Voltage limiter for 1v DC and 20v/50v AC - Page 1
Forum section: electronics / projects

author: cowasaki
author_rank: Frequent Contributor
timestamp: January 16, 2020, 07:14:15 am
content: I’m building a device which will have three gauges on it to measure inputs.A - DC output normal range should be very small but it there is a fault with the device a larger value could appear. A 1v DC gauge would be more than enough for a working device.How can I let up to 1v through without changing it but limit this to 1v to avoid damage to the gauge.B - AC output likewise I need to limit an AC voltage without changing it. The limit here is 20 or 50vWhat would you suggest?I don't mind some roll off at the top end as after 0.5v we are just talking about it being faulty so 0.7 or 0.9 doesn't matter it's still too high.I would prefer the AC voltages to remain as accurate as they can be.Thanks all.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2020, 09:01:48 am
content: The usual way to limit analog meter movements is with a pair of diodes parallel to the movement (possibly with some of the series resistance). Normally there is some series resistor anyway to set the voltage range - this may need to be high enough power rating. If critical one could include a PTC or other current limiting, but this is not standard.The normal meter movements can tolerate quite a bit more than full scale current. So even if made for some 100 µA and 0.1 V it can very well survive 1 mA/1V and possibly even10 mA (and still only 100 mW).For AC movements things can get a little more complicated, as moving iron movements (often nonlinear scale) tend to use more power and thus may not have as much head room for overload. The idea is still the same: limit the voltage with something like diodes and limit the current through the diode with a resistor, PTC or if need electronic (e.g. 2 depletion mode MOSFETs).

author: cowasaki
author_rank: Frequent Contributor
timestamp: January 16, 2020, 10:08:16 am
content: Quote from: Kleinstein on January 16, 2020, 09:01:48 amSo on the 1v DC meter put a diode parallel to the meter. Would that affect the reading? I’m only really bothered by the first half volt or so. I can’t find any information about maximum voltage for the 1v meter but they do 5, 10, 20, 50v versions.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2020, 11:34:16 am
content: The normal moving coil DC movements itself are for some 100 µA and a low voltage like 100 or 200 mV. For voltage reading meters there is a series-resistor. The limiting diodes would be only in parallel to the actual movement and maybe a small fraction of the resistance. The main part of the resistor that sets the range is still in series and would limit the current. Usually the diodes directly parallel to the movement are not effecting the reading as the voltage is low (less than some 300 mV) and sufficient limit to the movement.It depends on the range and exact type how much voltage the resistor can stand. This may not be that much for a 1 V meter. With the 100 µA example and a 1 V range the resistor would be 10 K (minus some 1 K for the movement). So 10 V would result in some 1 mA and 10 mW which should be OK. 100 V would likely be too much for more than a brief moment.Is there a picture or more data for the meters ?

author: cowasaki
author_rank: Frequent Contributor
timestamp: January 16, 2020, 01:34:54 pm
content: https://www.ebay.co.uk/itm/DC-0-600V-Analog-Volt-Voltage-Needle-Panel-Meter-Voltmeter-Gauge-85C1-UK-Seller/372856563246?hash=item56cffba62e:m:mjaq6DiyLqOK6TfQmFaZcYAIt’s not expensive but the idea is that I have three of these meters to show DC voltage, AC voltage and current (via 1ohm resistor) relating to an amps output. The machine I’m building has shielded banana sockets on the rear for output to a more accurate DMM when I need to.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2020, 02:34:12 pm
content: For the 1 V meter it is not clear if one has easy access to the very likely internal series resistor. Protection gets a lot easier (and less effect on the reading) with that access.A 1 Ohms resistor to also read 0-1 A could be tricky for 2 reasons. The first is that at only 1 V the 1 Ohms resistor would have quite some effect on the current. The 2nd point is that 1 W of power at the shunt would need high power shunt to keep the self heating effect at bay. Even with only the not so accurate needle this could be a a slight problem, as power resistors tend get some 200 K of temperature rise at full power. So possibly a few percent of change in resistance. Normally there are similar meters with some 75 or 100 mV full scale for current measurment - just keep in mind they may not include the shunt.The simple movements are for DC only. AC reading would need a rectifier. Just a passive rectifier would not be linear. So with a linear scale it needs an active precision rectifier with additional supply.

author: cowasaki
author_rank: Frequent Contributor
timestamp: January 16, 2020, 03:04:50 pm
content: Quote from: Kleinstein on January 16, 2020, 02:34:12 pmThe DC measurement is across the speaker. The 1ohm resistor is to measure the current from the amp using a volt meter. I use that method already and it works well. I use 2 x 1 ohm resistors then a 2, 4 & 8 allowing me to simulate a 4, 8 or 16 ohm speaker (or 2!). Then another AC volt meter across the speaker out for output voltage. With the output voltage and the known resistance I can show the power output. The DC voltage is to check for its existence as there shouldn’t be any/much at all. In reality I could just have an LED to light when there is more than there should be but I like the analogue meters.