Thanks for all of your opinion =)

Alright, my application is electrokinetics in which I am trying to control the movement of a particle suspended in liquid media (both are dielectrics).

Hence, you can expect the impedance of the system to be quite high, and correspondingly, the current to be quite low (I do not have any figure how much of these currently).

The reason I want to have quick response is because I want the system to be triggered by certain event, and respond to that event very quickly. Hence, this is the reason why I need short response time.

As for the repetition rate, the application requires only intermittent switching: the AC bias is OFF in normal times --> when a certain event is happening, apply the AC bias for, say, 2 ms --> turn OFF AC bias again, until the next event triggers the system. The ON/OFF transition should takes place 2 ms or less. (I guess, now you can see why I can not have the PC to control the generator, because this method would takes forever for this kind of application).


Judging from your advises, it seems that a relay may not be a wise option in the first place. I am open to explore all possibilities (I have quite a bit tolerance window for the AC bias parameter to play around with). Or, alternatively, I would be very thankful if you could suggest me a device which could turn ON/OFF AC bias instead of a relay.

Many thanks,

1MK5

 

[dlgoff]

I did a google search = fast switching high voltage transistors and this is the first one on the list.

http://www12.fairchildsemi.com/ds/FJ/FJD5304D.pdf"

However the maximum Collector-Emitter Voltage is only 400volts. But there are others in the list.

 

[sophiecentaur]

But, if you use a suitable transformer, you can operate your transistor at a much more reasonable voltage. This is really not a problem - except that it's unfamiliar to you. The solution just may not be what you expected.

 

[dlgoff]

Good point sophiecentaur.