# Designing signal conditioner to convert current source to voltage output

I've seen a lot of signal conditioners online for converting a 4-20 mA current loop signal to a 0-10 voltage output. However, they seem extremely expensive for the task they are achieving. Are the high costs due to protection against spikes in the input/other extreme events?

• packaging , volume and distributor [profits] make all the difference – Tony Stewart Sunnyskyguy EE75 Jul 20 at 15:24

Are the high costs due to protection against spikes in the input/other extreme events?

Yes, this would be a standard expectation in industrial control systems. Many also include isolation between the input and output signals and this will push the cost up significantly. This eliminates ground loops and allows, for example, the PLC to be electrically isolated from the sensor loop.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Conversion doesn't have to be expensive.

If you can tolerate 2 - 10 V from the conversion then a 500 Ω precision resistor will do the job. These are available in 0.1% tolerance from the big suppliers such as RS.

Just as the 4 mA gives a "live zero" (which means that you can tell the difference between a sensor giving a zero indication as opposed to a sensor that has become disconnected) the 2 - 10 V can be used in the same way if you can set the scale and span of the input you are feeding.

• Would the drop with a precision resistor be sufficient to drive a solid state relay's input? I wasn't sure how an applied voltage works with current draw for something like an opto-isolator when there is a current source instead of voltage source – coolro Jul 20 at 15:39
• The circuit is not reversible for V to I but I to V on a PCB costs~\$4 wiki.dfrobot.com/… then V to I could be similar – Tony Stewart Sunnyskyguy EE75 Jul 20 at 16:07
• @coolro: A solid state relay is an on-off device. You don't drive it directly with a 4 - 20 mA signal (or 0 - 10 V either). That's not how SSRs are designed to be operated. What are you really trying to do? By SSR do you mean a dimmer-style AC phase-angle controller? – Transistor Jul 20 at 16:14
• @Transistor I have a 4-20 mA output on my controller that I'd like to use to control a random-fire SSR. I don't intend to use the whole range of the 4-20 output just min and max values to turn the SSR on and off – coolro Jul 20 at 16:24
• Then you need a major edit to your question. It has little to do with the problem tou are trying to solve and my answer is irrelevant. – Transistor Jul 20 at 16:54