Circuits FAQ

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Friday, June 10, 2016

SSR Open Circuit Leakage Voltage

hi been trying to use a dc ssr relay to switch 24 volt supply on and off for a unit i use that needs to see 0v or 24v but when the switch voltage is applied there is still 18v showing on the load side not zero ..does the ssr have to have a load on it to fully switch off and on from 0v to 24v ?

Mail from PST in UK

You are right you need a bleeder resistor of 10k or 100k to ensure the collector leakage current of SSR is bypassed to make you see a '0' when the SSR is off. If the SSR is working well, and no input or '0' control input then SSR output is high impedance and only a collector or diode leakage will hold the SSR output at the Full Voltage but it cannot drive any current. Hence a bleeder across output will show you a '0'. Use it if the bleeder does not cause any problems.

Solid State Relays or SSR

If you were driving a 5A Solenoid, then you would not see this problem. If you are driving a Solid State Load, such effects are natural. It is because the Solid State Load like a Electronic Circuit will draw current only on demand.

These instruction is for a Normally Open Output, NO. A Normally Closed NC is vice versa. And also some Special SSRs may have sensitive inputs, So a low impedance '0v' at input has to be ensured.

The Bleeder should bypass the leakage current, so one end of bleeder goes to SSR output and other end of bleeder goes to the ground or 0V point of your circuit which is controlled by SSR.


They are called SSR as there are made of semiconductors and have no moving parts to wear out. They do generate heat depending on load and switching. Thyristors are used in most as they are rugged, mosfets and IGBT may also be used for various applications.

Solid State Relays or SSR

 In this section you will find SSR designs and PCB layouts which you can study. These were made and sold  by me years ago. So the PCB layouts have gone thru some revisions and they worked quite well.

Wednesday, June 01, 2016

LM3914 meter for 25 V DC

"I wish to assemble a bargraph display to indicate approximately 25volts dc.How many LM3914 I should cascade? How to calculate resistor value which is grounded from the junction of Pin 6 & 7 (Rhi &Ref.out)? Assuming i need to cascade 3 LM3914s,what is the method to cascade these ics? The display need not be precise or very bright. I hope you can help me out."

Mains Voltage monitor using LM3914

"I shall be very grateful. Can we do it in such a way that the first LM3914 displays 1-10 v & second 11-20v & the third 21-30v?"

Thank you again - Mail from AD

Reply from delabs

Have a look at the TI application note which has many examples. Understanding this will help you make your own solution.

LM3914 - Dot/Bar Display Driver

20-Segment Meter with Mode Switch

20-Segment Meter with Mode Switch

Analog and 3-1/2 digital display

Cascading is required if you want more resolution. Example for measuring 20V, you want to show in 1V per LED steps. It can withstand +/- 35 Volts input upper limit.

The inbuilt reference is 1.2V use it as shown in the Typical Application. This also helps in setting the brightness of LED display. You may need to use external attenuators, buffers or precision  rectifiers to measure AC voltages or High Level DC voltages.