AC device similar to a mosfet?

On a sunny day (Sun, 30 Jan 2011 17:12:16 -0600) it happened "Jeff Johnson" wrote in :

MOSFET in a bridge rectifier. So the MOSFET drain on the +, the source on the -, and the AC on the ~ and ~.

so now I need 4 instead of 2... is that suppose to be an improvement?

On a sunny day (Sun, 30 Jan 2011 17:41:18 -0600) it happened "Jeff Johnson" wrote in :

Not if you cannot count to 2.

1 MOSFET expensive 1 Bridge rectifier very cheap

------------ +

2 components total

You saved money. Idiot.

Hello Jeff,

LDR + light source, if bandwidth of controlling signal is not of importance

Best regards,

Wim

Why use two FETs?

A fet is bi-directional. For a variable resistor, just one will do.

........... Zim

Why this outbreak of name calling? There's no need to cause hard feelings over such trivial things.

mike

On a sunny day (Mon, 31 Jan 2011 21:26:27 -0700) it happened m II wrote in :

It is not name calling, it was an objective observation, or if you prefer that word: 'measurement'.

"Graeme Zimmer" wrote in news:VLL1p.8987$ snipped-for-privacy@newsfe11.iad:

Body diode!

And, even in single quadrant operation, there is limited dynamic range.

The problem should be more completely specified :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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On a sunny day (Tue, 01 Feb 2011 13:32:32 -0700) it happened Jim Thompson wrote in :

Dunno what you mean by that. The range is from full on to completely off. There is a non-linearity issue. The simplest solution is to put the MOSFET in a diode bridge, and that bridge in series with the AC and load.

I have done that, you can add a resistor divider from drain to base to make it more linear. I sort of assume high power. But why use that? Switchers are relatively simple and do not dissipate that much, and can be made with any curve you like.

For low level AC signal level control the JFET method works too.

Absolutely. But the OP cannot count to 2 (he thinks it is 4), so that would be a big challenge.

The JFET is the classic example of an AC voltage controlled resistor.

That said, I still don't see the need to dis' the poster. Well if it's Phil, then dis away.

How about wrapping the mosfet in a bridge rectifier?

--

John Devereux

And the bridge impedance is ? ? ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

John Devereux wrote in news: snipped-for-privacy@devereux.me.uk:

If he uses a bridge then its unipolar and all you need is one FET.

It'll have two problems...

(1) Setting gate drive

(2) Diode curvature

I got the idea that the OP is trying to make a voltage variable attenuator.

Somewhere, in some recent prior posts, I've shown that the curvature compensation scheme usually applied to JFET's works equally well with power MOSFET's.

I'll see if I can resurrect that post. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

He mention's in his other post "High side active load" that its part of a test fixture for a device he does'nt specify.

I think I seen this question somewhere else and it had something to do with relays.

I thought I saw somewhere it was for 110VAC, so diode drops should not matter much, The OP wanted something similar to a mosfet so I proposed.... a mosfet :)

--

John Devereux

CdS photocells give the best dynamic range and range of control. When integrated with an LED sometimes called "analog optocouplers". e.g. Silonex.

If the range of control needed is slim to moderate and the analog signal is above a few tens of Hz, incadescent lamps are a clever solution in some situations. e.g. HP 200A. But unlike CdS this does not meet your requirement for semiconductor, or does it? :-)